Endocrine System

Endocrine system is a collection of glands that work interdependently and produce hormones that regulate the body’s growth, metabolism, and sexual development and function. The Endocrine system influences almost every cell, organ, and function of our bodies. It is instrumental in regulating mood, growth and development, tissue function, metabolism, and sexual function and reproductive processes.

Hormones

Hormones, the chemical messengers are carried in the bloodstream to a target tissue elsewhere in the body, where they must be absorbed into the tissue before they can have an effect. There are two groups of Hormones viz. Peptide Hormones and Steroid Hormones. Peptide hormones, such as insulin, are short chains of amino acids. Peptide hormones are hydrophilic (water loving), so they don’t pass easily through cell membranes. The receptors for peptide hormones are embedded in the plasma membranes of target cells. Steroid hormones, such as testosterone and estrogen, are lipids, so they’re hydrophobic (water fearing) and can pass easily through the hydrophobic layer of the plasma membrane and enter cells. Thus, the receptors for steroid hormones are located inside the cell.

The Pineal Gland

The pineal gland or pineal body or epiphysis is located in the center of the head. It secretes the hormone melatonin, a hormone produced at night and related to the regulation of circadian rhythm (or the circadian cycle, the wakefulness-sleep cycle). Melatonin may also regulate many body functions related to the night-day cycle.

Pituitary Gland

The pituitary gland or hypophysis is located in one of the bones at the base of the skull. It has two portions viz. anterior hypophysis and posterior hypophysis. The anterior part produces two hormones that work directly viz. growth hormone (GH) and prolactin; and four tropic hormones {tropic hormones regulate the other endocrine glands} viz. adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The posterior part releases two hormones produced in the hypothalamus viz. oxytocin and antidiuretic hormone (ADH or vasopressin).

  • Growth Hormone (GH) acts on bones, cartilage and muscles to promote the growth of these tissues. During childhood, GH secretion deficiencies may lead to dwarfism. Excessive production of GH in children may cause exaggerated bone growth and gigantism.
  • Prolactin stimulates the production and secretion of milk by the mammary glands in women.
  • ACTH stimulates the cortical portion of the adrenal gland to produce and secrete cortical hormones called glucocorticoids.
  • TSH stimulates the activity of the thyroid gland, increasing the production and secretion of its hormones T3 and T4.
  • FSH is a gonadotropic hormone {gonadotropic means it stimulates the gonads} and acts on the ovaries to induce the growth of follicles and, in men, it stimulates spermatogenesis.
  • LH is also a gonadotropic hormone; it acts upon the ovaries of women to stimulate ovulation and the formation of the corpus luteum (which secretes estrogen); in men, it acts on the testicles to stimulate the production of testosterone.
  • Oxytocin is secreted in women during delivery to increase the strength and frequency of uterine contractions and therefore to help the baby’s birth. During the lactation period, the infant’s sucking action on the mother’s nipples stimulates the production of oxytocin, which then increases the secretion of milk by the mammary glands.
  • Vasopressin, or ADH, participates in the regulation of water in the body and therefore in the control of blood pressure, since it allows the reabsorption of free water through the renal tubules. As water goes back into circulation, the volume of blood increases.

Thyroid Gland

The thyroid is located in the anterior cervical region (frontal neck), in front of the trachea and just below the larynx. It is a bilobed mass below the Adam’s apple. It secrets thyroxine (T4), triiodothyronine (T3) and calcitonin.

  • T3 and T4 act to increase the cellular metabolic rate of the body (cellular respiration, metabolism of proteins and lipids, etc.) Goiter. the abnormal enlargement of the thyroid gland, can occur as a result of hypothyroidism or hyperthyroidism. Endemic goiter is caused by a deficiency in iodine consumption. Hypothyroidism caused by deficient iodine ingestion is more frequent in regions far from the coast because sea food is rich in iodine.
  • Calcitonin inhibits the release of calcium cations by bones, thus controlling the level of calcium in the blood.

