|
Unlimited Tutoring & Homework Help
|
|
Introduction
A cell, a tissue or an organ, which secretes certain useful chemical compounds required for a particular function, is called a Gland.
Endocrine Glands - Meaning and Types
A wide variety of physiological processes are carried out unconsciously by the endocrine system through chemical messengers called "hormones." The endocrine system is a collection of glands that produces these hormones, which are necessary for normal bodily functions. The hormones regulate metabolism, growth and sexual development. These glands release the hormones directly into the bloodstream, where they are transported to organs and tissues throughout the entire body.
Thyroid
The thyroid gland is butterfly-shaped gland and surrounds the front of the larynx and upper part of the trachea in the neck. The thyroid is the largest endocrine gland. The right and the left lobes are connected by a mass of tissue called isthmus. It is composed of number of vesicles or thyroid follicles lined with cuboidal epithelium abundantly supplied with blood and held together by connective tissue. These cells secrete a sticky fluid called the colloid of the thyroid compored of the glycoprotein, thyroglobulin.
The Parathyroid Gland
The parathyroids are four small, flat, oval glands placed two on each side of the thyroid gland in the neck. The cells of the parathyroids are arranged in a compact mass, not in vesicles like those of the thyroid. The cells are of 2 types: small chief cells and large oxyphil cells.
The Thymus Gland
The thymus gland is situated in the upper chest near the front side of the heart. It is pinkish-grey in colour and consists of two lobes. At birth the gland is quite small, weighting about 10 grams, and increases in size in the adult. It is composed of tiny lobules held together by connective tissue. It is also called as "the throne of immunity" or traning school of T-lymphocyte.
The Adrenal Glands
The adrenal or suprarenal glands are paired glands placed on the upper part of each kidney. The adrenal glands consist of an outer yellowish part, the cortex and inner adrenal medulla. The gland is shaped like a tricorn hat.
Hypothalamus
The hypothalamus is a part of the brain, situated at the base of the fore brain. It consists of several masses of the grey matter called hypothalamic nuclei, scattered in the white matter. It is connected to the anterior lobe of the pituitary gland by hypophysal portal blood vessels and with the posterior lobe of pitutary gland by axons of its neurons. The portal vessels and axons pass through a stalk to the pitutary lobes thus forming a hypothalamo-hypophysial axis. This is most important for homeostatsis as it regulates the most major physiological activities in the body. It also indicates a close connection and co-ordination between the hormonal and nervous systems.
The Pituitary Gland (Hypophysis)
The pituitary gland is a small, reddish grey, pea-shaped gland attached to the hypothalamus of the brain by a stalk or infundibulum in front of the pons. It is called the master gland and consists of three lobes namely- anterior lobe, intermediate lobe and posterior lobe. The anterior and posterior lobes are connected by hypothalamo hypophysial portal system and by axons of hypothalamic neurons respectively.
Pancreas
Pancreas is a mixed gland as it performs both endocrine and exocrine functions. Pancreas is an elongated, yellowish gland. It consists of lobules that secrete pancreatic juice. Interspersed at random among the lobules are Islets of langerhans, which produce hormones.
Gonads
The gonads include the testes in males and ovaries in females. They are mixed organs. They produce reproductive cells and secrete hormones, which control reproductive organs. These hormones are collectively called sex hormones. They are secreted from the age of puberty or sexual maturity. The testes and ovaries secrete different hormones.
Role Of Hormones As Messengers And Regulators
Maintenance of internal chemical environment of the body to a constant is called homeostasis. Hormones play a major role in maintaining homeostasis by their intergrated action and feed back controls.
Summary
Thyroid secretes thyroxine, triiodothyronine and calcitonin, which enhance the metabolic rate, promote body growth and tissue differentiation. Failure of thyroid secretion produces cretinism in young age and myxoedema in adults. Excess secretion of thyroid hormone causes Graves disease.
