Endocrine glands secrete hormones instead of ducts into the interstitial fluid that surrounds the secretory cells. Hormones spread from the interstitial fluid into blood capillaries, and blood carries them to target cells throughout the body. Because most hormones are required in very small amounts, typically low circulation levels are found.
The endocrine glands also include the pituitary, thyroid, parathyroid, adrenal and pineal glands. In addition, many organs and tissues are not exclusively classified as endocrine glands but contain cells that secrete hormones. The endocrine system consists of all endocrine glands and hormone-secreting cells.
As endocrine glands secretions are released directly into the blood, not into tubes or ducts, they are also referred to as ductless glands. The major endocrine glands include the hypothalamus, the pituitary gland, pineal body, thyroid, parathyroid, adrenal glands, pancreas, and gonads.
• Pituitary gland
• Pineal body
• Adrenal glands
The hypothalamus is a functionally diverse forebrain region that exerts a deep regulatory influence on survival-critical physiological and behavioral processes. Directionally, the diencephalon contains the hypothalamus. It is lower than the thalamus, later than the optic chiasm, and the temporal lobes and optic tracts border on the sides. The location of the hypothalamus, in specific its proximity and interaction with the thalamus and pituitary gland, allows it to behave as a bridge between the nervous and endocrine systems.
The various functions of the hypothalamus in the body are as follow:
• Autonomic Function Control
• Endocrine Function Control
• Motor Function Control
• Food and Water Intake Regulation
• Sleep-Wake Cycle Regulation
Various hormones of the hypothalamus and their functions are as follows:
• Anti-Diuretic Hormone (Vasopressin) –It regulates water levels and influence blood volume and blood pressure.
• Corticotropin-Releasing Hormone –It acts on the pituitary gland causing the release of hormones in response to stress.
• Oxytocin –It influences sexual and social behavior.
• Gonadotropin-Releasing Hormone –It stimulates the pituitary to release hormones that influence the development of reproductive system structures.
• Somatostatin - It inhibits the release of thyroid-stimulating hormone (TSH) and growth hormone (GH).
• Growth Hormone-Releasing Hormone –It stimulates the release of growth hormone by the pituitary.
• Thyrotropin-Releasing Hormone –It stimulates the pituitary to release thyroid-stimulating hormone (TSH). The body metabolism, growth, heart rate and body temperature is majorly regulated by TSH
The pineal gland is a small endocrine gland that is attached to the midline roof of the brain's third ventricle. Part of the epithalamus is placed between the top two colliculi, has a weight of 0.1–0.2 g, and is covered by a capsule formed by the pia mater.
The gland consists of masses of pinealocytes called neuroglia and secretory cells. The pineal gland is secreting melatonin, a serotonin-derived amine hormone. Melatonin seems to help set the biological clock of the body, which is controlled by the hypothalamus ' suprachiasmatic nucleus.
Several functions of pineal gland are:
• Secretion of the hormone melatonin
• Regulation of endocrine functions
• Transformation of entire nervous system signals to endocrine signals
• Causes sleepiness
• Influences sexual development
• Influences immune system function
• Antioxidant activity
Several small, round masses of tissue called the parathyroid glands are partially embedded in the back surface of the lateral lobes of the thyroid gland. Everyone weighs approximately 40 mg (0.04 g). Usually, each lateral thyroid lobe is attached to one superior and one inferior parathyroid gland for a total of four. Microscopically, there are two types of epithelial cells in the parathyroid glands. The more numerous cells, known as chief (main) cells, produce parathyroid hormone (PTH), also known as parathormone. The other cell type's function called an oxyphil cell, is unknown.
Several functions of parathyroid glands are as mentioned below:
• The parathyroids release PTH, which regulates how much calcium is in the blood.
• TH also indicates that the kidneys and small intestines save calcium from digestion rather than release it into the urine.
The thymus is between the lungs behind the sternum. Thymosin-produced hormones— thymosin, thymichumoral factor (THF), thymic factor (TF), and thymopoietin — promote T cell maturation and may delay aging. The thymus in the upper chest cavity is a two-lobed structure which partially extends into the neck. The thymus is above the heart's pericardium, the aorta, the lungs, the thyroid, and the breastbone. The thymus has a thin outer cover called a capsule and consists of three cell types: epithelial cells, lymphocytes, and cells called Kulchitsky, or neuroendocrine.
