Preksha Dhyana: Human Body Part I (Anatomy And Physiology): [9] Endocrine System

Glands

A gland is composed of a cluster or accumulation of cells. Every gland might be likened to a chemical factory in which all cells are working and the production of the factory is their secretions. Some glands, such as those of the skin and the gastro-intestinal tract, produce secretions which drain out to the surface or into a body cavity via a duct and affect only near where they are released. These are called exocrine glands (exo=outside). They are—liver, pancreas, kidneys, salivary glands etc

In contrast, the endocrine glands (endo=inside), specialized organs, ductless and their production passes directly into the blood-stream, circulates all over the body and acts at places far from where it originated.

We have already dealt with the important exocrine glands in the previous chapters. In this chapter we shall, therefore, deal with the ductless endocrine glands, their secretions called hormones and their profound effects on the physical functions, mental states and behavioural patterns of an individual. Unlike continuous anatomical structures of most of the systems of the body, the glands of the endocrine system are scattered through the body like islands. Nevertheless, they are unified into a finely coordinated system, function in a marvellously harmonious fashion and coordinate the activities of the body.

Endocrinology is one of the latest and the fastest deve­loping areas of bio-medical research. The endocrine system, at its simplest, works like a thermostat. Just as, the thermo­stat may instruct the central heating to switch on or off, the endocrine system regulates various functions of the vital organs of the body to match the external conditions.

This analogy is, however, over simple. The endocrine system is, in fact, an elegant system which controls numerous positive-negative systems to keep the conditions inside the body stable

The main endocrines are: the pineal, pituitary, thyroid, parathyroid, thymus, adrenals, islets of Langerhans and gonads (sex-glands). All these glands are comparatively small, are devoid of ducts and have access to very rich vascular supply. The products of these glands are organic chemical compounds called hormones. They are effective in very small quantities. They regulate such important body-processes, as growth and development, sexual activity, pregnancy and birth, metabolism and maintain homeostasis in the organism. Secretions of one gland profoundly influence and control the activities of others.

Regulation of body-processes depends on regulation of hormone production. For the most part, hormone-secretion of endocrine system is under the control of pituitary and hypothalamus. Variety of trophic hormones secreted by the pituitary reach thyroid, adrenals and gonads via the blood stream and stimulate each of them to secrete specific (peripheral) hormones in definite quantities. An efficient feedback system of plus-minus or minus-plus chemical interaction regulates the function of these three glands as well as the trophic function of the pituitary. The feedback system works like this: with the increased level of a peripheral hormone, the secretion of the stimulant—the trophic hormone—is inhibited and vice versa.

A brief discussion of individual members of the system now follows

(i) The Pineal Gland

Until recently, the status of the pineal gland as an endocrine organ was highly controversial. Despite greatly increased interest, many mysteries of its functions still remain to be unravelled.

The pineal gland or body (epiphysis cerebri, corpus pineale) lies near the centre of the brain. It is a cone-shaped structure resembling a pine cone. It is 7-10 mm long, 4-6 mm wide and 2-3 mm thick and weighs 0.2 gm. It is greyish-red in colour. It is hidden away at the base of the brain in a tiny cave behind and above the pituitary gland. It is composed, in part, of nerve-cells containing pigment similar to that present in the cells of retina.

Although it is located within the brain, the gland is innervated doubly by central and autonomic systems. Impulses from these nerve-fibres, transmitted to the gland by neuro-transmitter norepinephrine, stimulate the secretion of the pineal hormone—melatonin. It acts as a general supervisor over all the other glands, and its secretions act as a regulator for all endocrines. During the first two-three years of life, its chief duty is to give the baby time to grow in bulk and put on weight. An important effect of the pineal secretion is to inhibit the production of gonadotropic hormones, thus reducing sexual excitation. Puberty (both in males and females) is delayed when melatonin production is high. Thus the pineal is believed to be the seat of the body's biological clock, which signals the onset of maturation.

