Anatomy and Physiology

Integumentary: skin, hair, nails; protective barrier against infection and environment, temperature regulation, sensory detection. Skeletal: bones, cartilage, ligaments; structure, support, movement (via muscle attachment), protection of organs, blood cell and mineral storage. Muscular: skeletal, smooth, cardiac muscle; movement, posture, heat generation, circulation. Nervous: brain, spinal cord, nerves; perception, processing, motor control, autonomic regulation (heart rate, digestion). Cardiovascular: heart, blood vessels, blood; oxygen and nutrient delivery, waste removal, immune function. Respiratory: lungs, airways, diaphragm; gas exchange (O2 in, CO2 out), pH regulation. Digestive: mouth, esophagus, stomach, intestines, pancreas, liver; break down food, absorb nutrients, eliminate waste. Urinary: kidneys, ureters, bladder, urethra; filter waste, regulate fluid and electrolytes, produce urine. Endocrine: pituitary, thyroid, adrenal, pancreas, gonads; hormone production, regulation of metabolism, growth, reproduction, stress response. Reproductive: testes/ovaries, penis/uterus, mammary glands; production of gametes (sperm/eggs), sexual function, lactation. Immune/Lymphatic: lymph nodes, spleen, thymus, white blood cells; defense against infection, antigen processing, antibody production. All systems are interconnected; disease in one system affects others.

The heart is a four-chambered pump: right atrium receives deoxygenated blood from the body via the superior and inferior vena cava, right ventricle pumps blood to the lungs via the pulmonary artery, left atrium receives oxygenated blood from the lungs via the pulmonary veins, left ventricle pumps blood to the body via the aorta. Blood flow path: deoxygenated body blood → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs (gas exchange) → pulmonary veins → left atrium → mitral (bicuspid) valve → left ventricle → aortic valve → aorta → body arteries (spiral down to capillaries for oxygen/nutrient delivery and waste/CO2 pickup) → veins → back to right atrium. Coronary arteries supply the heart muscle itself. Valves ensure one-way flow and prevent backflow. Heart sounds (S1 'lub' from AV valve closure, S2 'dub' from semilunar valve closure) occur in predictable sequence. A murmur is turbulent flow, often from a leaky or stenotic valve. Understanding this path is essential for recognizing how cardiac disease (valve disease, stenosis, MI) affects circulation and what clinical findings to expect.

The respiratory system moves air in and out (ventilation) and exchanges oxygen and carbon dioxide between air and blood (diffusion). Pathway: nose/mouth → pharynx → larynx → trachea → bronchi (left and right) → bronchioles → alveoli (tiny air sacs where gas exchange occurs). Inspiration (inhalation): diaphragm and intercostal muscles contract, thoracic cavity enlarges, air pressure inside drops, air flows in. Expiration (exhalation): muscles relax, thoracic cavity shrinks, air flows out. At the alveoli, oxygen diffuses from air into the bloodstream (where it binds hemoglobin on red blood cells), and CO2 diffuses from blood into the alveoli and is exhaled. Oxygen travels to body tissues where it is released and used for cellular metabolism; CO2 is produced and returned to the lungs to be exhaled. Impaired ventilation (COPD, asthma, pneumonia, muscle weakness) reduces oxygen uptake and CO2 elimination, leading to hypoxemia and hypercapnia. Understanding this is essential for recognizing respiratory disease and the rationale for treatments (oxygen therapy, bronchodilators, mechanical ventilation).

The nervous system has two main divisions: central (brain and spinal cord) and peripheral (nerves extending to all body parts). The brain controls consciousness, movement, sensation, emotion, memory, and autonomic functions. Major brain regions: cerebrum (thinking, language, motor/sensory), cerebellum (balance, coordination), brainstem (breathing, heart rate, basic reflexes). The spinal cord relays signals between brain and periphery and coordinates reflexes. Nerves are bundles of axons that carry electrical signals (action potentials) to and from the central nervous system. Sensory nerves bring information to the brain (touch, pain, temperature, proprioception); motor nerves carry commands from the brain to muscles and organs. The autonomic nervous system has two opposing branches: sympathetic (fight-or-flight: increases heart rate, breathing, blood glucose) and parasympathetic (rest-and-digest: decreases heart rate, increases digestion). Neurotransmitters (acetylcholine, dopamine, serotonin, norepinephrine) are chemicals that relay signals between neurons. Dysfunction in any part (stroke = lost blood to brain tissue, spinal cord injury = lost signal transmission, seizure = abnormal electrical activity) causes specific deficits.