Several functions of the thymus are:
• It mainly works for the development of T lymphocytes. These cells, once fully mature, leave the thymus and are transported to the lymph nodes and spleen via the blood vessels.
• Hormone-like proteins that assist in maturation and differentiation of T cells.
• Differentiation in T lymphocytes and enhancement of T-cell function is induced by Thymopoietin and thymulin.
• Thymosin significantly increases the immune reaction and induces certain other hormones of the pituitary glands
The paired adrenal glands have a flattened pyramidal shape, one of which is superior to each kidney in the retroperitoneal space. In an adult, each adrenal gland has a height of 3–5 cm, a width of 2–3 cm, and a thickness of slightly less than 1 cm, with a mass of 3.5–5 g, at birth only half.
Cortical secretions include mineralocorticoids, glucocorticoids, and androgens, Mineralocorticoids (mainly aldosterone) increase sodium and water reabsorption and decrease potassium reabsorption. Secretion is controlled by the renin-angiotensin-aldosterone (RAA) pathway and by K+ level in the blood.
Glucocorticoids (mainly cortisol) promote protein breakdown, gluconeogenesis, and lipolysis; help resists stress, and serve as anti-inflammatory substances. Their secretion is controlled by ACTH.
Androgens secreted by the adrenal cortex stimulate the growth of axillary and pubic hair, aid the prepubertal growth spurt, and contribute to libido.
The adrenal medulla secretes epinephrine and norepinephrine(NE), which are released during stress and produce effects similar to sympathetic responses.
The pancreas is a gland that is both endocrine and exocrine. A flattened organ measuring approximately 12.5–15 cm (4.5–6 in.) in length, the pancreas is located in the duodenum curve, the first part of the small intestine, consisting of a head, a body, and a tail. Approximately 99% of the pancreatic cells are arranged in clusters called acini. The acini produce digestive enzymes that flow through a network of ducts into the gastrointestinal tract. One to two million tiny clusters of endocrine tissue called pancreatic islets or Langerhans islets are scattered among the exocrine acini.
The exocrine and endocrine portions of the pancreas are served by a large number of capillaries.
There are four types of hormone-secreting cells in the pancreas. Alpha or A cells make up about 17% of pancreatic islet and excrete glucagon cells. Beta or B cells makes up the majority of cells i.e. about 70% of pancreatic islet cells and excrete insulin. Delta or D cells make up only 7% of pancreatic islet cells and secrete somatostatin (it is similar as of growth hormone– an hormone secreted by the pituitary glands). F cells makes up the remaining of the pancreatic islet cells and are majorly involved in the secretion of pancreatic polypeptide.
Gonads are the organs that make gametes— male sperm and female oocytes. The gonads secrete hormones in addition to their reproductive function. In the female pelvic cavity, the ovaries, paired oval bodies, produce several steroid hormones including two estrogens (estradiol and estrone) and progesterone. Together with anterior pituitary FSH and LH, these female sex hormones regulate the menstrual cycle, maintain pregnancy, and prepare the mammary glands for lactation. They also promote breast enlargement and hip widening at puberty, as well as helping to maintain these secondary female sex features. Also, the ovaries produce inhibin, a protein hormone that inhibits follicle-stimulating hormone (FSH) secretion. The ovaries and placenta produce a peptide hormone called relaxin during pregnancy, which increases the flexibility of pubic symphysis during pregnancy and helps to dilate the cervix during labor and delivery.
The male gonads, the testes, are in the scrotum oval glands. Testosterone, an androgen or male sex hormone, is the main hormone produced and secreted by the testes. Testosterone stimulates pre-born testing, regulates sperm production, and stimulates the development and maintenance of male secondary sex characteristics, such as beard growth and voice deepening. Inhibin is also produced here, that inhibits further secretion of FSH.
Other Endocrine hormone excretory organs
Body tissues other than those normally classified as endocrine glands contain endocrine tissue and secrete hormones, including the gastrointestinal tract, placenta, kidneys, skin, and heart.
The endocrine system made up of endocrine glands and other tissues that secrete hormones. Hormones only affect specific target cells with receptors to recognize (bind) a particular hormone. The number of hormone receptors may decrease or increase (up-regulation) (down-regulation). Circulating hormones enter the bloodstream; local hormones affect neighbouring cells locally. Hormones are chemically either lipid-soluble (steroids, thyroid hormones, nitric oxide) or water-soluble (amines; peptides, proteins, and glycoproteins; and eicosanoids).