Ageing and Death
Category : NEET
Ageing
(i) Definition: Ageing is the show deterioration in the structure and function of body cells tissues and organs of an animal and starts after the adulthood.
(ii) Gerontology: The field of developmental biology that deal with the process and problems of ageing is known as gerontology - (Gr. geron = old man; logos = discourse). The scientists involved in the science of ageing are called gerontologist.
(iii) Life cycle and life span: In all metazoan animals, the life cycle includes two developmental period; embryonic period (pre-natal developmental period) which extends from zygote to offspring till hatching or birth, and post embryonic period (post-natal developmental period), which includes growth, adulthood, reproduction, ageing. Thus, the life cycle comprises five main events: birth, growth, maturity, old age and death, that follow in the sequence named. Maximum life span is the maximum number of years survived by any member of a species, while average life span is the number of years survived by members of a population. Life expectancy is the age at which half the population still survives. The life span varies greatly in different organisms:
|
S.No. |
Animals name |
Life span |
|
(1) |
Mayfly |
24 hours |
|
(2) |
Silk moth |
2-3 days |
|
(3) |
Mouse |
3.5 years |
|
(4) |
Rats |
4.6 years |
|
(5) |
Humming bird |
8 years |
|
(6) |
Rabbits |
13 years |
|
(7) |
Monkeys |
26 years |
|
(8) |
Dog |
20-30 years |
|
(9) |
Bullfrog and Lion |
30 years |
|
(10) |
Toads |
36 years |
|
(11) |
Cat |
35-40 years |
|
(12) |
Chimpanzee |
45 years |
|
(13) |
Horses |
60 years |
|
(14) |
Man |
60.4 years (during 1988-95 period - WHO report) |
|
(15) |
Elephant |
70 years |
|
(16) |
Turkey |
118 years |
|
(17) |
Parrots |
140 years |
|
(18) |
Tortoise and banyan tree |
200 years |
|
(19) |
Sequoia |
About 3000-4000 years (longest life span) |
Maximum life span of human has been found to be about 121 years. Shirechiyo Izumi of Japan died due to pneumonia at the age of 120 years and 237 days in 1986.
Average life span of women is longer than men while the biological process of ageing is faster in human male than in human female. No organism lives for ever. (It is believed that Hydra is immortal as it does not undergo ageing). Every organism dies of old age if not killed earlier by an accident, a predator, aparasite or a disease.
(iv) Why old age ends in a natural death: Though it is difficult to give a categorical answer, certain factors that lead to death in old age are known. During the growth period, new cells are formed faster than the rate of death of old cells. But after the maximum growth, the metabolic rate declines and rate of formation of new cells is lower than the rate of death of body cells. So the repair of damaged cells is not complete and a slow deteriorative process starts. In human beings, it starts after the age of 30 years. But in certain cases, deterioration in structure and function of cells may start during childhood or even during prenatal life e.g. hearing efficiency of ear and atresia in ovary. As the organism grows older,
(a) The metabolic activities gradually decline.
(b) Capacity to replace the worn out cells decreases.
(c) The power to repair the worn out tissues and organs decreases.
(d) Resistance power to diseases is lowered.
(e) Lowered adaptability.
(f) Finally, non-functioning of some vital organs like heart, brain, kidneys, lungs or liver.
So with ageing, there is an impairment of physiological functions, called senescence and ultimately leads to death.
Peak of activity: In humans, generally speaking, the peak of activity is thought to reach at about 30 years of age. Thereafter, the body shows signs of senescence, i.e., deterioration and loss of normal function. From the age of about 30 years, the human body becomes functionally less efficient by about 0.8% every year. It is a biological process.
(v) Deterioration data of an old person: A 75 years old man, for instance, has, as compared to a 30 years old person, about 64% less taste buds, about 44% less renal glomeruli about 20% less nerve cells in the brain, and about 37% less axons in the spinal nerves. His heart pumps 35% less blood and sends 20% and 58% less blood to the brain and the kidneys respectively. His lungs have 44% less vital capacity and provide about one-third less oxygen to the blood per minute. His kidneys have 31% lower rate of glomerular filtration. His nerve impulses are propagated at a rate about 10% slower. At the age of 30, the height starts decreasing indefectibly, it decreases by 0.3 cm. at 40, by 2 cm. at 50 and by 2.5 cm. at 70.
(vi) Changes in ageing or symptoms of ageing: Gerontologist have worked out a large number of changes that accompany ageing. These are discussed below under three heads -
(a) Changes at organ level: During ageing, different organs and organ systems show different rates of decline e.g.
