Endocrine glands are ductless glands that secrete Hormones and other substances directly into the blood stream. The main endocrine Glands are the Pancreas, Pituiatary, Thyroid, and Para thyroid, Hypothalamas, Pineal Gland, Adrenal Gland, Ovaries and the Placenta. Together they make up the Endocrine system and all are influenced by one another. Their secretory capability is interlinked and this controls many body functions including reproduction. The interdependent relationship between the glands results in the changes in endocrine activity that happen at Puberty, Conception, Pregnancy and the Menopause.
Glands are solid organs which produce secretions. They do so from the substances brought to them through the blood which is richly supplied to them. As diagrammatically represented in figure, glands in the body can be divided into two types. One the left is depicted a gland like the salivary gland. The secretion leaves the gland through a tiny tube called a duct which carries it to its destination, Thus, saliva formed by a salivary gland is carried through the salivary duct to the mouth. Its action is local, viz to keep the mouth moist and to help in the digestion of starchy foods. Such a gland with a duct is known as a gland of external secretion or an exocrine gland. There is a large number of such glands in the body, particularly in the digestive tract and in the skin.
On the other hand,there are in the body a few glands which have no such ducts. The secretion of this gland is put back into the blood stream. This type of a gland is, therefore, known as a duct-less gland, gland of internal secretion or an endocrine gland and its secretion is known as a hormone. Through the blood stream hormones circulate all over the body and reach all the body cells and tissues. Thus, the action of internal secretion is more generalised. The hormones influence the metabolic processes of the cells and the tissues. Their site of action is referred to as the target. Different hormones have different targets. Ductless glands constitube the endocrine system.
Types : The endocrine gland are of two types: purely endocrine party endcrine.
- Purely Endocrine Gland. These are devoted entirely to the secretion of hormones. They include the hypothalamus, thyroid, parathyroid pineal.
- Partly Endocrine Glands. These are party endocrine and partly exocrine they are heterocrine. They include the kidneys : pancreas, gonads, mucous membrane of alimentary canal and placenta.
Endocrinology continuous to make very rapid progress. One index of its progress is that it has also reached the common man,for, most people today seem to be concious of glands and hormones. To them glandular trouble is perhaps as famaliar as gastric trouble and heart trouble. The following description of the endocrine system is meant to give the reader a fair idea of the working of this fascanating system. The six principle endocrine glands in the body of man and women are illustrated with the exception of the sex glands’ the other glands are common in the both sexes. Each gland of the system nowbe considered individually.
Situated in the head beneath is the pituitary gland. Oval shape and measuring about 10 mm in diameter, this tiny gland performs very important functions.
Its rear portion or the posterior lobe secretes two hormones and these are the constituents of what is commonly known as pituitrin. Action on the kidney, the first hormones reduces the quantity of urine formed and thus regulates the amount of body water. This is known as the antidiuretic hormones. It is common knowledge if one drinks a large quantity of water, one passes a large quantity of urine. On the other hand, if one does not drink water for a long time or loses water by other channels like excessive perspiration in hot weather, the quantity of urine falls. In the former case, the secretion of this hormones is decreased or stopped; in the latter case it is increased.
The second hormone of the posterior lobe is oxytocin. This hormone stimulates involuntary muscles to contract. However, its most important action is stimulation of contraction of the uterus during labour. It also helps in the ejection of milk from a lactating breast. Milk secretion itself is stimulated by mammotrophic hormones from the anterior pituitary. Oxytocin stimulates the anterior pituitary to secrete this hormone. Oxytocin thus, has. important functions to perform in connection with the birth of the baby and its nourishment by the mother thereafter.
The front part of the pituitary gland is known as the anterior lobe and it secretes a large number of hormones. Firstly, it secretes the growth hormone which stimulates skeletal growth and development in childhood. Both growth hormone and mammotrophic hormone act directly on their targets, viz. the bones and the breasts, respectively. There are other hormones secreted by the anterior lobe whose target is not the whole body or any peripheral tissue but is the other endocrine glands. Such hormones are known as trophic hormones and they act on the endocrine glands in a reciprocal or regulatory fashion. The thyroid, adrenal cortex and the sex glands are thus under the influence of the anterior pituitary. How the anterior pituitary regulates the activity of these glands may be understood by knowing the relationship between this gland and say, the thyroid.
Thyrotrophic hormone stimulates the thyroid to produce increased secretion of thyroxine. In its turn thyroxine secreted by the thyroid depresses the anterior pituitary and thus, reduces secretion of the thyrotrophic hormone. Apparently, there are no trophic hormones of the anterior, pituitary to act on the parathyroid, pancreas and the adrenal medulla.
Control of activity of the other endocrine glands by its trophic hormones gives the anterior pituitary a distinguished place in the endocrine system and this gland is, therefore, often described as the leader of the endocrine orchestra. Pituitary gets this power to control the activity of other glands in virtue of its proximity to and its connections with a part of the brain called the hypothalamus. The whole endocrine system is thus, indirectly brought under control of the nervous system.
