The human skeleton is divided into the axial skeleton (80 bones: skull, vertebral column, rib cage) and appendicular skeleton (126 bones: limbs, shoulder girdle, pelvic girdle). Long bones (femur, humerus) contain red marrow for blood cell production – producing about 500 billion blood cells daily. Bones are living tissues, constantly remodeled through resorption (osteoclasts break down old bone) and formation (osteoblasts build new bone). The femur (thigh bone) is the longest and strongest bone. The smallest bone is the stapes (ear bone, 3mm). Joints connect bones: fibrous (no movement, skull sutures), cartilaginous (limited movement, spine), and synovial (free movement, knees, shoulders). Synovial joints contain lubricating fluid. Ligaments connect bone to bone; tendons connect muscle to bone. Common bone conditions include osteoporosis (decreased bone density), fractures (breaks), and arthritis (joint inflammation). Calcium, vitamin D, and weight-bearing exercise are essential for bone health.

Blood flow through the heart: Deoxygenated blood from the body enters the right atrium via the superior and inferior vena cavae, passes through the tricuspid valve to the right ventricle, then is pumped through the pulmonary valve to the lungs via the pulmonary artery. Oxygenated blood returns from the lungs via pulmonary veins to the left atrium, passes through the mitral valve to the left ventricle, then is pumped through the aortic valve to the body via the aorta. The left ventricle has the thickest muscle because it must pump blood throughout the entire body. The heart's electrical system (sinoatrial node = natural pacemaker, atrioventricular node, Purkinje fibers) coordinates contractions. An electrocardiogram (ECG/EKG) records this electrical activity. Average heart rate is 60-100 beats per minute at rest. The heart pumps about 1.5 gallons of blood per minute (2,000 gallons daily). Cardiovascular diseases (heart attacks, stroke, high blood pressure) are leading causes of death worldwide. Lifestyle factors like diet, exercise, and not smoking significantly affect heart health.

At rest, humans breathe about 12-20 times per minute (17,000-30,000 breaths daily). Each breath moves about 500 mL of air (tidal volume). Total lung capacity is about 6 liters in adult males, 4.2 liters in females. The alveoli number about 300-500 million per lung, providing a surface area of about 70-100 square meters (roughly the size of a tennis court) for gas exchange. Oxygen diffuses across alveolar walls into capillaries, binding to hemoglobin in red blood cells. Carbon dioxide diffuses from blood into alveoli to be exhaled. Surfactant (a fatty substance) prevents alveolar collapse. Common respiratory conditions include asthma (airway inflammation), COPD (chronic obstructive pulmonary disease), pneumonia (infection), and lung cancer. Smoking is the leading preventable cause of respiratory disease. The respiratory and cardiovascular systems work together – oxygenated blood is delivered to tissues, and deoxygenated blood returns to the lungs.

The cerebrum is the largest part, responsible for higher functions. The cerebral cortex (outer layer, 2-4 mm thick) has four lobes: frontal (executive functions, personality, speech), parietal (touch, temperature, pain), temporal (hearing, memory, language comprehension), and occipital (vision). The left hemisphere typically controls language and logic; the right hemisphere spatial abilities and creativity – connected by the corpus callosum. The limbic system (hippocampus, amygdala, hypothalamus) processes emotion, memory, and motivation. The brain uses about 20% of the body's oxygen and calories despite being only 2% of body weight. Neurons communicate via electrical impulses and chemical neurotransmitters (dopamine, serotonin, acetylcholine). Neuroplasticity – the brain's ability to reorganize by forming new neural connections throughout life. Common brain disorders include Alzheimer's disease (memory loss), Parkinson's disease (movement problems), stroke (blood flow interruption), epilepsy (seizures), and traumatic brain injury. The blood-brain barrier protects the brain from pathogens and toxins.

Accessory organs include the liver (produces bile for fat digestion, processes nutrients, detoxifies chemicals), gallbladder (stores and concentrates bile), and pancreas (produces digestive enzymes and bicarbonate, also produces insulin and glucagon for blood sugar regulation). The small intestine has three sections: duodenum (most chemical digestion), jejunum, and ileum. Its inner surface has villi and microvilli (fingerlike projections) that increase surface area for absorption to about 250-300 square meters. The large intestine (about 5 feet long) houses about 100 trillion bacteria (gut microbiome) that help digest fiber, produce vitamins (K, B7, B12), and support immune function. Digestion time varies by food type: liquids pass quickly, solids take 24-72 hours from mouth to elimination. Common digestive disorders include gastroesophageal reflux disease (GERD), irritable bowel syndrome (IBS), inflammatory bowel disease (Crohn's, ulcerative colitis), celiac disease (gluten intolerance), and gallstones. Fiber, hydration, and probiotics support digestive health.