(1) Heart: With increasing age the efficiency of heart decreases. In a man of 70 years, the heart pumps only 65 per cent blood per minute as compared to a 30 years old man. Consequently, the blood going to the brain and kidney is reduced to 80 percent and 40 percent respectively.
(2) Oxygen uptake by blood : At the age of 20 blood takes about 4 litres of oxygen per minute, while in a man of 75 years, it takes only about 1.5 litres of oxygen in the same period.
(3) Decrease of blood volume: The production of new RBCs from the bone marrow declines and consequently the volume of blood also decreases.
(4) Kidney: The number of kidney tubules is found to reduce to half in the old age. As a result the volume of urine decreases. This creates lots of other urinary troubles and also causes body ache, low back and difficulty in passing urine.
(5) Lungs: The capacity of lungs for intake of air decreases. This leads to reduction in the oxygen supply to different tissues. Therefore, old persons suffer from breathlessness and inflammation of mucous membrane.
(6) Digestive system: The number of taste buds on tongue reduces to about one-third. The secretion of digestive juices also decreases with old age. This may result in indigestion, loss of appetite, dyspepsia, constipation and gas formation.
(7) Retention of water: The capacity of body cells to retain water also decreases with the result, the skin in old persons is dry and wrinkled.
(8) Nerve impulse: The rate of nerve impulse propagation reduces with age. The decline is about ten percent in man of 75 years as compared to that of 50 years old person.
(9) Increased mineral deposition in the bones which become brittle and easily factorable.
(10) Muscle tissue degenerates due to biochemical changes in the muscle cells or neuromuscular junction. This greatly reduces the muscular strength. Without exercise, estimated muscle mass declines 22 percent for women, and 23 percent for men. It can be prevented by regular exercise.
(11) Thymus is almost microscopic by the age of 70 years. This lowers the number and functioning of T-lymphocytes which lowers the immunity against the antigens.
(12) Hair start greying or falling at the age of 40 due to reduced rate of protein synthesis.
(13) Menopause in female at an average age of 52 years.
(14) Body becomes thin, shriveled and stooping.
(15) Decline in hearing power begins after the age of 10 years. It declines steadily up to 50 years of age after which the rate of decline is much slower. Hearing declines faster in men than in women.
(16) Eyes: Accommodation power of eye starts declining in the 40s; ability to distinguish fine details may begin to decline in the 70s while there is increased susceptibility of eyes to glare and more difficulty in detecting moving objects from 50 years onward. It is generalised that the human body becomes functionally less efficient from the age of 30 years onward by about 0.8% every year. So, the age of 30 years is a turning point in the process of development.
(b) Cellular changes: Cellular changes are of two types
Morphological changes:
(1) Accumulation of exhaustion pigments: The exhaustion pigment lipofuscin, yellow pigment and brown deposits are byproducts of unsaturated lipid oxidation. It is especially obvious in nerve and heart muscle cells but is present in almost all other cells of the body, though to a lesser degree. The accumulation of this pigment represents the failure of some excretory mechanism and has important implications in cellular senescence.
(2) Appearance of lipid vacuoles: Small lipid vacuoles appear in the cytoplasm.
(3) Decline in cell volume: The cells showing ageing exhibit hypertrophy or decrease in cell volume.
(4) Nuclear pyknosis: With advancing age, the nucleus becomes shrunken and stains deeply. Such a nucleus is called pyknotic and the degenerative process is known as nuclear pyknosis. This is caused by the condensation of the nuclear material rather than an increase in chromatin.
(5) Hypotrophy (decreased volume) of body cells: With the increasing age other changes associated with aging are - (i) increase in cholesterol levels, (ii) increase in blood globulin, (iii) decrease in alkaline and acid phosphatases (iv)decreases in cellular respiration.
Physiological changes:
(1) Accumulation of chromosomal aberrations and gene mutation in the nuclei with advancing age. These change the transcribed RNA which leads to the synthesis of defective proteins. These also retard the replication of DNA.
(2) Decrease in semipermeability of cell membrane due to deposition of calcium in the peripheral part of cytoplasm.
(3) Decrease in the rate of metabolism due to decreased number of mitochondria with advancing age.
(4) Decreased rate of protein synthesis is due to decrease in RER in cells.
(5) Increased inactivity of aldolase enzyme in the liver cells with advancing age.
