Human Body Systems
Whether you're researching a specific condition or looking for a specialist, it's helpful to understand the basic categorization of the body's organ 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.
The skeletal system is the framework of the body. Bones provide rigidity and strength that support the body and allow movement and mobility. They also serve as a protective shield for internal organs. Bones connect at joints. The hip and shoulder are examples of a ball and socket joint, while the knee and elbow are examples of hinge joints. Other joints have limited movement ability or are immovable. The spinal vertebrae are an example of a joint with little movement, while the bones in the skull are so tightly fused that they are considered to be a single unit. Ligaments are tough bands of connective tissue that connect bones at joints. They maintain joint stability while still allowing for free movement. Inside most bone we find a soft tissue called bone marrow. 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. Small amounts of adipose tissue (fat) is also stored in bone marrow.
Primary function: Physically supports and protects the body and enables movement.
Key parts: Bones, joints, and ligaments.
The respiratory system is comprised of the lungs and the airways. It works together with the cardiovascular system to provide oxygen to the body and to remove carbon dioxide from the blood. External respiration, 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 allows the oxygen in the air to move into the tiny blood vessels (capillaries) found in the lungs. At the same time, carbon dioxide moves out of the blood vessels in the lung and into the air in the lungs. Expiration (exhalation) moves this carbon dioxide-rich air out of the lungs.
Internal respiration is the exchange of oxygen and carbon dioxide that takes place between the blood and metabolizing cells throughout the body.
Primary function: Absorbs oxygen from the air and removes carbon dioxide from the body.
Key organs: Mouth, nose, nostrils, sinuses, pharynx, cilia, trachea, larynx, diaphragm, lungs, bronchi, alveoli
The muscular system is responsible for the movement of the body and internal organs. Skeletal (striated) muscles control voluntary movement. Smooth muscles control involuntary motion in blood vessels and throughout the digestive tract. Cardiac muscle is responsible for the rhythmic contraction of the heart. Tendons are fibrous connective tissue that bind muscles to bones and stabilize joints during movement.
Primary function: Voluntary and involuntary movement of body parts and organs.
Key organs: Muscles and tendons.
The integumentary system refers to the structures that serve as the outer protection for the body. Skin helps to protect fragile underlying tissues and organs and acts as a barrier against infection and trauma.
The skin contains nerves that allow the body to experience touch, temperature and pain. It also plays a role in body temperature regulation and synthesizes vitamin D from sun exposure. In addition, the skin produces hair across most of the body. Hair serves to retain heat and plays a role in perceiving touch. Skin is the largest body organ.
Primary function: Protective barrier for the body against infection and trauma; is an important sensory organ.
Key organs: Skin, hair, and nails.
The cardiovascular system consists of the heart and the blood vessels (arteries, veins, and capillaries). This system supplies oxygen-rich blood to all cells of the body. The left side of the heart contains oxygenated blood that originates from the lungs. This blood is transported throughout the body in large blood vessels called arteries. Arteries branch into arterioles and then into capillaries where oxygen, carbon dioxide, water, and nutrients are exchanged with surrounding tissue. Veins then transport the deoxygenated blood back to the right side of the heart, where it is pumped back to the lungs for reoxygenation and carbon dioxide removal.
Primary function: Supplies oxygen-rich blood to the body and removes carbon dioxide waste from the bloodstream to prevent toxic accumulation.
Key organs: Heart, blood vessels (arteries, veins, and capillaries), and blood.
The lymphatic system collects and transports lymph from tissues back into the bloodstream. Lymph is formed from the fluid that moves out of the bloodstream and the body's cells into the tissue. It is rich in nutrients and white blood cells.
Lymph circulates from the tissue back into the bloodstream through lymph channels. At regular points along the way, lymph nodes monitor and filter the lymph, looking for toxins and infections. The lymph channels merge to form bigger lymphatic vessels that join the subclavian veins in the neck.
The largest organ in the lymphatic system is the spleen, which filters the 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 infective agents such as viruses, bacteria, 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 function: Filters out lymph fluid to help the body to maintain proper fluid balance and to help maintain the body's immunity function.
Key organs: Spleen, thymus, tonsils, lymph nodes, and lymphatic vessels.
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.
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 and 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 occurs. The process is aided by bile made in the liver. Indigestible food is then transported to the large intestine (colon) for final water and salt electrolyte absorption. The large intestine also absorbs vitamins produced by resident bacteria before passing the waste to the rectum and anus for evacuation.
Primary function: Breaks down food into usable components for the body's cells; eliminates toxic waste from the body.
Key organs: Mouth, teeth, tongue, esophagus, stomach, pancreas, liver, small intestine, large intestine (colon), rectum, and anus.
The endocrine system is composed of the pancreas and glands that secrete hormones. Hormones are proteins that regulate various body processes. 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 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, stress response, and sexual development. The pancreas is located in the abdominal cavity and regulates insulin, which is a hormone that controls blood sugar levels.
Primary function: Controls various metabolic functions of the body through hormones.
Key organs: Pituitary gland, thyroid gland, adrenal glands, and pancreas.
The nervous system serves as the body's main messaging center. It controls the body's interaction with its environment by allowing thought, memory, and motion. It also enables the functioning of the senses such as 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 of the 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 function: Control and regulate conscious and unconscious functioning of the body; allows sensing and interaction with the environment.
Key organs: Brain, spinal cord, ganglia, nerves, and sensory organs.
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 ureter 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 function: Filters waste products from the blood and forms urine which can then be excreted by the body.
Key organs: Kidneys, ureters, bladder, and urethra.
The reproductive system enables reproduction and sexual functioning. In a female, ovaries are small glands that contain eggs and produce female hormones. An egg can be fertilized by sperm 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.
In a male, the testes produce sperm, which can fertilize an egg. The vas deferens transports mature sperm into the urethra. The prostate provides extra fluid for the sperm to move easily. During sexual intercourse, sperm exits through the urethra.
Primary function: The reproductive system is responsible for sexual functioning and reproduction. It also produces sex hormones such as estrogen and testosterone.
Key organs: Fallopian tubes, uterus, vagina, testes, ductus (vas) deferens, urethra, penis, and prostate.
- 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