Thus, in childhood, the pineal gland helps in holding the sex in abeyance, and later aids in the maturing processes of sex after puberty. In adult life, it regulates the proper nourishment to muscles and controls the action of light on the pigment of skin. It produces the normal physical and mental development of the brain cells. The rich blood supply to the pineal is suggestive of its significant functioning.

Some experimental evidence indicates that pineal hormones inhibit ACTH secretion, and thus indirectly help to regulate the secretions of the adrenal gland. Modern pineologists call pineal gland a regulator of regulators.

(ii) The Pituitary Gland

The pituitary gland (also called the hypophysis) is about the size of a pea, situated almost exactly in the centre of the head at the base of the brain and just behind the root of the nose. It hangs suspended from the underside of the brain in a little cup or cradle. It has a greyish yellow colour. It increases in size until about 30th year, and in the adult male, weighs about 600 mgms (slightly more in women). No part of the body is exempt from its influence.

To understand how the endocrine system works, we must look at the pituitary as well as the hypothalmus which controls it. Together they constitute a single interconnected system and control production of hormones from the other glands.

The pituitary is traditionally called the master gland or 'the conductor of the endocrine orchestra'. However, an even grander analogy must be found for the hypothalamus since it controls the pituitary. It is here that the nervous and the endocrine systems are co-ordinated.

The pituitary is composed of an anterior and a posterior part. The former produces six different hormones and is controlled by hormone-like releasing factors from the hypothalamus. Four of these are trophic hormones, that is, they regulate the action of other endocrine glands:

Thyroid-stimulating hormone (TSH) stimulates the thyroid.

Adrenocortico trophic hormone (ACTH) stimulates the adrenal gland.

Follicle-stimulating hormone (FSH) and luteinising hormone (LH) are both gonadotropins i.e. stimulate gonads (sex glands).

The hypothalamus receives inputs from parts of the body including the emotional centres of the brain. Whenever required it produces releasing factors which stimulates the pituitary to produce trophic hormones, which in turn stimulate the other glands to release the hormone needs for the occasion.

The other hormones are growth hormone and prolactine which stimulates the secretion of milk in the breasts. Growth hormone is still rather a mystery. Its job is to stimulate growth in the growing years but it works on many systems even after adolescence. At various times of life, the thyroid and sex-hormones also share a growth promoting role.

Functions of the two posterior pituitary hormones are more specific. Antidiuretic hormone (ADH) is responsible for the regulation of water loss from the kidneys through braking effect.

The other hormone—oxytocine is most important during child-birth and breast-feeding. It causes the uterus to contract which will result in the expulsion of the fetus.

The posterior pituitary is not really an endocrine but a storage depot of the hormones secreted by the hypothalamus.

(iii) The Thyroid Gland

The thyroid gland consists of two lateral maroon-coloured masses astride the upper end of the trachea, close to the larynx. The two lobes are connected with a narrow strip of the same tissue just below the Adam's apple. Each lobe is about 5 cms. in length, 3 cms. in width and about 1 to 2 cms. thick. The normal thyroid of an adult weighs 25 to 40 gms. but its size fluctuates with age, habitation and diet. It is heavier in the female than in the male, and becomes enlarged during sex-excitement, menstruation and pregnancy. The vascular supply to this gland is by four main arteries and is exceptionally rich. Probably more blood flows through it in proportion to its size than through any other organ of the body with the possible exception of the adrenal gland; e.g., it receives four times as much blood as do the kidneys. This gland is innervated by sympathetic and parasympathetic nerves.

Its most astonishing feature is its ability to take up and concentrate large amounts of iodine which, within the gland, may be 50 to 300 times more than in the blood-plasma. Iodine ions are very effectively removed from the circulating blood by an active transport mechanism (an iodide pump). Iodine is used to produce hormones which regulate protein-, carbohydrate-, and fat-metabolism. They are essential for growth and mental development. Another hormone which does not contain iodine is concerned in the body's calcium metabolism.^[*]

The rate of release of thyroid hormones is principally regulated by the thyrotrophic hormone—TSH—of the anterior pituitary. The role of thyroid hormones lies in growth and development.

