Regulation of Internal Environment II – SS3 Biology Lesson Note (First Term)

The endocrine system is a group of ductless glands that release chemical messengers called hormones directly into the blood.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define the endocrine system and hormones;
2. state the functions of hormones;
3. name the hormone produced by the thyroid gland, islets of Langerhans, adrenal medulla, ovaries and testes, and state what each does;
4. describe the effects of too much or too little thyroxine and insulin;
5. list common diseases of the endocrine system and their causes.

What is the endocrine system?

The endocrine system is made up of ductless glands whose hormones travel in the blood to other parts of the body.

• Endocrine gland: a gland with no duct, so its product passes straight into the blood.
• Hormone: a chemical messenger that circulates in the blood and makes other parts of the body act in a certain way.
• Target organ: the part of the body that a hormone acts on.

What is the difference between endocrine and exocrine glands?

• Endocrine gland: has no duct and releases hormones into the blood, e.g. thyroid gland.
• Exocrine gland: has a duct that carries its product to a particular place, e.g. salivary glands.

What are the functions of hormones?

Hormones control growth, development and metabolism.

1. They stimulate growth and development.
2. They stimulate the basic rate at which the body works, i.e. metabolism.
3. They control the development of sexual organs and secondary sexual characteristics.
4. They help the body respond to stress, e.g. adrenaline.
5. They regulate blood glucose level, e.g. insulin.

• Pituitary gland: found at the base of the brain and called the master gland because it controls many other endocrine glands.

Which glands make up the endocrine system?

The main endocrine glands in this topic are the pituitary, thyroid, islets of Langerhans, adrenal medulla, ovaries and testes.

What does the thyroid gland do?

The thyroid gland secretes thyroxine, which controls the basal metabolic rate of the body.

• Shape and position: butterfly-shaped and found in the neck.
• Basal metabolic rate: the rate at which oxygen and food are used by the body tissues.
• Effect on growth: thyroxine affects both physical and mental growth.
• Iodine: needed to make thyroxine.

• Why goitre appears: where water lacks iodine, the thyroid gland grows bigger to make up for the insufficient iodine.

What do the islets of Langerhans do?

The islets of Langerhans produce insulin, which regulates the level of blood glucose.

• Position: groups of special cells in the pancreas, not connected to the pancreatic ducts.
• Action of insulin: controls the rate at which glucose is taken up from the blood by surrounding tissues.
• Too little insulin: glucose is not taken into the tissues but appears in the urine and is lost from the body. This is diabetes.
• Diabetic coma: a diabetic who does not receive regular insulin injections may fall into a coma, because brain cells are not receiving any glucose.
• Too much insulin: blood glucose falls too low, causing weakness, dizziness and possible fainting.

What do the adrenal glands do?

The adrenal medulla produces adrenaline, which prepares the body for action in stress.

• Position: the adrenal glands sit above each kidney.
• Two tissues: the inner adrenal medulla and the outer adrenal cortex.
• Link to nerves: adrenaline works with the sympathetic nerves of the autonomic nervous system (ANS).

Adrenaline prepares the body for action by:

1. increasing the breathing rate;
2. raising the blood sugar level;
3. increasing the heartbeat rate;
4. toning the muscles and directing blood to where it is needed most, e.g. the leg muscles when running away, or the skin when one is getting hot.

What do the ovaries do?

The ovaries produce two hormones, oestrogen and progesterone.

• Oestrogen: controls female secondary sexual characteristics, such as development of mammary glands, hair under the armpit and groin, and widening of the hip.
• Progesterone: prepares the lining of the uterus to receive a fertilised egg for implantation and helps to continue the pregnancy to full term.
• If no pregnancy occurs: the ovary stops producing progesterone, the uterine lining breaks down and is lost as menstrual flow, and the monthly menstrual cycle starts again.

What do the testes do?

The testes produce testosterone, which controls the male secondary sexual characteristics.

• Growth of hair on the face, armpit and groin.
• Increased muscle development.
• Deepening of the voice.
• Enlargement of the penis and scrotum.

What do the sex hormones do together?

Sex hormones promote the growth of the sexual organs and the appearance of secondary sexual characteristics.

• Normal levels: lead to full development of the sexual organs and arousal of interest in the opposite sex.
• Underdevelopment of the hormones: results in underdevelopment of the sexual organs and a lack of interest in the opposite sex.

What are the diseases of the endocrine system?

Endocrine diseases result from too much or too little of a hormone, or from lack of a raw material such as iodine.

Evaluation

1. What is a hormone?
2. State the function of thyroxine and name two conditions caused by too little of it.
3. Explain what happens to blood glucose when the insulin level is low.
4. List four ways adrenaline prepares the body for action.
5. Name one hormone produced by the ovaries and one produced by the testes, and state one function of each.

Assignment

Answer these three parts in your notebook:

1. Explain why goitre may appear where water lacks iodine.
2. Describe what happens to the lining of the uterus when pregnancy does not occur.
3. State three male secondary sexual characteristics controlled by testosterone.