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Homeostasis is the activity of cells throughout the body to maintain the physiological state within a narrow range that is compatible with life. Homeostasis is regulated by negative feedback loops and, much less frequently, by positive feedback loops. 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.
Homeostasis is the collection of processes that maintain a stable and constant internal environment in the human body. It allows all organs, cells, and other parts of the body to function as efficiently as possible and requires constant adjustments to hormones, temperature, acidity, and other factors. Homeostasis in humans creates an internal balance in response to changes in the external environment that is vital to an individual’s health and well-being. The primary areas in which homeostatic regulation occurs are body temperature, PH, blood sugar levels, and blood pressure.
Maintaining homeostasis requires that the body continuously monitor its internal conditions. From body temperature to blood pressure to levels of certain nutrients, each physiological condition has a particular set point. A set point is the physiological value around which the normal range fluctuates.
A normal range is the restricted set of values that is optimally healthful and stable. 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. Control centers in the brain and other parts of the body monitor and react to deviations from homeostasis using negative feedback.
Body temperature must remain nearly constant in the body to ensure both individual comfort and the efficient completion of all bodily processes. Physical activity and liver functions are the main producers of heat, which is counteracted by mechanisms such as sweating and dilation of blood vessels in order to bring the body temperature back to its normal level. If the body temperature rises too high, tissue and cell damage can occur. Temperature is regulated by a series of feedback circuits that react to signals about temperature change. These feedback circuits are a key feature of many internal systems that maintain homeostasis in humans.
The PH level, or acid-base balance of the blood, is constantly monitored and controlled by the kidneys. Some organs require more acidic environments than others to perform essential functions, but blood plasma must maintain a constant PH level. Similarly, the blood sugar level or the amount of glucose in the blood must be regulated. This is controlled by the pancreas and involves a delicate balance between the release of two key hormones: insulin and glucagon. Failure of glucose regulation mechanisms can result in diabetes.
Blood pressure is controlled not only by the functions of the heart, but also by the kidneys and other organs. The amount of fluid inside and outside the cells is monitored to ensure that blood flows efficiently throughout the body. Medical management is often needed to help the body maintain blood pressure homeostasis in humans, particularly in people of advanced age or with additional medical problems.
A lack of homeostasis in humans can be catastrophic or even life-threatening. Problems with regulatory function can lead to kidney, liver, or heart failure, severe dehydration, and many other types of disease. Generally, however, humans are able to adapt to a wide variety of different environmental conditions as a result of intricate balancing systems within the body. While most homeostatic operations go unnoticed, they are constantly taking place and are vital to health and vitality.
Negative feedback is a mechanism that reverses a deviation from the set point. Therefore, negative feedback maintains body parameters within their normal range. 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. A negative feedback system has three basic components.
- A sensor, also referred to a receptor, is a component of a feedback system that monitors a physiological value. This value is reported to the control center.
- The control center is the component in a feedback system that compares the value to the normal range. If the value deviates too much from the set point, then the control center activates an effector.
- 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.
In order to set the system in motion, a stimulus must drive a physiological parameter beyond its normal range (that is, beyond homeostasis). This stimulus is “heard” by a specific sensor. For example, in the control of blood glucose, specific endocrine cells in the pancreas detect excess glucose (the stimulus) in the bloodstream. These pancreatic beta cells respond to the increased level of blood glucose by releasing the hormone insulin into the bloodstream. The insulin signals skeletal muscle fibers, fat cells (adipocytes), and liver cells to take up the excess glucose, removing it from the bloodstream. 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. This prevents blood sugar levels from continuing to drop below the normal range.
Humans have a similar temperature regulation feedback system that works by promoting either heat loss or heat gain. 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:
- 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.
- As blood flow to the skin increases, sweat glands are activated to increase their output. As the sweat evaporates from the skin surface into the surrounding air, it takes heat with it.
- The depth of respiration increases and a person may breathe through an open mouth instead of through the nasal passageways. This further increases heat loss from the lungs.
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 arrangement traps heat closer to the body core and restricts heat loss. If heat loss is severe, the brain triggers an increase in random signals to skeletal muscles, causing them to contract and producing shivering. The muscle contractions of shivering release heat while using up ATP. 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 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 breakdown of glycogen into glucose also results in increased metabolism and heat production.
Positive feedback intensifies a change in the body’s physiological condition rather than reversing it. A deviation from the normal range results in more change and the system moves farther away from the normal range. Positive feedback in the body is normal only when there is a definite endpoint. 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.
Childbirth at full term is an example of a situation in which the maintenance of the existing body state is not desired. Enormous changes in the mother’s body are required to expel the baby at the end of pregnancy. 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. The extreme muscular work of labor and delivery are the results of a positive feedback system.
The first contractions of labor (the stimulus) push the baby toward the cervix (the lowest part of the uterus). The cervix contains stretch-sensitive nerve cells that monitor the degree of stretching (the sensors). 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. Oxytocin causes stronger contractions of the smooth muscles in the uterus (the effectors), pushing the baby further down the birth canal. This causes even greater stretching of the cervix. The cycle of stretching, oxytocin release, and increasingly more forceful contractions stops only when the baby is born. At this point, the stretching of the cervix halts, stopping the release of oxytocin.
The second example of positive feedback centers on reversing extreme damage to the body. Following a penetrating wound, the most immediate threat is excessive blood loss. Less blood circulating means reduced blood pressure and reduced perfusion (penetration of blood) to the brain and other vital organs. If perfusion is severely reduced, vital organs will shut down and the person will die. The body responds to this potential catastrophe by releasing substances in the injured blood vessel wall that begin the process of blood clotting. As each step of clotting occurs, it stimulates the release of more clotting substances. This accelerates the processes of clotting and sealing off the damaged area. Clotting is contained in a local area based on the tightly controlled availability of clotting proteins. This is an adaptive, life-saving cascade of events.