Thyroxine is the main secretion of this gland. It is a gelatenous substance. Besides a large percentage of iodine, it also contains iron, arsenic and phosphorous—iron for the general system and to aid in the electric energy and conductivity of the system; phophorous for the nerve and brain centre; and arsenic for the skin. The hormone is the builder of the nerve and brain tissue. Tri-iodo-thyronine is the other hormone which regulates the metabolic rate of the body.

Thyroid gland is essentially an energy producing organ and its output is the controller of the rate of metabolism or the speed of living. Any disturbance in the thyroid secretion in quantity, i.e., too much or too little, or in quality, i.e., an abnormal change in the chemical composition, produces serious results. Thyroid is very closely related to other glands. It is the other great link between the brain and the organs of generation; and its acceleration is necessary to give balance to the brain.

The effects of thyroid gland on other functions are widespread:

• aids in normal mental and physical development;
• secretes the iodine-containing hormones which combats the poison of the body;
• gives nerve stability;
• helps to control the amount of fat stored in the body and the basal metabolic rate;
• prevents and cures goitre.

(iv) The Parathyroid Glands

Parathyroids are four minute yellowish brown ovoid bodies about 6 mm long and 3 mm wide, embedded in the lobes of the thyroid gland (two in each lobe). Sometimes-the lower two are found much further down and in the chest. The hormone secreted by these glands is called parathormone (PTH). Its action sets the long term calcium levels in the body.

The overall effects of parathyroid hormone are to increase the calcium levels in the extra-cellular fluids and decrease the phosphate concentration. The hormone is secreted in a self-regulating feedback system, independent of pituitary control. In exerts its effects in three major sites:

1. In the bones, where it promotes reabsorption of calcium and phosphate into the extra-cellular fluid by increasing the number and activity of the osteoclasts^[**]
2. In the intestine, where it enhances the absorption of calcium and phosphate, and
3. In the kidneys, where it increases the reabsorption of calcium but enhances excretion of phosphate with the-urine.

The functioning of the hormone PTH is dependent on adequate supply of Vitamin-D.

(v) The Thymus Gland

Unlike other glands, the thymus gland is not universally accepted as a member of the endocrine system. It is generally described under "organ of uncertain endocrine function". However, there cannot be much doubt that it is the source of one or more hormone-like factors.

A lymphoid, two lobed structure, this gland is situated in the chest between the two lungs and extends up into the neck. It descends and comes to the upper portion of the heart. A brownish mass, it reaches its largest size at the beginning of puberty, when it is about 5 cms. long, 3.5 cms. wide and 6 mm. thick, and weighs 25-35 gms. It grows rapidly during the first two years of the child's life and it gradually shrinks after the 20th year. By the age of 75 years, it weighs only 6 to 10 gms. Some of the secreting cells, however, remain throughout life. This gland is believed to control the physical growth of children, the greater part of which takes place before the 14th year of age. During this time, it holds other glands, particularly the sex-glands, in check, delays puberty and furthers normal brain development.

The thymus is a lymphoid organ, since it contains closely packed lymphocytes. Besides the function mentioned above, the thymus exerts an influence on the lymph nodes, spleen and other lymphatic tissues so that they too gain the ability to produce lymphocytes and foster the development of immuno-competent cells by means of a hormone.

An interesting correlation that has recently attracted -the attention of researchers is the fact that there is a dramatic increase in auto-immune diseases such as arthritis, anemia, (conditions in which the body attacks its own useful cells as a result of errors in recognition mechanism) with increasing age precisely at the time when the thymus is losing its functional capacity.

It is believed that this is the gland that keeps children childish and sometimes makes adults childlike. During childhood, it promotes growth of bones, but at puberty, its functioning begins to decrease, i.e., rise in the functioning level of sex-glands exerts restraining influence on the thymus, but many times the thymus gland does not cease its action. The continuance of its activity after puberty causes peculiar actions of sex-development and stops the process of transforming into positive sex-expression, either of male or female.

