Physiological Conditions for Growth

Growth takes place due to various chemical changes in the cells collectively called metabolism. The interaction between catabolism and anabolism results in growth, which can be positive, negative or zero.
Positive Growth : When anabolism exceeds the catabolism, there is increase in size or growth.
Negative Growth : If the catabolism exceeds the rate of anabolism, it will result in decrease in growth i.e., degrowth as during starvation.
Zero growth : When anabolism and catabolism processes of the body are balanced.

Hormonal Control of Growth Rate

Upto 10-13 years of age thymosin controls the growth rate but with slow rate.Towards the end of childhood (5^th – 6^th years) STH takes over the control. Thyroxine at the same time also plays the role as it controls the BMR and important for mental and sexual maturity. Between 14 to 18 years of age the growth rate is  rapid and reaches the peak. At this point of time the action of testosterone in male and estrogen and progesterone in female add in the process for appearance of secondary sexual characters and puberty marks the fully grown up stage. Thereafter the rate of growth declines and stops completely after 22-23 years of age. Repair and Regeneration