A gland is a group of cells organized together with the ability of extracting certain materials from the blood and lymph, and concentrating the materials or manufacturing from them a specific chemical compound. Glands are mainly of two types in the body. In one type the product of the gland is passed into ducts or tubes which deliver it to the surface or the digestive tract; these are exocrine or duct glands, such as the sweat glands, the breast, the salivary glands, the pancreas, and the liver.
What is the Endocrine System?
Another type of gland in the body is the endocrine or ductless gland. These have no collecting tubes to deliver their products to a specific site, but rather liberate their secretions directly into the blood stream. These are commonly referred to as the glands of internal secretion. The product of the endocrine gland is called a hormone. It is transported in the blood stream throughout the body to exert a specific influence upon cells, organs, or systems remotely removed from the site of its origin.
There is a basic similarity of function between the endocrine glands and the nervous system. Both function in an integrating role, co-ordinating the activities of the various parts of the body into unified purpose. But there are certain differences by which this correlation of function is accomplished. Nervous action begins and ends instantaneously, while gland action is more gradual for a long sustained effect. Nerve action may be directed to one specific area, while endocrine secretions are distributed to all tissues, the response being dependent on the capabilities of the tissues to respond to such chemical stimulation. Nerve impulses may initiate new actions, while endocrine secretions merely accelerate or slow down actions which are already present. Nervous and endocrine controls may modify each other in their co-ordination of the body’s activities as a unified whole.
THE PITUITARY GLAND
The pituitary gland, or hypophysis, lies in the center of the head at the base of the brain encased in a bony shell. Underproduction and overproduction of the gonadotropic hormones results in underdevelopment of the sex glands or disturbances in the menstrual and sexual activities. The lactogenic hormone is normally secreted in the last few hours of pregnancy and throughout the nursing period. Disturbances in the amount of secretion or thyrotropic and adrenotropic hormones affect the normal function of the thyroid and adrenal glands; these in turn will be overactive or underactive. Abnormalities of secretion of the anterior lobe hormones which control body metabolism result in various but specific diseases of the body’s general metabolic co-ordinated activities.
THE PARATHYROID GLANDS
Lying on the back side of the thyroid gland are the parathyroid glands. These are usually four in number, but there may be several. The product of these small pea-sized glands is the hormone known as parathormone, which plays an important role in the control of calcium metabolism.
Underfunction, or hypoparathyroidism, results in a deficient amount of calcium circulating throughout the body; the deposition of calcium in the teeth and bones is decreased, resulting in softer structure of these parts, a condition resembling rickets. Severe deficiency of the parathyroid hormone results in a condition called tetany, characterized by periodic muscle spasms. This occurs as the result of the decreased calcium supply to muscles, since calcium is important in the chemistry of muscle contraction. Deficient production of the hormone may be the result of inadvertent removal of the parathyroids at the time of thyroidectomy, or less frequently by tumor. In such states the hormone may be administered artificially to correct the deficiency.
Hyperparathyroidism, or excessive production of parathyroid hormone, gives rise to abnormal calcification throughout the body. The bones may become decalcified, resulting in skeletal deformities and easy fractures; calcium deposits may occur at other sites such as tendons, kidneys, etc. This disease ensues when the parathyroids are the seat of tumors, most often benign. Recovery usually follows with surgical removal of the tumor.
The pancreas gland lying in the center of the abdomen serves dual purpose. Its functions in supplying necessary digestive juices. As an organ of internal secretion, it manufactures the hormone insulin. This is produced in small islets of gland tissue dispersed throughout the main gland substance. The hormone passes directly into the blood stream to be utilized by all tissues in carbohydrate metabolism. Deficiency of insulin causes the disease diabetes mellitus.
Tumors of the pancreas may occur and give rise to an excess production of insulin, a condition known as hyperinsulinism. Removal of the tumor is necessary, but large portions of the pancreas should not be unnecessarily removed, lest diabetes ensue.
THE PINEAL BODY
Within the center of the brain lies a small structure called the pineal body, or pineal gland. There has been much speculation regarding the function and importance of this structure, and it is usually classified with the endocrine glands. However, no specific hormone has been isolated from the pineal, nor has any function been proved. It is most likely a vestigial structure such as the appendix. At the present time it has no surgical import.
Usually classified as an endocrine organ is the structure which lies in the middle of the chest called the thymus. Its appearance suggests that it has a glandular function. However, no hormone secretion or other function has been proved. It has been suggested that it produces a growth-stimulating hormone, this theory based on the fact that it is largest during the developmental years and shrinks in size after puberty. But little evidence of any effect of the thymus on growth has been produced; moreover, no stunting or other detrimental effects have been ascribed to its surgical removal or x-radia- tion at any year of life. Frequently, however, the gland is found on x-ray film to appear markedly enlarged in infants; associated often is a decreased resistance to infection and other related symptoms so that it is often deemed wise to shrink the gland by a series of x-ray treatments. This is empirical treatment but probably of benefit and of no known harm.
OTHER ENDOCRINE ORGANS
Other organs in the body may be the source of a hormone. For some there is convincing evidence; for others, only speculation.
It is quite evident that the lining of the duodenum (first portion of small intestine) secretes a hormone (secretin) into the blood stream which in turn causes the pancreas to excrete its digestive juices.
There is some evidence that the stomach liberates a hormone (gastrin) which acts upon its own lining cells, stimulating the production of stomach acids.
The largest gland in the body, the liver, has been awarded an endocrine function by many investigators. It is most probable that there are one or several endocrine duties of the liver. This important organ manufactures and secretes many substances, stores many substances, and chemically changes many substances; many varied functions have been ascribed. No doubt the liver has an endocrine role. However, this organ does not lend itself well to methods of endocrine research since its removal causes instant death and ordinary endocrine studies are based on the effect of absence of a gland. Liver extract has found its usefulness in therapeutics, perhaps through hormonal content. Future investigation may label the liver an endocrine gland.
Research in the field of endocrinology will no doubt isolate other hormones and other organs of internal secretion.