Islets of Langerhans

Endocrine tissue making up the islets of Langerhans accounts for about 1 to 2 % of the total weight of the pancreas. They are supplied with blood more abundantly than other parts of the pancreas. Three kinds of cells are distinguished in the islets: (i) beta-cells—located near the centre of the islets and comprising 60 to 70 % of all its cells; (ii) delta-cells which are predecessors of other cells (2 to 8 %); (iii) alpha-cells—located near the periphery.

Glucagon is formed in the alpha-cells, insulin in the beta-cells, and somatostatin in the delta-cells. We have already discussed about the glucagon and insulin in the 8th chapter. Somatostatin is one of the regulators of the secretion of insulin and glucagon.

(vi) The Adrenal Glands

Adrenals are a pair of three- cornered hat-shaped glands capping the upper end of the kidneys. They are about as big as the ends of one's fingers. The weight of both adrenals ranges from 6 to 12 gms. Their length is 40-60 mm, breadth 20-35 mm, and thickness 6-10 mm. Each adrenal is a double gland, composed of a cortex—an outer layer, and a medulla—an inner layer. The cortex makes up the bulk of the gland, which is bright yellow outside and reddish brown inside. The medulla is much thinner and grey in colour. Cortex itself is divided into three zones:

1. a narrow outer,
2. a wide heavier middle, and
3. an irregular inner.

In childhood and youth, the glands are relatively larger and more prominent than in the adult life. At all ages, the amount of blood passing through the adrenals is very great compared to their size. They are supplied with blood by the superior, middle, and inferior adrenal arteries. The tremendous importance of these glands is better understood when it is known that death occurs very quickly after removal. They are innervated by sympathetic and parasympathetic nerves.

Adrenal Cortex

Probably more hormones—more than three dozens - are produced by the adrenal cortex than by any other endocrine gland of the body. A number of these hormones are essential to life. The cortex is closely related to the organs of reproduction. The secretions of the cortex stimulate the healthy growth of the brain and sex-cells, develop great mental concentration and physical endurance and generate a vigorous muscular and nervous constitution. So close are the brain and adrenal cortex related that a normal human brain never develops without a normal adrenal cortex.

Fifty steroid compounds have been isolated from the adrenal cortex to date. Functionally, the adrenal-cortical hormones can be grouped into three major classes:

1. Mineralocorticoids which influence primarily the electrolytes of the extra-cellular fluids, regulate the quantity of salt and water and maintain the electrolyte balance in the body.
2. Glucocorticoids which act primarily on the carbohydrate-, protein- and fat-metabolism and help fight stressful conditions (infection toxicosis, injury etc.) by raising the blood-glucose-level. They are secreted in response to ACTH released from the pituitary.
3. The sex hormones (both male and female).

Two of the most important hormones are:
1. Aldosterone, belonging to the first group, and
2. Cortisone which is converted to hydrocortisone, from the second one. Cortisone is nowadays synthesized and used in treatment of many different disorders mostly to suppress inflammation. Pain in muscles and joints are often dramatically affected and relieved by the use of this drug.

The Adrenal Medulla

The adrenal medulla has a loose structure; it is about one-tenth the size of the cortex. It secretes two important hormones:

1. Epinephrine—also called adrenaline.
2. Norepinephrine (noradrenaline).

Noradrenaline is the precursor of adrenaline.

Fear, pain, exposure to cold, low blood-pressure, emotional upsets and other challenging experiences stimulate the release of these hormones.

They are secreted in response from sympathetic nerves. Their effects are basically similar, but there are some key differences. Norepinephrine which is also a neurotransmitter is also released by the ends of the sympathetic nerve-fibres.

