The human body has 11 organ systems when the integumentary system is added to the ten on the National Cancer Institute's SEER list. They are the integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary and reproductive systems. SEER defines a system as a group of organs arranged so that together they perform complex functions for the body. Other references count 10 or 12 because they group the same organs differently.
The 11 organ systems, what each does and its main organs
| System | What it does | Main organs |
|---|---|---|
| Integumentary | Covers and protects the body, holds sensory nerve endings and helps regulate body temperature | Skin, hair, nails, sweat and oil glands |
| Skeletal | Supports and protects the soft organs, works with muscles to move the body and stores calcium | Bones, cartilage, ligaments, tendons |
| Muscular | Produces movement, posture, joint stability and body heat | Skeletal muscles, tendons |
| Nervous | Controls and communicates: senses changes, integrates them and signals muscles and glands | Brain, spinal cord, nerves, ganglia |
| Endocrine | Releases hormones that influence growth, development and metabolism | Pituitary, thyroid, pancreas, adrenal glands, testes, ovaries |
| Cardiovascular | Pumps blood around a closed circuit that delivers nutrients and carries wastes away | Heart, arteries, veins, capillaries |
| Lymphatic | Returns excess fluid to the blood, absorbs fats and defends against microorganisms | Lymph nodes, spleen, thymus, tonsils |
| Respiratory | Supplies oxygen, removes carbon dioxide and helps regulate blood pH | Nose, pharynx, larynx, trachea, bronchi, lungs |
| Digestive | Breaks food down, absorbs it and removes the waste | Mouth, esophagus, stomach, small and large intestine; liver, gallbladder, pancreas |
| Urinary | Keeps the volume and composition of body fluids within normal limits and removes wastes | Kidneys, ureters, bladder, urethra |
| Reproductive | Produces egg and sperm cells and hormones; in females, nurtures the developing offspring | Testes, ovaries and the ducts and glands that serve them |
Two systems are easy to put numbers on. SEER says the adult skeleton usually has 206 named bones, set out region by region in how many bones are in the human body. The skin is the largest organ, and it leads the list of the largest organs in the human body.
To see where the parts sit, open the 3D human anatomy explorer. The bones are named on the 3D skeleton, the muscles are on the 3D muscles, and single organs can be found with the organ locator.
Why the count is 10, 11 or 12
| Reference | Systems | How its list differs |
|---|---|---|
| National Cancer Institute, SEER training modules | 10 | No integumentary system on the list |
| SEER's ten with the integumentary system added | 11 | Adds the system of the skin, hair and nails |
| Cleveland Clinic | 12 | Adds an exocrine system to the eleven |
| Washington State University, medical school | 12 | Merges muscles and bones; adds immune and behavioral systems |
The SEER module names ten major systems: skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary and reproductive. Adding the integumentary system, the skin with its hair, nails and glands, reaches eleven. SEER itself covers the exocrine glands, such as the sweat, sebaceous and mammary glands, inside its endocrine module.
Cleveland Clinic says the number runs from 10 to 12 depending on who does the grouping. Its own list of 12 is those eleven plus an exocrine system, the glands that make secretions such as mucus and tears. It also names merged labels that shorten other lists. Musculoskeletal covers the muscular and skeletal systems, genitourinary covers the reproductive and urinary systems, and cardiorespiratory covers circulation and breathing.
Washington State University's medical school reaches 12 by another route. Its definitions join muscles, bones and cartilage in one musculoskeletal system, group blood formation with the lymphatic system, and list separate immune and behavioral systems.
How organ systems work together: keeping blood calcium steady
The SEER modules follow calcium across four systems. Bone holds more calcium than any other organ, and it acts as the store that the others draw on.
- When blood calcium drops under the normal range, the parathyroid glands, part of the endocrine system, secrete parathyroid hormone. SEER calls it the most important regulator of blood calcium, and its effect is to raise the level.
- Calcium is released from the skeleton into the blood so that metabolic needs have an adequate supply.
- When blood calcium climbs past the normal range, the excess is stored in the bone matrix.
- The thyroid gland also secretes calcitonin, which opposes the parathyroid glands and keeps lowering the level until it is back to normal.
SEER says the release and storage go on almost continuously. The nervous and muscular systems feel the result when the supply runs short. In hypoparathyroidism, too little parathyroid hormone lowers blood calcium, which triggers spontaneous nerve impulses and muscle contraction.
Organs that belong to more than one system
Assigning organs to systems is not exact, because many organs contribute to more than one. SEER's own pages show this. The pancreas is listed as an accessory organ of digestion and as an endocrine gland. The ovaries and testes are endocrine glands and also the primary organs of reproduction. The urinary module notes that the lungs, the skin, the liver and the intestines also excrete wastes.
From cells to organ systems
The SEER module describes four kinds of structure, each built from the one before it: cells, tissues, organs and systems. Its example is the stomach, an organ made of muscle, connective, epithelial and nervous tissue.