Human Body Systems
Organs of the human body are commonly grouped into eleven systems. Each body system includes organs and structures that serve a common purpose. The systems are highly interdependent, working together to sustain life and enable interaction with the surrounding environment.
Cardiovascular
The cardiovascular system, also known as the circulatory system, consists of the heart and a network of blood vessels—arteries, veins, and capillaries. This essential system delivers oxygen-rich blood to every cell in the body. The right side of the heart pumps blood to the lungs for reoxygenation and carbon dioxide removal. Subsequently, the left side of the heart receives this oxygenated blood from the lungs and circulates it throughout the body via arteries. These arteries narrow into arterioles and further into capillaries, where oxygen, carbon dioxide, water, and nutrients are exchanged with cells in surrounding tissues. Veins then carry deoxygenated blood back to the right side of the heart.
Primary functions: Supplies oxygen-rich blood to the body and removes carbon dioxide waste from the bloodstream to prevent toxic accumulation. Transports nutrients and hormones throughout the body.
Key organs: Heart, blood vessels (arteries, veins, and capillaries), and blood.
In depth: Blood Components
Digestive
The digestive system handles ingestion of food and liquids, breaking these down into their nutritional components to fuel the body. It also separates and eliminates waste products.
The digestive system is responsible for the ingestion of food and liquids, breaking these down mechanically and chemically into their nutritional components to fuel the body. It also separates and eliminates waste products.
Digestion starts in the mouth when food is chewed and mixed with saliva. Saliva contains enzymes that break down starches into simple sugars. After swallowing, food passes through the esophagus into the stomach, where digestive enzymes break down proteins, fats, and carbohydrates. Food then enters the small intestine, where further digestion and most nutrient absorption occur. This process is aided by bile, which is made in the liver and stored in the gallbladder. Bile breaks down fats and carries away waste. The pancreas (also part of the endocrine system) produces pancreatic juices that contain enzymes that aid in digestion. Indigestible food is then transported to the large intestine for final water and electrolyte (salt) absorption. Part of the large intestine, the colon, also absorbs vitamins produced by resident bacteria before passing the waste to the rectum and anus for evacuation.
Primary functions: Breaks down food into usable components for the body’s cells and eliminates toxic waste from the body.
Key organs: Mouth, teeth, tongue, esophagus, stomach, pancreas, liver, gallbladder, small intestine, and large intestine (cecum, colon, rectum, and anal canal).
Endocrine
The endocrine system is composed of several organs and numerous glands that produce and secrete hormones. Hormones are released into the bloodstream and travel to various organs and tissues, where they regulate bodily processes, such as growth, metabolism, and reproduction. Hormones have very narrow optimum concentrations, so too much or too little can significantly affect the body’s functioning. The pineal gland produces and regulates melatonin, the hormone that controls sleep patterns. The pituitary gland produces the hormones that control the thyroid and adrenal glands. The thyroid gland is a small gland in the front of the neck that helps regulate metabolism, growth, and development. The adrenal glands sit on top of the kidneys and are involved in blood pressure control, the stress response, and sexual development. The pancreas is located in the abdominal cavity and produces insulin, which is a hormone that controls blood sugar levels.
Primary functions: Produces and secretes hormones that regulate various metabolic functions.
Key organs: Pituitary gland, pineal gland, thyroid gland, parathyroid glands, adrenal glands, pancreas, testes, and ovaries.
In depth: Endocrine System
Integumentary
The integumentary system includes structures that serve as outer protection for the body. It includes the skin, hair, nails, and glands.
Skin helps protect fragile underlying tissues and organs, acting as a barrier against physical damage and infection. It is composed of multiple layers and contains nerves that allow the body to feel temperature, pressure, and pain. It also plays a role in body temperature regulation and synthesizes vitamin D from sun exposure. The skin is the largest organ in the body.
The skin produces hair across most of the body. Hair serves to retain heat and plays a role in perceiving touch. The glands within the integumentary system include sweat glands, which help regulate body temperature, and sebaceous glands, which produce protective oils.
Primary functions: Protection against external damage and infection, regulation of body temperature, and sensory reception.
Key organs: Skin, hair, nails, sebaceous and sweat glands.
Lymphatic and Immune
The lymphatic system collects and transports lymph (a fluid) from the tissues back into the bloodstream through lymph channels. Lymph is formed from the fluid that moves out of the bloodstream and the body’s cells into tissues. It is rich in nutrients and white blood cells. At regular points along the way, lymph nodes monitor and filter the lymph, checking for toxins and infectious agents. The lymph channels merge to form larger lymphatic vessels that join the subclavian veins in the neck.
The largest organ in the lymphatic system is the spleen, which filters blood and produces lymphocytes (white blood cells that fight infection). Lymphocytes include T cells, B cells, and natural killer (NK) cells. These cells work to identify infectious agents, such as viruses, bacteria, parasites, and fungi. They are able to neutralize them by attacking the infectious agent and triggering the formation of antibodies that assist in destroying the infection.
Primary functions: Filters out lymph fluid to maintain proper fluid balance and helps maintain the body's immune function.
Key organs: Spleen, thymus, tonsils, lymph nodes, lymphatic vessels, and bone marrow.
