The internal environment of the human body is tightly controlled and kept almost constant by the process called homeostasis.
Without homeostasis, it would be difficult for an animal to survive. When a person is under stress or pain, the chances of deviation in normal physiology is averted.
What is homeostasis
Homeostasis is a dynamic body process that tries to maintain nearly constant conditions in the internal environment of a biological system at any point in time.
For example, a man has a blood pressure of 120/80. This is always kept constant by the body within a narrow range of deviation like ±10. Similarly, the pH of body fluids like tears, saliva, blood, electrolyte levels is kept constant.
|Sl. No||Body physiological variable||Homeostatic Value or Range|
|1||Temperature||36.5–37.5 °C / (97.7–99.5 °F)|
|2||Blood (pH)||7.35 – 7.45|
|3||Saliva (pH)||5.8 – 7.4|
|4||Bile (pH)||6 – 8.5|
|5||Urine (pH)||4.5 – 8|
|6||Sweat (pH)||4.5 -7|
|7||Blood glucose levels||60-100mg/100ml of plasma|
|8||Sodium (Electrolyte)||135 -143 mmol/liter|
|9||Chloride||97-106 mmol/liter of plasma|
|10||Iron||14-35 mmol/ liter|
|11||Blood pulse||60 – 80 per minute|
|12||Blood pressure||120/80 ±10 mmHg systolic/diastolic|
When there is a disturbance to these values, the body tries to bring it to normal by homeostasis.
So, it is a dynamic and ever-changing process that tries to keep the body under normal levels at all the possible conditions.
This dynamic control is brought about by important organs like the brain, heart, lungs, kidneys, etc.
For example, there is constant ion concentration by kidneys and constant oxygen levels by the respiratory system.
Since a living body is subjected to many external and internal stresses, the homeostasis is ever active.
It does so by trying to correct a small change in the range of regular physiological conditions.
Mechanism of homeostasis:
Homeostasis is regulated by control systems that detect any changes in physiology and respond accordingly. The control system has parts like
a) Control center
b) Sensor or Detectors
This is also called as the regulating center. It determines the levels within which a physiological variable has to be maintained.
It receives input from a sensor or detector. Once it determines the needed change, it sends a response to the effector.
The control center provides nervous stimuli or hormonal stimuli as an effector response to bring desired homeostatic change.
Sensors are the ones that first detect the change in the physiological variable and send the signal to the control center.
Examples of sensors:
- Baroreceptors that detect the changes in B.P.
- Chemoreceptors detect the carbon dioxide levels in the body.
- Temperature-sensitive nerve centers determine the peripheral temperature.
Effectors carry out the signal provided by the control center and induce the desired changes in body physiology.
Example for effectors corresponding to the sensors above are
- Heart, blood vessels and also kidneys as they can alter blood volume
- Lungs, bone marrow (for erythropoiesis)
- Skeletal muscles (generate heat), blood vessels of skin (minimize heat loss by contraction)
This is a constant process and requires a readjustment of many physiological variables.
Types of homeostatic mechanisms
A feedback mechanism is one where a system’s response is controlled based on the input it receives.
The homeostasis occurs basically by either of the two methods.
- Negative feedback mechanism
- Positive feedback mechanism
1. Negative feedback mechanism:
The negative feedback mechanism is an inhibitory function. Here, the effector’s responses decrease the effect of the original stimulus to bring back the homeostasis.
Examples of negative feedback include the control of body temperature.
It prevents the body temperature from getting too high or too low.
When the temperature is high, the system is inhibited to prevent a further rise in temperature. The stimulation of skeletal muscles, constriction of blood vessels in the skin is inhibited. This leads to a lowering of body temperature.
If the temperature is low, the system is stimulated to enhance the temperature rise. The temperature regulating center in the hypothalamus of the brain is activated. This leads to the stimulation of skeletal muscles to produce more heat. Further, the peripheral blood vessels get constrict to minimize heat loss to the surroundings.
If you notice clearly, the body’s intent is always to generate heat and the hypothalamus keeps the body cool.
When a person suffers from sunstroke, the temperature center is deranged so there is a constant rise in the body heat. The hypothalamus is unable to determine the comfortable levels of body temperature and give a required effector signal.
In a human body, most of the homeostatic mechanisms are of negative feedback type.
2. Positive feedback mechanism
This is an amplifier type of process. Here the stimulation progressively increases the response. So, as long as the stimulation is present, the response is also amplified.
This type of mechanism is seen in few systems like the blood clotting and the uterus contraction at the time of delivery.
When the head of the fetus in the womb enters into the cervix, this leads to the stretch sensitization of nerve of uterine walls. The nerve signal on reaching the posterior hypothalamus, oxytocin is released which further precipitates the uterine contractions.
This cycle of oxytocin secretion due to stretch reflex and enhanced uterine contraction occur until the baby is delivered.
The key physiological process under Homeostasis:
Homeostasis of blood pressure
Normal blood pressure in humans is 120/80mmHg. If one is anxious or even irritated, his blood pressure tends to rise to 140/90. Again when he cools down, it returns to normal. Similarly, many other body organs like heart, lungs also increase in their function.
This rise is meant to adopt the body to the excess demand like more oxygen, energy to cope with the situation.
This shift in balance is quite normal and is within a few points. If it goes beyond, mostly it leads to disorders and even death based on the intensity and duration of deviation from homeostatic range.
Body systems like the nervous system, endocrine system are involved in the maintenance of homeostasis. This is done through nerve stimulation or changes in hormonal levels.
Other examples of correction of homeostasis are
- Sweating when the body temperature rises due to heat in summer.
- Gushing and intense breathing to give excess oxygen while running or jogging.
In the above conditions, the body tries to keep the system normal. In conditions of failure to keep up the body temperature within limits, the condition results in sunstroke. So the ability of the body to maintain homeostasis is essential to keep the body healthy.