Q.1 Define Homeostasis? Discuss about the feedback system in detail.

Homeostasis is defined as maintenance of a constant internal environment. The term homeostasis was coined by W B Cannon.

Internal environment refers to Extracellular Fluid and all the tissues are dependent on extracellular fluid for their survival. Hence maintenance of internal environment / extracellular fluid is very much essential for survival of life. The extracellular fluid includes blood and interstitial fluid.

The term internal environment or mileu interior was coined by Claude Bernard.

Homeostasis is maintained in the body by Feedback mechanisms and Adaptive control system.

Feedback mechanisms

Feedback mechanisms are of two types

1.    Negative feedback mechanism

2.    Positive feedback mechanism

Negative feedback mechanism

If the activity of a particular system is increased or decreased, a control system initiates a negative process which brings about the change towards normal. The action is negative to the initial response. Most of the regulatory mechanism in the body is by negative feedback.

Eg:

1.    Regulation of blood pressure

2.    Secretion of thyroid hormone

Regulation of blood pressure

When the blood pressure suddenly rises or lowers, it initiates a series of reactions that tries to bring the blood pressure to normal levels.

Regulation of thyroid hormone secretion

When thyroid hormone secretion is more, it inhibits the secretion of thyroid stimulating hormone from pituitary so that thyroid hormone is not secreted from the thyroid gland.

Positive feedback mechanism

In this mechanism the system reacts in such a way to increase the intensity of change in the same direction. Usually it is harmful.

Eg:

1.    Formation of blood clot

2.    Uterine contractions during labour

Formation of blood clot

Injury to the blood vessel and bleeding causes formation of prothrombin activator which converts prothrombin into thrombin. Thrombin accelerates formation of prothrombin activator as well as converts fibrinogen to fibrin which arrests bleeding

Uterine contractions during labour

During onset of labor contraction of uterus cause movement of fetus into cervix leading to its dilation. Movement of fetus through cervix stimulates the receptors present there and the impulse generated stimulates hypothalamus to release oxytocin which further contracts the uterus.

Adaptive control system

It is also known as feed-forward control or servo mechanism. It is a delayed type of negative feedback mechanism. In a feed-forward system, the output of one stage of the processing of the control system is sent to a later stage of the process to affect later activity. Eg:

Preadaptation for exercise: changing the activity of postural muscles and of the vascular system in order to ready the body for the movement when it occurs.