Muscular
The muscular system is responsible for the movement of the body and internal organs. Skeletal (striated) muscles control voluntary movement, in addition to maintaining posture. Tendons are fibrous connective tissue that bind muscles to bones and stabilize joints during movement. Smooth muscles in the walls of blood vessels and the digestive tract control involuntary movement that supports blood flow and digestion. Cardiac muscle is responsible for the rhythmic contraction of the heart.
Primary functions: Provides voluntary and involuntary movement of body parts and organs.
Key organs: Skeletal muscle and tendons, smooth muscle, and cardiac muscle.
Nervous
The nervous system serves as the body’s main messaging center by sending and receiving signals throughout the body. It controls the body’s interaction with its environment by allowing thought, memory, and motion. It also enables the functioning of the senses, including taste, touch, sight, hearing, and smell. These processes occur through messages sent along specialized nerve cells called neurons.
The central nervous system consists of the brain and spinal cord. The peripheral nervous system consists of nerves that lie outside the central nervous system. It sends information to and from the central nervous system. The peripheral nervous system has two parts. The autonomic nervous system controls the involuntary functions of the heart, smooth muscle, and many glands in the body. The somatic nervous system sends signals from the central nervous system to the skeletal muscles to mediate voluntary movement. Ganglia are nerve clusters that function to relay nerve impulses by connecting the central and peripheral nervous systems.
Primary functions: Controls and regulates conscious and unconscious functioning of the body. Sends and receives signals throughout the body to allow sensing of and interaction with the environment.
Key organs: Brain, spinal cord, ganglia, nerves, and sensory organs.
In depth: Nervous System
Reproductive
The reproductive system enables reproduction and sexual functioning.
In females, the ovaries are small glands that contain eggs and produce female hormones. An egg can be fertilized by sperm that is introduced into the vagina during sexual intercourse. Fertilization usually occurs in the fallopian tubes, small tubes connecting the uterus to the ovaries. The fertilized egg then implants into the uterus, where it develops into a fetus and is eventually delivered during childbirth.
In males, the testes produce sperm that can fertilize an egg. The vas deferens transports mature sperm into the urethra. The prostate provides extra fluid for the sperm to move easily. Sperm exits through the urethra during sexual intercourse.
Primary functions: The reproductive system is responsible for sexual functioning and reproduction. It produces and transports gametes and develops offspring. It also produces sex hormones, such as estrogen and testosterone.
Key organs: Ovaries, fallopian tubes, uterus, vagina, testes, ductus (vas) deferens, urethra, penis, and prostate.
Respiratory
The respiratory system consists of the lungs and airways. It works together with the cardiovascular system to provide oxygen to the body and remove carbon dioxide from the blood. Ventilation, or breathing, is the movement of air into and out of the lungs through the airways. Inspiration (breathing in) pulls air into the lungs, and expiration (breathing out) moves carbon dioxide-rich air out of the lungs. Within the lungs, external respiration takes place. Oxygen in the air moves into capillaries (tiny blood vessels) found in the lungs. At the same time, carbon dioxide moves out of the blood vessels in the lungs and into the air in the lungs. In the purview of the circulatory system, internal respiration is the exchange of oxygen and carbon dioxide, which takes place between the blood and metabolizing cells throughout the body.
Primary functions: Absorbs oxygen from the air and removes carbon dioxide from the body.
Key organs: Mouth, nose, sinuses, pharynx, cilia, trachea, larynx, diaphragm, lungs, bronchi, alveoli.
In depth: Respiratory System
Skeletal
The skeletal system is the framework of the body. Bones provide rigidity and strength that support the body, protect internal organs, and allow movement and mobility. Bones connect at joints. The hip and shoulder are examples of ball and socket joints, and the knee and elbow are examples of hinge joints. Other joints have limited movement or are immovable. The individual joints between spinal vertebrae allow limited movement, while the bones in the skull are so tightly fused that they are considered a single unit. Ligaments are tough bands of connective tissue that connect bones at joints. They maintain joint stability while still allowing free movement. The soft tissue within some bones, called bone marrow, produces red and white blood cells in a process called hematopoiesis. Bones store minerals such as calcium and phosphorus, which are released into the bloodstream as needed. Bone marrow also stores small amounts of adipose tissue (fat).
Primary functions: Physically supports and protects the body and enables movement.
Key organs: Bones, cartilage, and ligaments.
In depth: Skeletal System
Urinary
The urinary system filters and removes extra water, salts, and waste produced by the body. The primary filtering process occurs in the kidneys, which produce urine. Urine is composed of urea (a waste product), water, and electrolytes. Urine is transported through the ureters to the urinary bladder for storage and is expelled through the urethra during urination. The urinary system also helps regulate blood volume, blood pressure, blood pH, and electrolyte levels.
Primary functions: Filters waste products from the blood and forms urine, which can then be excreted by the body.
Key organs: Kidneys, ureters, bladder, and urethra.
Additional resources:
- Introduction to the Human Body by the National Cancer Institute
- Human Body Systems at Kenhub
- The Human Body at Healthline
- Human Body Organ Systems at Merck Manuals
- Shani Saks, MD
- Reitze Rodseth MBBS, PhD
- Lynette Stewart, BSN
- Brian Sullivan, MD