The endocrine system: glands, hormones and chemical coordination

Learning objectives

By the end of this lesson, you should be able to:

• define the endocrine system, a hormone and a target organ
• explain how endocrine glands differ from exocrine glands
• describe how hormones are transported and produce specific effects
• identify the major endocrine glands in the human body
• compare nervous and hormonal coordination
• explain why hormones are important in growth, metabolism, reproduction and homeostasis

Why the body needs an endocrine system

The body must coordinate activities that involve different organs. Some responses need to happen very quickly, such as withdrawing a hand from a hot surface. Others involve slower changes that may continue for hours, days or years, such as growth, development, reproduction and adjustment of blood glucose concentration.

The endocrine system is a group of ductless glands and hormone-producing tissues that release chemical messengers called hormones directly into the blood. The blood carries hormones around the body. A hormone affects only cells or organs that have the appropriate receptors; these are called its target cells or target organs.

A hormone is effective even when present in a small concentration. Its effect depends on the target tissue, the number and activity of receptors, and the concentration of the hormone. Hormones may alter the activity of cells, stimulate secretion, influence growth or change the rate of chemical reactions.

Endocrine glands and exocrine glands

An endocrine gland has no duct. It releases its secretion into the surrounding tissue fluid and then into the blood. The blood transports the hormone to its target.

An exocrine gland releases its secretion through a duct onto a surface or into a body cavity. Examples include salivary glands, sweat glands and glands that release digestive enzymes into the alimentary canal.

The pancreas is an important exception to a simple either-or classification because it has both endocrine and exocrine functions. Its islets of Langerhans release hormones into the blood, while other pancreatic cells produce digestive juice that passes through a duct into the small intestine.

How hormones work

Hormonal coordination follows a general sequence:

1. A change in the internal or external environment is detected.
2. An endocrine gland or hormone-producing cell releases a hormone.
3. The hormone enters the blood and is transported around the body.
4. The hormone reaches target cells that possess suitable receptors.
5. Receptor binding changes the activity of the target cell.
6. The response helps regulate a body process or correct the original change.

A hormone does not affect every cell it passes. For example, insulin circulates in the blood, but its important effects occur in target tissues such as the liver, skeletal muscle and fat tissue. Cells without the relevant receptor do not respond in the same way.

Hormones may act by changing the activity of enzymes, opening or closing membrane channels, influencing the movement of substances into cells, or switching particular genes on or off. The precise mechanism depends on the hormone.

Major endocrine glands

The principal endocrine glands commonly studied in human biology include the pituitary, thyroid, adrenal glands, pancreas, ovaries and testes. The hypothalamus also has a central coordinating role because it links the nervous and endocrine systems.

The table gives the major functions at syllabus level. Each gland may release more than one hormone, and a hormone may have more than one effect.

The pituitary gland and the hypothalamus

The pituitary gland is often called the master gland because several of its hormones regulate other endocrine glands. This description is useful but incomplete: the pituitary itself is controlled by the hypothalamus, and some glands are also regulated directly by changes in the blood.

The hypothalamus receives information about the body's internal condition and controls the pituitary using releasing and inhibiting hormones. It also produces antidiuretic hormone (ADH), which is stored and released by the posterior pituitary, and oxytocin, which is also released from the posterior pituitary.

Growth hormone from the anterior pituitary stimulates growth of bones and other tissues, especially during childhood and adolescence. Too little growth hormone during childhood can cause unusually slow growth; excessive secretion during childhood can cause excessive linear growth. Hormone disorders must be understood in relation to the age of the person and the hormone involved.

Hormonal coordination compared with nervous coordination

Both systems coordinate the body, but they communicate in different ways.

These are general differences, not absolute rules. Some hormones act rapidly, and nervous activity can produce effects that last. The two systems work together. For example, the hypothalamus links nerve information to the release of hormones by the pituitary gland.

WASSCE examination focus

• A hormone is a chemical messenger released into the blood; it is not an electrical impulse.
• Endocrine glands are ductless. Do not describe the blood as a duct.
• A target organ responds because its cells possess suitable receptors.
• Do not confuse the pituitary gland with the hypothalamus. The hypothalamus controls the pituitary.
• The pancreas has both endocrine and exocrine functions.
• When comparing nervous and endocrine coordination, compare the communication method, speed, distribution and duration of effects.

Quick check

1. Define a hormone.
2. State two differences between endocrine and exocrine glands.
3. Why does a hormone not affect every cell it passes in the blood?
4. Name the gland that produces thyroxine.
5. Explain why the pituitary gland is sometimes called the master gland.
6. Give two differences between nervous and endocrine coordination.

Answers

1. A chemical messenger released by endocrine cells into the blood and carried to target cells or organs.
2. Endocrine glands are ductless and release hormones into the blood; exocrine glands use ducts to carry secretions to a surface or cavity.
3. Only cells with suitable receptors for that hormone respond to it.
4. The thyroid gland.
5. It releases hormones that regulate the activity of several other endocrine glands, although it is itself controlled by the hypothalamus.
6. Nervous coordination uses impulses along neurones and is usually rapid; endocrine coordination uses hormones transported in blood and is often slower but longer-lasting.