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Anatomical Position & Directional Terms

Standard anatomical position (standing, facing forward, arms at sides, palms forward) is the reference point for all directional terminology in anatomy.
  • Superior (toward the head) vs inferior (toward the feet)
  • Anterior/ventral (front) vs posterior/dorsal (back)
  • Medial (toward the midline) vs lateral (away from the midline)
  • Proximal (closer to trunk) vs distal (farther from trunk), used for limbs
  • Superficial (near the surface) vs deep (farther from the surface)
  • Palms face forward in anatomical position, not at the sides as in a relaxed stance

Body Planes and Sections

Planes are imaginary flat surfaces used to describe cuts or views through the body.
  • Sagittal plane divides the body into left and right parts
  • Midsagittal (median) plane creates exactly equal left and right halves
  • Frontal (coronal) plane divides the body into anterior and posterior parts
  • Transverse (horizontal/axial) plane divides the body into superior and inferior parts
  • CT and MRI scans are typically read as transverse sections

Body Cavities and Membranes

The body's internal organs are housed in cavities lined by protective membranes.
  • Dorsal cavity contains the cranial cavity (brain) and vertebral canal (spinal cord)
  • Ventral cavity contains the thoracic cavity and abdominopelvic cavity
  • The diaphragm separates the thoracic cavity from the abdominopelvic cavity
  • Thoracic cavity contains two pleural cavities (lungs) and the mediastinum (heart, esophagus, trachea)
  • Serous membranes (pleura, pericardium, peritoneum) reduce friction with a thin fluid layer between parietal and visceral layers

Levels of Structural Organization

Anatomy studies structure at increasing levels of complexity, from atoms to the whole organism.
  • Chemical level: atoms combine into molecules (e.g., DNA, proteins)
  • Cellular level: the basic structural and functional unit of life
  • Tissue level: groups of similar cells performing a common function
  • Organ level: two or more tissue types working together (e.g., the heart)
  • Organ system level: organs working together for a common purpose (e.g., cardiovascular system)
  • Organismal level: all systems functioning together as a whole living being

Homeostasis and Feedback Loops

Homeostasis is the body's ability to maintain a stable internal environment despite external changes.
  • Negative feedback loops reverse a change and are the most common regulatory mechanism (e.g., blood glucose regulation by insulin)
  • Positive feedback loops amplify a change until a specific endpoint is reached (e.g., labor contractions via oxytocin)
  • A feedback loop has three components: receptor (detects stimulus), control center (processes information), and effector (produces the response)
  • Failure of homeostatic control contributes to disease states

The Four Primary Tissue Types

All organs are built from combinations of four basic tissue types, each with a distinct structure and function.
  • Epithelial tissue covers body surfaces and lines cavities/organs; it is avascular and regenerates quickly
  • Connective tissue supports, binds, and protects (e.g., bone, cartilage, blood, adipose tissue)
  • Muscle tissue contracts to produce movement (skeletal, cardiac, smooth)
  • Nervous tissue generates and transmits electrical impulses (neurons and neuroglia)
'Anterior' and 'ventral' are synonyms in humans, not opposites
A sagittal cut does not have to be down the midline — only the midsagittal plane splits the body into exact mirror halves
The stomach is an organ, not a tissue — it is made of all four tissue types working together
Positive feedback is not 'bad'; it is a normal, self-limiting mechanism (e.g., childbirth, blood clotting), not a runaway malfunction

Bone Structure and Classification

Bones are classified by shape and built from distinct tissue layers that combine strength with light weight.
  • Long bones (femur, humerus) have a diaphysis (shaft) and epiphyses (ends)
  • Short bones (carpals, tarsals) are roughly cube-shaped and provide stability with limited motion
  • Flat bones (skull, sternum, scapula) protect organs and provide broad muscle attachment surfaces
  • Irregular bones (vertebrae, facial bones) have complex shapes that fit specific functions
  • Compact bone forms the dense outer layer; spongy (cancellous) bone forms the lighter, porous interior
  • The medullary cavity of long bones stores yellow bone marrow (fat) in adults

Axial Skeleton

The axial skeleton forms the body's central axis and protects the brain, spinal cord, and thoracic organs.
  • The skull consists of 8 cranial bones and 14 facial bones
  • The vertebral column has 33 vertebrae grouped as 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4 fused coccygeal
  • The rib cage includes 12 pairs of ribs, the sternum, and thoracic vertebrae
  • True ribs (1-7) attach directly to the sternum via costal cartilage; false ribs (8-12) do not attach directly
  • The hyoid bone is unique because it does not articulate with any other bone

Appendicular Skeleton

The appendicular skeleton includes the limbs and the girdles that attach them to the axial skeleton, enabling movement.
  • The pectoral (shoulder) girdle consists of the clavicle and scapula
  • The upper limb includes the humerus, radius, ulna, carpals, metacarpals, and phalanges
  • The pelvic girdle consists of the fused hip bones (ilium, ischium, pubis), which attach to the sacrum
  • The lower limb includes the femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges
  • The femur is the longest and strongest bone in the human body

Joints (Articulations)

Joints are classified by the tissue connecting the bones and by the degree of movement they allow.
  • Fibrous joints (e.g., skull sutures) are immovable (synarthroses)
  • Cartilaginous joints (e.g., between vertebrae, pubic symphysis) allow slight movement (amphiarthroses)
  • Synovial joints (e.g., knee, shoulder) are freely movable (diarthroses) and contain synovial fluid within a joint capsule
  • Types of synovial joints include hinge (elbow, knee), ball-and-socket (shoulder, hip), pivot (atlas-axis), and saddle (thumb)
  • Ligaments connect bone to bone and stabilize joints

Bone Growth, Remodeling, and Repair

Bone is a dynamic, living tissue that grows, remodels, and heals throughout life through the coordinated action of specialized cells.
  • Osteoblasts build new bone matrix; osteocytes maintain mature bone; osteoclasts break down (resorb) bone tissue
  • Longitudinal (lengthwise) bone growth occurs at the epiphyseal plate (growth plate) during childhood and adolescence
  • Ossification (osteogenesis) is the process of bone formation, occurring via intramembranous (flat bones) or endochondral (most other bones) pathways
  • Bone remodeling continuously replaces old bone tissue in response to mechanical stress and calcium needs
  • Fracture repair proceeds through a hematoma, fibrocartilaginous callus, bony callus, and final remodeling

Skeletal Functions and Common Disorders

Beyond providing structure, the skeleton performs critical physiological roles and is subject to specific pathologies.
  • Functions include support, protection, movement (via muscle attachment), mineral storage (calcium, phosphorus), and blood cell formation (hematopoiesis in red marrow)
  • Osteoporosis is a decrease in bone density/mass that increases fracture risk, common in postmenopausal women due to lower estrogen
  • Osteoarthritis involves the breakdown of articular cartilage at joints, causing pain and stiffness
  • Rickets (children) and osteomalacia (adults) result from vitamin D deficiency causing softened bones
  • Scoliosis is an abnormal lateral curvature of the spine
Cartilage is not bone — cartilage lacks blood vessels and calcified matrix, while bone is highly vascularized and mineralized
The epiphyseal plate is cartilage, not bone, until it fully ossifies (closes) after puberty, ending height growth
Ligaments connect bone to bone; tendons connect muscle to bone — these are commonly reversed by students
Not all joints are synovial (freely movable) — sutures of the skull are fibrous and immovable, not 'stiff synovial joints'

Types of Muscle Tissue

The body contains three distinct types of muscle tissue, each suited to a specific function and control mechanism.
  • Skeletal muscle is voluntary, striated, and multinucleated; it attaches to bone via tendons
  • Cardiac muscle is involuntary, striated, and found only in the heart; cells are connected by intercalated discs
  • Smooth muscle is involuntary, non-striated, and lines hollow organs (intestines, blood vessels, bladder)
  • Intercalated discs contain gap junctions that allow electrical signals to spread rapidly between cardiac cells
  • Skeletal muscle fibers are multinucleated because they form from the fusion of many myoblasts during development

