Which systems maintain homeostasis

In response, pancreas cells are stimulated to secrete insulin, which enables sugar uptake by cells and the storage of sugar in the liver and muscles. In effect, insulin decreases blood sugar levels to normal Tortora and Anagnostakos, The respiratory system provides an example of homeostatic regulation by the nervous system. In normal breathing there is a state of homeostasis. During exercise the respiratory system must work faster to keep the O2 in the extracellular fluid and in the cells within normal limits, preventing excessive build-up of CO2 and disturbance to the blood pH through the accumulation of acid Tortora and Anagnostakos, Because muscles require more O2 during exercise, more CO2 is released and therefore also needs to be excreted Tortora and Anagnostakos, These chemical changes are detected in certain nerve cells, which send this message to the cardio-respiratory centre in the medulla oblongata in the base of the brain Docherty, The brain sends a message to the heart to increase its pumping action heart rate to take on more O2 and enable the blood to give up excess CO2.

Respiratory muscles also receive instructions from the brain to contract faster, enabling a rise in both O2 delivery and CO2 exhalation. Homeostasis is constantly disturbed by external factors, which can be described as a form of stress on the internal environment Tortora and Anagnostakos, These stresses can be:.

Since these stresses affect the chemical reactions sustaining life, there are built-in physiological mechanisms to maintain or return them to desirable levels. Each organ or structure has its own intrinsic way of keeping the internal environment within normal limits.

When homeostasis is altered there are two possible responses:. As this tends to keep things constant, it allows the maintenance of homeostasis. For example, if there is a fall in calcium in the blood, the parathyroid glands sense the decrease and secrete more parathyroid hormone, thereby increasing calcium release from the bones;. Positive feedback is used when rapid change is needed. For example in childbirth the hormone oxytocin is produced to stimulate and enhance labour contractions Bodyguide, A receptor detects external changes that could influence the internal environment.

The final piece of the homeostasis puzzle was supplied by the application of control theory from systems engineering to explain self-regulation in biological systems. The interaction of these regulatory mechanisms not only increases the stability of the system but provides redundancy back-up such that failure of one component does not necessarily lead to catastrophe.

Thus, from its inception physiological investigations have been directed toward understanding the organism be it microbe, plant, animal, or human as a single functional entity.

Both feedback and feedforward are the mechanisms by which homeostasis is obtained. I shall begin this section with a discussion of the contribution of feedback to homeostatic regulation and then briefly discuss feedforward also known as central command mechanisms. A feedback system is a closed loop structure in which the results of past actions changes in the internal environment of the system are fed into the system via information, feedback to control future action; the system affects its own behavior modified from Forrester, There are two types of feedback systems: negative feedback that seeks a goal and responds as a consequence of failure to meet this goal maintains a stable range of values and positive feedback that produces growth processes wherein the actions build on the results that then generate still greater action a growth cycle.

These feedback systems are themselves subject to higher levels of control; that is, the operational range of the regulated variables can be adjusted to support the behavioral response to environmental stimuli.

Homeostasis is the result of the complex interaction and competition between multiple negative and positive feedback systems and provides the basis for physiological regulation. Once again we can trace the origin of self-regulatory systems to the ancient Greeks. A water clock depends upon a steady flow of water to measure an unvarying flow of time. If the water level is not relatively constant, the water outflow will vary depending on the height of the water column supplying the clock faster with a full container and slower as the water level in the container falls.

The water clock designed by Ktesibios used a float valve similar to that used in the modern flush toilet to maintain a constant water level in the clock water reservoir. As water levels fall, the float also falls thereby opening a valve that allows water to flow into the clock reservoir and to replenish the water level. Then, as the water returns to the desired level, the float rises and closes the valve.

Thus, the clock water reservoir could be regulated such that there is no net gain or loss in the water level and thereby it maintains a constant water outflow rate from which an accurate estimate of time can be obtained. The accuracy of this type of water clock was not supplanted until the 17th century when a pendulum was employed to regulate the clock mechanism.

