The protein produced from the gene expression would then exert its effect on the target organ. Homeostatic mechanisms that respond to a perturbation may be in the form of a looping mechanism called feedback mechanism that may be positive or negative.
Positive feedback maintains the direction of the stimulus. It tends to accelerate or promote the effect of the stimulus. Examples are labor contractions, blood clotting, and action potential generation. Negative feedback is a self-regulatory system and is employed in various biological systems.
It reverses the direction of the stimulus and tends to inhibit the source of stimulus or slow down the metabolic process. Examples include thermoregulation, blood glucose regulation, baroreflex in blood pressure, calcium homeostasis, potassium homeostasis, and osmoregulation. Labor contraction during childbirth is positive feedback since the initial contraction of the uterine muscle leads to further contractions.
Rather than inhibiting the contraction, the body tends to produce more contractions. At labor, the posterior pituitary gland releases oxytocin which stimulates muscle contraction. At child delivery, oxytocin release is further augmented, intensifying muscle contractions until the neonate is pushed outside the birth canal.
The formation of a blood clot is an example of positive feedback. The conversion of blood from a liquid into a solid form entails series activations of clotting factors. As soon as one clotting factor is activated, the next clotting factor is activated, resulting in the formation of a fibrin clot.
In this process, the direction of the stimulus is maintained. In neuron signaling, positive feedback is demonstrated during membrane depolarization. As nerve impulse is relayed along the axon of the neuron, voltage-gated sodium channels open in a series down the axon. The first set of voltage-gated sodium channels open, resulting in the influx of sodium ions. This, in turn, causes the depolarization of the surrounding area, which means the next set of voltage-gated sodium channels will open.
Thermoregulation is an example of negative feedback. It refers to the homeostatic regulation of body temperature. The human body tends to maintain an internal temperature of about The core temperature is regulated chiefly by the nervous system, particularly the anterior hypothalamus and the preoptic area of the brain. When the ambient temperature is less than the skin temperature, heat loss occurs. This means that in colder surroundings e. As a result, the core temperature falls.
This is picked up by the thermoregulatory center of the brain and initiates control mechanisms to return the core temperature to the set point. One of the homeostatic mechanisms is by shivering to generate heat.
The thermoregulatory center in the brain sends signals to the muscles to shiver. Since the body remains still while shivering less heat will be dissipated to the environment. On the other way around, when the ambient temperature is higher than the skin temperature, the body gains heat and consequently, the core temperature rises. This occurs during the hot summer days. The thermoregulatory control center in the brain responds, for example by stimulating the eccrine sweat glands to secrete sweat to cool the body off by evaporative cooling.
Thermoregulation is an important homeostatic mechanism not just in humans but also in mammals. Mammals maintain a constant body temperature that makes them characteristically warm-blooded. The body maintains an optimal core temperature through internal regulation by a bodily system comprised of thermoreceptors in the hypothalamus, the brain, the spinal cord, the internal organs, and the great veins. During the cold season, they look for warm spots and they tend to increase their activity.
Some species, such as birds, huddle or nestle together for warmth. For instance, radiant heating in the form of steam radiators, radiant in-floor heating, in-wall heating, masonry heaters, and passive solar heating, can warm up surfaces and objects efficiently and produce an even and comfortable heat. Read more facts on Radiant Heating. Human blood is comprised of cellular elements and plasma. The levels of these components in the blood plasma go through homeostatic regulation.
For example, blood sugar level is regulated to set the blood glucose concentration within the tolerable limit. The body maintains homeostasis in this regard largely through the pancreas. The pancreas is a glandular structure made up of two major types of cells: alpha and beta cells. The alpha cells produce and secrete glucagon whereas the beta cells, insulin.
Glucagon and insulin are hormones from the pancreas that regulate glucose concentration in the blood. Insulin , in particular, lowers blood sugar levels by inciting the skeletal muscles, and the fat tissues to take up glucose from the bloodstream.
It also incites the liver cells to take glucose in and store it into glycogen. Conversely, glucagon raises blood sugar level by stimulating the liver to convert its stored glycogen into glucose by glycogenolysis or produce glucose by gluconeogenesis and release it into the bloodstream. Thus, when the glucose level is high in the blood circulation e. But when glucose level drops e.
Another instance of negative feedback is the homeostatic regulation of blood pressure. Blood pressure is the force exerted by the circulating blood as it hits the arterial walls.