(6) Decreased rate of cell mitosis. The non-dividing nerve cells and muscle cells start ageing earlier than the dividing cells of spleen and liver.
(7) Size of the nucleus decreases.
(8) Breakdown of cellular membrane. With advancing age, the lipids of biological membranes, that surround the cells and certain cell organelles, breakdown, forming a fatty, brown pigment called lipofuscin. The lipofuscin granules accumulate in ageing muscle and nerve cells, and interfere in their functioning.
(c) Extracellular change: The intercellular fibrous protein collagen forms about 40% of the total protein content of the body. It plays a significant role in the process of ageing. Collagen in young animals is permeable, flexible and soluble. As the age advances, it becomes less permeable, rigid and insoluble. On account of these changes in the surrounding collagen, it becomes progressively difficult for the diffusion of food and oxygen from the blood capillaries to the cells, and for the nitrogenous wastes and carbon dioxide to diffuse from the cells into the blood capillaries. The barrier of aged collagen between the cells and the blood results in decline of metabolic activities of ageing of the cells, and also of the animal.
(vii) Theories of ageing: Biological phenomena leading to ageing are not fully known. Several theories have been proposed to explain various aspects of ageing. These theories of ageing have been divided into two categories -
(a) Programmed theories: These theories state that ageing is due to certain “internal biological clocks” and follows a definite time table.
(b) Damage or Error theories: These theories state that ageing in the living organism is induced by certain external or environmental factors which cause damage to cell and organism. Several theories have been put forward to explain the process of ageing.
(1) Environmental or Mutation theory: It was proposed by Szilard (1959) and was supported by Curtis (1963). It states that ageing is largely due to adverse changes in the environment, especially radiations (cosmic rays, X-rays etc.) which induce gene mutations in the somatic cells by errors in DNA duplication. These mutations accumulate and result in the synthesis of defective polypeptide chains and defective proteins. Consequently, this reduces cellular efficiency and results in ageing. The view is supported by observations on ageing human cells, which produce defective enzymes. Moreover, sublethal exposure of individuals to radiation lowers their life expectancy.
(2) Gene theory of ageing: It states that ageing is an intrinsic phenomenon and is controlled by some genetic time table, called ageing genes, of the genome which induces ageing at specific period. The presence of a genetic time table was proposed by Pal. This theory states that ageing is the result of switching on of these ageing genes at the specific period of life span and induce the age associated symptoms, called senescence, followed by cell-death.
(3) Environmental-cum-genetic theory: Some biologists hold that interaction between environmental and genetic factors brings about ageing. This theory is supported by the fact that domestication, which changes the environment of animals, increases their life span.
(4) Neurohormonal or endocrine theory: According to this theory, ageing produces primary defects in certain centres of the central nervous system which control the functioning of the endocrine glands. Finch suggested that there is a brain-endocrine masterplan that states that biological clock operates through hormones to control the pace of ageing e.g.
(i) Level of hGH (Human Growth Hormone) decreases in about half of all adults.
(ii) In females, there is decline in level of female hormones, estrogens with age. Main source of estrogens in this period is fat tissue and not the ovaries.
(iii) In males, there is decline in the level of male hormone, testosterone, so causing decreased strength of muscles.
(iv) Level of melatonin hormone of pineal gland also falls with increasing age which may initiate changes throughout the endocrine system.
(v) There is also decreased secretion of DHEA (dehydroepiandrosterone) of adrenal glands so there is decreased synthesis of sex-hormones like testosterone and estrogens.
(5) Immunity theory: This theory suggests a link between ageing and disappearance of the thymus gland by late middle age in man. Thymus stimulates the proliferation of lymphocytes, increasing resistance to infection. Absence of this gland affects in two ways:
(i) It weakens the body's natural defence against foreign germs.
(ii) It increases the number of abnormal (defective or harmful) cells formed in the body itself. This destroys the tissues. The neuro-hormonal and immunity theories are collectively called pacemaker theories of ageing.
(6) Wear-and-Tear or stress theory: It states that the cells and tissues of the body continuously wear out due to internal and external stress factors. This coupled with the fact that the regenerative capacity progressively declines with age, causes ageing and finally death.
(7) Cross linkage theory: According to this theory ageing is caused by the increase of bonds between protein and nucleic acid molecules in the cell. These bonds alter the functional characteristic of these important cellular components leading to non-availability of certain functional proteins and resulting in malfunctioning of the cell.
(8) Waste product theory: According to this theory the accumulation of waste products are considered to poison the cell gradually, resulting in their ageing and death.
