Homeostasis: Definition, Mechanisms, and Importance

Definition of Homeostasis

• Homeostasis means the maintenance of the nearly constant internal environment of a cell. It can also be known as the constancy of the internal environment of a cell.
• Homeostasis of the cell depends on the following:
  • According to Bernard’s concept of Milieu interieur, or homeostasis, it is mainly dependent on the interstitial fluid (ISF).
  • The first concept of homeostasis was given by the scientist, Claude Bernard. He introduced the term ‘Milieu interieur’ meaning interior environment. 
  • According to Claude Bernard, the cell maintains its internal environment constant, which, in turn, depends on the immediate external environment of the cell, which is the interstitial fluid (ISF). If the composition of the interstitial fluid is changed, the internal environment of the cell will also change. Thus, according to Bernard’s concept of Milieu interieur, or homeostasis, is mainly dependent on the interstitial fluid (ISF). 
• It is to be noted that although Claude Bernard introduced the term Milieu interior, he did not coin the term homeostasis. Walter Bradford Cannon was the first coin this term.
• Our body contains a control system for maintaining homeostasis. There are two types of control systems in our bodies: feedback and feedforward.

Also Read: Proprioceptive Reflexes: Mechanisms and Functions

The Feedback Control System

• This contains two types, 
  • Positive feedback control system 
  • Negative feedback control system

Positive Feedback Control System

• Suppose there is a reaction where A produces B and B produces C. This makes C the output. A feedback system is a system where the output returns to give input so that the next action will change. C, which is the output, is returning to give some input either to A or B so that the next action changes. This sort of system is known as the feedback system. 
• If this input that has returned is of stimulatory type, it will be known as the positive feedback control system. So, if C is stimulating B and again B will produce C, the pathway will move in the same direction. This system is the positive feedback control system and is also known as Vicious Cycle.
• According to the positive feedback control system, an example is taken where the blood pressure increases due to certain reasons. Then, according to the positive feedback control system, the increase in blood pressure will stimulate further rise of blood pressure through which it increases further. The blood pressure will, in this manner, continue to rise, leading to disastrous consequences. However, this is not seen to be the case generally. This is because, in the blood pressure feedback control system it is not the positive feedback control system working. This is why it is said that positive feedback control system destabilizes our control system.
• Thus, a system is required where an increase in blood pressure will not stimulate further rise but rather decrease it to stabilise it. This system is known as the negative feedback control system. In the negative feedback control system, the C that has been produced will give inhibitory input to B. The C may return to and try to inhibit A so that the next action changes. 
• Negative feedback control systems are more prevalent in our bodies, and they try to stabilize our control systems by stabilizing different parameters in the body. Thus, most control systems are negative feedback control systems, whereas positive feedback control systems are limited.
• There are certain conditions where positive control feedback systems are beneficial to the body. 
• Here are a few examples of the positive feedback control system:  

1. The process of blood clotting:
   • In the intrinsic pathway of blood coagulation, when factor X is produced, it stimulates the prothrombin, and prothrombin produces thrombin. 
   • Whenever thrombin is generated, it can activate multiple clotting factors, such as factor V and factor VIII. All of these clotting factors, in turn, will activate factor X, leading to the production of more thrombin. 
   • Thus, whenever a small amount of thrombin is created, it gives positive feedback on factor V and factor VIII. 
2. A Spike in Oestrogen and LH: 
   • A spike in oestrogen LH is observed during the follicular phase of the menstrual cycle, causing ovulation. It is known that oestrogen has positive feedback on LH just before the ovulation. 
   • Otherwise, during the whole menstrual cycle, oestrogen has a negative feedback on FSH and LH. 
   • Oestrogen stimulates LH and FSH production 1-2 days before ovulation and helps during the ovulation spike. 
3. Uterine Contraction During Childbirth:
   • Whenever uterine contraction occurs, it pushes the foetal head towards the cervix, causing stretching of the cervix. 
   • This stretch generates a neural signal that ascends to the hypothalamus. This stimulates the synthesis of the oxytocin hormone. This oxytocin hormone causes more uterine contraction. 
   • When there is a larger uterine contraction, the foetal head will be pushed towards the cervix, and the cervix will be stretched.
4. Lactation:
   • A similar mechanism occurs during lactation. When the baby suckles near the areola region of the breast, a positive feedback control system is initiated. 
   • This will produce oxytocin and this hormone will cause more ejection of the milk. 
5. Generation of Nerve Impulses/Nerve Action Potential:
   • When action potential occurs, one requires a channel known as a voltage-gated sodium channel. The opening of this channel is an example of a positive feedback control system. This channel, which is open-gated, opens in the presence of depolarization. 
   • When depolarization occurs, it stimulates the opening of the sodium channel. Through this channel, sodium enters the cell. This positive-charged sodium will produce more depolarization. More depolarization means more opening of the voltage-gated sodium channel. Thus, opening one sodium channel causes depolarization. This stimulates the opening of another sodium channel; hence, it is a positive feedback control system.
6. Release of Sarcoplasmic Calcium through Ryanodine Receptors:
   • Release of Sarcoplasmic Calcium through Ryanodine Receptors during excitation-contraction coupling in cardiac muscle is an example of the positive feedback control system. 
   • Here in the diagram, there is a representation of the sarcoplasmic reticulum with two spherical ends. 
   • At the spherical end, a ryanodine receptor, the RyR, can be seen. If this receptor is opened, calcium is released from the sarcoplasmic reticulum into the cytoplasm. 
   • This calcium acts as a stimulus for the opening of more RyR channels. When this channel is opened a little, an influx of calcium into the cytoplasm will further stimulate the opening of the RyR channel. 
   • A positive feedback mechanism then sets in, causing the intracellular calcium to rise to a very high level, producing muscle contraction. This sort of mechanism is present in the cardiac muscle.

