Principles of Human Anatomy

1. The Body is Organized Hierarchically from Chemicals to Organisms

The levels of structural organization are chemical, cellular, tissue, organ, system, and organismal.

Building blocks. The human body is built from the ground up, starting with the smallest units of matter. Atoms combine to form molecules, which are the chemical basis of life. These molecules then assemble into the smallest living units.

Cells form tissues. Cells, the fundamental structural and functional units, group together with surrounding materials to form tissues. There are four basic tissue types: epithelial (covering/lining), connective (support/bind), muscular (movement), and nervous (control/communication). Tissues work together to perform specific functions.

Organs and systems. Different tissues combine to form organs, structures with specific functions and shapes (like the stomach or heart). Organs then cooperate in systems (like the digestive or cardiovascular systems) to perform complex tasks, culminating in the complete living individual.

• Chemical level: Atoms, molecules (e.g., DNA, glucose)
• Cellular level: Cells (e.g., muscle cell, nerve cell)
• Tissue level: Groups of cells (e.g., smooth muscle tissue)
• Organ level: Different tissues combined (e.g., stomach)
• System level: Related organs (e.g., digestive system)
• Organismal level: All systems together

2. The Skeletal System Provides the Body's Framework and Protection

Bone tissue is a complex and dynamic living tissue.

Support and structure. The skeletal system, composed of bones and cartilage, forms the body's internal framework, supporting soft tissues and providing attachment points for muscles. It protects vital organs like the brain, spinal cord, heart, and lungs. Bones are classified by shape: long, short, flat, irregular, and sesamoid.

Movement and storage. Bones act as levers, allowing muscles to produce movement. They also store essential minerals like calcium and phosphorus, releasing them into the blood as needed. Red bone marrow within certain bones is the site of blood cell production.

• Bone shapes: Long (femur), Short (carpals), Flat (skull), Irregular (vertebrae), Sesamoid (patella)
• Functions: Support, Protection, Movement, Mineral Storage, Blood Cell Production, Triglyceride Storage

Dynamic tissue. Bone is constantly being remodeled through bone resorption (breakdown by osteoclasts) and bone deposition (formation by osteoblasts). This process allows bone to adapt to stress, repair fractures, and maintain mineral homeostasis. Aging can lead to bone mass loss and increased brittleness.

3. Muscles Generate Force for Movement and Maintain Posture

Most of the work done by the body, such as pumping blood through the blood vessels, eating, breathing, moving food through the gastrointestinal tract, moving urine out of the urinary bladder, generating heat, speaking, standing up straight, and getting our skeletons to move, is a result of the activity of muscles.

Types of muscle. The body contains three types of muscular tissue: skeletal, cardiac, and smooth. Skeletal muscle is primarily attached to bones for voluntary movement. Cardiac muscle forms the heart wall for involuntary pumping. Smooth muscle is in internal organs for involuntary functions like digestion and blood vessel control.

Properties and functions. Muscular tissue is electrically excitable, contractible (generates force), extensible (stretches), and elastic (returns to shape). Beyond movement, muscles stabilize body positions, store and move substances (like blood and food), and generate heat (thermogenesis).

• Muscle types: Skeletal (voluntary, striated), Cardiac (involuntary, striated, autorhythmic), Smooth (involuntary, nonstriated, some autorhythmic)
• Functions: Movement, Stabilization, Storage/Movement of substances, Heat production

Contraction mechanism. Muscle contraction involves the sliding filament mechanism, where myosin heads pull thin actin filaments towards the center of the sarcomere. This process is triggered by calcium ions released from the sarcoplasmic reticulum, initiated by a muscle action potential from a neuron at the neuromuscular junction.

4. The Nervous System Controls and Communicates Throughout the Body

The brain is the center for registering sensations, correlating them with one another and with stored information, making decisions, and taking actions.

Neurons and neuroglia. The nervous system is a complex network of neurons (nerve cells) and neuroglia (supporting cells). Neurons are electrically excitable, transmitting nerve impulses. Neuroglia support, nourish, and protect neurons. The system is organized into the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves outside the CNS).

Functions and divisions. The nervous system performs sensory input, integrative processing, and motor output. The PNS is divided into the somatic nervous system (voluntary control of skeletal muscles, somatic/special senses) and the autonomic nervous system (involuntary control of smooth muscle, cardiac muscle, glands, visceral senses). The ANS has sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions.

• CNS: Brain, Spinal Cord
• PNS: Cranial nerves, Spinal nerves, Ganglia, Sensory receptors
• Functions: Sensory, Integrative, Motor
• PNS Divisions: Somatic (voluntary), Autonomic (involuntary), Enteric (GI tract)

Brain regions. The brain has four major parts: the brainstem (medulla, pons, midbrain - vital functions, relay), cerebellum (coordination, balance), diencephalon (thalamus - relay, hypothalamus - homeostasis, epithalamus - melatonin), and cerebrum (largest part - thought, sensation, movement control). Cranial nerves emerge from the brain, serving the head and neck.