White blood cells (leukocytes) are the immune system's main cellular component, produced in bone marrow. Types include: neutrophils (most abundant, first responders), lymphocytes (B cells, T cells, natural killer cells), monocytes (become macrophages that engulf pathogens), eosinophils (fight parasites), and basophils (involved in allergic reactions). The lymphatic system (lymph nodes, spleen, thymus, tonsils, lymph vessels) transports lymph fluid containing immune cells. Vaccines work by exposing the immune system to harmless pathogen components, creating memory cells that provide rapid protection upon actual exposure. Active immunity results from infection or vaccination (long-lasting). Passive immunity results from receiving antibodies (temporary, as in breast milk or antibody treatments). Allergies are overreactions to harmless substances (allergens). Autoimmune diseases (rheumatoid arthritis, type 1 diabetes, multiple sclerosis) occur when the immune system attacks the body's own tissues. Immunodeficiency (HIV/AIDS, chemotherapy side effects) reduces immune function. The inflammatory response (redness, heat, swelling, pain) helps isolate and eliminate pathogens.

Skeletal muscles work in antagonistic pairs (biceps contracts to bend elbow, triceps contracts to straighten). Muscles contract through the sliding filament theory – actin and myosin filaments slide past each other using ATP energy. Muscle fibers are either slow-twitch (Type I, endurance, red due to myoglobin) or fast-twitch (Type II, rapid powerful movements, white). The smallest functional unit is the sarcomere. Common muscle disorders include muscular dystrophy (genetic muscle wasting), myasthenia gravis (autoimmune affecting nerve-muscle communication), and muscle strains. Exercise increases muscle mass through hypertrophy (enlargement of fibers). Muscles require calcium, magnesium, and potassium for proper function. The strongest muscle by force is the masseter (jaw muscle). The longest muscle is the sartorius (thigh). Muscles produce heat – shivering increases heat production when cold.

The epidermis is composed of stratified squamous epithelium, with keratinocytes (produce keratin for waterproofing) and melanocytes (produce melanin for UV protection). The outermost stratum corneum has dead cells that flake off – humans shed about 500 million skin cells daily. The dermis contains collagen and elastin for strength and elasticity. Sweat glands (eccrine for cooling, apocrine in armpits/genitals) help regulate temperature and excrete waste. Sebaceous glands produce sebum (oil) to lubricate skin and hair. Nerve endings detect touch, pressure, pain, temperature, and vibration. The hypodermis stores fat, provides insulation, and absorbs shock. Skin functions include protection against pathogens and UV radiation, temperature regulation (sweating and blood vessel dilation/constriction), sensation, vitamin D synthesis (from sunlight), and immune defense (Langerhans cells). Common skin conditions include acne (clogged follicles), eczema (inflammation), psoriasis (autoimmune overproduction), skin cancer (basal cell, squamous cell, melanoma), and burns. Sunscreen, moisturizers, and hydration maintain skin health.

Hormones are either steroid (derived from cholesterol, can enter cells, e.g., estrogen, testosterone, cortisol) or non-steroid (peptides/proteins, bind to cell surface receptors, e.g., insulin, growth hormone). The hypothalamus links the nervous and endocrine systems. The pituitary gland, located at the base of the brain, produces growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, prolactin, and antidiuretic hormone. The thyroid produces thyroxine (T4) and triiodothyronine (T3) – regulating metabolic rate; calcitonin lowers blood calcium. Parathyroid hormone (PTH) raises blood calcium. The adrenal medulla produces epinephrine (adrenaline) and norepinephrine – rapid stress response. The adrenal cortex produces cortisol (stress, metabolism) and aldosterone (blood pressure, salt balance). The pancreas produces insulin (lowers blood glucose) and glucagon (raises blood glucose). Diabetes mellitus results from insufficient insulin (Type 1) or insulin resistance (Type 2). The pineal gland produces melatonin (sleep-wake cycles). Feedback loops (negative feedback most common) maintain hormone balance. Endocrine disruptors (certain chemicals) can interfere with normal hormone function.

Neurons are specialized cells that transmit electrical impulses. A typical neuron has dendrites (receive signals), a cell body (contains nucleus), an axon (transmits impulses away), and axon terminals (release neurotransmitters at synapses). Myelin sheaths (produced by Schwann cells in PNS, oligodendrocytes in CNS) insulate axons and speed impulse transmission via saltatory conduction (impulses jump between nodes of Ranvier). The spinal cord transmits information between brain and body and controls reflexes (automatic responses without brain involvement, e.g., knee-jerk reflex). The sympathetic nervous system prepares the body for stress – increases heart rate, dilates pupils, slows digestion, releases adrenaline. The parasympathetic nervous system promotes "rest-and-digest" – slows heart rate, constricts pupils, stimulates digestion, promotes salivation. Neurotransmitters include acetylcholine (muscle contraction, memory), dopamine (reward, movement), serotonin (mood, sleep, appetite), norepinephrine (alertness, attention), GABA (inhibition, calming), and glutamate (excitation, learning). Nervous system disorders include multiple sclerosis (demyelination), Parkinson's disease (dopamine loss), Alzheimer's disease (neurodegeneration), spinal cord injuries, and peripheral neuropathy.