Endocrine System

Endocrine System

After studying this chapter, readers should be able to:

1. List the hormones released from the anterior and posterior lobes of the pituitary gland.

2. Discuss from where the various types of hormones are derived.

3. Describe the location of the thyroid gland and identify the hormones produced by this gland.

4. Explain the functions of parathyroid hormones.

5. Describe the location, structure, and general functions of the adrenal glands.

6. Identify the hormones produced by the adrenal cortex and medulla.

7. Identify the hormones produced by the pancreas and specify the functions of those hormones.

8. Describe the functions of the hormones produced by the kidneys, heart, and thymus.

9. Identify the hormones produced by the testes and ovaries.

10. Describe the hormones of special importance to normal growth.

Overview

The endocrine system works along with the nervous system to regulate the functions of the human body to maintain homeostasis. However, the endocrine system works much more slowly than the nervous system, which can cause responses to occur within milliseconds. The endocrine system and its widely scat-tered glands secrete hormones that diffuse from the interstitial fluid into the bloodstream. The hormones, which are chemical messengers, act on target cells, regulating their metabolic functions. Hormones affect most body cells, regulating growth and development, balance of blood components, body defenses, cellular metabolism, energy balance, and even reproduction. Hormones are considered “long-distance” chemical signals.

However, there are also “short-distance” chemi-cal signals: the paracrines and autocrines. Paracrine secretions are those that affect only neighboring cells.

An example of paracrine actions is when somatostatin from certain pancreatic cells stops insulin release by other pancreatic cells. Autocrine secretions are those that affect the secreting cell only. For example, these chemicals are specific prostaglandins that are released by smooth muscle cells and cause contraction of these cells. Exocrine glands are those that secrete nonhor-monal substances outside the body through ducts, and include the sweat and salivary glands. The endocrine system controls body processes for long periods of time.

Endocrine System

The endocrine system, like the nervous system, uses chemical signals that bind to receptor molecules. Similarities and differences between the two sys-tems are summarized in TABLE 16-1. The glandular cells of the endocrine system release hormones into the bloodstream, carrying messenger molecules throughout the body. Endocrine glands are ductless glands. They help regulate metabolism, controlling chemical ­reactions, transporting substances, regulating water and electrolyte­ balances, and aiding in reproduction, growth, and development. Endocrine glands release hormones into the surrounding tis-sue fluid. These glands usually have a large amount of both vascular and lymphatic drainage of their hormones.

The major endocrine glands include the pitu-itary gland, thyroid gland, parathyroid glands, adre-nal glands, pancreas, pineal gland, thymus gland, and reproductive glands (FIGURE 16-1). The endocrine functions of the hypothalamus include the production and release of hormones, so it is considered to be a neuroendocrine organ.Also, several organs maysecrete hormones, including the stomach, small intes-tine, kidneys, and heart. (These are discussed in detail later in this chapter.)