Hormone interactions

Multiple hormones may affect a single target tissue simultaneously. Therefore, the response of the target tissue depends not only on the effects of each hormone individually, but also on the nature of the interaction of the hormones at the tissue. The three types of hormone interactions include:

• Permissiveness

• Antagonism

When two hormones interact at the target tissue such that the combination of their effects is more than additive, synergism occurs. In other words, their combined effect is greater than the sum of their separate effects. For example, epinephrine, cortisol, and glucagon are three hormones that each increase the level of blood glucose. The magnitude of their individual effects on glucose levels tends to be low to moderate. However, the simultaneous activity of all three hormones results in an increase in blood glucose that is several times greater than the sum of their individual effects.

In permissiveness, one hormone enhances the responsiveness of the target tissue to a second hormone; in other words, the first hormone increases the activity of the second. For example, the normal maturation of the reproductive system requires reproductive hormones from the hypothalamus, pituitary, and gonads as well as the presence of thyroid hormone. Although thyroid hormone by itself has no effect on the reproductive system, if it is absent the development of this system is delayed. Therefore, thyroid hormone is considered to have a permissive effect on the reproductive hormones, facilitating their actions causing sexual maturation.

When the actions of one hormone oppose the effects of another, the result is antagonism. For example, insulin decreases blood glucose and promotes the formation of fat. Glucagon, on the other hand, increases blood glucose and promotes the degradation of fat. Therefore, the effects of insulin and glucagon are antagonistic.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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