10. Endocrine Glands — "The Producers of Chemical Messengers"

10.1 Need for the Regulation of Body Activities

The activities in our body are highly complex and they need to be so regulated that every activity takes place at a proper time and in a correct sequence. This regulation is done by the nervous system and by chemical regulators called hormones (horma: to stir up / to excite / to put into action).

Table 10.1 : Differences between hormonal control and nervous control

Hormonal control	Nervous control
1. Usually slow.	1. Immediate/Rapid.
2. Transmitted chemically through blood.	2. Transmitted electro-chemically through the nerve fibres and chemically across synapses.
3. Affects different organs (widespread in body).	3. Affects only the particular muscles or the gland (local).
4. Effect is short term or long-lasting.	4. Effect only short-lived.
5. Can affect growth.	5. Cannot affect growth.
6. Can bring about specific chemical changes and regulates metabolism.	6. Does not influence chemical changes and cannot regulate metabolism.
7. Cannot be modified by learning from previous experience.	7. Can be modified to some extent by learning from previous experience.

Hormone: A hormone is a secretion from some glandular part of the body, which is poured into blood and which acts on the target organs or cells of the same individual, bringing about coordination between distant parts of the body.

10.2 General Properties of Hormones

Hormones are secreted from their source directly into the blood (and not into lymph).
They regulate the physiological processes by chemical means. They affect the enzyme systems of the body.
They act on target organs or cells usually away from their source.
Hormones produced in one species usually show similar influence in other species.
They are produced in very small quantities and are biologically very active.
Chemically, some hormones are peptides (proteins such as insulin) which are water soluble, some are amines (derived from amino acids such as adrenaline) again water-soluble, and some are steroids derived from cholesterol such as testosterone which are lipid-soluble.
Their excess (hypersecretion/oversecretion) or deficiency (hyposecretion/undersecretion), both may lead to serious consequences.
Hormones are not stored in the body and are excreted from the system.

10.3 Endocrine Glands

Endocrine glands are glands which secrete hormones. They are called ductless glands because their secretions are poured directly into the blood and not through any special duct.

The principal endocrine glands in the human body included in the syllabus are:

Adrenal
Pancreas
Thyroid
Pituitary

Fig 10.1: The principal endocrine glands.

1. Adrenal Glands

The adrenal glands are like caps above the kidneys (ad: near, renal: kidney). Each adrenal gland consists of two parts — (i) a central medulla and (ii) a peripheral cortex.

Fig 10.2: Position of adrenal on kidney, and its two parts in cross section which secrete respective hormones.

(i) Adrenal medulla

Secretes adrenaline (also called epinephrine and noradrenaline). Adrenaline is a hormone which prepares the body to meet any emergency situation, for "fight" i.e. to face danger or for "flight", to run away from it. Extra energy and strength is provided to the body in that situation.

Functions of Adrenaline—The Emergency Hormone

It increases heart beat accompanied by an increase in blood pressure.
It increases blood supply to the muscles while decreasing it to skin and visceral organs.
More glucose is released into the blood by the liver (like putting more fuel into the engine).

Table 10.2 : Responses to Adrenaline

Body part	Effects of adrenaline	Biological advantage
Heart	Beats faster Blood pressure increases	Sends more glucose and oxygen to the muscles
Breathing centre of the brain	Faster and deeper breathing	Increased oxygenation of the blood; rapid removal of carbon dioxide
Arterioles of the skin	Constricts them	Less blood going to the skin means more is available to the muscles
Arterioles of the digestive system	Constricts them	Less blood for the digestive system, allows more to reach the muscles
Muscles of body	Tenses them	Ready for immediate action
Liver	Conversion of glycogen to glucose	Glucose available in blood for energy production

(ii) Adrenal cortex

Secretes many hormones but the best known hormone is cortisone (which suppresses inflammation).

The cortical hormones are categorised as:

Mineralocorticoids: regulates mineral metabolism, specially Na+ and K+ ions.
Glucocorticoids: regulates carbohydrate, protein and fat metabolism.

Women with beard & Men with breasts!

If there is an overgrowth of adrenal cortex in a mature woman, she develops certain male characteristics, such as a beard, moustaches and deep male voice. The condition is known as adrenal virilism. If the overgrowth occurs in mature men, they may develop some feminine characteristics, such as enlargement of breasts.

Disorders of Adrenal Cortex

Hyposecretion (Addison's disease): Symptoms are loss of energy, skin pigmentation, loss of weight, nausea, hypoglycemia (low blood sugar), sensitivity to cold and pain, increased susceptibility to infections.
Hypersecretion (Cushing's syndrome): Symptoms are obesity, hyperglycemia (higher blood sugar), osteoporosis, weakness, salt and water retention.

2. Pancreas

Pancreas is both a duct gland as well as a ductless gland. As a duct gland, its secretion (pancreatic juice) is poured into the duodenum for digestion. As a ductless gland, it has special groups of hormone-secreting cells called Islets of Langerhans, which are scattered in the entire gland. The islet cells produce three hormones — insulin, glucagon and somatostatin from three different kinds of cells called beta, alpha and delta cells respectively. (Note: Somatostatin from Delta cells is excluded from syllabus).

Hormones of Islets of Langerhans (in Pancreas)

Hormone	Source	Action
1. Insulin	beta cells	- Promotes glucose uptake by body cells - Stimulates deposition of extra glucose (as glycogen) in liver and muscles
2. Glucagon	alpha cells	Stimulates liver to convert glycogen into glucose

Fig 10.4: The role of pancreatic hormones (insulin and glucagon) in regulating blood sugar (glucose) level.

