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Although an organ has a specific function, organs also function as part of a group, called an organ system. The organ system is the organizational unit by which medicine is studied, diseases are generally categorized, and treatments are planned. This book is, in large part, organized around the concept of organ systems.

An example of an organ system is the cardiovascular system, which includes the heart (cardio) and blood vessels (vascular). The cardiovascular system is responsible for pumping and circulating the blood. The digestive (or gastrointestinal) 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 musculoskeletal system includesthebones,muscles,ligaments, tendons, and joints, which support and move the body

Of course, organ systems do not function alone. For example, after a large meal is eaten, the digestive system needs more blood to perform its functions. Therefore, it enlists the aid of the cardiovascular system and 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 work. 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 activity.

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. The heart must know when the body is resting so that it can slow down and when organs need more blood so that it can speed up. The kidneys must know when the body has too much fluid, so that they can produce more dilute urine, and when the body is dehydrated, so that they can conserve water.

Through communication, the body keeps itself in balance—a concept called homeostasis. Through homeostasis, organs neither underwork nor overwork, and each organ facilitates the functions of every other organ.

Communication to maintain homeostasis can occur through the nervous system or through chemical stimulation. One part of the nervous system, 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. Chemicals used to communicate are called transmitters. Transmitters that are produced by one organ and travel to other organs through the bloodstream are called hormones. Transmitters that conduct messages between parts of the nervous system are called neurotransmitters.

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.

The body also has a group of organs—the endocrine system—whose primary function is to produce hormones that regulate the function of other 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; and the adrenal glands produce epinephrine, which stimulates many organs to prepare the body for stress.

Source : http://www.merckmanuals.com