ICSE Class 8 Biology Chapter 07 Endocrine System Hormones

Endocrine System - Hormones

Syllabus

The endocrine system - names of endocrine glands - action of some hormones like adrenalin, thyroxine, insulin and pituitary hormone.

Teacher's Note

The endocrine system works silently in our bodies, controlling everything from growth to how we react during exciting or stressful moments. Understanding these chemical messengers helps us appreciate why we feel the way we do.

Our body has two types of glands - the exocrine glands and the endocrine glands. The exocrine glands are salivary glands, pancreas, etc. These glands send their secretions through ducts directly to the target organ. On the other hand, the endocrine glands are ductless glands. Their secretions are called hormones, which are poured directly into the blood and are thus carried to the target organs.

Endocrine Glands

The hormone-producing glands are called endocrine glands such as pituitary gland, thyroid gland, etc. These glands have no duct but they pour their secretions directly into the blood (Fig. 7.1). Therefore, they are also called ductless glands. Though the secretion of these glands, i.e., hormone, is secreted in very small quantity, yet it is carried to all parts of the body. Each hormone has a specific function and its effect is produced in one or more specific part only.

Hormone is a chemical substance secreted from a gland, which is directly poured into the blood stream and which acts on the target organs or cells. It coordinates the functioning of our body.

In this chapter, we shall describe only important glands, prescribed in the syllabus, namely thyroid, adrenal, pancreas, etc.

Thyroid Gland

Thyroid is a bilobed gland situated on either side of the wind pipe, just below the larynx.

It secretes thyroxine which is an iodine containing protein. This hormone speeds up the rate of most of the activities of the body. It stimulates growth in young ones. The insufficient secretion or oversecretion of this gland has harmful effects on the body.

Insufficient Secretion of Thyroxine

Insufficient secretion of thyroxine causes three ailments:

1. Simple goitre - It is the ailment caused due to the enlargement of thyroid. It shows swelling in the neck. It occurs due to the insufficient iodine in our daily food.

2. Cretinism - It is the ailment which brings about abnormal development in the child. It may cause dwarfism and mental retardation of the child.

3. Myxoedema - It is a defective state in an adult if his thyroid is not functioning properly. It shows general sluggishness of metabolism with swelling on the face and the hands.

Thyroid deficiencies are common in many parts of India. The use of iodised (iodine-added) salt in the food is recommended because iodine is required in the production of thyroxine.

Oversecretion of Thyroxine

Oversecretion of thyroxine causes enlargement of gland, an increased metabolic rate, increased heart beat and general restlessness.

Adrenal Glands

The adrenal glands are like caps over the two kidneys. Each adrenal gland consists of two parts - a central medulla and a peripheral cortex.

Medulla secretes adrenaline (emergency hormone). This hormone is secreted when a person is under severe stress (physical or emotional), fearful or angry. This hormone prepares the body to meet any emergency situation for fight i.e., to face the danger. This hormone provides an extra energy and strength to the body in any adverse situation. It increases the heart beat and the rate of respiration accompanied by rise in blood pressure, increase in blood supply to the muscles and release of more glucose into the blood by the liver (like putting more fuel into the engine). Thus, adrenaline energises the person to face any situation.

The adrenal cortex secretes many hormones, but the well known hormone is cortisone. In general, the cortical hormones influence carbohydrate, fat, and protein metabolism. Also, it regulates salt and water balance in the body. They adapt the body to stresses such as extreme heat, cold, burns, infections, etc.

Pancreas

The pancreas is situated behind the stomach. Although it is mainly a digestive gland, it has cluster of hormone secreting cells called Islets of Langerhans. They secrete two hormones - insulin and glucagon.

Insulin removes excess glucose from the blood stream in three ways:

(i) Insulin converts extra glucose to glycogen which is ultimately stored in the liver and muscles.

(ii) Insulin induces the cells to burn extra glucose to produce heat and energy for the body.

(iii) Insulin causes the cells to convert extra sugar into fat.

Insufficient secretion of insulin causes diabetes (more correctly called diabetes mellitus). A diabetic person has a high concentration of sugar in the blood. He excretes a great deal of urine loaded with sugar and feels thirsty because of the loss of water through too much of urination. Usually, he loses weight and tends to become weak.

In certain persons, the glucose level comes down due to oversecretion of insulin. This is very harmful and may have serious consequences.

Glucagon stimulates the breakdown of glycogen in the liver to glucose. Thus it raises the sugar level in the blood.

Some Other Hormone-Producing Glands

Pituitary gland is located below the brain. It secretes several hormones, some of which regulate the activity of other endocrine glands. Pituitary gland is a sort of master gland.

Wall of the stomach produces a hormone that stimulates secretion of gastric juice.

Inner lining of duodenum produces a hormone which stimulates the flow of bile juice into the duodenum.

The table given on the next page the four major hormones, their secreting glands and their main functions.

Teacher's Note

The pituitary gland is often called the master gland because it controls other endocrine glands, much like how a conductor controls an orchestra to produce harmonious music.

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