Homeostasis :- More than Constant Internal Environment

Homeostasis:-

From the earliest days of physiology—at least as early as the time of Aristotle—physicians recognized that good health was somehow associated with a balance among the multiple lifesustaining forces (“humours”) in the body.

It was further determined by careful observation that most of the common physiological variables found in healthy organisms such as humans—blood pressure; body temperature; and blood-borne factors such as oxygen, glucose, and sodium ions, for example—are maintained within a predictable range. This is true despite external environmental conditions that may be far from constant. Thus was born the idea, first put forth by Claude Bernard, of a constant internal environment that is a prerequisite for good health, a concept later refined by the American physiologist Walter Cannon, who coined the term homeostasis.

 Originally, Homeostasis was defined as a state of reasonably stable balance between physiological variables such as those just described

However, this simple definition cannot give one a complete appreciation of what homeostasis entails. There probably is no such thing as a physiological variable that is constant over long periods of time. In fact, some variables undergo fairly dramatic swings around an average value during the course of a day, yet are still considered to be in balance. That is because homeostasis is a dynamic, not a static, process.

"Homeostasis, therefore, does not imply that a given physiological function or variable is rigidly constant with respect to time".

Consider swings in the concentration of glucose in the blood over the course of a day. the blood glucose concentration increases considerably within a short time after eating. Clearly, such a large change in the blood concentration of glucose is not consistent with the idea of a stable or static internal environment. What is important is that once the concentration of glucose in the blood increases, compensatory mechanisms restore it toward the concentration it was before the meal.

Blood glucose concentrations, as we have seen, may vary considerably over the course of a day. Yet, if the daily average glucose concentration was determined in the same person on many consecutive days, it would be much more predictable over days or even years than random, individual measurements of glucose over the course of a single day. In other words, there may be considerable variation in glucose values over short time periods, but less when they are averaged over long periods of time. 

This has led to the concept that homeostasis is a state of dynamic constancy. In such a state, a given variable like blood glucose may vary in the short term but is stable and predictable when averaged over the long term.

It is also Important to realize that a person may be homeo static for one variable but not homeostatic for another. Homeo stasis must be described differently, therefore, for each variable

Like I can be Homeostatic for Sodium but at the same time can be non-Homeostatic for calcium.

To elaborate on our earlier definition of therefore, when homeostasis,  is maintained, we refer to physiology; when it is not, we refer to pathophysiology (from the Greek meaning “suffering” pathos, or “disease”).

Cutting in short Properties of Homeostasis:-

Homeostatic Control Systems:-

The activities of cells, tissues, and organs must be regulated and integrated with each other so that any change in the extracellular fluid initiates a reaction to correct the change. The compensating mechanisms that mediate such responses are performed by "homeostatic control systems"

Imagine that we rapidly decrease the temperature of the room, say to 5°C, and keep it there. This immediately increases the loss of heat from our subject’s warm skin, upsetting the balance between heat gain and loss. The body temperature therefore starts to decrease. Very rapidly, however, a variety of homeostatic responses occur to limit the decrease

Feedback Systems:-

Negative feedback system:-

Negative feedback system, in which an increase or decrease in the variable being regulated brings about responses that tend to move the variable in the direction opposite (“negative” to) the direction of the original change. Thus, in our example, a decrease in body temperature led to responses that tended to increase the body temperature—that is, move it toward its original value.

Another example is :- Excess of ATP blocks production of further ATP inside cell.

Positive feedback system:-

Not all forms of feedback are negative. In some cases, positive feedback accelerates a process, leading to an “explosive” system. This is counter to the general physiological principle of homeostasis, because positive feedback has no obvious means of stopping. Not surprisingly, therefore, positive feedback is much less common in nature than negative feedback.

Some of the examples of this feedback are :-

1. Clot producing more clots.
2. High spike of Oxytocin hormone during parturition.

Feedforward Regulation:-

Another type of regulatory process often used in conjunction with feedback systems is feedforward, in which changes in regulated variables are anticipated and prepared for before they actually occur.

Examples:-

1. The smell of food triggers nerve responses from odor receptors in the nose to the cells of the digestive system. The effect is to prepare the digestive system for the arrival of food before we even consume it, for example, by inducing saliva to be secreted in the mouth and causing the stomach to churn and produce acid.
2. Familiar form of this is the increased heart rate that occurs in an athlete just before a competition begins

Importance - "Feedforward regulation improves the speed of the body’s homeostatic responses and mini mizes fluctuations in the level of the variable being regulated— that is, it reduces the amount of deviation from the set point"

Concept of set point /Steady state in Homeostasis:-

A set point is the physiological value around which the normal range fluctuates.

Resetting of Set Points:-

The set points for many regulated variables can be physiologically reset to a new value. A common example is fever, the increase in body temperature that occurs in response to infection and that is somewhat analogous to raising the setting of a thermostat in a room. The homeostatic control systems regulating body temperature are still functioning during a fever, but they maintain the temperature at an increased value. This regulated increase in body temperature is adaptive for fighting the infection, because elevated temperature inhibits proliferation of some pathogens. In fact, this is why a fever is often preceded by chills and shivering. The set point for body temperature has been reset to a higher value, and the body responds by shivering to generate heat.

The example of fever may have left the impression that set points are reset only in response to external stimuli, such as the presence of pathogens, but this is not the case. Indeed, the set points for many regulated variables change on a rhythmic basis every day. For example, the set point for body temperature is higher during the day than at night

Components of Homeostatic Control system:-

There are 3 components of hoemostatic system :-

1. Afferent one/ Receptor
2. Integrating System
3. Efferent one / Effector

• A receptor detects the environmental change.
• An integrating center often receives signals from many receptors, some of which may respond to quite different types of stimuli. Thus, the output of an integrating center reflects the net effect of the total afferent input; that is, it represents an integration of numerous bits of information.
• The output of an integrating center is sent to the last component of the system, whose change in activity constitutes the overall response of the system. This component is known as an effector. The information going from an integrating center to an effector is like a command directing the effector to alter its activity. This information travels along the efferent pathway

The classical example of integration of all the three component in forming a homeostatis control system in maintaing body temperature..

That's all from my side
Signing Off--
Bhavesh saini 

It's always better to practise what you just learnt :-
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