(9) Clinker's theory: It states that ageing is due to accumulation of metabolic wastes inside the body cells. These wastes, beyond some limit, poison the body cells and decline the metabolic rate and induce ageing.
(10) Error catastrophy theory: It was proposed by Orgeld (1963 A.D.). It states that errors in reading genetic code results in defective proteins which form defective enzyme leading to catastrophic damage to cells, tissues and organs, so induce senescence.
(11) Free radical theory: A free radical is a molecule with an unpaired and highly reactive electron. The oxygen-free radicals are formed as the by-product of normal cellular respiration. These free radicals take electrons from other molecules of a biological system so making them unstable and combine readily with other molecules. These free radicals initiate a chemical instability. These free radicals act as cross-linkers and inactivate functional molecules like DNA, RNA, enzymes, etc. and cause deterioration of functions of organisms. These free radicals are also involved in degenerative disorders, including cancer, atherosclerosis, cataract and neuro-degeneration.
(12) Metabolic or living theory : It states that those organisms (e.g. rats, mice, insects, birds, etc.) which have higher metabolic rate, mature, age and die earlier than those organisms (e.g. human beings) which have low metabolic rate and take years to mature and years to age.
(13) Collagen theory: It was proposed by F. Verzar (1964). It states that ageing is induced by changes in the collagen protein in the interstitial fluid surrounding the body cells (explained in extracellular changes in ageing).
(14) Reconciliation of theories of ageing: B.L. Strehler postulated that the ageing is programmed by the action of 'on-off switches' that reside in the genetic machinery. The mechanism activates first one set of genes, then another to produce special products (the enzymes, hormones, antibodies etc.) as the individual matures, ages and dies. The specific off switch prevents the key body cells found in the thymus, brain, heart and endocrine glands from dividing once the animal has attained maturity.
(15) Integrated theory: This theory assumes that ageing may be caused by many interacting mechanisms that occur in various cell types and at various times in the life of an individual. It seems likely that the "ageing genes" control many minor degenerations that accumulate to cause ageing. For instance, degenerations in the control systems (immune, nervous and hormonal systems) cause ageing due to loss of adaptability to survive in changing conditions. Thus, there are too many theories to explain the phenomenon of ageing. Each theory explains a particular aspect of ageing. It is long way to formulate comprehensive theory for explaining different aspects of ageing in all kinds of cells and organisms.
Death
(i) Definition: Death may be defined as the permanent cessation of all the vital function in an organism.
(ii) Characteristics:
(a) It is the last event in the degenerative processes of ageing.
(b) Death of an organism involves the death of the body cells. But all the cells of the body do not die at the same rate e.g. ciliated cells lining the respiratory tract of mammals continue to beat their cilia for a long time even after animal's death. Brain cells of body are last to die.
(c) There is no natural death in the protozoans e.g. Amoeba.
(d) Death involves widespread cell breakdown and cell death.
(e) It usually occurs due to lack of oxygen supply to body tissues.
(iii) Causes of death: Causes of death are many. These can be separated into following main categories –
(a) The weakening of the body tissues and of vital organs like heart, lungs, liver, kidneys, etc. which cause physiological and metabolic disorders of permanent nature leading to death. Death, in some cases, occurs due to sudden stoppage of the circulation of blood, food and oxygen to heart and brain leading to immediate death.
(b) The immune system (A system that provide resistance against disease – causing microbes) of the body is gradually impaired with advancing age. This increases the chances of infection in old age. Many old persons die of infectious diseases.
(c) Sudden blockage in the circulation of blood to heart, lungs and brain. This causes instantaneous death.
(iv) Brain or cerebral death: In the presence of cardiac activity, the permanent loss of cerebral functions, manifested clinically by absence of responses to external stimuli, lack of breath, and absence of cerebral reflexes is called brain death.
(v) The only truth of life: The death is an inevitable reality of life, and should be gladly accepted. It is a biological necessity for the maintenance of the balance of nature. Old organisms must make room for new ones.
(vi) Habit that influence life span: Life-style habits can influence life span considerably. Although a healthful diet does not guarantee immortality, regular exercise and avoiding of alcohol, smoking and drugs, contentment and freedom from stress can make a person's last year’s more pleasant.
(vii) Significance: Death is an essential and inescapable biological phenomenon which helps in maintaining ecological balance or homeostasis in nature. It prevents overcrowding of the members of a specific species and justifies the ‘continuity of life’ on earth.
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