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27.02.2016


Maintaining homeostasis requires that the body continuously monitor its internal conditions. In order to set the system in motion, a stimulus must drive a physiological parameter beyond its normal range (that is, beyond homeostasis).
Humans have a similar temperature regulation feedback system that works by promoting either heat loss or heat gain ([link]b).
Blood vessels in the skin begin to dilate allowing more blood from the body core to flow to the surface of the skin allowing the heat to radiate into the environment.
The depth of respiration increases, and a person may breathe through an open mouth instead of through the nasal passageways. In contrast, activation of the brain’s heat-gain center by exposure to cold reduces blood flow to the skin, and blood returning from the limbs is diverted into a network of deep veins. Positive feedback intensifies a change in the body’s physiological condition rather than reversing it. Childbirth at full term is an example of a situation in which the maintenance of the existing body state is not desired.
The first contractions of labor (the stimulus) push the baby toward the cervix (the lowest part of the uterus). Homeostasis is the activity of cells throughout the body to maintain the physiological state within a narrow range that is compatible with life. After you eat lunch, nerve cells in your stomach respond to the distension (the stimulus) resulting from the food. Which of the following is an example of a normal physiologic process that uses a positive feedback loop?
Identify the four components of a negative feedback loop and explain what would happen if secretion of a body chemical controlled by a negative feedback system became too great. The four components of a negative feedback loop are: stimulus, sensor, control center, and effector. What regulatory processes would your body use if you were trapped by a blizzard in an unheated, uninsulated cabin in the woods? From body temperature to blood pressure to levels of certain nutrients, each physiological condition has a particular set point. A sensor, also referred to a receptor, is a component of a feedback system that monitors a physiological value. As the sweat evaporates from the skin surface into the surrounding air, it takes heat with it.


A person’s body retains very tight control on water levels without conscious control by the person.
A deviation from the normal range results in more change, and the system moves farther away from the normal range. Enormous changes in the mother’s body are required to expel the baby at the end of pregnancy.
The cervix contains stretch-sensitive nerve cells that monitor the degree of stretching (the sensors).
Homeostasis is regulated by negative feedback loops and, much less frequently, by positive feedback loops. If too great a quantity of the chemical were excreted, sensors would activate a control center, which would in turn activate an effector.
This would reduce blood flow to your skin, and shunt blood returning from your limbs away from the digits and into a network of deep veins. For example, in the control of blood glucose, specific endocrine cells in the pancreas detect excess glucose (the stimulus) in the bloodstream. If heat loss is severe, the brain triggers an increase in random signals to skeletal muscles, causing them to contract and producing shivering. And the events of childbirth, once begun, must progress rapidly to a conclusion or the life of the mother and the baby are at risk. These nerve cells send messages to the brain, which in turn causes the pituitary gland at the base of the brain to release the hormone oxytocin into the bloodstream.
Less blood circulating means reduced blood pressure and reduced perfusion (penetration of blood) to the brain and other vital organs. Both have the same components of a stimulus, sensor, control center, and effector; however, negative feedback loops work to prevent an excessive response to the stimulus, whereas positive feedback loops intensify the response until an end point is reached. Your brain’s heat-gain center would also increase your muscle contraction, causing you to shiver. The control center is the component in a feedback system that compares the value to the normal range. These pancreatic beta cells respond to the increased level of blood glucose by releasing the hormone insulin into the bloodstream. Childbirth and the body’s response to blood loss are two examples of positive feedback loops that are normal but are activated only when needed. The extreme muscular work of labor and delivery are the result of a positive feedback system ([link]).


Oxytocin causes stronger contractions of the smooth muscles in of the uterus (the effectors), pushing the baby further down the birth canal.
For example, the set point for normal human body temperature is approximately 37°C (98.6°F) Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a normal range a few degrees above and below that point. If the value deviates too much from the set point, then the control center activates an effector. The insulin signals skeletal muscle fibers, fat cells (adipocytes), and liver cells to take up the excess glucose, removing it from the bloodstream. The brain triggers the thyroid gland in the endocrine system to release thyroid hormone, which increases metabolic activity and heat production in cells throughout the body. The body responds to this potential catastrophe by releasing substances in the injured blood vessel wall that begin the process of blood clotting. Your body would also produce thyroid hormone and epinephrine, chemicals that promote increased metabolism and heat production. Control centers in the brain and other parts of the body monitor and react to deviations from homeostasis using negative feedback. An effector is the component in a feedback system that causes a change to reverse the situation and return the value to the normal range. As glucose concentration in the bloodstream drops, the decrease in concentration—the actual negative feedback—is detected by pancreatic alpha cells, and insulin release stops.
The brain also signals the adrenal glands to release epinephrine (adrenaline), a hormone that causes the breakdown of glycogen into glucose, which can be used as an energy source. The cycle of stretching, oxytocin release, and increasingly more forceful contractions stops only when the baby is born.
The breakdown of glycogen into glucose also results in increased metabolism and heat production.
Clotting is contained in a local area based on the tightly controlled availability of clotting proteins. The maintenance of homeostasis by negative feedback goes on throughout the body at all times, and an understanding of negative feedback is thus fundamental to an understanding of human physiology.




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