Skeletal Muscle Structure

Skeletal muscle is organized hierarchically from whole muscle down to the contractile proteins that generate force.
  • A muscle is made of fascicles, which are bundles of individual muscle fibers (cells)
  • Each muscle fiber contains many myofibrils, composed of repeating sarcomeres
  • The sarcomere is the basic functional contractile unit, bounded by Z-discs
  • Thick filaments are made of myosin; thin filaments are made of actin
  • The sarcoplasmic reticulum stores calcium ions needed for muscle contraction
  • Connective tissue wrappings include the epimysium (whole muscle), perimysium (fascicles), and endomysium (individual fibers)

The Sliding Filament Mechanism

Muscle contraction occurs when thin filaments slide past thick filaments, shortening the sarcomere without the filaments themselves changing length.
  • A nerve impulse triggers acetylcholine release at the neuromuscular junction
  • Acetylcholine binding causes depolarization and release of calcium from the sarcoplasmic reticulum
  • Calcium binds troponin, shifting tropomyosin to expose myosin-binding sites on actin
  • Myosin heads bind actin, forming cross-bridges and pulling thin filaments toward the sarcomere's center (power stroke)
  • ATP is required to detach myosin from actin, allowing the cycle to repeat or the muscle to relax
  • During contraction, the H zone and I band narrow, but the A band length stays the same

Muscle Metabolism and Fatigue

Muscles generate ATP through multiple pathways depending on the intensity and duration of activity.
  • The phosphocreatine system provides immediate, short-term ATP for the first ~10-15 seconds of intense activity
  • Anaerobic glycolysis produces ATP quickly without oxygen but yields lactic acid as a byproduct, useful for short bursts
  • Aerobic (cellular) respiration produces the most ATP but requires oxygen and is used during sustained, lower-intensity activity
  • Muscle fatigue results from ATP depletion, lactic acid accumulation, and ion imbalances
  • Slow-twitch (Type I) fibers are fatigue-resistant and rely on aerobic metabolism; fast-twitch (Type II) fibers fatigue quickly but generate more power

Muscle Actions and Terminology

Muscles work in coordinated groups, and their names often describe their location, shape, or action.
  • The prime mover (agonist) is the main muscle producing a movement; the antagonist opposes/reverses it
  • Synergists assist the prime mover; fixators stabilize the origin bone so force is applied effectively
  • The origin is the fixed attachment point; the insertion is the movable attachment point that moves toward the origin during contraction
  • Muscle naming conventions include location (biceps brachii), shape (deltoid = triangular), size (maximus/minimus), and number of origins (biceps = two heads)
  • Isotonic contractions change muscle length (concentric = shortening, eccentric = lengthening); isometric contractions produce tension without changing length

Major Muscle Groups and Disorders

Understanding major superficial muscles and common muscular disorders connects structure to real-world function and pathology.
  • Major muscles include the biceps brachii/triceps brachii (elbow flexion/extension), quadriceps/hamstrings (knee extension/flexion), deltoid (shoulder abduction), and gastrocnemius (plantarflexion)
  • The diaphragm is the primary muscle of respiration
  • Muscular dystrophy is a group of genetic disorders causing progressive muscle weakness and degeneration
  • Myasthenia gravis is an autoimmune disease that attacks acetylcholine receptors, causing muscle weakness and fatigue
  • Tetanus (the sustained contraction, not the disease) occurs when stimulation frequency is high enough that the muscle cannot relax between twitches
Muscles only pull, they never push — movement in the opposite direction requires an antagonist muscle, not the same muscle reversing
Rigor mortis occurs because of a LACK of ATP after death, preventing myosin from detaching from actin — not from excess calcium alone
The sarcomere shortens during contraction, but the individual thick and thin filaments do NOT shorten — they slide past each other
'Muscle tone' refers to a constant, low-level partial contraction at rest, not muscle size or strength

Neuron Structure and Function

Neurons are the specialized cells that generate and transmit electrical and chemical signals throughout the nervous system.
  • Dendrites receive incoming signals and conduct them toward the cell body
  • The cell body (soma) contains the nucleus and organelles that maintain the cell
  • The axon conducts electrical impulses away from the cell body toward the axon terminals
  • The myelin sheath, produced by Schwann cells (PNS) or oligodendrocytes (CNS), insulates axons and speeds signal conduction
  • Nodes of Ranvier are gaps in the myelin sheath where saltatory conduction allows the impulse to 'jump,' greatly increasing speed
  • Neuroglia (glial cells) support, protect, and nourish neurons but do not transmit impulses themselves

The Action Potential

An action potential is a rapid, temporary reversal of membrane voltage that allows neurons to transmit signals over long distances.
  • Resting membrane potential is about -70 mV, maintained by the sodium-potassium pump and selective membrane permeability
  • Depolarization occurs when voltage-gated sodium channels open, allowing Na+ to rush into the cell
  • Repolarization occurs when sodium channels close and voltage-gated potassium channels open, allowing K+ to exit
  • The action potential is 'all-or-none' — it either fires at full strength once threshold (~-55 mV) is reached, or it does not fire at all
  • The refractory period ensures the action potential travels in one direction and limits the maximum firing rate of a neuron

Synaptic Transmission

Neurons communicate with each other and with effector cells across synapses, most commonly using chemical neurotransmitters.
  • An action potential reaching the axon terminal triggers voltage-gated calcium channels to open
  • Calcium influx causes synaptic vesicles to release neurotransmitter into the synaptic cleft via exocytosis
  • Neurotransmitter binds receptors on the postsynaptic membrane, causing excitatory (depolarizing) or inhibitory (hyperpolarizing) effects
  • Common neurotransmitters include acetylcholine, dopamine, serotonin, norepinephrine, and GABA
  • Neurotransmitter is cleared from the synapse by reuptake, enzymatic breakdown, or diffusion, ending the signal

Central Nervous System (CNS)

The CNS, consisting of the brain and spinal cord, integrates sensory information and coordinates the body's responses.
  • The cerebrum, the largest brain region, is divided into frontal, parietal, temporal, and occipital lobes, each with specialized functions
  • The cerebellum coordinates balance, posture, and fine motor movement
  • The brainstem (midbrain, pons, medulla oblongata) controls vital, automatic functions like heart rate and breathing
  • The hypothalamus regulates homeostasis (temperature, hunger, thirst) and links the nervous and endocrine systems via the pituitary gland
  • The spinal cord conducts signals between the brain and body and mediates reflexes independently of the brain
  • Meninges (dura mater, arachnoid mater, pia mater) and cerebrospinal fluid protect and cushion the CNS

Peripheral Nervous System (PNS)

The PNS connects the CNS to the rest of the body and is divided into sensory and motor divisions with further subdivisions.
  • The sensory (afferent) division carries information from receptors toward the CNS
  • The motor (efferent) division carries commands from the CNS to muscles and glands
  • The somatic nervous system controls voluntary movement of skeletal muscle
  • The autonomic nervous system controls involuntary functions and is divided into sympathetic and parasympathetic divisions
  • The sympathetic division triggers 'fight-or-flight' responses (increased heart rate, dilated pupils, redirected blood flow)
  • The parasympathetic division promotes 'rest-and-digest' functions (decreased heart rate, increased digestion)

Reflexes and Neurological Disorders

Reflexes are rapid, automatic responses that protect the body, and disruptions to neural structures cause characteristic disorders.
  • A reflex arc includes a receptor, sensory neuron, integration center (often the spinal cord), motor neuron, and effector
  • The patellar (knee-jerk) reflex is a classic example of a monosynaptic reflex arc
  • Multiple sclerosis (MS) is an autoimmune disease that destroys myelin in the CNS, slowing nerve conduction
  • Parkinson's disease results from the loss of dopamine-producing neurons, causing tremors and movement difficulty
  • Alzheimer's disease involves progressive neuron loss and is associated with amyloid plaques and neurofibrillary tangles
Nerve impulses are electrical within the neuron but chemical between neurons at the synapse — not electrical the entire way
The action potential does NOT get bigger with a stronger stimulus — it is all-or-none; a stronger stimulus increases firing frequency, not amplitude
The sympathetic division does not 'turn off' the parasympathetic division and vice versa — both often act simultaneously with one usually dominating
Neuroglia (glial cells) are far more numerous than neurons in the brain, but they do not directly generate or transmit nerve impulses