Abnormalities in Insulin

Insufficient secretion (Hyposecretion) causes diabetes (more correctly diabetes mellitus or hyperglycemia). The word "mellitus" means honey, referring to the passage of sugar (glucose) in urine.

A diabetic person:

has high concentration of sugar in blood (hyperglycemia).
excretes a great deal of urine loaded with sugar.
feels thirsty because of the loss of water through too much urination.
loses weight and becomes weaker and weaker.

Over-secretion of insulin (or insulin shock): Sugar level in the blood is lowered (hypoglycemia). Brain may enter a state of coma if the level becomes too low. A prompt bite of sweet biscuits or sugar candy is helpful.

3. Thyroid

The thyroid is a bilobed (butterfly-shaped) structure situated in front of the neck just below the larynx. The two lobes are joined by a narrow isthmus. It secretes two hormones thyroxine and calcitonin. [Note : Calcitonin is not included in the syllabus].

Thyroxine regulates the basal metabolism, i.e., the rate of cellular oxidation resulting in heat production at rest. It also influences the general growth of the body, ossification of bones, body temperature, mental development, etc.

Improper Functioning of Thyroid

A. Undersecretion (Hypothyroidism): Insufficient secretion of thyroxine may lead to three conditions:

Simple goitre: Enlargement of the thyroid and is visible as a swelling in the neck. This is due to insufficient quantity of iodine in food. Common in hilly regions.
Cretinism: Affects the growth of children showing dwarfism and mental retardation. This is due to defective development or early atrophy of the thyroid.
Myxoedema: Condition that affects an adult. The person becomes sluggish with swelling of the face and hands.

B. Oversecretion (Hyperthyroidism): Excess of thyroxine secretion may cause a kind of goitre called exophthalmic goitre (exo: outward, ophthalmos: eye). A person has a marked increase in metabolic rate, rapid heart beat, shortness of breath, protruding eyes, and forms a goitre in the neck.

4. Pituitary

The pituitary gland is a small projection (about the size of a pea) which hangs from the base of the mid-brain below hypothalamus. It is popularly called the master gland because it seems to control practically all other endocrine glands.

A. Hormones from Anterior Pituitary

Growth hormone (GH): Essential for normal growth. Also called somatotropin.
- Deficiency in childhood results in dwarfism.
- Oversecretion in childhood results in gigantism.
- Oversecretion suddenly in an adult results in extra growth of bones in the face and hands/feet (acromegaly).

Fig 10.8: These two fully-grown men are victims of malfunctioning of the pituitary gland (giant and dwarf).

Thyroid stimulating hormone (TSH): Activates thyroid to secrete thyroxin.
Gonad stimulating (Gonadotropic) hormones: Regulate the activities of the testes and ovaries.
Adrenocorticotropic hormone (ACTH): Regulates the activity of adrenal cortex.

B. Hormones from Posterior Pituitary

Antidiuretic hormone (ADH) / Vasopressin: Constricts blood vessels with rise in blood pressure. It acts on the kidney increasing reabsorption of water from the kidney tubules. Deficiency of ADH causes diabetes insipidus (water diabetes) in which urination is frequent and copious, resulting in loss of water and excessive thirst.
Oxytocin: Stimulates vigorous contractions of the uterus in a pregnant mother, leading to the birth of the baby. Also stimulates milk ejection.

"Diabetes" means passing out excessive amounts of urine.

The two types of "diabetes" : In diabetes mellitus, the urine contains sugar caused due to insufficient insulin, but in diabetes insipidus, there is no sugar in urine. ("Insipid" means tasteless, "melli" means honey/sweet).

10.4 Control of Hormonal Secretions [Feedback Mechanism]

The body has mechanisms to maintain a normal state. Whenever there is a change in this state there are "messages" through the body systems to "increase" if there is a fall below the normal, or to "decrease" if there is a rise above the normal. This kind of ordering for the opposite is "negative feedback".

Most feedback mechanisms in our body are of this negative type (Example : Blood sugar level). Positive feed-back mechanisms are very few (Example : uterine contractions during child birth).

Table 10.4 Hormones from four major endocrine glands and their principal actions

Endocrine Gland	Hormones Secreted	Principal Actions	Disorders (Undersecretion / Oversecretion)
1. Adrenals (i) Cortex	i. Mineralocorticosteroids ii. Glucocorticoids	Regulate plasma sodium & potassium Regulate carbohydrate/lipid metabolism	Addison's disease / Adrenal virilism
(ii) Medulla	Adrenaline	Prepares body for any emergency	-
2. Pancreas (Islets of Langerhans)	i. Insulin ($\beta$ cells) ii. Glucagon ($\alpha$ cells)	Lowers blood sugar level Raises blood sugar level	Diabetes mellitus / -
3. Thyroid	Thyroxine	Promotes tissue metabolism, growth and differentiation	Simple goitre, cretinism, myxoedema / Exophthalmic goitre
4. Pituitary (i) Anterior	i. Growth hormone (GH) ii. TSH iii. ACTH	Body growth Stimulates thyroxine secretion Stimulates secretion from adrenal cortex	Dwarfism / Gigantism, acromegaly
(ii) Posterior	i. Vasopressin (ADH) ii. Oxytocin	Increases reabsorption of water from kidneys Causes contraction of uterine muscle	Diabetes insipidus / -