Lesson 10
The Human Endocrine System

10.1 Introduction

The endocrine glands are glands of internal secretion (rather than external, as seen with the sweat glands and digestive glands.This internal secretion results from the fact that these glands have no ducts. Thus, they are often referred to as the ductless glands.The secretions produced by the endocrine glands are called hormones. Hormones are carried by the bloodstream to specific organs or tissues, which are then called the target organs. The activity of the target organ, in turn, affects the activity of the endocrine organ. Thus, it is a reverse or feedback mechanism.

Control "Systems" of the Human Body

The structure and function of the human body is controlled and organized by several different "systems." 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.

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.

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

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.

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

Pituitary body

Thyroid gland

Parathyroid glands

Pancreatic islets (islands of Langerhans)

Suprarenal (adrenal) glands

Gonads (female--ovaries; male--testes)

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.

Figure 10-1. The endocrine glands of the human body and their locations.