Negative Feedback Control System

• Typical examples of the negative feedback control system can be seen in endocrinology.  
• For example, the hypothalamus produces the thyrotropin-releasing hormone, which stimulates the pituitary gland. TSH stimulates the thyroid gland, which in turn stimulates the production of T3 and T4. Then, T4 and T3 have a negative feedback effect on TSH production and on TRH production.
• Gain
  • In the negative feedback control system, the gain of the control system must be studied. With the help of gain, we can understand the effectiveness of the control system. As can be understood from the equation above, gain is equal to corrections upon error.
  • Gain is calculated so that the effectiveness of the control system can be assessed. The more the gain, the more is effectiveness of the control system. If there were a condition where the correction is 100 and the error is 0, the gain in such a condition will be infinitive. When a control system is so effective that the residual error is 0, then the gain of the control system is known as infinite gain. An example of such a control system is the Kidney. 
  • The Kidney is a very slow system for the regulation of blood pressure and for the regulation of blood volume. But given enough time, the kidney can maintain blood pressure without residual error. The kidney will excrete excess water from the body and the blood pressure will return to 100 mm Hg, which makes the error 0. Thus, for the purpose of regulation of blood pressure and blood volume, the gain of the kidney is infinite gain with the residual error being equal to 0. 
• Regulation Factor
  • Apart from gain, certain books provide another factor known as regulation factor. Regulation factor checks the accuracy of the control system. Gain checks the effectiveness and regulation factor checks the accuracy of the control system. Regulation factor is equal to change with control system upon change without control system.

Also Read: Different Types Of Hypoxia

The Feedforward Control System 

• The feedforward control system is anticipatory control system. When the control system predicts that some change is going to happen and corrective measure are being taken before the change, that is known as the anticipatory control system. 
• When one is returning home from work and it is winter, the room can be expected to be cold when entered. So, if there is a system by which one can switch on the heater in the room, before entering the room, taking a precautionary measure so that the room temperature is raised. This is a typical demonstration of how the feedforward control system works. 

There are some typical examples of this system. They are as follows:

1. The Thermoregulation System
   • This is also known as body heat regulation system.
   • In this system both the feedforward and the feedback components are present. There is something called the core body temperature. It is the internal temperature of the body; it is in and around the vital organs of the body. It remains constant at 37 degrees centigrade. The thermoregulation system always tries to maintain the temperature at constant. Apart from core body temperature, there also lies skin temperature, also known as shell temperature. This skin temperature can change according to environmental temperature. But the prime target of the thermoregulation system is to maintain the core body temperature constant.
   • For example, in the winter season, the environmental temperature is very low, 4 or -4, for instance, it will decrease the skin temperature. There are thermoreceptors present on the skin which will detect the fall of temperature of skin and then send a signal to the thermoregulatory centre of the hypothalamus. The hypothalamus then activates certain systems which produces heat such as shivering, vasoconstriction, etc. This heat production will rise the core body temperature ahead of decreasing core body temperature. The thermoregulation system has taken the measure in such case because if the skin temperature remains low it would cause the core temperature to drop eventually. This signal is delivered to the hypothalamus, and it then produces heat. The temperature thus rises ahead of fall. This is known as feedforward loop of thermoregulation. 
   • Thus, whenever there is drop in environmental temperature, the core body temperature is not going to decrease because of the anticipatory feedforward control system. However, when exposed to very low environmental temperature when feedforward system is not strong enough, the decrease in skin temperature will subsequently decrease the core body temperature. Now when the core body temperature is decreased, there are certain receptors at the level of the internal organs which will send signal to the hypothalamus, which will in turn activate the heat production system. This will try to raise the fall of core body temperature. Here, when the temperature is already decreased this mechanism takes place to get the temperature to rise again. This is an example of the negative feedback type of control system of thermoregulation.
2. Increase in Heart Rate and Respiratory Rate Even Before the Start of Exercise
   • When one is asked to start exercising on a treadmill and they take precautions, the system is activated inside the body causing an increase of heart rate and respiratory rate. This is known as psychic stimulation. Just the mere thought of exercise is rising the heart rate, the blood pressure and respiratory rate.
3. Cephalic Phase of Gastric Secretion
   • Cephalic phase means that the food is at the level of the mouth, or when one is looking at the food when hungry, the acid secretion will start in the stomach. Here this happens because the stomach is taking anticipatory precautions.
4. Receptive Relaxation of Stomach
   • Two types of relaxations of the stomach take place,
     • Receptive relaxation
     • Adaptive relaxation
   • When food is at the level of the mouth and one is swallowing it and it enters the upper parts of the oesophagus, the upper part of the stomach relaxes or dilates. This happens ahead of the food reaching, in order to receive the food. This is known as receptive relaxation of the stomach and is typical of the feedforward control system.
   • When the food is already inside the stomach, the stomach will again relax to adjust the volume of the food. This is adaptive relaxation and is not a part of the feed-forward control system.
5. Cerebellum
   • The granular cells present at the level of the cerebellum stimulate the basket cells. The basket cells are inhibitory in kind and inhibit the Purkinje cells. The Purkinje cells have no control on the basket cell or on the granular cells. This makes the output always go in one direction, from granular to basket and then to Purkinje, but there is no return of input. This is an example of the feedforward or anticipatory control system.

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