Although each organ Tissues and Organs Tissues are related cells that are joined together. The cells in a tissue are not identical, but they work together to accomplish specific functions. A sample of tissue removed for examination... read more has its specific functions, organs also function together in groups, called organ systems (see Table: Major Organ Systems Major Organ Systems Although each organ 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... read more ). Doctors categorize disorders and their own medical specialties according to organ systems.
Some examples of organ systems and their functions include the digestive system, the cardiovascular system, and the musculoskeletal system.
The digestive (or gastrointestinal) system Overview of the Digestive System The digestive system, which extends from the mouth to the anus, is responsible for receiving food, breaking it down into nutrients (a process called digestion), absorbing the nutrients into... read more , 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 Biology of the Heart The heart and blood vessels constitute the cardiovascular (circulatory) system. The heart pumps the blood to the lungs so it can pick up oxygen and then pumps oxygen-rich blood to the body.... read more includes the heart (cardio) and blood vessels (vascular). The cardiovascular system is responsible for pumping and circulating the blood.
The musculoskeletal system Introduction to the Biology of the Musculoskeletal System The musculoskeletal system provides form, stability, and movement to the human body. It consists of the body's bones (which make up the skeleton), muscles, tendons, ligaments, joints, cartilage... read more includes the bones, muscles, ligaments, tendons, and joints, which support and move the body.
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 Overview of the Digestive System The digestive system, which extends from the mouth to the anus, is responsible for receiving food, breaking it down into nutrients (a process called digestion), absorbing the nutrients into... read more obtain more blood to perform its functions. The digestive system enlists the aid of the cardiovascular system Biology of the Heart The heart and blood vessels constitute the cardiovascular (circulatory) system. The heart pumps the blood to the lungs so it can pick up oxygen and then pumps oxygen-rich blood to the body.... read more and the nervous system Overview of the Nervous System The nervous system has two distinct parts: the central nervous system (the brain and spinal cord) and the peripheral nervous system (the nerves outside the brain and spinal cord). The basic... read more . 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 Kidneys The kidneys are bean-shaped organs that figure prominently in the urinary tract. Each is about 4 to 5 inches (12 centimeters) long and weighs about one third of a pound (150 grams). One lies... read more 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 Overview of the Autonomic Nervous System The autonomic nervous system regulates certain body processes, such as blood pressure and the rate of breathing. This system works automatically (autonomously), without a person’s conscious... read more 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 Endocrine Glands The endocrine system consists of a group of glands and organs that regulate and control various body functions by producing and secreting hormones. Hormones are chemical substances that affect... read more consists of various glands that produce chemical transmitters called hormones. Hormones travel to other organs through the bloodstream and regulate the function of those organs. For example, the thyroid gland Overview of the Thyroid Gland The thyroid is a small gland, measuring about 2 inches (5 centimeters) across, that lies just under the skin below the Adam’s apple in the neck. The two halves (lobes) of the gland are connected... read more produces thyroid hormone, which controls the metabolic rate (the speed at which the body's chemical functions proceed). The pancreas Pancreas The pancreas is an organ that contains two types of glandular tissue: Pancreatic acini Islets of Langerhans (See also Overview of the Digestive System.) The acini produce digestive enzymes.... read more 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 Overview of the Adrenal Glands The body has two adrenal glands, one near the top of each kidney. They are endocrine glands, which secrete hormones into the bloodstream. Each adrenal gland has two parts. Medulla: The inner... read more , 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 Overview of the Pituitary Gland The pituitary is a pea-sized gland that is housed within a bony structure (sella turcica) at the base of the brain. The sella turcica protects the pituitary but allows very little room for expansion... read more , 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.