The secretion of these two hormones is controlled by the sympathetic nervous system and the higher centres in the cerebral cortex and hypothalamus. Thus the functioning of the adrenal medulla is intimately linked with the sympathetic nervous system Both the release and resynthesis of the hormones are triggered by nerve impulses. The amount of adrenaline, in circulation in general, is about 1 part to 2) millions, while a hundred thousand times more is stored in the glands as reserve.

Adrenaline has been called the emergency hormone. Its entry into the blood causes a tremendous heightening of vigour and tensing of the nervous system. More sugar is sent into the blood from the liver and more red blood-cells are forced into circulation from the blood reservoirs of the liver and spleen The heart beats more strongly, the blood-pressure and temperature rises, breathing is more rapid, and the blood rushes in the brain as well as to the skeletal muscles of the limbs. On the other hand, the digestion and everything non-essential is inhibited, arrested and suppressed. All this is in preparation of 'fight or flight' in response to the emergency.

Through repeated excitement, anger, fear etc the adrenal glands may be exhausted of their reserve supply of adrenaline secretion. If enough time is not allowed for the glands to recuperate, amount of the secretion will be insufficient to meet the demand. The result would be temporary or chronic adrenal deficiency. It is characterized by indecision, a tendency to worry and an inclination to weep for the slightest provocation.

(vii) The Gonads (Sex-glands)

The term "gonad" literally means "seed", and the male and female sex-organs—the gonads—produce the seeds of the new generation. The main gonads of the female are the ovaries and in the male they are the testes. Their principal function is to produce the germ-cells (ova and sperm) that can fuse with the germ-cells of the opposite sex to produce a new life. Elaborate system of ducts and glands, for the conveyance of the germ-cells towards the exterior* has been evolved. These systems constitute accessories.

The gonads also double as potent endocrine glands, secreting hormones that condition the functional state and influence the psycho-biological phenomena involved in the sexual act. Thus besides producing the ovum, ovaries also produce endocrine substances that vitalize a woman and make her feminine.

The testes have semen as their external secretion which carries the sperm and which is stored at the prostate gland. Its internal secretions are the male energising forces and what makes him really male. They are the male endocrines. These hormones have profound influences not only on the sexual lives but also a number of body-organs and functions of an individual.

The Ovaries;

The ovary is one of the paired organs situated in the pelvis. It is 3-4 cms. long 2-2.5 cms. wide and weighs 6-7 gms.

Secretions of a woman's sex-hormones are in a large measure responsible for the characteristics that distinguish her from a man. These are secreted mainly by the ovaries which are two small glands of the size of a bean which lie inside the pelvis supported by ligaments. They are touched by the fronded ends of the fallopian tubes—the ducts for the passage of ova to uterus.

The major hormones produced by the ovaries are estrogens and progesterone. At or about the age of 10, the biological clock (pineal gland) in the girl's body seems to set off an alarm that wakes up a specific portion of the hypothalamus. Releasing factor is despatched to the anterior pituitary and the secretion of follicle-stimulating hormone (FSH) begins. Now the dormant endocrine function of the ovaries awakens and they begin to secrete their own hormones, estrogens.

The Testes

Testes are the chief male gonads like the ovaries in the female, having two major functions: they produce the germ cells that act in reproduction and they also are powerful endocrine organs secreting male sex hormones. The testes in the adult are small ovoid structures about 3 to 5 cms. long, 2-3 cms. wide and weighs 20-30 gms. suspended in the scrotum.

The male sex hormones are called androgens and testosterone is the major androgen produced by the testes. Other androgens are produced by testes as well as adrenal glands but all of them are slight in comparison with testosterone. The function of the testes is controlled by the hypothalamic-pituitary system, and its regulation by the gonadotrophic hormones is accomplished according to the feedback principle. As in the case of female, pituitary hormones play a profound role in controlling the functions of the testes in male. At least some differences in the typical behaviour and personality between men and women seem to be linked with the secretion of testosterone.

The secretion of testosterone begins in the male foetus. Small quantities of testosterone continue to be secreted during childhood, but there is a dramatic increase at puberty. This sparks a furious growth—spurt along with the development of the mature sexual organ.