Heart Structure and the Cardiac Cycle

The heart is a four-chambered muscular pump that circulates blood through the body via a precisely coordinated sequence of contractions.
  • The heart has two atria (receiving chambers) and two ventricles (pumping chambers)
  • The right side pumps deoxygenated blood to the lungs (pulmonary circuit); the left side pumps oxygenated blood to the body (systemic circuit)
  • Atrioventricular (AV) valves (tricuspid, mitral/bicuspid) prevent backflow from ventricles to atria
  • Semilunar valves (pulmonary, aortic) prevent backflow from arteries back into the ventricles
  • The cardiac cycle consists of systole (contraction) and diastole (relaxation) of the heart chambers
  • The left ventricle has the thickest wall because it must generate enough pressure to pump blood throughout the entire body

Cardiac Conduction System

The heart's rhythmic contraction is coordinated by a specialized electrical conduction system, allowing it to beat independently of the nervous system.
  • The sinoatrial (SA) node, located in the right atrium, is the heart's natural pacemaker
  • The electrical signal spreads through the atria, causing atrial contraction, then reaches the atrioventricular (AV) node
  • The AV node briefly delays the signal, allowing the atria to finish emptying before ventricular contraction
  • The signal travels down the Bundle of His and Purkinje fibers, triggering coordinated ventricular contraction
  • An electrocardiogram (ECG/EKG) records this electrical activity; the P wave, QRS complex, and T wave correspond to atrial depolarization, ventricular depolarization, and ventricular repolarization

Blood Vessels and Blood Composition

Blood travels through a closed system of vessels, and blood itself is a specialized connective tissue with distinct cellular and liquid components.
  • Arteries carry blood away from the heart and have thick, muscular, elastic walls to withstand high pressure
  • Veins carry blood toward the heart and contain valves to prevent backflow against gravity
  • Capillaries are single-cell-thick vessels where gas, nutrient, and waste exchange occurs
  • Blood consists of plasma (55%, mostly water) and formed elements (45%): red blood cells, white blood cells, and platelets
  • Red blood cells (erythrocytes) contain hemoglobin, which binds and transports oxygen
  • White blood cells (leukocytes) fight infection; platelets (thrombocytes) are essential for blood clotting

Blood Types and Clotting

Blood type compatibility and the clotting cascade are critical clinical concepts governed by antigens on red blood cells and a cascade of clotting factors.
  • The ABO blood group system is based on A and B antigens present or absent on red blood cell surfaces
  • Type O negative is the universal donor; type AB positive is the universal recipient
  • The Rh factor (positive or negative) is an additional antigen that must be matched, especially important in pregnancy
  • Hemostasis (clot formation) involves vascular spasm, platelet plug formation, and the coagulation cascade producing a fibrin mesh
  • Vitamin K is required for the liver to synthesize several clotting factors

Respiratory Structures and Ventilation

The respiratory system moves air into and out of the lungs and provides the surface for gas exchange with the blood.
  • Air passes through the nasal cavity/pharynx, larynx, trachea, bronchi, bronchioles, and into alveoli
  • The epiglottis prevents food from entering the trachea during swallowing
  • Alveoli are tiny air sacs surrounded by capillaries where gas exchange occurs across a thin respiratory membrane
  • Inhalation occurs when the diaphragm contracts and flattens, increasing thoracic volume and decreasing pressure, drawing air in
  • Exhalation is typically passive, occurring as the diaphragm relaxes and the elastic lungs recoil
  • Surfactant, produced by cells in the alveoli, reduces surface tension and prevents alveolar collapse

Gas Exchange, Transport, and Disorders

Oxygen and carbon dioxide move by diffusion along pressure gradients, and disruptions to this system cause well-known respiratory and cardiovascular diseases.
  • External respiration: oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli
  • Internal respiration: oxygen diffuses from blood into tissues; carbon dioxide diffuses from tissues into blood
  • Most oxygen is transported bound to hemoglobin; most carbon dioxide is transported as bicarbonate ions in plasma
  • The medulla oblongata controls the basic rhythm of breathing, primarily responding to blood CO2 (and pH) levels
  • Atherosclerosis (arterial plaque buildup) and hypertension increase the risk of heart attack and stroke
  • Asthma causes bronchiole constriction and inflammation; COPD (emphysema/chronic bronchitis) causes progressive, largely irreversible airflow limitation
Arteries do not always carry oxygenated blood — the pulmonary arteries carry deoxygenated blood to the lungs; it is direction (away from heart), not oxygen content, that defines an artery
The AV node does not initiate the heartbeat — the SA node is the primary pacemaker; the AV node only relays and delays the signal
Inhalation is active (requires diaphragm contraction); quiet exhalation is passive (elastic recoil), not muscular effort
Blood 'type O' still has antigens for the immune system to consider (the Rh factor) — 'universal donor' refers specifically to ABO/Rh compatibility, not total absence of all markers

Overview of the Digestive Tract

The digestive system is a long muscular tube (the alimentary canal) plus accessory organs that mechanically and chemically break down food for absorption.
  • The alimentary canal runs: mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus
  • Accessory organs (salivary glands, liver, gallbladder, pancreas) secrete substances into the canal but food does not pass through them
  • Peristalsis is the wave-like smooth muscle contraction that propels food through the digestive tract
  • Digestion includes both mechanical breakdown (chewing, churning) and chemical breakdown (enzymes)
  • The four layers of the GI tract wall, from innermost to outermost, are mucosa, submucosa, muscularis, and serosa

Mouth, Esophagus, and Stomach

Digestion begins in the mouth and continues through the esophagus into the stomach, where food is churned and chemically broken down.
  • Salivary amylase in saliva begins the chemical digestion of starches in the mouth
  • The esophagus uses peristalsis to move the food bolus to the stomach; the lower esophageal sphincter prevents reflux
  • The stomach secretes hydrochloric acid (HCl) and pepsinogen, which is activated to pepsin to begin protein digestion
  • Mucus-secreting cells protect the stomach lining from its own acidic, enzymatic environment
  • Food leaves the stomach as a semi-liquid mixture called chyme, released gradually into the small intestine via the pyloric sphincter

Small Intestine and Accessory Organs

Most chemical digestion and virtually all nutrient absorption occur in the small intestine, with essential help from the liver, gallbladder, and pancreas.
  • The small intestine has three sections: duodenum, jejunum, and ileum
  • Villi and microvilli dramatically increase the small intestine's surface area for nutrient absorption
  • The liver produces bile, which emulsifies (breaks up) fats to aid digestion by lipase
  • The gallbladder stores and concentrates bile, releasing it into the duodenum when fatty food is present
  • The pancreas secretes digestive enzymes (amylase, lipase, proteases) and bicarbonate to neutralize stomach acid entering the duodenum
  • The pancreas also has an endocrine role, producing insulin and glucagon from the islets of Langerhans

Large Intestine and Elimination

The large intestine completes the digestive process by absorbing water and electrolytes and compacting waste for elimination.
  • The large intestine (colon) absorbs water and electrolytes from indigestible food matter
  • Beneficial gut bacteria in the colon synthesize some vitamins (like vitamin K) and ferment undigested material
  • Sections of the colon include the ascending, transverse, descending, and sigmoid colon, ending at the rectum and anus
  • Feces consist of indigestible fiber, bacteria, and water
  • The internal anal sphincter is involuntary; the external anal sphincter is under voluntary control

