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What is homeostasis?

The term homeostasis was coined by Cannon and this term is derived from ‘homoios’ meaning similar and ‘stasis’ meaning position.

It is defined as the maintenance of a nearly constant internal environment of the cell in the body is called homeostasis. This term is used to define the constancy of the Milieu interior.

The internal environment of the cell

The component of extracellular fluid is considered to be the internal environment of the cell. Our body cells can live and function only when the ECF is compatible with their survival.

The internal environment must be kept relatively stable for the functioning of the cell. Thus, the chemical composition, as well as the physical state of this internal environment of the body, must be maintained within narrow limits.

Important constituents and physical characteristics of extracellular fluid (ECF)

Normal Value Value  of normal range
Oxygen 40 35–45 mmHg
Carbon dioxide 40 35–45 mmHg
Sodium ion  142 138–146 mmol/l
Potassium ion 4.2 3.8–5.0 mmol/L
Calcium ion 1.2 0.5–2.0 mmol/L
Chloride ion 108 103–112 mmol/L
Bicarbonate ion 28 24–32 mmol/L
Glucose 85 75–95 mg/dl
Body temperature 98.4 (37.0) 98–98.8 (37.0) °F ( °C)
Acid-base 7.4 7.3–7.5


So homeostasis is not a fixed rigid state rather it is a range. Systems such as the digestive, respiratory and excretory contribute directly to homeostasis.

The endocrine and the nervous systems provide an important contribution to homeostasis by coordinating the activities of all the other systems that participate more directly in homeostatic mechanisms.

The endocrine system takes time to initiate action, and once the action has been initiated, it continues for quite long.

In comparison, the nervous system initiates action much speedy or faster, and its action can also be switched off promptly

Role of various body systems in homeostasis

Every part of the body makes some specific contribution to the survival of the whole organism. It can be done by maintaining the composition, pH, and temperature of the internal environment at an optimal level.

The optimal level is that at which the enzyme functions the best. The role of various systems are described below:

The circulatory system

The main role is to transport materials such as nutrients compounds, oxygen (O2), carbon dioxide (CO2), wastes, electrolytes, and hormones from one part of the body to another.

The 1st stage is the movement of blood in the blood vessels in the body, and the second is the movement of fluid between the blood capillaries & the intercellular spaces between the tissue cells in the body.

As blood passes through the blood capillaries, a continuous exchange of extracellular fluid also occurs between the plasma portion of the blood and the interstitial fluid that fills the intercellular spaces.

The walls of the capillaries are more permeable to most molecules in plasma except plasma protein molecules, which are too large to pass it.

The process of diffusion is caused by the kinetic motion of the molecules in both the plasma and the interstitial fluid.

So, the fluid and dissolved molecules are continually moving and bouncing in all directions within the plasma and the fluid in the intercellular spaces, as well as through the capillary pores.

The digestive system

Its main role is to break down dietary food into small nutrient molecules that can be absorbed into the plasma for distribution to the entire body cells.

The dissolved nutrients such as carbohydrates compounds, fatty acids, & amino acids, are absorbed from the ingested food materials into the extracellular fluid of the blood.

It also eliminates undigested food residues to the external environment in the feces.

The respiratory system in Homeostasis

The main role of it is to get O2 from the lungs and eliminate CO2 from the external environment. Each time the blood passes through the lungs, it picks up oxygen in the alveoli, thus providing needed cells.

The membrane between the alveoli and the lumen of the pulmonary capillaries in the alveolar membrane ( thickness is 0.4 to 2.0 micrometers). So it is also important in maintaining the proper pH of the internal environment.

The urinary system

Its major function is to remove excess water, salt, acid, and other electrolytes from the plasma and eliminate them in the urine, along with waste products other than CO2.

These substances include end products of cellular metabolism like urea, uric acid, excesses of ions and water from the food, etc.

Initially, plasma filters through the glomeruli into the tubules and then reabsorption of substances needed by the body like glucose, amino acids, appropriate amounts of water, and many of the ions occur.

