a. Control "Systems" of the Human Body. The structure and function of the human body is controlled and organized by several different "systems."

(1) Heredity/environment. The interaction of heredity and environment is the fundamental control "system." Genes determine the range of potentiality and environment develops it. For example, good nutrition will allow a person to attain his full body height and weight within the limits of his genetic determination. Genetics is the study of heredity.

(2) Hormones. The hormones of the endocrine system serve to control the tissues and organs in general. (Vitamins have a similar role.) Both hormones and vitamins are chemical substances required only in small quantities.

(3) Nervous system. More precise and immediate control of the structures of the body is carried out by the nervous system.

b. The Endocrine System. In the human body, the endocrine system consists of a number of ductless glands producing their specific hormones. Because these hormones are carried to their target organs by the bloodstream, the endocrine organs (glands) are richly supplied with blood vessels.

c. Better Known Endocrine Organs of Humans. The better known endocrine organs are the:

(1) Pituitary body.

(2) Thyroid gland.

(3) Parathyroid glands.

(4) Pancreatic islets (islands of Langerhans).

(5) Suprarenal (adrenal) glands.

In addition, there are several other endocrine organs, less well understood, and other organs suspected to be of the endocrine type. See figure 10-1, which shows the better known endocrine glands and their locations.

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