Kidney Structure and Urine Formation

The kidneys filter blood to remove wastes and regulate fluid and electrolyte balance through the functional unit called the nephron.
  • The nephron is the functional unit of the kidney, consisting of the glomerulus and renal tubule
  • Glomerular filtration: blood pressure forces water, ions, and small molecules from the glomerulus into Bowman's capsule
  • Tubular reabsorption: needed substances (glucose, water, ions) are reabsorbed from the filtrate back into the blood, mostly in the proximal convoluted tubule
  • Tubular secretion: additional wastes and excess ions are actively secreted from blood into the filtrate
  • The loop of Henle creates a concentration gradient in the kidney medulla that allows the kidney to concentrate urine
  • Antidiuretic hormone (ADH) increases water reabsorption in the collecting duct, concentrating urine when the body needs to conserve water

The Urinary System and Digestive/Excretory Disorders

Beyond the kidneys, the urinary system stores and eliminates urine, and both systems are subject to well-known clinical conditions.
  • Urine travels from the kidneys through the ureters to the bladder, then out through the urethra
  • The kidneys also regulate blood pressure (via renin), blood pH, and red blood cell production (via erythropoietin)
  • Gastroesophageal reflux disease (GERD) occurs when the lower esophageal sphincter fails to prevent stomach acid from entering the esophagus
  • Peptic ulcers are erosions in the stomach or duodenal lining, often caused by H. pylori infection or NSAID overuse
  • Kidney stones form when minerals crystallize in the urinary tract, and chronic kidney disease reflects progressive, often irreversible loss of nephron function
The gallbladder does not produce bile — the liver produces bile; the gallbladder only stores and concentrates it
'Stomach acid digests protein' is incomplete — HCl activates pepsinogen into pepsin and denatures proteins, but pepsin (an enzyme) does the actual chemical digestion
Most nutrient absorption happens in the small intestine, not the large intestine — the large intestine mainly absorbs water and electrolytes
Urine is not simply filtered blood — it is the filtrate remaining after reabsorption of most water, glucose, and ions back into the blood

Endocrine System Overview and Hormone Action

The endocrine system uses glands that secrete hormones directly into the bloodstream to regulate body processes slowly and over a long duration, in contrast to the fast, localized signals of the nervous system.
  • Hormones are chemical messengers released by endocrine glands that travel through the blood to act on distant target cells
  • Target cells respond to a hormone only if they have the matching receptor, either on the cell surface (peptide hormones) or inside the cell (steroid hormones)
  • Peptide/protein hormones (e.g., insulin) are water-soluble and bind surface receptors, triggering a second-messenger cascade
  • Steroid hormones (e.g., cortisol, testosterone, estrogen) are lipid-soluble, cross the cell membrane, and bind receptors inside the cell to directly affect gene expression
  • The hypothalamus links the nervous and endocrine systems by controlling the pituitary gland
  • Effects of hormones tend to be slower to start but longer-lasting than nerve impulses

The Pituitary and Thyroid Glands

The pituitary gland, controlled by the hypothalamus, is often called the 'master gland' because it releases hormones that regulate other endocrine glands, including the thyroid.
  • The anterior pituitary secretes tropic hormones such as TSH (thyroid-stimulating hormone), ACTH (adrenocorticotropic hormone), FSH, LH, and growth hormone (GH)
  • The posterior pituitary stores and releases two hormones made by the hypothalamus: antidiuretic hormone (ADH) and oxytocin
  • Growth hormone stimulates growth and cell reproduction; oversecretion in childhood causes gigantism, undersecretion causes pituitary dwarfism
  • The thyroid gland, in the neck, produces thyroxine (T4) and triiodothyronine (T3), which set the body's basal metabolic rate
  • Thyroid hormone production requires dietary iodine; iodine deficiency can cause an enlarged thyroid (goiter)
  • Hypothyroidism (too little thyroid hormone) causes fatigue, weight gain, and cold intolerance; hyperthyroidism (too much, as in Graves' disease) causes weight loss, rapid heart rate, and heat intolerance

Adrenal Glands and the Pancreas

The adrenal glands sit atop the kidneys and manage the body's stress response, while the pancreas manages blood sugar through two antagonistic hormones.
  • The adrenal medulla (inner region) releases epinephrine (adrenaline) and norepinephrine for the rapid 'fight-or-flight' response
  • The adrenal cortex (outer region) releases cortisol, a steroid hormone that raises blood glucose and suppresses the immune system during prolonged stress
  • The adrenal cortex also releases aldosterone, which increases sodium (and water) reabsorption in the kidneys to help regulate blood pressure
  • The pancreas is both an exocrine gland (digestive enzymes) and an endocrine gland (hormones from clusters of cells called the islets of Langerhans)
  • Beta cells of the pancreas secrete insulin, which lowers blood glucose by promoting its uptake into cells and storage as glycogen in the liver
  • Alpha cells secrete glucagon, which raises blood glucose by stimulating the breakdown of glycogen into glucose

Hormone Feedback Loops

Most hormone levels are controlled by negative feedback loops that keep the body in a stable internal state (homeostasis) by counteracting change.
  • In negative feedback, a rise in a hormone or its effect suppresses further release of that hormone, keeping levels within a narrow range
  • Example: rising thyroid hormone (T3/T4) feeds back to inhibit the hypothalamus and anterior pituitary, reducing TRH and TSH release
  • Example: rising blood glucose triggers insulin release; falling blood glucose triggers glucagon release, so the two hormones oppose each other to keep glucose stable
  • Positive feedback loops amplify a change instead of reversing it and are rarer; oxytocin during childbirth is a classic example, where uterine contractions trigger more oxytocin release until delivery
  • Type 1 diabetes results from autoimmune destruction of insulin-producing beta cells, so blood glucose cannot be lowered normally
  • Type 2 diabetes results from cells becoming resistant to insulin's effects, often linked to obesity and lifestyle factors

Innate (Nonspecific) Immunity

Innate immunity is the body's fast-acting, nonspecific defense system, active from birth, that responds the same way to any pathogen.
  • The first line of defense includes physical and chemical barriers: intact skin, mucous membranes, stomach acid, and enzymes in tears and saliva
  • The second line of defense activates once a pathogen breaches the first line, including phagocytic white blood cells, the inflammatory response, fever, and the complement system
  • Neutrophils are the most abundant white blood cell and are usually the first phagocytes to arrive at a site of infection
  • Macrophages engulf pathogens by phagocytosis and also display pieces of the pathogen (antigens) on their surface to alert the adaptive immune system
  • Natural killer (NK) cells patrol the body and destroy virus-infected and cancerous cells without needing to recognize a specific antigen
  • Inflammation (redness, heat, swelling, pain) increases blood flow and vessel permeability to bring immune cells to the injury site; fever raises body temperature to slow pathogen reproduction

Adaptive Immunity, Antibodies, and Vaccination

Adaptive immunity is slower to activate than innate immunity but is highly specific to a particular pathogen and creates lasting memory.
  • B lymphocytes (B cells) drive humoral immunity by differentiating into plasma cells that secrete antibodies specific to one antigen
  • T lymphocytes (T cells) drive cell-mediated immunity; helper T cells coordinate the immune response by activating B cells and cytotoxic T cells, while cytotoxic T cells directly destroy infected or abnormal cells
  • An antibody (immunoglobulin) is a Y-shaped protein that binds a specific antigen, marking it for destruction or neutralizing it directly
  • After an infection, some B and T cells become long-lived memory cells, allowing a faster, stronger secondary immune response if the same pathogen is encountered again
  • Active immunity develops when the body makes its own antibodies, either from natural infection or from vaccination with a weakened, inactivated, or partial pathogen/antigen
  • Passive immunity comes from receiving ready-made antibodies (e.g., from mother to infant across the placenta or in breast milk); it acts immediately but is temporary since no memory cells are formed
Insulin lowers blood glucose and glucagon raises it, not the other way around — students frequently swap these two
The adrenal medulla releases epinephrine for short-term 'fight-or-flight' response, while the adrenal cortex releases cortisol for longer-term stress response — they are different regions with different hormones
Vaccination provides active immunity (the body makes its own antibodies and memory cells), not passive immunity — passive immunity comes from receiving pre-made antibodies and does not last
Innate immunity is fast but nonspecific and has no memory; adaptive immunity is slower to start but is pathogen-specific and creates memory cells for faster future responses

