Anatomy & Physiology Study Guide

The 11 Organ Systems at a Glance

The human body is organized into 11 organ systems, each a network of organs and tissues working toward a shared function: integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic/immune, respiratory, digestive, urinary, and reproductive. HOSA questions can draw from any of these systems, and all of them appear regularly in competition question sets. A few quick baseline facts you should have memorized cold: adults have 206 bones, over 640 named skeletal muscles, and approximately 37 trillion cells in the human body. The largest organ is the skin (integumentary system); the largest internal organ is the liver.

Cardiovascular System

The heart has four chambers: the right atrium, right ventricle, left atrium, and left ventricle. Deoxygenated blood from the body enters the right atrium via the superior and inferior venae cavae, passes to the right ventricle, and is pumped to the lungs via the pulmonary arteries — this is pulmonary circulation. Oxygenated blood returns from the lungs via the pulmonary veins into the left atrium, then the left ventricle, which pumps it out through the aorta to the body — this is systemic circulation. The heart's valves (tricuspid, pulmonary, mitral/bicuspid, aortic) prevent backflow. The sinoatrial (SA) node, located in the right atrium, is the heart's natural pacemaker. A normal resting heart rate is 60–100 beats per minute.

Nervous System

The nervous system divides into the central nervous system (CNS) — brain and spinal cord — and the peripheral nervous system (PNS) — all other nerves. The brain has four lobes: the frontal lobe (executive function, voluntary movement, speech production), parietal lobe (sensory processing, spatial awareness), temporal lobe (hearing, memory, language comprehension), and occipital lobe (vision). The cerebellum coordinates balance and fine motor control; the brainstem regulates involuntary functions like breathing and heart rate. The basic functional unit of the nervous system is the neuron. Neurons communicate via electrical signals called action potentials and chemical messengers called neurotransmitters across gaps called synapses.

Skeletal & Muscular Systems

The adult human skeleton contains 206 bones, down from approximately 270–300 at birth (many fuse during development). Bones are classified as long (femur, humerus), short (carpals, tarsals), flat (skull plates, sternum), irregular (vertebrae), and sesamoid (patella). The axial skeleton comprises the skull, vertebral column, and rib cage (80 bones); the appendicular skeleton comprises the limbs and girdles (126 bones). Joints are classified as fibrous (immovable, e.g., sutures of the skull), cartilaginous (slightly movable, e.g., intervertebral discs), and synovial (freely movable, e.g., knee, shoulder). Skeletal muscle fibers are striated and under voluntary control; cardiac muscle is striated but involuntary; smooth muscle is non-striated and involuntary.

Respiratory & Digestive Systems

Air enters via the nose or mouth, passes through the pharynx, larynx, and trachea, and enters the lungs through the left and right bronchi, which branch into bronchioles and terminate in tiny air sacs called alveoli — the site of gas exchange. Oxygen diffuses into surrounding capillaries while carbon dioxide diffuses out. The diaphragm is the primary muscle of breathing; its contraction causes inhalation. In the digestive system, chemical digestion of carbohydrates begins in the mouth (salivary amylase), protein digestion begins in the stomach (pepsin, hydrochloric acid), and lipid digestion occurs primarily in the small intestine (pancreatic lipase, bile from the gallbladder). Nutrient absorption occurs mainly in the small intestine (duodenum, jejunum, ileum); water is reabsorbed in the large intestine.

Endocrine & Urinary Systems

The endocrine system uses hormones — chemical messengers secreted directly into the bloodstream — to regulate growth, metabolism, reproduction, and homeostasis. Key glands and their hormones: the pituitary ("master gland") secretes growth hormone, TSH, and ADH; the thyroid secretes thyroxine (T4) and triiodothyronine (T3) regulating metabolism; the adrenal glands secrete cortisol (stress response) and adrenaline (epinephrine); the pancreas secretes insulin (lowers blood glucose) and glucagon (raises blood glucose); the gonads (testes and ovaries) secrete sex hormones. The functional unit of the kidney is the nephron, which filters blood, reabsorbs needed substances, and excretes waste as urine. The kidneys maintain fluid balance, electrolyte levels, and acid-base balance — all central to the concept of homeostasis, the body's maintenance of a stable internal environment.