A major limitation of early steam engines was that their speed was affected by both the steam pressure generated by the boiler and work load placed upon the engine. James Watt — vastly improved the efficiency and safety of the steam engine by the development of a centrifugal feedback valve that controlled the speed of the engine Rosen, This, in turn, opened a valve to decrease the flow of steam into the engine and a slower speed was restored.

Conversely, as the engine speed decreased, so also would the rotation of the flyweights, thereby decreasing the outward centrifugal force. The flyweights would drop pulled down by gravity closer together, closing the steam valve so more steam could enter into the engine and increase its speed. As with the water clock and its water reservoir level, a constant engine speed could be maintained despite fluctuating steam pressure and changing work load without the constant supervision of a human monitor.

Figure 4. See text for details. Source: public domain, as modified from, https:www. In , Harold S. Black — applied feedback regulation to electrical circuits to amplify transatlantic telephone signals Black, His negative feedback amplifier patented in can be considered to be one of the most important developments in the field of electronics.

Further advances in systems control theory were achieved during World War II with the development of servo-control negative feedback mechanisms for anti-aircraft weapons. In , two influential papers were published that established that the mathematical principles of control theory, as first described by Maxwell, could be applied to explain behavior in living organisms.

Interestingly, Rosenblueth worked closely with Cannon and undoubtedly was influenced by his ideas. In his book Cybernetics, Wiener developed the first formal mathematical analysis of feedback control in biological systems, concepts that have subsequently been extensively applied in modeling physiological systems as, for example, by Arthur Guyton — and his many students with regard to cardiovascular regulation.

Thus, the concept of feedback regulation in living organisms may be said to have co-evolved with the mathematical concepts of control theory in mechanical systems. Negative feedback regulation is a particularly important mechanism by which homeostasis is achieved, as will be described in the following paragraphs.

The water clock and centrifugal steam governor described in the preceding paragraphs provide classic examples of negative feedback systems. Thus, the float simultaneously affects the water levels and is affected by water level forming a circular causality or a cycle of causation. It is important to emphasize that this is an automatic self-regulatory system, meaning that it requires no external adjustment once the operating level around which the variable is regulated has been set. A simplified general form of a closed loop feedback system is illustrated in Figure 5.

Effector activity opposes and thereby buffers against changes in the variable. A solid line is used in this diagram to indicate a direct relationship increase leads to increase, decrease leads to decrease between the components, while a dashed line represents an inverse relationship increase leads to a decrease and vice versa.

Negative feedback regulation must contain an odd number of dashed lines in order to maintain the variable within a narrow range of the desired value.

Figure 5. A schematic representation of negative feedback regulation. A solid line indicates that the connected components are directly related an increase in one component leads to increase the connected component, while a decrease will lead to decrease in the connected components. A dashed line indicates the connected components are inversely related an increase in one component leads to a decrease in the connected component while a decrease will lead to an increase in the connected component.

An odd number of dashed lines are a necessary condition for any negative feedback cycle of causation. Negative feedback acts to maintain the controlled variable within a narrow range of values see text for a detailed description.

A commonly used example of negative feedback is the regulation of room temperature by a thermostatically controlled heating and cooling system as displayed in Figure 6. Room temperature is the regulated variable, the sensor is a thermometer, the comparator is the thermostat—the device that compares the desired temperature operating point with the actual temperature error detection , and the effector is the heating or cooling system. In this example, an increase in outside heat is detected by the sensor and the information is conveyed to the thermostat.

The temperature information is compared to operating point and if there is sufficient difference between actual and desired temperature, the cooling system is activated and the heating system is inactivated reducing the error signal. The converse would happen if environmental temperature should fall, the cooling system would be turned off and the heating units activated. Thus, stable room temperatures can be maintained despite a wide range of fluctuating external conditions.

