The cervix contains stretch-sensitive nerve cells that monitor the degree of stretching (the sensors). The first contractions of labor (the stimulus) push the baby toward the cervix (the lowest part of the uterus). A positive feedback loop results in a change in the body’s status, rather than a return to homeostasis. Normal childbirth is driven by a positive feedback loop. The breakdown of glycogen into glucose also results in increased metabolism and heat production. 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 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 muscle contractions of shivering release heat while using up ATP. If heat loss is severe, the brain triggers an increase in random signals to skeletal muscles, causing them to contract and producing shivering. This arrangement traps heat closer to the body core and restricts heat loss. 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. This further increases heat loss from the lungs. The depth of respiration increases, and a person may breathe through an open mouth instead of through the nasal passageways. As the sweat evaporates from the skin surface into the surrounding air, it takes heat with it. As blood flow to the skin increases, sweat glands are activated to increase their output.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.When the brain’s temperature regulation center receives data from the sensors indicating that the body’s temperature exceeds its normal range, it stimulates a cluster of brain cells referred to as the “heat-loss center.” This stimulation has three major effects: Humans have a similar temperature regulation feedback system that works by promoting either heat loss or heat gain ( b). This prevents blood sugar levels from continuing to drop below 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 insulin signals skeletal muscle fibers, fat cells (adipocytes), and liver cells to take up the excess glucose, removing it from the bloodstream. These pancreatic beta cells respond to the increased level of blood glucose by releasing the hormone insulin into the bloodstream. For example, in the control of blood glucose, specific endocrine cells in the pancreas detect excess glucose (the stimulus) in the bloodstream. This stimulus is “heard” by a specific sensor. In order to set the system in motion, a stimulus must drive a physiological parameter beyond its normal range (that is, beyond homeostasis). (b) Body temperature is regulated by negative feedback. (a) A negative feedback loop has four basic parts. In a negative feedback loop, a stimulus-a deviation from a set point-is resisted through a physiological process that returns the body to homeostasis. 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. Therefore, negative feedback maintains body parameters within their normal range. Negative feedback is a mechanism that reverses a deviation from the set point. Control centers in the brain and other parts of the body monitor and react to deviations from homeostasis using negative feedback. For example, the set point for normal human body temperature is approximately 37☌ (98.6☏) Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a normal range a few degrees above and below that point. A normal range is the restricted set of values that is optimally healthful and stable. A set point is the physiological value around which the normal range fluctuates. From body temperature to blood pressure to levels of certain nutrients, each physiological condition has a particular set point. Maintaining homeostasis requires that the body continuously monitor its internal conditions. Contrast negative and positive feedback, giving one physiologic example of each mechanism.Discuss the role of homeostasis in healthy functioning.By the end of this section, you will be able to:
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