Table of Contents

Homeostasis | The Internal Environment | Control Systems | Feedback Systems in Homeostasis

Body Systems and Homeostasis | Links

Animal organs are usually composed of more than one cell type. Organs perform a certain function. Most organs have functions in only one organ system. Organ systems are composed of organs, and perform a major function for the organism.

Homeostasis | Back to Top

Homeostasis is the maintenance of a stable internal environment. Homeostasis is a term coined in 1959 to describe the physical and chemical parameters that an organism must maintain to allow proper functioning of its component cells, tissues, organs, and organ systems. Single-celled organisms are surrounded by their external environment. Most multicellular organisms have most of their cells protected from the external environment, having them surrounded by an aqueous internal environment. This internal environment must be maintained in such a state as to allow maximum efficiency. The ultimate control of homeostasis is done by the nervous system. Often this control is in the form of negative feedback loops. Heat control is a major function of homeostatic conditions that involves the integration of skin, muscular, nervous, and circulatory systems.

Multicellular organisms have a series of organs and organ systems that function in homeostasis. Changes in the external environment can trigger changes in the internal environment as a response.

The Internal Environment | Back to Top

There are two types of extracellular fluids in animals:

Internal components of homeostasis:

  1. Concentration of oxygen and carbon dioxide
  2. pH of the internal environment
  3. Concentration of nutrients and waste products
  4. Concentration of salt and other electrolytes
  5. Volume and pressure of extracellular fluid

Control Systems | Back to Top

Open systems are linear and have no feedback, such as a light switch. Closed Systems has two components: a sensor and an effector, such as a thermostat (sensor) and furnace (effector). Most physiological systems in the body use feedback to maintain the body's internal environment.


Most homeostatic systems are extrinsic: they are controlled from outside the body. Endocrine and nervous systems are the major control systems in higher animals.

The nervous system depends on sensors in the skin or sensory organs to receive stimuli and transmit a message to the spinal cord or brain. Sensory input is processed and a signal is sent to an effector system, such as muscles or glands, that effects the response to the stimulus.

The endocrine system is the second type of extrinsic control, and involves a chemical component to the reflex. Sensors detect a change within the body and send a message to an endocrine effector (parathyroid), which makes PTH. PTH is released into the blood when blood calcium levels are low. PTH causes bone to release calcium into the bloodstream, raising the blood calcium levels and shutting down the production of PTH.

Some reflexes have a combination of nervous and endocrine response. The thyroid gland secretes thyroxin (which controls the metabolic rate) into the bloodstream. Falling levels of thyroxin stimulate receptors in the brain to signal the hypothalamus to release a hormone that acts on the pituitary gland to release thyroid-stimulating hormone (TSH) into the blood. TSH acts on the thyroid, causing it to increase production of thyroxin.


Local, or intrinsic, controls usually involve only one organ or tissue. When muscles use more oxygen, and also produce more carbon dioxide, intrinsic controls cause dilation of the blood vessels allowing more blood into those active areas of the muscles. Eventually the vessels will return to "normal".

Feedback Systems in Homeostasis | Back to Top

Negative feedback control mechanisms (used by most of the body's systems) are called negative because the information caused by the feedback causes a reverse of the response. TSH is an example: blood levels of TSH serve as feedback for production of TSH.

Positive feedback control is used in some cases. Input increases or accelerates the response. During uterine contractions, oxytocin is produced. Oxytocin causes an increase in frequency and strength of uterine contractions. This in turn causes further production of oxytocin, etc.

Homeostasis depends on the action and interaction of a number of body systems to maintain a range of conditions within which the body can best operate.

Body Systems and Homeostasis | Back to Top

Eleven major organ systems are present within animals, although some animals lack one or more of them. The vertebrate body has two cavities: the thoracic, which contains the heart and lungs; and the abdominal, which contains digestive organs. The head, or cephalic region, contains four of the five senses as well as a brain encased in the bony skull. These organ systems can be grouped according to their functions.

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Links | Back to Top

Text ©1992, 1994, 1997, 1998, 1999, 2000, M.J. Farabee, all rights reserved, although use for educational purposes is very much encouraged and appreciated!

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