Figure 6. A schematic representation of the regulation of room temperature to illustrate the concept of negative feedback regulation. A solid line indicates that the connected components are directly related an increase in one component leads to an increase the connected components, while a decrease will lead to a decrease in the connected components.

A dashed line indicates that the connected components are inversely related an increase in one component leads to a decrease in the connected component while a decrease will lead to an increase in the connected component.

Negative feedback acts to maintain the room temperature within a narrow range of values despite changes in ambient temperature see text for a detailed description. With this caveat firmly in mind, the concept of self-regulation in biological system can be illustrated by the regulation of blood pressure. As early as the midth century, it became obvious that arterial blood pressure was maintained within a narrow range of values via the activation of neutrally mediated reflex adjustments Adolph, However, it was not until to s that the principles of negative feedback were applied to explain the homeostatic regulation of arterial blood pressure.

A detailed description of intricacies of blood pressure regulation is beyond the scope of the present essay for a recent review see Dampney, Nonetheless, a simplified feedback cycle, analogous to the one we used for room temperature, is seen in Figure 7.

Figure 7. A simplified schematic representation of the regulation of arterial blood pressure as a physiological example of negative feedback regulation. Negative feedback regulation acts to maintain the arterial blood pressure within a narrow range of values see text for a detailed description. Algebraically, blood pressure BP — analogous to voltage, E, in an electrical circuit is the product of the cardiac output CO — analogous to current, I, in an electrical circuit and systemic vascular resistance also known as total peripheral resistance TPR — analogous to electrical resistance, R.

Cardiac output is itself the product of the amount of blood ejected per beat [stroke volume SV ] multiplied by the number of beats per minute [heart rate HR ]. Returning to Figure 7 , the sensors are receptors baroreceptors located in arterial blood vessels aortic arch and carotid sinuses that respond to changes in arterial pressure increases in BP increase receptor activity. The comparator function is performed by a cluster of nerve cells within the medulla of brain [nucleus tractus solitarius NTS ] where the signal is processed to affect the output of the effector system.

The signal is processed at the NTS and then effects excitatory [rostral ventral lateral medulla RVLM via interneuron connections] and inhibitory [nucleus ambiguus NA , monosynaptically] areas within the medulla to elicit the motor response see Figure 8 for more details. The motor output from the central nervous system to target organs is conducted by means of two sets of nerves to the heart: parasympathetic nerves originating in the NA that decrease HR and sympathetic nerves originating in the intermediolateral column, IML of the spinal cord, regulated by neurons from the RVLM that increase HR and SV.

The sympathetic nerves also go to blood vessels, the activation of which decreases vessel diameter and thereby increases TPR. Thus, if BP should increase, the so-called baroreceptor reflex is activated. An increase in parasympathetic activity coupled with a decrease in sympathetic activity would reduce cardiac output decreasing HR and SV and decrease TPR.

The opposite changes would occur if blood pressure should decrease. Thus, negative feedback regulation buffers against transitory changes and thereby helps maintain a stable blood pressure on a beat-by-beat basis throughout the day despite changing environmental or behavioral conditions.

Figure 8. A simplified schematic representation of the central neural structures involved in baroreceptor reflex regulation of arterial blood pressure. Arterial pressure receptors located in the carotid sinuses and aortic arch nerve firing increases as arterial pressure increases convey afferent information via the glossopharyngeal IXth and vagus Xth nerves to the brain, respectively. This information is first processed by neurons located in the nucleus tractus solitarius NTS. The NTS then alters parasympathetic and sympathetic efferent nerve activity.

Specifically, the NTS alters the activity of neurons monosynaptically located in the nucleus ambiguus NA, parasympathetic pre-ganglionic neurons and neurons polysynaptically, via interneuron connections in the caudal ventrolateral medulla CVLM.