Parathyroids

The parathyroids are four small glands, two of which are embedded in each posterior face of one lobe of the thyroid. The parathyroids secrete parathormone, a hormone that, along with calcitonin and vitamin D, regulates calcium levels in the blood.

Pancreas

Pancreas is a mixed gland because it produces both endocrine and exocrine secretions. It releases pancreatic juice as exocrine gland while insulin, glucagon and somatostatin hormones as endocrine gland. These hormones are produced in so called “islets of Langerhans”.

Glucose Regulation

For normal body functions, the Blood glucose levels must be maintained. If blood glucose levels are abnormally low, it shall not be able to supply the energy metabolism of cells. If it is too high, it causes severe harm to peripheral nerves, the skin, the retina, the kidneys and other important organs, and may cause cardiovascular diseases.

  • The pancreatic hormone Glucagon increases blood glucose while Insulin reduces it. Glucagon stimulates glycogenolysis, thus forming glucose from the breakdown of glycogen. Insulin is the hormone responsible for the entrance of glucose from blood into cells.
  • When glucose is low (for example during fasts), glucagon is secreted and insulin is inhibited. When glucose is high (for example after meals) glucagon is inhibited and insulin secretion is increased.

While glucagon targets the liver, insulin works in all cells.  Somatostatin inhibits both insulin and glucagon secretions.

Diabetes Mellitus

Diabetes mellitus is caused by deficient production or action of insulin and, as a result, characterized by a low glucose uptake by cells and a high blood glucose level. This diseases is identified by a so called diabetic triad called  polyuria (excessive urine) , polydipsia (excessive thirst)  and polyphagia (excessive hunger). Diabetic persons are advised to take less carbohydrates because these substances are broken down into glucose and this molecule is absorbed in the intestines.

Type-I and Type-II diabetes

Type I or juvenile diabetes or insulin-dependent diabetes is the impaired production of insulin by the pancreas and is caused by destruction of the cells of the islets of Langerhans by autoantibodies (autoimmunity). Type II diabetes occurs adults. In this, the pancreas secretes normal or low levels of insulin, but the main cause of the high glucose sugar is the peripheral resistance of the cells to the action of the hormone.

Type I diabetes is treated with the parenteral administration of insulin. Insulin must be administered intravenously or intramuscularly because, as a protein, it will be digested if ingested orally. In type II diabetes, treatment is done with oral drugs that regulate glucose metabolism or, in more severe cases, with parenteral insulin administration. The moderation of carbohydrate ingestion is an important aid in diabetes treatment.

Diabetes insipidus is the disease caused by deficient ADH secretion by the pituitary gland. In diabetes insipidus, blood lacks ADH and, as a result, the reabsorption of water by the tubules in the kidneys is reduced, and a large volume of urine is produced. The patient urinates in large volumes and many times a day.

Adrenal Glands

Each adrenal gland is located on the top of each kidney (forming a hat-like structure on the top of the kidneys). Each adrenal gland has two parts viz. outer cortical portion and inner adrenal cortex. Further, there is a central part called adrenal medulla.

  • Adrenal medulla releases adrenaline (aka epinephrine) and noradrenaline (aka norepinephrine). These two hormones increase the breakdown of glycogen into glucose, thus increasing blood sugar and metabolic rate. They are released during situations of danger (fight or flight response) and they intensify the strength and rate of the heartbeat and selectively modulate blood irrigation in some tissues.
  • The adrenal cortex releases cortical hormones viz. glucocorticoids, mineralocorticoids and cortical sex hormones. Glucocorticoids stimulate the formation of glucose and as immunosuppressive role, meaning that they reduce the action of the immune system and for this reason are used as medicine to treat inflammatory and autoimmune diseases and the rejection of transplanted organs.
  • Mineralocorticoids regulate the concentration of sodium and potassium in the blood and, as a result, control the water level in the extracellular space.
  • Cortical sex hormones are Androgens. They promote secondary male and female features.

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