Male Reproductive Anatomy and Spermatogenesis

The male reproductive system is built to produce, mature, and deliver sperm, with testosterone driving both sperm production and male secondary sex characteristics.
  • The testes are the primary male reproductive organs, producing sperm (spermatogenesis) and the hormone testosterone
  • Sperm are produced in the seminiferous tubules inside the testes, then travel to the epididymis to mature and be stored
  • The vas deferens transports mature sperm from the epididymis toward the urethra during ejaculation
  • The seminal vesicles and prostate gland add fluid containing nutrients (like fructose) and buffers, which together with sperm make up semen
  • Testosterone, produced mainly by the testes, drives sperm production and the development of secondary sex characteristics such as facial hair and a deeper voice
  • Spermatogenesis is a continuous process that begins at puberty and produces millions of sperm daily throughout adult life

Female Reproductive Anatomy and Oogenesis

The female reproductive system produces eggs, provides the site for fertilization, and supports a developing fetus if pregnancy occurs.
  • The ovaries are the primary female reproductive organs, producing eggs (oocytes) and the hormones estrogen and progesterone
  • Unlike sperm production, oogenesis begins before birth; a female is born with all the immature egg cells she will ever have
  • The fallopian tubes (oviducts) carry an egg from the ovary toward the uterus and are the usual site of fertilization
  • The uterus is a muscular organ lined by the endometrium, which thickens each cycle to potentially support a fertilized egg
  • The cervix is the narrow lower portion of the uterus that opens into the vagina, the muscular canal that receives sperm and serves as the birth canal
  • Only one egg is typically released per menstrual cycle (ovulation), compared to the continuous mass production of sperm in males

The Menstrual Cycle

The menstrual cycle is a roughly month-long hormonal cycle that prepares an egg for release and the uterus for possible pregnancy, driven by feedback between the pituitary and ovaries.
  • The cycle (about 28 days) has phases: the follicular phase, ovulation, the luteal phase, and menstruation
  • Follicle-stimulating hormone (FSH) from the anterior pituitary stimulates growth of an ovarian follicle containing the developing egg
  • The growing follicle secretes rising estrogen, which builds up the uterine endometrium and eventually triggers a surge of luteinizing hormone (LH)
  • The LH surge triggers ovulation, the release of the mature egg from the ovary, typically around day 14 of a 28-day cycle
  • After ovulation, the ruptured follicle becomes the corpus luteum, which secretes progesterone to maintain the endometrium in case of pregnancy
  • If fertilization does not occur, the corpus luteum degenerates, progesterone and estrogen drop, and the endometrium sheds as menstruation

Fertilization and Early Development

Fertilization combines genetic material from sperm and egg to form a new individual, which then undergoes early development and implants in the uterus.
  • Fertilization occurs when a sperm nucleus fuses with an egg nucleus, typically in the fallopian tube, forming a single-celled zygote
  • The zygote undergoes rapid cell division (cleavage) as it travels down the fallopian tube toward the uterus
  • By about day 5-6, the developing embryo forms a blastocyst, a hollow ball of cells, which implants into the endometrium
  • The placenta develops from both embryonic and maternal tissue, allowing exchange of gases, nutrients, and wastes between mother and fetus without their blood directly mixing
  • Human chorionic gonadotropin (hCG), produced by the developing placenta, maintains the corpus luteum early in pregnancy and is the hormone detected by pregnancy tests
  • Human development is divided into three trimesters, with major organ formation occurring in the first trimester

Homeostasis and Negative Feedback

Homeostasis is the body's maintenance of a stable internal environment despite external changes, achieved mainly through negative feedback loops.
  • Homeostasis relies on receptors (sensors that detect change), a control center (that compares the change to a set point), and effectors (that carry out a response)
  • In negative feedback, a deviation from the set point triggers a response that pushes the variable back toward the set point, which is the most common regulatory mechanism in the body
  • Positive feedback loops amplify a change rather than reversing it and are used only in specific processes, such as childbirth or blood clotting, because they must eventually be shut off
  • Blood glucose regulation by insulin and glucagon is a classic example of negative feedback maintaining homeostasis
  • Loss of homeostatic control, such as failure to regulate blood glucose or body temperature, can quickly become life-threatening

Thermoregulation and Osmoregulation

Thermoregulation keeps body temperature within a narrow range, and osmoregulation keeps water and solute concentrations balanced; both are controlled by the hypothalamus using negative feedback.
  • The hypothalamus acts as the body's thermostat, comparing blood temperature to a set point (around 37°C/98.6°F in humans)
  • When body temperature rises, the hypothalamus triggers vasodilation (blood vessels widen to release heat) and sweating (evaporative cooling)
  • When body temperature falls, the hypothalamus triggers vasoconstriction (blood vessels narrow to conserve heat) and shivering (muscle contractions generate heat)
  • Osmoregulation is the control of water and solute balance, carried out mainly by the kidneys
  • Osmoreceptors in the hypothalamus detect blood solute concentration; when blood becomes too concentrated (dehydration), the posterior pituitary releases more ADH, which increases water reabsorption in the kidneys
  • Aldosterone, from the adrenal cortex, increases sodium reabsorption in the kidneys, which indirectly promotes water retention and helps regulate blood pressure
Ovulation is triggered by a surge in luteinizing hormone (LH), not follicle-stimulating hormone (FSH) — FSH's main job is stimulating follicle growth earlier in the cycle
Fertilization normally occurs in the fallopian tube, not the uterus — the uterus is where implantation of the resulting blastocyst happens several days later
Positive feedback amplifies a change (as in childbirth contractions), while negative feedback reverses it — most homeostatic regulation in the body relies on negative, not positive, feedback
Sweating and vasodilation are the body's responses to being too hot, while shivering and vasoconstriction respond to being too cold — students often mix up which response goes with which temperature direction
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Unit 1: Intro & Terminology

Anatomical Position
Standard reference stance: standing upright, facing forward, arms at sides, palms facing forward.
Sagittal Plane
A vertical plane dividing the body into left and right portions.
Frontal (Coronal) Plane
A vertical plane dividing the body into anterior and posterior portions.
Transverse Plane
A horizontal plane dividing the body into superior and inferior portions.
Homeostasis
The body's ability to maintain a stable internal environment despite changes in the external environment.
Negative Feedback Loop
A regulatory mechanism that reverses a change to bring a variable back toward its set point; the most common form of physiological control.
Positive Feedback Loop
A regulatory mechanism that amplifies a change until a specific endpoint is reached, such as labor contractions or blood clotting.
Parietal vs Visceral Membrane
Parietal layers line the walls of a body cavity; visceral layers cover the surface of the internal organ within that cavity.
Mediastinum
The central compartment of the thoracic cavity containing the heart, esophagus, trachea, and great vessels.
Diaphragm
The dome-shaped skeletal muscle that separates the thoracic cavity from the abdominopelvic cavity and drives breathing.

Unit 2: Skeletal System

Diaphysis
The shaft of a long bone, composed primarily of compact bone surrounding the medullary cavity.
Epiphyseal Plate
A layer of hyaline cartilage near each end of a long bone in children that allows lengthwise bone growth; it ossifies into the epiphyseal line after puberty.
Osteoblast
A bone cell that synthesizes and secretes new bone matrix (osteoid), promoting bone formation.
Osteoclast
A large, multinucleated cell that breaks down (resorbs) bone tissue, releasing calcium into the bloodstream.
Axial Skeleton
The 80 bones forming the body's central axis: skull, vertebral column, sternum, and ribs.
Appendicular Skeleton
The 126 bones of the limbs and the pectoral and pelvic girdles that attach them to the axial skeleton.
Synovial Joint
A freely movable joint (diarthrosis) containing synovial fluid within a joint capsule, such as the knee or shoulder.
Ligament
A band of dense connective tissue connecting bone to bone, stabilizing joints.
Osteoporosis
A condition of reduced bone density and mass that increases the risk of fractures, common in postmenopausal women.
Hematopoiesis
The production of blood cells, occurring in the red bone marrow found in flat bones and the epiphyses of long bones.

