Ageing is defined as the process of progressive deterioration with the passage of time, in the structure and functions of cells, tissues, organs and organism. It finally leads to death. The branch of biology dealing with the study of the processes of ageing is known as "Gerontology". The following changes are associated with the process:

I. Morphological and Physiological changes
a) With the advancing age as compared to a 30 year old person, in a man of 70 years the heart pumps only 65% blood per minute and the blood going to the brain and kidneys is reduced to 80% and 42% respectively. At the age of 20 years, blood takes about 4 litres of oxygen per minute from the lungs, but in a 75 year old man, it takes only about 1.5 litres of oxygen in one minute.
b) With the advancing age, the number of kidney tubules is reduced to half. The number of taste buds in the tongue is reduced to about one-third.
c) The production of new R.B.C from bone marrow also decreases.
d) Cells gradually loose the capacity to retain water, blood volume decreases and the tissues become dry. This results in dry and wrinkled skin, flabby and weak muscles, brittle bones, reduced blood circulation, decreased formation of urine and a thin, shriveled and stooping body.

II. Cellular Changes

1. Chromosomal aberrations and gene mutations in somatic cells cause changes in DNA structure, as a result of which enzyme synthesis is impaired. For example, with an increase in the age, the liver cells show more of chromosomal aberrations.
2. An enzyme aldolase is found to become inactive in the liver cells of mice with the advancement of age.
3. The size of nucleus decreases.
4. Large quantities of defective proteins accumulate in the cells.
5. DNA duplication is very much reduced.
6. Body cells gradually loose the power of multiplication.
7. Mitochondria decrease and lysosomes increase in number.

III. Extracellular Changes

An extracellular protein called collagen is found to be associated with ageing. In young organisms, it is permeable, flexible and soluble but in older organisms, it becomes less permeable, rigid and insoluble. Because of this permeability of cells to oxygen, carbon dioxide and nutrients is affected as it offers physical barriers.

Theories of Ageing
The following theories have been proposed to explain the causes of ageing:
1. Ageing involves the accumulation of somatic mutations caused due to radiations present in the environment. It leads to a gradual decline in the synthesis of functional enzymes and disturbs the normal working of the cell leading to its death.
2. According to another theory it is the intrinsic genetic property of cells to age.It is based on the fact that even when the environment conditions remain suitable for an organism, it undergoes ageing process at a rate determined by its genes.
3. According to wear and tear theory of ageing, the cells and tissues of the body are continuously subjected to wear-and-tear processes due to changes in the extrinsic and intrinsic factors. Thus, animals gradually exhibit the signs and symptoms of ageing and ultimately die.
4. According to immunity theory, ageing is associated with the disappearance of thymus gland by the late middle age in man. It causes a weakening in the defense mechanisms of the body and in the course of time, leads to a large scale decay and destruction of cells and tissues.
5. According to another theory, ageing is due to changes in the parts of central nervous system that regulate the functioning of endocrine glands. Absence of hormones causes the malfunctioning of tissues. It has been proved that reduction in the production of sex hormones leads to ageing in mammals including man.
6. According to Clinker theory, each cell accumulates poisonous substances in its cytoplasm as a result of its metabolic activities. These substances reduce the life of the cell. It is supported by the fact that large quantities of pigments containing proteins and enzymes are accumulated in older people. It is true for nerve and muscle cells which do not undergo mitosis.

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