Although each organ (see Tissues and Organs) has its specific functions, organs also function together in groups, called organ systems (see Table: Major Organ Systems). Doctors categorize disorders and their own medical specialties according to organ systems, and this book also is structured largely around the concept of organ systems.
Some examples of organ systems and their functions include
The digestive (or gastrointestinal) system (see Overview of the Digestive System), extending from the mouth to the anus, is responsible for receiving and digesting food and excreting waste. This system includes not only the stomach, small intestine, and large intestine, which move and absorb food, but associated organs such as the pancreas, liver, and gallbladder, which produce digestive enzymes, remove toxins, and store substances necessary for digestion.
The cardiovascular system (see Biology of the Heart) includes the heart (cardio) and blood vessels (vascular). The cardiovascular system is responsible for pumping and circulating the blood.
The musculoskeletal system (see Introduction to the Biology of the Musculoskeletal System) includes the bones, muscles, ligaments, tendons, and joints, which support and move the body.
Major Organ Systems
Organ systems often work together to do complicated tasks. For example, after a large meal is eaten, several organ systems work together to help the digestive system obtain more blood to perform its functions. The digestive system enlists the aid of the cardiovascular system and the nervous system (see Overview of the Nervous System). Blood vessels of the digestive system widen to transport more blood. Nerve impulses are sent to the brain, notifying it of the increased digestive activity. The digestive system even directly stimulates the heart through nerve impulses and chemicals released into the bloodstream. The heart responds by pumping more blood. The brain responds by perceiving less hunger, more fullness, and less interest in vigorous physical (musculoskeletal system) activity, which preserves more blood to be used by the digestive system instead of by skeletal muscles.
Communication between organs and organ systems is vital. Communication allows the body to adjust the function of each organ according to the needs of the whole body. In the example above, the heart needs to know when the digestive organs need more blood so that it can pump more. When the heart knows that the body is resting, it can pump less. The kidneys (see Kidneys) must know when the body has too much fluid, so that they can produce more urine, and when the body is dehydrated, so that they can conserve water.
Homeostasis is the term used to describe how the body maintains its normal composition and functions. Because organ systems communicate with each other, the body is able to maintain stable amounts of internal fluids and substances. Also, the organs neither underwork nor overwork, and each organ facilitates the functions of every other organ.
Communications to maintain homeostasis occur by means of the autonomic nervous system and the endocrine system. Special chemicals called transmitters carry out the communications.
The autonomic nervous system (see Overview of the Autonomic Nervous System) largely controls the complex communication network that regulates bodily functions. This part of the nervous system functions without a person's thinking about it and without much noticeable indication that it is working. Transmitters called neurotransmitters conduct messages between parts of the nervous system and between the nervous system and other organs.
The endocrine system (see Endocrine Glands) consists of various glands that produce transmitters called hormones. Hormones travel to other organs through the bloodstream and regulate the function of those organs. For example, the thyroid gland produces thyroid hormone, which controls the metabolic rate (the speed at which the body's chemical functions proceed). The pancreas produces insulin, which controls the use of sugar.
One of the best known transmitters is the hormone epinephrine (adrenaline). When a person is suddenly stressed or frightened, the brain instantly sends a message to the adrenal glands, which quickly release epinephrine. Within moments, this chemical has the entire body on alert, a response sometimes called the fight-or-flight response. The heart beats more rapidly and powerfully, the eyes dilate to allow more light in, breathing quickens, and the activity of the digestive system decreases to allow more blood to go to the muscles. The effect is rapid and intense.
Other chemical communications are less dramatic but equally effective. For example, when the body becomes dehydrated and needs more water, the volume of blood circulating through the cardiovascular system decreases. This decreased blood volume is perceived by receptors in the arteries in the neck. They respond by sending impulses through nerves to the pituitary gland, at the base of the brain, which then produces antidiuretic hormone. This hormone signals the kidneys to concentrate urine and retain more water. Simultaneously, the brain senses thirst, stimulating a person to drink.