5. The Cardiovascular System Transports Blood for Life's Processes

The heart beats about 100,000 times every day, which adds up to about 35 million beats in a year, and approximately 2.5 billion beats in an average lifetime.

Blood composition. Blood is a liquid connective tissue consisting of blood plasma (watery extracellular matrix with proteins and solutes) and formed elements (red blood cells, white blood cells, platelets). Red blood cells transport oxygen via hemoglobin. White blood cells defend against disease. Platelets are involved in clotting.

Heart as a pump. The heart, a four-chambered muscular organ, pumps blood through the body's vessels. The right side pumps deoxygenated blood to the lungs (pulmonary circulation) for gas exchange. The left side pumps oxygenated blood to the rest of the body (systemic circulation).

• Blood components: Plasma (water, proteins, solutes), Formed elements (RBCs, WBCs, Platelets)
• Heart chambers: 2 Atria (receiving), 2 Ventricles (pumping)
• Circulation routes: Pulmonary (heart to lungs and back), Systemic (heart to body and back)

Blood vessels. A network of vessels carries blood: arteries (away from heart), arterioles (smaller arteries), capillaries (exchange vessels), venules (smaller veins), and veins (toward heart). Vessel walls have three layers (tunics), with variations in thickness and elasticity reflecting function. Valves in veins prevent backflow.

6. The Lymphatic System Defends Against Disease and Manages Fluids

The lymphatic (lymphoid) system is one of the principal body systems that helps to defend us against disease-producing microbes.

Lymph and vessels. The lymphatic system includes lymph (fluid), lymphatic vessels (transport lymph), lymphatic tissue (with lymphocytes), and red bone marrow. Lymphatic capillaries collect excess interstitial fluid and return it to the bloodstream, preventing edema and maintaining blood volume.

Immune defense. Lymphatic tissue, found in organs like lymph nodes, the spleen, and the thymus, houses lymphocytes (B cells and T cells) and macrophages. These cells carry out immune responses against pathogens and abnormal cells (antigens).

• Components: Lymph, Lymphatic vessels, Lymphatic tissue/organs, Red bone marrow
• Functions: Drain excess interstitial fluid, Transport dietary lipids, Carry out immune responses

Lymphatic organs. Primary lymphatic organs (red bone marrow, thymus) are where lymphocytes mature. Secondary lymphatic organs (lymph nodes, spleen, lymphatic nodules like tonsils) are where immune responses occur. Lymph nodes filter lymph, and the spleen filters blood.

7. The Endocrine System Regulates Body Functions Through Hormones

Together, the nervous and endocrine systems coordinate functions of all body systems.

Hormone secretion. Endocrine glands secrete hormones directly into the interstitial fluid, which then diffuse into the bloodstream. Hormones travel throughout the body but only affect target cells that possess specific receptors for that hormone. Exocrine glands, in contrast, secrete products into ducts.

Endocrine glands. Major endocrine glands include the pituitary (often called the "master gland"), thyroid, parathyroid, adrenal, and pineal glands. Other organs like the hypothalamus, pancreas, ovaries, and testes also contain hormone-secreting cells.

• Major glands: Pituitary, Thyroid, Parathyroid, Adrenal, Pineal
• Other endocrine tissues: Hypothalamus, Pancreas, Ovaries, Testes, Kidneys, etc.

Hormone actions. Hormones regulate a wide range of physiological activities, including metabolism, energy balance, growth, development, reproduction, and responses to stress. The hypothalamus links the nervous and endocrine systems, controlling pituitary hormone release via releasing and inhibiting hormones transported through the hypophyseal portal system.

8. The Respiratory System Facilitates Gas Exchange for Cellular Energy

The respiratory system is responsible for gas exchange—intake of O2 and elimination of CO2—and the cardiovascular system transports blood containing the gases between the lungs and body cells.

Airways and lungs. The respiratory system includes the nose, pharynx, larynx, trachea, bronchi, and lungs. The conducting zone (nose to terminal bronchioles) filters, warms, and moistens air. The respiratory zone (respiratory bronchioles to alveoli) is where gas exchange occurs.

Gas exchange. The lungs contain millions of tiny air sacs called alveoli, surrounded by capillaries. Oxygen from inhaled air diffuses across the thin respiratory membrane into the blood, while carbon dioxide from the blood diffuses into the alveoli to be exhaled.

• Conducting zone: Nose, Pharynx, Larynx, Trachea, Bronchi, Bronchioles
• Respiratory zone: Respiratory bronchioles, Alveolar ducts, Alveoli
• Core function: Gas exchange (O2 in, CO2 out)

Breathing mechanics. Pulmonary ventilation (breathing) involves inhalation (air in) and exhalation (air out), driven by pressure differences created by changes in thoracic cavity volume. The diaphragm and intercostal muscles are key respiratory muscles. Breathing is regulated by the respiratory center in the brainstem, influenced by factors like blood CO2 and O2 levels.

