Hormones are secreted in response to specific chemical stimuli. A rise in the plasma glucose concentration, for example, stimulates insulin secretion from structures in the pancreas known as the pancreatic islets, or islets of Langerhans. Hormones are also secreted in response to nerve stimulation and to stimulation by other hormones.
The secretion of a hormone can be inhibited by its own effects, in a negative feedback manner. Insulin, as previously described, produces a lowering of blood glucose. Since a rise in blood glucose stimulates insulin secretion, a lowering of blood glucose caused by insulin’s action inhibits further insulin secretion. This closed-loop control system is called negative feedback inhibition.
Homeostasis of blood glucose is too important—the brain uses blood glucose as its primary source of energy—to entrust to the regulation of only one hormone, insulin. So, during fasting, when blood glucose falls, it is prevented from falling too far by several mechanisms. First, insulin secretion decreases, preventing muscle, liver, and adipose cells from taking too much glucose from the blood. Second, the secretion of a hormone antagonistic to insulin, called glucagon, increases. Glucagon stimulates processes in the liver that cause it to secrete glucose into the blood. Through these and other antagonistic negative feedback mechanisms, the blood glucose is maintained within a homeostatic range.
Negative feedback control of blood glucose.
The rise in blood glucose that occurs after eating carbohydrates is corrected by the action of insulin, which is secreted in increasing amounts (a) at that time. During fasting, when blood glucose falls, insulin secretion is inhibited and the secretion of an antagonistic hormone, glucagon, is increased (b). This stimulates the liver to secrete glucose into the blood, helping to prevent blood glucose from continuing to fall. In this way, blood glucose concentrations are maintained within a homeostatic range following eating and during fasting.
Although physiology is the study of function, it is difficult to properly understand the function of the body without some knowledge of its anatomy, particularly at a microscopic level. Microscopic anatomy constitutes a field of study known as histology. The anatomy and histology of specific organs will be discussed together with their functions in later chapters. In this section, the common “fabric” of all organs is described.
Cells are the basic units of structure and function in the body. Cells that have similar functions are grouped into categories called tissues. The entire body is composed of only four major types of tissues. These primary tissues include (1) muscle, (2) nervous, (3) epithelial, and (4) connective tissues. Groupings of these four primary tissues into anatomical and functional units are called organs. Organs, in turn, may be grouped together by common functions into systems. The systems of the body act in a coordinated fashion to maintain the entire organism.