This gland consists of two pyramid- shaped lobes joined by a bridge giving the gland an appearance like the English letter H. Thyroid is situated in the lower part of the front of the neck and is in close related with the larynx and the windpipe, Hormone secreted by the thyroid gland is called thyroxine and it is rich in iodine. Iodides in food and water are essential for the production of thyroxine.
Thyroxine stimulates the metabolism and energy production of all the body cells. This is manifested by over production of body heat when the hormone is secreted in excess.
These tiny glands are situated in the neck, two on each side, in close relationship with the back of the thyroid gland. The secretion of the parathyroid glands is known as parathormone. This hormone influences the metabolism of two important elements in the body, calcium and phosphorous. These elements are principally found in the bones and teeth, and are also present in the body fluids. Phosphorus is an important constituent of the protoplasm of all body cells. Activity of parathormone is reflected by alterations in the level of calcium and phosphorus in the blood.
There are two adrenal glands which are situated one on each side in the abdomen in relation to the upper ends of the kidney. For this reason, they are also called the suprarenal glands. Each adrenal gland is composed of two parts inner portion called the medulla and an outer portion called the cortex. Both the parts perform important functions;
Adrenal medulla secretes hormones called adrenaline and noradrenaline. These hormones have several important actions, such as, constriction of blood vessels of the skin and abdominal viscera, and dilatation and increased T blood supply to the muscles, increase in heart rate and blood pressure, relaxation of smooth muscles and conversion of liver glycogen into glucose. Function of the adrenal medulla is thus intimately related to a part of the nervous system called the sympathetic nervous system which also functions by release of the adrenergic hormones at the nerve endings. Secretion of these glands rapidly prepares the body for muscular activity-in what is called as the fight or fight response. Emotional stimuli quickly cause secretion of the adrenergic hormones.
Adrenal cortex secretes a number of hormones called corticosteroids. Some corticosteroids like aldosterone act mainly on the metabolism of mineral or electrolytes like sodium and potassium. They are, therefore, known as mineral corticoids. For normal cellular and tissue function, it is essential to have optimal quantites of the electrolytes in the body.
Other corticosteroids secreted by the adrenal cortex mainly affect carbohydrate metabolism. They raise the blood sugar and have an action opposite to that of insulin. Besides carbohydrate metabolism, they also influence protein and fat metabolism and are antiinflammatory. For this last action, a familiar substance which is used. In the treatment of inflammatory conditions is known as cortisone.
These corticoids thus, control vital body functions and the adrenal cortex, therefore, occupies a unique place in the endocrine system. Life rapidly wanes and death results if the adrenal cortex is removed.
Adrenal cortex also secretes male and female sex hormones, actions of which are similar to the hormones of the testis and the ovary.
Situated in the abdomen behind the stomach is this large gland. Pancreas is a mixed gland. Its external secretion known as the pancreatic juice is useful for the digestion of food. Its endocrine secretion is the hormone called insulin. This is secreted by clumps of small cells in the pancreas which are known as the islets of Langerhans. Insulin helps in the metabolism of carbohydrates. It does so by increasing the utilization of glucose by the body cells and secondly by converting it in the liver into glycogen in which form glucose is stored for future use. Insulin action, therefore, reduces the glucose in the blood. Deficiency of insulin produces the familiar disease diabetes mellitus.
Another hormone produced by the pancreatic islets is glucagon whose actions are opposite to those of insulin.
Sex Gland or Gonads
The male sex gland is the testis which is located in the scrotum and the female sex gland is the ovary which is located in the pelvis. There is one gland on each side. The gonads have two types of functions. Firstly, there is production of germ cells which are the spermatozoa in case of the testis and ova in case of the ovary. The second function is the production of hormones.
Activity of the sex gland start about the period of puberty or coming of age and the growing adolescent starts becoming an adults. This is manifested by the development of the external and internal sexual organs and by the appearance of characters of maleness or femaleness known as the secondary sexual characters. In both sexes, the puberty starts about the age of 13 years and proceeds in an orderly fashion. Girls develop faster and their pubertal development may be complete by the age of 16 years; on the other hand, development is somewhat slower in the boys and the process may continue till the age of 20 years. Both sexes also undergo a rapid increase in height and weight during this period and this is known as the growth spurt . This is more spectacular in boys who ultimately excel the girls in physical stature.
In the male, the hormones secreted by the testis is testosterone. This hormone stimulates growth and development of the male external and internal genital organs and determines the male secondary sex characters like muscular build, breaking of the voice, appearance of the facial hair mustache and beard, etc. In the female, the ovary secretes two types of hormones called oestrogen and progesterone. These determine the development of the external and internal genital organs, and their cyclical activity produces cyclical changes in the uterus and the appearance of the mensstrual period. The hormones also cause the development of female secondary sexual characters like breasts, feminine distribution of fat and feminine contours etc. Some secondary sexual characters which are common to both sexes like the axillary hair and public hair are apparently under the influence of male and female sex hormones secreted by the adrenal cortex.