9. The Digestive System Breaks Down Food for Absorption

The digestive system... is composed of two groups of organs: the gastrointestinal (GI) tract and the accessory digestive organs.

GI tract. The gastrointestinal (GI) tract is a continuous tube from mouth to anus, including the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Its wall has four layers: mucosa (lining), submucosa (connective tissue, nerves, vessels), muscularis (muscle for mixing/propulsion), and serosa/adventitia (outer covering).

Accessory organs. Accessory digestive organs (teeth, tongue, salivary glands, liver, gallbladder, pancreas) aid digestion but are not part of the tube. They physically break down food (teeth, tongue) or produce/store secretions (saliva, bile, enzymes) that enter the GI tract.

• GI tract organs: Mouth, Pharynx, Esophagus, Stomach, Small Intestine, Large Intestine
• Accessory organs: Teeth, Tongue, Salivary glands, Liver, Gallbladder, Pancreas

Digestive processes. The system performs ingestion, secretion, mixing/propulsion (motility), digestion (mechanical and chemical), absorption (into blood/lymph), and defecation (waste elimination). Most digestion and absorption occur in the small intestine, which has structural adaptations like circular folds, villi, and microvilli to increase surface area.

10. The Urinary System Filters Blood and Eliminates Wastes

While the respiratory system rids the body of carbon dioxide, the urinary system disposes of most other unneeded substances.

Kidneys. The kidneys are the primary organs, filtering blood to produce urine. They are retroperitoneal, protected by ribs and surrounded by tissue layers (renal capsule, adipose capsule, renal fascia). Internally, they have a cortex and medulla containing functional units.

Nephrons. The microscopic functional units are nephrons, consisting of a renal corpuscle (filtration) and a renal tubule (reabsorption and secretion). Blood flows into the glomerulus (capillary network) for filtration, then through peritubular capillaries and vasa recta for exchange with the tubule.

• Organs: Kidneys, Ureters, Urinary bladder, Urethra
• Kidney regions: Cortex, Medulla, Pyramids, Columns, Calyces, Pelvis
• Nephron parts: Renal corpuscle (glomerulus, capsule), Renal tubule (PCT, loop, DCT)

Urine formation. Urine production involves glomerular filtration (plasma filtered into tubule), tubular reabsorption (useful substances returned to blood), and tubular secretion (wastes/excess added to filtrate). Urine drains from nephrons into collecting ducts, papillary ducts, calyces, renal pelvis, ureters, urinary bladder (storage), and exits via the urethra.

11. Reproductive Systems Ensure the Continuation of the Species

Humans produce offspring by a process called sexual reproduction in which haploid sperm cells produced by the testes of males fertilize the haploid secondary oocytes produced by the ovaries of females.

Gonads. The primary reproductive organs (gonads) are the testes in males and ovaries in females. Testes produce sperm (spermatogenesis) and male hormones (testosterone). Ovaries produce secondary oocytes (oogenesis) and female hormones (estrogens, progesterone). Gametes are haploid (n), combining to form a diploid (2n) zygote during fertilization.

Ducts and glands. Ducts transport and store gametes (epididymis, ductus deferens, uterine tubes). Accessory glands produce fluids that support gametes and form semen in males (seminal vesicles, prostate, bulbourethral glands). The uterus in females is where embryonic and fetal development occurs.

• Male organs: Testes, Epididymis, Ductus deferens, Ejaculatory ducts, Urethra, Seminal vesicles, Prostate, Bulbourethral glands, Penis, Scrotum
• Female organs: Ovaries, Uterine tubes, Uterus, Vagina, Vulva, Mammary glands

Reproductive cycle. The female reproductive cycle involves cyclical changes in the ovaries (ovarian cycle) and uterus (uterine cycle), regulated by hormones from the hypothalamus, pituitary, and ovaries. Key events include follicular development, ovulation (release of oocyte), and preparation/shedding of the uterine lining.

12. Surface Anatomy Provides External Clues to Internal Structures

A knowledge of surface anatomy will not only help you to identify structures on the body’s exterior, but it will also assist you in locating the positions of various internal structures.

Visualization and palpation. Surface anatomy is the study of external landmarks. It involves visualizing (looking) and palpating (feeling) structures through the skin. This allows identification of bones, muscles, vessels, and projections that serve as guides.

Clinical application. Knowledge of surface anatomy is crucial for physical examinations and medical procedures. It helps locate pulse points, injection sites, surgical incision lines, and areas for listening to internal sounds (heart, lungs).

• Regions: Head, Neck, Trunk, Upper Limbs, Lower Limbs
• Techniques: Visualization, Palpation
• Applications: Pulse, Injections, Incisions, Auscultation

Mapping internal structures. By correlating external landmarks with underlying anatomy (often using transverse sections), clinicians can estimate the position of internal organs and structures that are not directly visible or palpable. This enhances diagnostic accuracy and procedural safety.

Last updated: May 20, 2025