The Primary Tissues

The organs of the body are composed of four different primary tissues, each of which has its own characteristic structure and function.The activities and interactions of these tissues determine the physiology of the organs.

Eating

Î Blood glucose

Î Blood glucose

Pancreatic islets (of Langerhans)

f Ins

ulin

Î Cellular uptake of glucose

Î Cellular uptake of glucose

J, Blood glucose

Fasting

Blood glucose

Blood glucose

Pancreatic islets (of Langerhans)

J, Ins Gluc

ulin agon

J, Cellular uptake of glucose

J, Cellular uptake of glucose

TGlucose secretion into blood by liver

Blood glucose

■ Figure 1.6 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.

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Blood Pressure Health

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