Homeostasis is the ability to maintain a stable internal body environment in an ever changing outside world. It can also be said to be the body’s attempt to maintain a relatively constant internal environment in the face of constantly changing external conditions. Homeostasis is used by physiologists to define maintenance of nearly constant in the internal environment. Chemical, thermal and neural factors interact to maintain homeostasis. Feedback controls distributed around the body help maintain the vital balances needed to maintain body function. The maintenance of constant conditions is achieved by the coordination of all the organs and tissues. In the body positive, negative and feed-forward feedback mechanisms occur. Generally to maintain homeostasis the body uses the negative feedback mechanisms.There is a general homeostatic mechanism. It involves disruptors or stimuli, detectors or sensors, control system, efferent pathway and effectors. Certain disruptors produce a change in variable. The sensors sense the change. The information about the change is then sent to the control system via the afferent pathway. From the control system there is output. The information concerning the output is sent along efferent pathway to the effectors. The effectors then respond and influence the degree to which the stimuli restore the affected variable to homeostasis.
Examples of homeostatic control systems
CELL VOLUME HOMEOSTASIS
All cells face a critical problem in the maintenance of a constant volume in the face of extracellular and intracellular fluctuations. These fluctuations can result in cell volume increase or decrease. These are generally swelling and shrinkage respectively. Activation of specific membrane transport coupled with metabolic processes serves to return cell volume to its normal resting point. The intracellular and extracellular solute environments can vary resulting in a transmembrane osmotic gradient. Inevitably, water will instantly flow into or out of the cell until equilibrium is regained. Animal cell membranes are unable to sustain significant hydrostatic pressure gradients therefore water flow cause cell swelling or shrinkageIn response to volume changes, cells activate regulatory mechanisms. Swollen cells undergo volume decrease and shrunken cells undergo volume increase. The gain or loss of ions such as Na+, K+, Cl- and organic osmolytes regulate cell volume. Regulatory volume decrease is facilitated by the loss of KCl via separate K+ and Cl- channels or the K-Cl co transporter. Volume increase on the other hand occurs by the uptake of both KCl and NaCl. The activation of the channels and pathways is rapid. In volume decrease, as the loss of KCl occurs water also tends to follow this ion movement. This is because the extracellular fluid will have a high osmotic pressure and so water moves into it. Water moves by osmosis into the extracellular fluid thereby reducing the volume in the intracellular fluid. Equilibrium is then attained. In volume increase as the ions move into the intracellular environment water also tends to move into the cell. This is due to the high osmotic pressure now existing in the intracellular fluid. The cell volume is thus returned to equilibrium.
Water Homeostasis (osmoregulation)
The body maintains a balance of water intake and output by a series of negative feedback loops involving the endocrine system and autonomic nervous system. The average adult body contains about 42 litres of water, so this must be maintained constant.
There are four primary mechanisms that regulate fluid homeostasis. These are antidiuretic hormone (ADH) or vasopressin, thirst mechanism,...
References: * Cellular volume homeostasis
Advan Physiol Educ 28:155-159, 2004. doi:10.1152/advan.00034.2004
* Ganong’s Review of Medical Physiology 23rd Edition
.Textbook of Medical Physiology 11th Edition, Guyton & Hall
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