Positive And Negative Feedback Loop Quiz

If blood glucose levels fall below normal levels (for instance, during the post-absorptive or fasting state, when nutrients from a recently digested meal are no longer circulating in the blood, or during starvation), insulin secretion is inhibited and, at the same time, the alpha cells of the pancreas respond by secreting glucagon, a hormone that has several important effects: (1) it accelerates the breakdown of glycogen to glucose in liver and skeletal muscle cells; (2) it increases the breakdown of fats to fatty acids and glycerol in adipose tissue and, consequently, the release of these substances into the blood (which cells can thus use for energy); and (3) it stimulates liver cells to increase glucose synthesis (from glycerol absorbed from the blood) and glucose release into the blood. These effects collectively cause an increase in blood glucose levels back to normal levels.

If blood glucose levels rise (for example, during the fed or absorptive state, when a meal is digested and the nutrient molecules are being absorbed and used), the beta cells of the pancreas respond by secreting insulin. Insulin has several notable effects: (1) it stimulates most body cells to increase their rate of glucose uptake (transport) from the blood; (2) it increases the cellular rate of glucose utilization as an energy source; (3) it accelerates the formation of glycogen from glucose in liver and skeletal muscle cells; and (4) it stimulates fat synthesis (from glucose) in liver cells and adipose (fat) tissue. These effects collectively cause a decrease in blood glucose levels back to normal levels.

An example of feedback is body temperature regulation. If blood temperature rises too high, this is sensed by specialized neurons in the hypothalamus of the brain. They signal other nerve centers, which in turn send signals to the blood vessels of the skin. As these blood vessels dilate, more blood flows close to the body surface and excess heat radiates from the body. If this is not enough to cool the body back to its set point, the brain activates sweating. Evaporation of sweat from the skin has a strong cooling effect, as we feel when we are sweaty and stand in front of a fan.

If the blood temperature falls too low, this is also sensed by the hypothalamus and signals are sent to the cutaneous arteries (those supplying the skin) to constrict them. Warm blood is then retained deeper in the body and less heat is lost from the surface. If this is inadequate, then the brain activates shivering. Each muscle tremor in shivering releases heat energy and helps warm the body back toward its 37 degrees Celsius set point.

An example of feedback is seen in blood clotting. Part of the complex biochemical pathway of clotting is the production of an enzyme that forms the matrix of the blood clot, but also speeds up the production of still more thrombin. That is, it has a self- catalytic , self-accelerating effect, so that once the clotting process begins, it runs faster and faster until, ideally, bleeding stops. Thus, this positive feedback loop is part of a larger negative feedback loop, one that is activated by bleeding and ultimately works to stop the bleeding.