Human Endocrine Glands!
Hormones are secreted by the endocrine glands, which are ductless glands.
We shall now loam about some important endocrine glands in the human body. These are shown in Figure 5.6.
Pituitary gland:
The pituitary is a small gland attached to the ventral side of the brain. The pituitary is the most important endocrine gland, as it secretes a number of hormones that regulate various functions of the body. It also controls the functioning of the other endocrine glands. Therefore, it is called the master gland of the body.
The pituitary gland consists of two main parts—the anterior lobe and the posterior lobe. The anterior lobe secretes various hormones. One of these is the growth hormone which regulates growth and development of the body. It promotes the growth of bones and muscles when the body is growing.
An excessive secretion of this hormone leads to gigantism, an abnormal condition of excessive growth. On the other hand, insufficient secretion of the growth hormone in childhood retards growth, leading to dwarfism, an abnormal condition of stunted growth.
The anterior lobe of the pituitary gland also secretes hormones that influence the secretion of milk in the mammary glands, the production of sperms in males and the maturing of ova (eggs) in females. Two types of hormones are secreted by the posterior lobe of the pituitary. One of these helps in childbirth and the other influences the reabsorption of water in the kidney.
Pineal gland:
It is a small gland attached to the dorsal side of the brain. It has light-sensitive cells. It controls the biological clock (the timing mechanism by which an organism controls regular activities such as sleeping).
Thyroid gland:
Thyroid is a large gland located behind the larynx (voice box) in the neck. The main hormone secreted by this gland is thyroxin, which contains iodine. Thyroxin controls the metabolism of carbohydrates, fats and proteins, and brings about balanced growth.
Excessive secretion of thyroxin is called hyperthyroidism. It increases the general metabolism of the body. As a result, fat stored in the body is depleted and there is a loss of body weight.
Insufficient thyroxin secretion is called hypothyroidism. It lowers the general metabolism of the body and increases body weight. By slowing down metabolic activity, hypothyroidism retards body growth and brain development in children.
When the thyroid gland becomes overactive and secretes excess thyroxin, it becomes enlarged. As a result, the neck swells up and the eyeballs bulge outward. This is called exophthalmia goiter. Swelling of the thyroid may also be due to the deficiency of iodine in the diet. This is called simple goiter. To prevent this it is important for us to have iodized salt in our diet. Iodine is needed for the synthesis of thyroxine.
Parathyroid glands:
These are two pairs of small glands buried in the thyroid gland. They secrete parathormone, which increases the level of calcium in the blood by taking out calcium from the bones. A certain amount of calcium in the blood is essential for functions such as muscular activity and blood clotting.
Thymus gland:
This gland, located near the heart, is present in new-born babies. It gradually becomes smaller with age and is degenerated or lost in the adult. It produces WBCs which fight infection.
Islets of Langerhans:
The pancreas is a digestive gland located in the C-shaped bend of the duodenum (Figure 5.6). Inside this gland there are groups of hormone-secreting cells. These groups are called the islets of Langerhans.
Among the hormones produced by them, insulin is the most important. Insulin controls the rate of oxidation of glucose. It helps the liver and muscle cells to absorb glucose from the blood. It also controls the formation of glycogen from glucose in the liver.
People who are unable to secrete sufficient insulin suffer from a condition called diabetes mellitus. The level of glucose in their blood keeps on rising, and after a limit the kidney lets the extra glucose be excreted with urine.
Doctors advise diabetics to take less sugar in their diet. Some diabetics are advised to take injections of insulin, if they have very high levels of blood sugar. High levels of blood sugar harm the body in many ways.
Adrenal glands:
We have two adrenal glands, one on each kidney. The adrenal glands secrete the hormone called adrenaline or epinephrine. This hormone is secreted when an individual is under great physical or emotional stress or feels threatened by some kind of danger.
Excitement generally stimulates adrenaline secretion. Adrenaline increases the heartbeat, rate of respiration and blood pressure. More air is inhaled as the diaphragm and the rib muscles contract, expanding the chest cavity.
Adrenaline constricts all the blood vessels except those that supply blood to the heart muscles and skeletal muscles. As the small arteries around the digestive organs constrict, blood is diverted to the skeletal muscles to carry out a response.
Adrenaline is called ‘fight and flight’ hormone because there is a surge of adrenaline when a person is fighting or preparing to fight or running away from danger. The changes caused by adrenaline prepare the body to react during an emergency. Hence, adrenaline is also called the ’emergency hormone’.
Testis:
The main function of the testis is to produce sperms. The testes also synthesize the male sex hormone testosterone. Testosterone secretion begins at the onset of puberty (age of sexual maturity), at 10-12 years of age. It helps in the development of secondary sexual characters in males, e.g., moustache, beard, etc.
Ovary:
At the onset of puberty the ovaries begin to secrete oestrogen, a female sex hormone. Oestrogen produces secondary sexual characters in females and prepares the body for pregnancy. During pregnancy, the ovaries secrete special hormones that help in the development of the baby.
Control of Hormone Secretion:
We have a feedback mechanism for controlling the precise quantity and timing of hormone secretion. For example, when we take a meal, our blood sugar level rises. The response to this stimulus is the secretion of the required amount of insulin. The insulin carries glucose to the tissues. As a result, the blood sugar level falls and insulin secretion is reduced. Such control of hormone secretion helps maintain a state of balance in the body.