Page 53 - 86 human physiology part-2

Chapter 14

These hormones from the hypothalamus cause release of the respective hormone from the
pituitary. The control of release of hormones from the pituitary happens when there is negative
feedback from the gland on which they act. Meaning that when the hormones increase on the effected
gland the pituitary will stop sending hormones to them.

Also, the heart, gastrointestinal tract, the placenta, the kidneys and the skin, whose major function
is not the secretion of hormones, nonetheless, contain some specialized cells that produce hormones.

In addition, all cells, except red blood cells secrete a class of hormones called eicosanoids. These
hormones are paracrines, or local hormones, that primarily affect neighboring cells. Two groups of
eicosanoids, the prostaglandins (PGs) and the leukotrienes (LTs), have a wide range of varying effects
that depend upon the nature of the target cell. Eicosanoid activity, for example, may impact blood
pressure, blood clotting, immune and inflammatory responses, reproductive processes, and the
contraction of smooth muscles.

Antagonistic Hormones

Maintaining homeostasis often requires conditions to be limited to a narrow range. When
conditions exceed the upper limit of homeostasis, specific action, usually the production of a hormone,
is triggered. When conditions return to normal, hormone production is discontinued. If conditions
exceed the lower limits of homeostasis, a different action, usually the production of a second hormone,
is triggered. Hormones that act to return body conditions to within acceptable limits from opposite
extremes are called antagonistic hormones. The two glands that are the most responsible for
homeostasis is the thyroid and the parathyroid.

The regulation of blood glucose concentration (through negative feedback) illustrates how the
endocrine system maintains homeostasis by the action of antagonistic hormones. Bundles of cells in the
pancreas called the islets of Langerhans contain two kinds of cells, alpha cells and beta cells. These
cells control blood glucose concentration by producing the antagonistic hormones insulin and
glucagon.

Beta cells secrete insulin. When the concentration of blood glucose rises such in after eating, beta
cells secret insulin into the blood. Insulin stimulates the liver and most other body cells to absorb
glucose. Liver and muscle cells convert glucose to glycogen, for short term storage, and adipose cells
convert glucose to fat. In response, glucose concentration decreases in the blood, and insulin secretion
discontinues through negative feedback from declining levels of glucose.

Alpha cells secrete glucagon. When the concentration of blood glucose drops such as during
exercise, alpha cells secrete glucagon into the blood. Glucagon stimulates the liver to release glucose.
The glucose in the liver originates from the breakdown of glycogen and the conversion of amino acids
and fatty acids into glucose. When blood glucose levels return to normal, glucagon secretion
discontinues through negative feedback.

Another example of antagonistic hormones occurs in the maintenance of Ca2+ concentration in the
blood. Parathyroid hormone (PTH) from the parathyroid glands increases Ca2+ in the blood by
increasing Ca2+ absorption in the intestines and reabsorption in the kidneys and stimulating Ca2+
release from bones. Calcitonin (CT) produces the opposite effect by inhibiting the breakdown of bone
matrix and decreasing the release of calcium in the blood.

268 | Human Physiology