Unit 3: Muscular System

Sarcomere
The basic contractile unit of skeletal muscle, extending between two Z-discs and containing overlapping actin and myosin filaments.
Myosin
The protein forming thick filaments in a sarcomere; its head forms cross-bridges with actin and pulls thin filaments during contraction.
Actin
The protein forming thin filaments in a sarcomere; it slides past myosin filaments to shorten the sarcomere during contraction.
Sliding Filament Theory
The model explaining muscle contraction: thin (actin) filaments slide past thick (myosin) filaments, shortening the sarcomere without the filaments themselves shortening.
Neuromuscular Junction
The synapse between a motor neuron's axon terminal and a skeletal muscle fiber, where acetylcholine triggers muscle excitation.
Prime Mover (Agonist)
The muscle primarily responsible for producing a specific movement.
Antagonist Muscle
A muscle that produces the opposite action of the agonist, helping control and reverse movement.
Origin and Insertion
The origin is a muscle's fixed attachment point; the insertion is the movable attachment point that is pulled toward the origin during contraction.
Phosphocreatine System
An immediate energy system that rapidly regenerates ATP for the first several seconds of high-intensity muscle activity.
Intercalated Discs
Specialized junctions connecting cardiac muscle cells, containing gap junctions that allow rapid electrical signal spread for coordinated heart contraction.

Unit 4: Nervous System

Neuron
The specialized nerve cell that generates and transmits electrical and chemical signals; consists of dendrites, a cell body, and an axon.
Myelin Sheath
A fatty insulating layer around axons, formed by Schwann cells (PNS) or oligodendrocytes (CNS), that speeds up nerve impulse conduction.
Action Potential
A rapid, all-or-none reversal of a neuron's membrane potential that propagates a nerve impulse along the axon.
Synapse
The junction between a neuron and another neuron or effector cell, where signals are transmitted, most often chemically via neurotransmitters.
Cerebrum
The largest part of the brain, divided into frontal, parietal, temporal, and occipital lobes, responsible for higher functions like reasoning, sensation, and voluntary movement.
Cerebellum
The brain region located below the cerebrum that coordinates balance, posture, and fine motor movements.
Hypothalamus
A brain structure that regulates homeostasis, including temperature, hunger, and thirst, and links the nervous and endocrine systems.
Sympathetic Nervous System
The division of the autonomic nervous system that triggers the 'fight-or-flight' response, increasing heart rate and redirecting blood flow to muscles.
Parasympathetic Nervous System
The division of the autonomic nervous system that promotes 'rest-and-digest' functions, such as slowing heart rate and stimulating digestion.
Reflex Arc
The neural pathway of a reflex, consisting of a receptor, sensory neuron, integration center, motor neuron, and effector, allowing rapid automatic responses.

Unit 5: Circulatory & Respiratory

Sinoatrial (SA) Node
The heart's natural pacemaker, located in the right atrium, which initiates the electrical impulse that sets heart rate.
Atrioventricular (AV) Node
A structure that briefly delays the electrical signal between the atria and ventricles, allowing the atria to finish emptying before ventricular contraction.
Hemoglobin
The iron-containing protein in red blood cells that binds and transports oxygen (and some carbon dioxide) throughout the body.
Systemic Circuit
The pathway of blood flow from the left ventricle through the body's tissues and back to the right atrium.
Pulmonary Circuit
The pathway of blood flow from the right ventricle to the lungs for gas exchange and back to the left atrium.
Alveoli
Tiny, thin-walled air sacs in the lungs surrounded by capillaries, serving as the primary site of gas exchange.
Diaphragm (Respiratory Role)
The dome-shaped muscle that contracts and flattens during inhalation, increasing thoracic volume and drawing air into the lungs.
Hemostasis
The physiological process of stopping bleeding, involving vascular spasm, platelet plug formation, and the coagulation cascade.
Atherosclerosis
A disease process in which fatty plaque builds up inside artery walls, narrowing vessels and raising the risk of heart attack and stroke.
Rh Factor
A red blood cell surface antigen (positive or absent/negative) that, along with ABO type, must be matched for safe blood transfusion and is important in pregnancy.

Unit 6: Digestive & Excretory

Peristalsis
Wave-like contractions of smooth muscle in the digestive tract that propel food forward through the alimentary canal.
Chyme
The semi-liquid, acidic mixture of partially digested food and gastric secretions released from the stomach into the small intestine.
Bile
A fluid produced by the liver and stored in the gallbladder that emulsifies fats, increasing their surface area for enzymatic digestion.
Villi
Finger-like projections lining the small intestine that, along with microvilli, greatly increase surface area for nutrient absorption.
Nephron
The functional unit of the kidney, consisting of the glomerulus and renal tubule, responsible for filtering blood and forming urine.
Glomerulus
A tuft of capillaries within Bowman's capsule where blood pressure drives filtration of water and small solutes into the nephron.
Loop of Henle
The U-shaped segment of the renal tubule that establishes the concentration gradient in the kidney medulla, enabling urine concentration.
Antidiuretic Hormone (ADH)
A hormone that increases water reabsorption in the kidney's collecting duct, concentrating urine and conserving body water.
Duodenum
The first section of the small intestine, where bile and pancreatic secretions mix with chyme to continue digestion.
Erythropoietin
A hormone released by the kidneys in response to low blood oxygen that stimulates red blood cell production in the bone marrow.

Unit 7: Endocrine & Immune

Pituitary gland
The 'master gland,' controlled by the hypothalamus; its anterior lobe releases tropic hormones (TSH, ACTH, FSH, LH, GH) and its posterior lobe releases ADH and oxytocin.
Thyroxine (T4)
Iodine-containing hormone from the thyroid gland that sets the body's basal metabolic rate; too little causes hypothyroidism, too much causes hyperthyroidism.
Cortisol
Steroid hormone from the adrenal cortex that raises blood glucose and suppresses immune activity during prolonged stress.
Epinephrine (adrenaline)
Hormone from the adrenal medulla that triggers the rapid 'fight-or-flight' response: increased heart rate, blood pressure, and blood glucose.
Insulin
Hormone from pancreatic beta cells that lowers blood glucose by promoting cellular glucose uptake and glycogen storage; absent or ineffective in diabetes.
Glucagon
Hormone from pancreatic alpha cells that raises blood glucose by triggering the breakdown of stored glycogen into glucose.
Negative feedback loop
A regulatory mechanism in which a change triggers a response that reverses the change, keeping a variable (like hormone level or blood glucose) stable.
Innate immunity
The body's fast, nonspecific defense present from birth, including physical barriers, phagocytes, inflammation, fever, and natural killer cells.
Adaptive immunity
A slower but highly specific immune response carried out by B and T lymphocytes that creates memory cells for faster future responses.
Antibody
A Y-shaped protein produced by plasma cells (from B cells) that binds a specific antigen to neutralize it or mark it for destruction.