The endocrine system consists of glands that produce hormones (chemical messengers) that regulate metabolism, growth, reproduction, mood, and stress response. Major endocrine glands: pituitary (anterior and posterior: releases hormones controlling growth, metabolism, reproduction, water balance); thyroid (regulates metabolism and body temperature); parathyroid (regulates calcium); adrenal (produces epinephrine/norepinephrine for fight-or-flight, cortisol for stress response); pancreas (produces insulin for glucose regulation, glucagon to raise glucose); and reproductive glands (testes/ovaries produce sex hormones). Hormones circulate in the bloodstream and bind to receptors on target cells, triggering cellular responses. Regulation occurs via negative feedback: when a hormone level is high, it shuts off further production (e.g., high thyroid hormone suppresses thyroid-stimulating hormone from the pituitary). Disruptions cause disease: hyperthyroidism (excess thyroid hormone = fast metabolism, anxiety, weight loss), diabetes (pancreatic insufficiency = high blood glucose), cortisol excess (Cushing's = high blood pressure, weight gain, mood changes). Hormones interact with the nervous system; for example, stress triggers adrenal epinephrine release, which activates sympathetic nerves.

Bones are hard structures made of collagen and mineral (calcium, phosphate) that provide rigid support and leverage. Joints are where bones meet; the type of joint (hinge, ball-and-socket, pivot) determines the range of motion. Muscles are tissues made of contractile fibers that shorten when stimulated by a motor nerve, pulling on bones via tendons (connective tissue anchors) and moving joints. A simple example: the biceps muscle crosses the elbow joint (a hinge); when the motor nerve signals, the biceps contracts and pulls on the radius bone (forearm), flexing the elbow and raising the forearm toward the shoulder. The triceps is the antagonist (opposite action): it extends the elbow. Movement is coordinated by the nervous system: motor cortex sends a signal, spinal cord relays it, motor nerve activates muscle fibers, muscle contracts and bone moves. Proprioceptors (sensory receptors in muscles and joints) send feedback to the brain about body position and movement, allowing precise control. Disruption at any level (muscle disease like muscular dystrophy, nerve damage like stroke, joint arthritis) impairs movement.

The immune system has innate (non-specific, immediate) and adaptive (specific, learned) branches. Innate defenses: skin barrier (prevents pathogen entry), stomach acid (kills ingested pathogens), white blood cells like neutrophils (engulf and kill bacteria immediately, first-line response), and complement proteins (tag pathogens for destruction). Adaptive defenses: B lymphocytes (produce antibodies, Y-shaped proteins that bind specific pathogens and mark them for destruction), T lymphocytes (killer T cells destroy infected cells; helper T cells coordinate immune response). When a pathogen enters the body, it is recognized by pattern-recognition receptors, triggering inflammation (swelling, redness, heat, pain) and an immune response. Immune memory: after an infection or vaccination, the adaptive immune system 'remembers' the pathogen via memory B and T cells, so a second encounter is fought off quickly (immunity). A weak immune system (HIV/AIDS, immunosuppression, chemotherapy) increases infection risk. An overactive immune system causes allergies (IgE antibodies to harmless substances like pollen) and autoimmune disease (immune system attacks self-tissue). Understanding the immune system is essential for interpreting lab tests (WBC count, antibodies), understanding vaccines, and recognizing immunodeficiency.

Electrolytes are minerals (sodium, potassium, calcium, magnesium, chloride, phosphate, bicarbonate) that dissolve in body water as ions and conduct electrical signals critical for nerve, muscle, and heart function. Sodium (Na+): main extracellular ion; maintains fluid balance and nerve signaling. Normal serum 135–145 mEq/L; hypernatremia (>145) causes dehydration and confusion; hyponatremia (<135) causes edema and seizures. Potassium (K+): main intracellular ion; critical for heart and muscle function. Normal serum 3.5–5.0 mEq/L; hyperkalemia (>5) causes cardiac arrhythmias and arrest; hypokalemia (<3.5) causes muscle weakness and arrhythmias. Calcium (Ca2+): critical for bone strength, muscle contraction, nerve function, and blood clotting. Normal serum 8.5–10.5 mg/dL; hypercalcemia (>10.5) causes kidney stones and arrhythmias; hypocalcemia (<8.5) causes muscle cramps and seizures. Magnesium and phosphate support similar functions. Bicarbonate (HCO3-) is the main buffer of blood pH (7.35–7.45). Electrolyte imbalances occur with dehydration, vomiting, diarrhea, medications (diuretics, ACE inhibitors), and kidney disease, and are detected via serum labs (basic metabolic panel). Understanding electrolytes is essential for IV fluid management, medication selection, and interpreting labs.