Preksha Dhyana Human Body Part 2

Author(s): Jethalal S Zaveri
Publisher: Jain Vishva Bharati

Page 86

tinues to fall after birth. In the nine months from conception to birth, a human fetus, increases its size and weight about 10 billionfold. In the first year after birth, it triples its weight and adds about 50 percent in height. Then follow a period of consolidation and the growth settles down for a time to a steady 2 to 3 kgs. and 5 to 7 cms. a year. The flow of sex hormones during adolescence sparks a sudden growth spurt adding as much as 15 to 18 cms. in a year. After this spurt, there is a rapid deceleration. Growth in height stops entirely by the age 18 to 21 in boys and 16 to 18 in girls. The onset of maturity may be delayed by a variety of factors such as undernourishment, but growth eventually ceases.
The programmed sequence of growth and its completion may hold important clues to the prolongation of vigorous life. Interactions of the hormones of the pituitary with other hormones and with control-centres in the brain may be key factors in the working of the built-in clocks that determine human growth and development. Thyroid hormone also stimulates growth, especially of the skeleton and nervous system through a general stimulation of metabolism. During the adolescent growth-spurt, sex hormones produced by the gonads and adrenal cortex further stimulate growth. Cells, too, seem to have a built-in clock that determines their specific life-span. Some believe that growth ceases when the hormonal processes that trigger D.N.A. synthesis and cell division somehow break down in aging cells. The maximum number of divisions, liver cells and other would undergo, in the human body, in a ife-time of 100 years is about 50. After this they cease dividing and ultimately die.
Although growth in height ceases at adulthood, there may be a net increase in weight in later years. The tendency to gain weight in middle age occurs, because there is usually a decrease in physical activity but no comparable reduction
1. This is known as “hayflick limit.”
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