The musculoskeletal system

Bones and joints provide support and protection for the soft tissues and organs. It also carries out as a storage reservoir for calcium (Ca2+) ions.

The muscular system includes skeletal muscles moves the bones to which the skeletal muscles are attached.

In absence of this, the body could not move to the appropriate place at the appropriate time to obtain the foods required for nutrition. This also provides motility for protection or preservation against adverse surroundings.

The integumentary system

The skin and its appendages like hair, nails, glands, etc. cover, cushion, & protect or preserve the deeper tissues and organs of the body.

This is also important for temperature regulation or maintenance & excretion of wastes and it provides a sensory interface between the body and the external environment.

So, This serves as an outer protective barrier that prevents internal fluid from being lost from the body and foreign microorganisms from entering.

Nervous system

It controls as well as coordinates body activities that require swift responses. It is especially important in detecting alters in the external environment and initiating reactions to them.

Also, it is responsible for higher functions such as consciousness, memory, and creativity. This is composed of three major parts such as the sensory input portion, the central nervous system (or integrative portion), & the motor output portion of the body.

Sensory receptors detect or perceive the state of the body or the state of the surroundings. For example, receptors in the skin sense whenever an object touches the skin.

The central nervous system can store information or details, generate thoughts or ideas, create ambition, and determine reactions.

Now, Appropriate signals are then transmitted through the motor output portion to carry out one’s desires.

Endocrine system

It is especially important in controlling or maintaining the blood concentration of nutrients and, by adjusting kidney function, controlling or maintaining the volume and electrolyte composition of the ECF.

Reproductive system

This is not considered a homeostatic function. However, help to maintain homeostasis by generating new beings to take the place of those that are dying.

Homeostatic/ Homeostasis control system

This is a functionally interconnected network of body components. It can be intrinsic as well as extrinsic.
To maintain homeostasis:

  • The control system must be able to detect the deviation in the internal environment factor.
  • Make appropriate adjustments in the activity of the body parts to restore this factor to its normal value.

Principles of regulation of Homeostasis

Most of the regulatory systems follow a similar characteristic pattern:

  • Change in the system is informed to the regulator.
  • The regulator initiates events that work in a direction opposite to the reported change.
  • The process is regulated returned to a set level recognized as normal.

Control system in Homeostasis

  • The control system is designed to maintain a controlled variable at a set point.
  • The value of the controlled variable or unsteady is continuously monitored by a sensor.
  • This is conveyed to the controller as a feedback signal.
  • The controller compares it with the set point.
  • The difference between the setpoint and the feedback signal is an error.
  • The output of the controller is conveyed to the effector.
  • Effector applies a correction that takes the controlled variable towards the set point.


  • Regulation of oxygen (O2) and carbon dioxide (CO2) concentration in the extracellular fluid.
  • Regulation of arterial blood pressure

Characteristics of the control system (in Homeostasis)

This can be described in the following 2 ways:

  1. Negative feedback mechanism
  2. Positive feedback mechanism

Negative feedback mechanism

Negative feedback mechanism -homeostasis

  • Most control systems of the body act by this feedback mechanism.
  • A slight change in a homeostatically controlled factor triggers a response.
  • The final response is to restore the factor to normal by moving the factor in the opposite direction of its initial change.
  • Corrective adjustments will oppose the original deviation from the normal level.
  • For example Temperature regulation


Positive feedback mechanism

  • In this, the output enhances or amplifies a change or alteration so that the controlled variable continues to move in the direction of the initial change.
  • Examples: parturition reflex, Blood clotting
Parturition reflex
  • The baby pushes against the cervix, that causing it to stretch.
  • Stretching of the cervix causes nerve impulses to be sent or transfer to the brain
  • The brain stimulates the pituitary to release oxytocin
  • Oxytocin causes the uterus to contract
Blood clotting
  • Injury to the blood vessel and bleeding
  • Formation of prothrombin activator
  • Conversion of prothrombin into thrombin
  • Conversion of fibrinogen into fibrin
  • Stoppage of bleeding




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