The CVLM, in turn, regulates the tonic sympathetic activity that originates in the rostral ventrolateral medulla [RVLM, that regulates sympathetic pre-ganglionic neurons located in the intermediolateral column IML of the spinal cord]. As an example, an increase in arterial blood pressure would increase baroreceptor nerve firing, increasing NTS neuron activity which, via interneurons, would trigger both an increase in the activity of the parasympathetic pre-ganglionic neurons located in the NA and decrease the firing of sympathetic pre-ganglionic neurons located in the IML less directly via CVLM mediated inhibition of the tonic activity of the RVLM.

The net result would be a decrease in heart rate? Reductions in arterial blood pressure would provoke changes in the opposite direction. Feedforward regulation is another mechanism by which homeostasis is modified and maintained as part of the behavioral response to environmental stimuli.

During feedforward regulation, which is also often referred to as central command, a response is elicited without feedback about the status of the regulated variable; that is, disturbances are evaluated and adjustments are made before changes in the regulated variable have actually occurred.

It should be emphasized that feedforward regulation, while acting independently of changes in the regulated variable, does require information about the nature and extent of the potential disturbance. For room temperature, the status of the windows and doors whether they are open or not must be monitored sensors placed on these openings.

Otherwise, a response would not be elicited until room temperature had deviated sufficiently from the set point to be detected by the thermostat and thereby activate the previously described negative feedback response. In living organisms, learning and experience provide the information necessary for feedforward control. The simple negative feedback schema described in the preceding paragraph cannot adequately convey the complexity of the homeostatic process that allows an organism to function and adapt to changing environmental conditions Carpenter, For example, the operating point or more accurately the operating range of the negative feedback regulation can be adjusted or even overridden by higher levels of control Goodman, These adjustments of the automatic e.

This hierarchical control is a multi-level, multi-goal seeking system as shown in Figure 9 modified from Goodman, In this schematic diagram, the first level represents the physiochemical processes, the organ and tissue functions, the component parts upon which homeostasis acts.

The second level is autonomous self regulation, homeostasis e. Here changes in a given variable are sensed and adjustments of the first level processes are initiated without input from higher levels of control. The third level is found in the central command and control centers central nervous system that process the information transmitted from the second level and integrates it with information from other sensory inputs to coordinate the physiological and behavioral response to changing environmental conditions.

This control can occur either at the conscious or unconscious level. An example of a conscious intervention would be the initiation of behaviors to cope with changing room temperature — adding or removing clothing, opening or closing windows seeking shade or sun, etc. As long as the pyrogen levels continue to increase and decrease you will feel like you are swinging back and forth.

Body functions such as regulation of the heartbeat, contraction of muscles, activation of enzymes, and cellular communication require tightly regulated calcium levels. Normally, we get a lot of calcium from our diet. The small intestine absorbs calcium from digested food. The endocrine system is the control center for regulating blood calcium homeostasis.

The parathyroid and thyroid glands contain receptors that respond to levels of calcium in the blood. In this feedback system, blood calcium level is the variable, because it changes in response to the environment. Changes in blood calcium level have the following effects:. Calcium imbalance in the blood can lead to disease or even death.

Hypocalcemia refers to low blood calcium levels. Signs of hypocalcemia include muscle spasms and heart malfunctions. Hypercalcemia occurs when blood calcium levels are higher than normal. Hypercalcemia can also cause heart malfunction as well as muscle weakness and kidney stones.

Improve this page Learn More. Skip to main content. Module Overview of Body Systems. Search for:. Maintaining Homeostasis Learning Outcomes Explain how different organ systems relate to one another to maintain homeostasis. Case Study: Fevers So what happens when you have a fever? The increase in pyrogen chemicals in the blood is stimulating the receptors that reset the upper temperature limit for a febrile response. Temperature is the variable during normal body temperature regulation, but not in this scenario.

Show Answer Answer d is correct. The hypothalamus is the control center for both normal body temperature homeostasis and febrile response. The skeletal muscle, sweat glands, and blood vessels are all effectors. Show Answer Option b is correct. This would increase the body temperature.

Option a would decrease the body temperature.