Unit 8: Reproductive & Homeostasis

Testes
Male gonads that produce sperm in the seminiferous tubules and secrete testosterone.
Ovaries
Female gonads that produce eggs (oocytes) and secrete estrogen and progesterone.
Ovulation
The release of a mature egg from the ovary, triggered by a surge in luteinizing hormone (LH), typically around day 14 of the cycle.
Corpus luteum
The structure formed from a ruptured ovarian follicle after ovulation; secretes progesterone to maintain the uterine lining.
Fertilization
The fusion of sperm and egg nuclei, typically in the fallopian tube, forming a single-celled zygote.
Placenta
An organ formed from embryonic and maternal tissue that exchanges gases, nutrients, and wastes between mother and fetus without mixing their blood.
Homeostasis
The maintenance of a stable internal environment despite external changes, primarily through negative feedback loops.
Negative feedback
A regulatory loop in which a deviation from a set point triggers a response that reverses the deviation, restoring balance.
Thermoregulation
Control of body temperature by the hypothalamus using responses like sweating/vasodilation (cooling) and shivering/vasoconstriction (warming).
Osmoregulation
Control of water and solute balance, mainly by the kidneys, regulated by hormones like ADH and aldosterone.
Press 1–4 to answer · Enter for next

Unit 1: Intro & Terminology

Anatomical Position & Directional Terms
Standard anatomical position (standing, facing forward, arms at sides, palms forward) is the reference point for all directional terminology in anatomy.
Body Planes and Sections
Planes are imaginary flat surfaces used to describe cuts or views through the body.
Body Cavities and Membranes
The body's internal organs are housed in cavities lined by protective membranes.
Levels of Structural Organization
Anatomy studies structure at increasing levels of complexity, from atoms to the whole organism.
Homeostasis and Feedback Loops
Homeostasis is the body's ability to maintain a stable internal environment despite external changes.
The Four Primary Tissue Types
All organs are built from combinations of four basic tissue types, each with a distinct structure and function.
Key fact
Anatomical position always assumes palms facing forward, regardless of the body's actual posture
Key fact
The abdominopelvic cavity is often divided into 9 regions or 4 quadrants for clinical description
Key fact
Anatomy studies structure while physiology studies function — the two are inseparable in practice
Key fact
Negative feedback maintains stability; positive feedback drives a process to completion, then stops

Unit 2: Skeletal System

Bone Structure and Classification
Bones are classified by shape and built from distinct tissue layers that combine strength with light weight.
Axial Skeleton
The axial skeleton forms the body's central axis and protects the brain, spinal cord, and thoracic organs.
Appendicular Skeleton
The appendicular skeleton includes the limbs and the girdles that attach them to the axial skeleton, enabling movement.
Joints (Articulations)
Joints are classified by the tissue connecting the bones and by the degree of movement they allow.
Bone Growth, Remodeling, and Repair
Bone is a dynamic, living tissue that grows, remodels, and heals throughout life through the coordinated action of specialized cells.
Skeletal Functions and Common Disorders
Beyond providing structure, the skeleton performs critical physiological roles and is subject to specific pathologies.
Key fact
The adult human skeleton has 206 bones total: 80 axial and 126 appendicular
Key fact
Red bone marrow (found in flat bones and epiphyses) produces blood cells; yellow bone marrow stores fat
Key fact
The femur is both the longest and strongest bone; the stapes (in the ear) is the smallest
Key fact
Calcium and phosphate ions give bone its hardness, while collagen fibers give it flexibility and resistance to fracture

Unit 3: Muscular System

Types of Muscle Tissue
The body contains three distinct types of muscle tissue, each suited to a specific function and control mechanism.
Skeletal Muscle Structure
Skeletal muscle is organized hierarchically from whole muscle down to the contractile proteins that generate force.
The Sliding Filament Mechanism
Muscle contraction occurs when thin filaments slide past thick filaments, shortening the sarcomere without the filaments themselves changing length.
Muscle Metabolism and Fatigue
Muscles generate ATP through multiple pathways depending on the intensity and duration of activity.
Muscle Actions and Terminology
Muscles work in coordinated groups, and their names often describe their location, shape, or action.
Major Muscle Groups and Disorders
Understanding major superficial muscles and common muscular disorders connects structure to real-world function and pathology.
Key fact
There are over 600 skeletal muscles in the human body, making up roughly 40% of body mass
Key fact
ATP is required for both contraction (myosin head movement) and relaxation (detaching myosin from actin)
Key fact
Calcium is the trigger that couples nervous stimulation to physical contraction (excitation-contraction coupling)
Key fact
Cardiac muscle is involuntary and self-exciting (autorhythmic), unlike skeletal muscle which requires nervous stimulation to contract

Unit 4: Nervous System

Neuron Structure and Function
Neurons are the specialized cells that generate and transmit electrical and chemical signals throughout the nervous system.
The Action Potential
An action potential is a rapid, temporary reversal of membrane voltage that allows neurons to transmit signals over long distances.
Synaptic Transmission
Neurons communicate with each other and with effector cells across synapses, most commonly using chemical neurotransmitters.
Central Nervous System (CNS)
The CNS, consisting of the brain and spinal cord, integrates sensory information and coordinates the body's responses.
Peripheral Nervous System (PNS)
The PNS connects the CNS to the rest of the body and is divided into sensory and motor divisions with further subdivisions.
Reflexes and Neurological Disorders
Reflexes are rapid, automatic responses that protect the body, and disruptions to neural structures cause characteristic disorders.
Key fact
The brain contains roughly 86 billion neurons, but far more glial cells supporting them
Key fact
Saltatory conduction along myelinated axons can be up to 100 times faster than conduction along unmyelinated axons
Key fact
The sympathetic and parasympathetic divisions typically have opposite effects on the same organs (antagonistic control)
Key fact
The blood-brain barrier, formed by tight junctions in brain capillaries, restricts what substances can pass from blood into brain tissue

Unit 5: Circulatory & Respiratory

Heart Structure and the Cardiac Cycle
The heart is a four-chambered muscular pump that circulates blood through the body via a precisely coordinated sequence of contractions.
Cardiac Conduction System
The heart's rhythmic contraction is coordinated by a specialized electrical conduction system, allowing it to beat independently of the nervous system.
Blood Vessels and Blood Composition
Blood travels through a closed system of vessels, and blood itself is a specialized connective tissue with distinct cellular and liquid components.
Blood Types and Clotting
Blood type compatibility and the clotting cascade are critical clinical concepts governed by antigens on red blood cells and a cascade of clotting factors.
Respiratory Structures and Ventilation
The respiratory system moves air into and out of the lungs and provides the surface for gas exchange with the blood.
Gas Exchange, Transport, and Disorders
Oxygen and carbon dioxide move by diffusion along pressure gradients, and disruptions to this system cause well-known respiratory and cardiovascular diseases.
Key fact
The heart beats roughly 100,000 times per day, pumping about 5 liters of blood per minute at rest
Key fact
The pulmonary artery is the only artery in the body that carries deoxygenated blood; the pulmonary vein is the only vein that carries oxygenated blood
Key fact
Hemoglobin's affinity for carbon monoxide is over 200 times greater than for oxygen, making CO poisoning extremely dangerous
Key fact
The SA node sets heart rate at rest (~60-100 bpm), but the autonomic nervous system and hormones like epinephrine can speed or slow it

Unit 6: Digestive & Excretory

Overview of the Digestive Tract
The digestive system is a long muscular tube (the alimentary canal) plus accessory organs that mechanically and chemically break down food for absorption.
Mouth, Esophagus, and Stomach
Digestion begins in the mouth and continues through the esophagus into the stomach, where food is churned and chemically broken down.
Small Intestine and Accessory Organs
Most chemical digestion and virtually all nutrient absorption occur in the small intestine, with essential help from the liver, gallbladder, and pancreas.
Large Intestine and Elimination
The large intestine completes the digestive process by absorbing water and electrolytes and compacting waste for elimination.
Kidney Structure and Urine Formation
The kidneys filter blood to remove wastes and regulate fluid and electrolyte balance through the functional unit called the nephron.
The Urinary System and Digestive/Excretory Disorders
Beyond the kidneys, the urinary system stores and eliminates urine, and both systems are subject to well-known clinical conditions.
Key fact
The small intestine is about 6 meters (20 feet) long and is the primary site of nutrient absorption due to its vast surface area from villi and microvilli
Key fact
The liver is the body's largest internal organ and performs bile production, detoxification, and nutrient storage/processing
Key fact
Each kidney contains roughly 1 million nephrons, and the kidneys filter the entire blood volume many times per day
Key fact
Bile does not chemically digest fat; it physically emulsifies fat into smaller droplets so pancreatic lipase can act on it more efficiently