Testosterone is responsible, to some degree, for sexual interest and sex drive, but an intricate complex of psychological factors is superimposed on this underlying endocrine mechanism.

Hormone and the Life Cycle

The growth and changes in the body are largely under hormonal control. So also is the variation that repeats its cycle regularly everyday the so-called 'circadian rhythms' to wake during the day and sleep at night.

Several hormones are active before birth e.g. those controlling metabolism. Growth hormone and thyroxine are especially influential in the period of rapid growth and development both in the womb and outside. On the other hand, sex hormones are inactive before puberty and again in old age. At puberty, the first signal is given by hypothalamus through gonadotropins from pituitary which stimulate the major producers of sex hormones—the testes and ovaries - to grow and begin to secrete. The effect is dramatic. In boys, hair grows on the face and elsewhere and the muscles strengthen. In girls, the breasts develop and menstruation begins. Both sexes become capable of reproduction. The endocrine rhythm is in-built.

For homeostasis, hormones are vital. Science cannot hope to mimic the fine control exercised by this powerful band of chemical messengers.

Interaction of Feeling and Behaviour

Having dealt with physical functions of the endocrine system, we shall now, briefly, discuss its action on the mental states and behavioural patterns of man.

The nervous system and the endocrine system are the two major control-systems of the body. The co-ordinating effects of the nervous system are transmitted nearly instantaneously by electro-chemical impulses; the endocrine glands secrete chemical regulators (hormones) which are carried through the body by the blood-stream. The action of the latter is more slowly established, but longer lasting than that of the former. While nerve-action is measured in milli­seconds, some hormones need several days to get started and then last for weeks, months or even years. Nerve-impulses control the function only of muscles and glands, while hormones may act on all the cells of the body.

A serious study of the endocrines and their hormones commenced about the beginning of the century. It has now been known that besides the old method of chemical intercommunication between the two systems, another method by nerve-action also exists. Lately, it has been realised that nervous and endocrine systems, both functioning to integrate the organism, are not as divergent as was formerly supposed. Many endocrine glands act on the nervous system through their hormones; on the other hand, endocrines are stimulated or inhibited by products of the nervous system.

Within the central nervous system, there are groups of nerve-cells, which are capable of functioning as glands. The chemical messengers released by these neuro-secretory cells are called neuro-hormones. These cells serve as link between the central nervous system and the endocrine system. With the help of these dual cells acting as go betweens, the central nervous system can control the functional activity of many endocrine glands, and adjust their activity in accordance with the requirements of varying internal and external environments. Equally important is the reverse relationship by which the endocrine system can influence the central nervous system- This concept of the reciprocal inter-relation of the two system is now generally accepted.

Recent studies on neuro-secretions leave no doubt that the nervous system has its own endocrine specialization for the release of hormones. The functional interlocking is so remarkable that nervous and endocrine elements are coming to be regarded as constituting a single integrated system called 'neuro-endocrine system'. As research deepens our knowledge of co-ordinate systems, it becomes increasingly apparent that their products participate not only in every bodily function, but have profound influence upon the mental states and behaviour of individuals.

The neuro-endocrine system is the seat of instincts and impulses of man. Impulses and urges not only generate feelings but also command appropriate action that satisfy the need. Animals just act out instinctive rituals of eating, courtship and fighting because they do not possess a reasoning mind. Man, on the other hand, because he has conscious reasoning, can control his responses to the insistence of the impulsive drives. Of course, man also does feel angry, hungry and sexually aroused. But he can modify his action. He could, for example, channel an erotic mood on to another creative track.

Love, hate and fear are endocrine impulses. It is the primitive urge of aggression from the endocrine, that -will start war and not the brain because no reasoning mind will ever wish to kill or injure. All the passions, emotions and impelling forces are the actions of the endocrine expre­ssions. The reasoning mind itself has no emotions but many a time the powerful impulses from the endocrine can overwhelm and continue to tinge the supposed reasoning.