Page 176 - 86 human physiology part-2

Development: Birth Through Death

Senses

Vision

Changes in vision begin at an early age. The cornea becomes thicker and less curved. The anterior
chamber decreases in size and volume. The lens becomes thicker and more opaque, and also increases
ridgity and loses elasticity. The ciliary muscles atrophy and the pupil constricts. There is also a
reduction of rods and nerve cells of the retina.

Hearing

Approximately one third of people over the age of 65 have hearing loss. The ability to distinguish
between high and low frequency diminishes with age. Loss of hearing for sounds of high-frequency
(presbycusis) is the most common, although the ability to distinguish sound localization also decreases.

Taste and Smell

Sensitivity to odors and taste decline with age. The sense of smell begins to degenerate with the
loss of olfactory sensory neurons and loss of cells from the olfactory bulb. The decline in taste
sensation is more gradual than that of smell. The elderly have trouble differentiating between flavors.
The number of fungiform papillae of the tongue decline by 50% by the age of 50. Taste could also be
affected by the loss of salivary gland secretions, notably amylase.

Cellular Aging

As people age, oxygen intake decreases as well as the basal metabolic rate. The decrease in the
metabolic rate, delayed shivering response, sedentary lifestyle, decreased vasoconstrictor reponse,
diminished sweating, and undernutrition are reasons why the elderly cannot maintain body temperature.
There is also a decrease in total body water (TBW). In newborns, TBW is 75% to 80%. TBW continues
to decline in childhood to 60% to 65%, to less than 60% in adults.

Organism Aging

Aging is generally characterized by the declining ability to respond to stress, increasing
homeostatic imbalance and increased risk of disease. Because of this, death is the ultimate consequence
of aging. Differences in maximum life span between species correspond to different "rates of aging".
For example, inherited differences in the rate of aging make a mouse elderly at 3 years and a human
elderly at 90 years. These genetic differences affect a variety of physiological processes, probably
including the efficiency of DNA repair, antioxidant enzymes, and rates of free radical production.

Aubrey de Grey

Aubrey David Nicholas Jasper de Grey, Ph.D., (born 20 April 1963 in London, England) is a

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