The pressure comes from the heart when it creates a pulsing act. This blood pressure is regulated within the homeostatic range through the cardiovascular center. This control center has three distinct activities related to blood pressure regulation 3 : 1 The cardiac center sending nerve impulses to the sympathetic cardiac nerves to increase cardiac output by increasing heart rate.
The cardiovascular center receives blood pressure changes information from receptors, e. The baroreceptors are the receptors that are mostly found in the carotid sinus. They are sensitive to blood pressure changes. For example, when the arterial wall stretches from an increased blood volume, the baroreceptors detect the consequential rise in blood pressure. They send signals to the atrial heart muscle cells to secrete atrial natriuretic peptide ANP into the bloodstream.
ANP is a potent vasodilator whose actions include lowering blood pressure. In this regard, its target organ is the kidney that apart from the major function of excreting wastes out of the body as urine it also plays an important role in managing blood volume through the renin-angiotensin-aldosterone system. In particular, ANP stimulates the kidney to stop secreting renin. Renin is an enzyme that converts angiotensinogen from the liver to angiotensin I.
The angiotensin I is converted next by the angiotensin-converting enzyme in the lungs into a potent vasoconstrictor peptide, angiotensin II. The latter causes the target blood vessel to constrict thereby raising peripheral resistance. A person with a high fever has hot, dry skin if they do sweat to help cool it.
Not only have the negative feedback systems shut down in such a case; the increased temperature speeds up the body chemistry, which causes the temperature to rise even more, which in turn speeds up the body chemistry even more, and so forth. This vicious cycle of positive feedback, a "runaway" process, can only end in death if not stopped. It is important to emphasize that homeostatic reactions are inevitable and automatic if the system is functioning properly, and that a steady state or homeostasis may be maintained by many systems operating together.
For example, flushing is another of the body's automatic responses to heating: the skin reddens because its small blood vessels automatically expand to bring more heated blood close to the surface where it can cool. Shivering is another response to chilling: the involuntary movements burn body tissue to produce more body heat. Negative feedback arises out of balances between forces and factors that mutually influence each other.
To illustrate several of its important characteristics, we can regard a car and its driver as a unified, complex, homeostatic or "goal-seeking" system--a cyborg, or "cybernetic organism," in that it seeks to keep the car moving on track. The driver does not steer by holding the wheel in a fixed position but keeps turning the wheel slightly to the left and right, seeking the wheel positions that will bring the naturally meandering car back on track.
Disturbance, or departure from equilibrium, is every bit as important as negative feedback: Systems cannot correct themselves if they do not stray. Oscillation is a common and necessary behavior of many systems. If the car skids, the driver automatically responds by quickly steering in the opposite direction. Such abrupt negative feedback, however, usually over-corrects, causing the car to move toward the other side of the road.
A negative feedback, if it is as large as the disturbance that triggered it, may become an impressed change in the direction opposite to that of the original disturbance. The car and driver recovers from the skid by weaving from side to side, swerving a little less each time. In other words, each feedback is less than the last departure from the goal, so the oscillations "damp out. The point may shift under the influence of circadian rhythms, menstrual cycles or daily fluctuations in body temperature.
Set points may also change in response to physiological phenomena, like fever, or to compensate for multiple homeostatic processes taking place at the same time, according to a review in Advances in Physiology Education. For example, in anticipation of a meal, the body secretes extra insulin, ghrelin and other hormones, according to a review in Appetite. This preemptive measure readies the body for the incoming flood of calories , rather than wrestling to control blood sugar and energy stores in its wake.
The ability to shift set points allows animals to adapt to short-term stressors, but they may fail in the face of long-term challenges, such as climate change. But they're not designed to last for long. Homeostatic systems may have primarily evolved to help organisms maintain optimal function in different environments and situations. The th eory posits that homeostasis makes it easier for organisms to extract important information from the environment and shuttle signals between body parts.
Regardless of its evolutionary purpose, homeostasis has shaped research in the life sciences for nearly a century. Though mostly discussed in the context of animal physiology, homeostatic processes also enable plants to manage energy stores, nourish cells and respond to environmental challenges.
Beyond biology, the social sciences, cybernetics, computer science and engineering all use homeostasis as a framework to understand how people and machines maintain stability despite disruptions.
Nicoletta Lanese is a staff writer for Live Science covering health and medicine, along with an assortment of biology, animal, environment and climate stories. She holds degrees in neuroscience and dance from the University of Florida and a graduate certificate in science communication from the University of California, Santa Cruz.
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