Unit 7: Endocrine & Immune

Endocrine System Overview and Hormone Action
The endocrine system uses glands that secrete hormones directly into the bloodstream to regulate body processes slowly and over a long duration, in contrast to the fast, localized signals of the nervous system.
The Pituitary and Thyroid Glands
The pituitary gland, controlled by the hypothalamus, is often called the 'master gland' because it releases hormones that regulate other endocrine glands, including the thyroid.
Adrenal Glands and the Pancreas
The adrenal glands sit atop the kidneys and manage the body's stress response, while the pancreas manages blood sugar through two antagonistic hormones.
Hormone Feedback Loops
Most hormone levels are controlled by negative feedback loops that keep the body in a stable internal state (homeostasis) by counteracting change.
Innate (Nonspecific) Immunity
Innate immunity is the body's fast-acting, nonspecific defense system, active from birth, that responds the same way to any pathogen.
Adaptive Immunity, Antibodies, and Vaccination
Adaptive immunity is slower to activate than innate immunity but is highly specific to a particular pathogen and creates lasting memory.
Key fact
HIV specifically infects and destroys helper T cells, which is why it progressively cripples the ability to coordinate both humoral and cell-mediated immunity
Key fact
The hypothalamus and pituitary form a control axis: the hypothalamus releases tropic hormones that tell the anterior pituitary which hormones to release, which in turn control glands like the thyroid, adrenal cortex, and gonads
Key fact
Insulin and glucagon are antagonistic hormones from the same organ (the pancreas), a clear example of how negative feedback keeps blood glucose within a narrow range
Key fact
A vaccine does not give a person the disease; it trains the immune system to produce memory cells so a real infection triggers a fast, strong secondary response

Unit 8: Reproductive & Homeostasis

Male Reproductive Anatomy and Spermatogenesis
The male reproductive system is built to produce, mature, and deliver sperm, with testosterone driving both sperm production and male secondary sex characteristics.
Female Reproductive Anatomy and Oogenesis
The female reproductive system produces eggs, provides the site for fertilization, and supports a developing fetus if pregnancy occurs.
The Menstrual Cycle
The menstrual cycle is a roughly month-long hormonal cycle that prepares an egg for release and the uterus for possible pregnancy, driven by feedback between the pituitary and ovaries.
Fertilization and Early Development
Fertilization combines genetic material from sperm and egg to form a new individual, which then undergoes early development and implants in the uterus.
Homeostasis and Negative Feedback
Homeostasis is the body's maintenance of a stable internal environment despite external changes, achieved mainly through negative feedback loops.
Thermoregulation and Osmoregulation
Thermoregulation keeps body temperature within a narrow range, and osmoregulation keeps water and solute concentrations balanced; both are controlled by the hypothalamus using negative feedback.
Key fact
A female is born with all the immature egg cells she will ever produce, while males continuously generate new sperm throughout adult life via spermatogenesis
Key fact
The corpus luteum's progesterone output is essential to maintaining the uterine lining; if it degrades without a rescuing pregnancy hormone (hCG), menstruation follows
Key fact
The placenta allows nutrient, gas, and waste exchange between mother and fetus but keeps their blood supplies separate — maternal and fetal blood do not normally mix
Key fact
The hypothalamus is the shared control center for both thermoregulation (body temperature) and osmoregulation (water/solute balance), each managed through negative feedback
Common mistakes for each unit — read the mistake, then make sure you know why it's wrong.

Unit 1: Intro & Terminology

Watch out
'Anterior' and 'ventral' are synonyms in humans, not opposites
Watch out
A sagittal cut does not have to be down the midline — only the midsagittal plane splits the body into exact mirror halves
Watch out
The stomach is an organ, not a tissue — it is made of all four tissue types working together
Watch out
Positive feedback is not 'bad'; it is a normal, self-limiting mechanism (e.g., childbirth, blood clotting), not a runaway malfunction

Unit 2: Skeletal System

Watch out
Cartilage is not bone — cartilage lacks blood vessels and calcified matrix, while bone is highly vascularized and mineralized
Watch out
The epiphyseal plate is cartilage, not bone, until it fully ossifies (closes) after puberty, ending height growth
Watch out
Ligaments connect bone to bone; tendons connect muscle to bone — these are commonly reversed by students
Watch out
Not all joints are synovial (freely movable) — sutures of the skull are fibrous and immovable, not 'stiff synovial joints'

Unit 3: Muscular System

Watch out
Muscles only pull, they never push — movement in the opposite direction requires an antagonist muscle, not the same muscle reversing
Watch out
Rigor mortis occurs because of a LACK of ATP after death, preventing myosin from detaching from actin — not from excess calcium alone
Watch out
The sarcomere shortens during contraction, but the individual thick and thin filaments do NOT shorten — they slide past each other
Watch out
'Muscle tone' refers to a constant, low-level partial contraction at rest, not muscle size or strength

Unit 4: Nervous System

Watch out
Nerve impulses are electrical within the neuron but chemical between neurons at the synapse — not electrical the entire way
Watch out
The action potential does NOT get bigger with a stronger stimulus — it is all-or-none; a stronger stimulus increases firing frequency, not amplitude
Watch out
The sympathetic division does not 'turn off' the parasympathetic division and vice versa — both often act simultaneously with one usually dominating
Watch out
Neuroglia (glial cells) are far more numerous than neurons in the brain, but they do not directly generate or transmit nerve impulses

Unit 5: Circulatory & Respiratory

Watch out
Arteries do not always carry oxygenated blood — the pulmonary arteries carry deoxygenated blood to the lungs; it is direction (away from heart), not oxygen content, that defines an artery
Watch out
The AV node does not initiate the heartbeat — the SA node is the primary pacemaker; the AV node only relays and delays the signal
Watch out
Inhalation is active (requires diaphragm contraction); quiet exhalation is passive (elastic recoil), not muscular effort
Watch out
Blood 'type O' still has antigens for the immune system to consider (the Rh factor) — 'universal donor' refers specifically to ABO/Rh compatibility, not total absence of all markers

Unit 6: Digestive & Excretory

Watch out
The gallbladder does not produce bile — the liver produces bile; the gallbladder only stores and concentrates it
Watch out
'Stomach acid digests protein' is incomplete — HCl activates pepsinogen into pepsin and denatures proteins, but pepsin (an enzyme) does the actual chemical digestion
Watch out
Most nutrient absorption happens in the small intestine, not the large intestine — the large intestine mainly absorbs water and electrolytes
Watch out
Urine is not simply filtered blood — it is the filtrate remaining after reabsorption of most water, glucose, and ions back into the blood

Unit 7: Endocrine & Immune

Watch out
Insulin lowers blood glucose and glucagon raises it, not the other way around — students frequently swap these two
Watch out
The adrenal medulla releases epinephrine for short-term 'fight-or-flight' response, while the adrenal cortex releases cortisol for longer-term stress response — they are different regions with different hormones
Watch out
Vaccination provides active immunity (the body makes its own antibodies and memory cells), not passive immunity — passive immunity comes from receiving pre-made antibodies and does not last
Watch out
Innate immunity is fast but nonspecific and has no memory; adaptive immunity is slower to start but is pathogen-specific and creates memory cells for faster future responses

Unit 8: Reproductive & Homeostasis

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Ovulation is triggered by a surge in luteinizing hormone (LH), not follicle-stimulating hormone (FSH) — FSH's main job is stimulating follicle growth earlier in the cycle
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Fertilization normally occurs in the fallopian tube, not the uterus — the uterus is where implantation of the resulting blastocyst happens several days later
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Positive feedback amplifies a change (as in childbirth contractions), while negative feedback reverses it — most homeostatic regulation in the body relies on negative, not positive, feedback
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Sweating and vasodilation are the body's responses to being too hot, while shivering and vasoconstriction respond to being too cold — students often mix up which response goes with which temperature direction