Topic 16 - Homeostasis

• Homeostasis is the maintenance of an internal environment within restricted limits in organisms.
• Homeostasis ensures that all the cells of the body are in an environment that meets their requirements and allows them to function normally despite external changes.
• Homeostatic control of any body system involves a series of stages featuring: the optimum point, a receptor, a coordinator, an effector and a feedback mechanism.
• The optimum point is the 'normal' point, at which the system operates most effectively.
• A receptor detects the stimulus of any deviation from the set point (norm).
• A coordinator coordinates information from various sources.
• An effector brings about the corrective measures needed to return the system to the optimum point (norm).
• A feedback mechanism is the process by which a receptor detects a stimulus created by the change to the system and the effector brings about the appropriate response.
• When an effector has corrected any deviation and returned the system to the optimum point, it is important that this information is fed back into the receptor. Otherwise, the receptor will continue to stimulate the effector, leading to an over-correction and causing a deviation in the opposite direction.
• There are two different types of feedback - negative feedback and positive feedback.
• Negative feedback occurs when the stimulus causes the corrective measures to be turned off. In doing so, this tends to return the system to its original (optimum) level (and prevents any overshoot).
• Having separate negative feedback mechanisms that control departures from the norm in either direction gives a greater degree of homeostatic control. This is because there are positive actions in both directions.
• The cells that make up the walls of the distal convoluted tubule have microvilli and many mitochondria, allowing them to rapidly reabsorb material from the filtrate by active transport.
• The main role of the distal convoluted tubule is to make final adjustments to the water and salts that are reabsorbed and to control the pH of the blood by selecting which ions to reabsorb.
• The permeability of the distal convoluted tubule is affected by various hormones.
• In the case of the loop of Henle, the counter-current flow means that the filtrate in the collecting duct (with a low water potential) meets interstitial fluid (with an even lower water potential). This means that although there is a fairly shallow concentration gradient between both regions, this gradient is maintained for the entire length of the collecting duct.
• Due to the counter-current flow in the loop of Henle, there is a steady flow of water into the interstitial fluid.
• If the both flows in the loop of Henle were parallel (i.e. not counter-current), less water would move into the blood.