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Biology - Hormones

A hormone is a complex chemical substance produced in one part or organ of the body that initiates or regulates the activity of an organ or a group of cells in another part of the body. Hormones are secreted by the endocrine glands and are carried through the bloodstream to the target organ. Some hormones have an effect on only one organ and some have an effect on many organs and cells. 

Pancreas :
The pancreas produces three digestive enzymes and sends them into the small intestine by way of the pancreatic duct. Those enzymes are not hormones because they don’t travel through the bloodstream. The pancreas does produce two hormones for this reason; the pancreas is an endocrine organ. These two hormones are insulin and glucagon. 

Insulin :
The pancreas produces insulin and secretes it into the bloodstream. Insulin has an effect on most of the body cells and that effect is to make them more permeable to glucose. So, by secretion of insulin into the bloodstream, the glucose that sitting around the blood suddenly starts moving into most of the body cells. This hormone causes the glucose concentration in the blood to go down. 

Glucagon :
The other hormone that pancreas produces is glucagon. Glucagon has an effect opposite to that of insulin. The target organ is the liver and glucagon causes the liver to release glucose into the blood. So, when the pancreas secretes glucagon, the blood glucose level goes up. 

Adrenal Gland :
The adrenal gland is actually two endocrine glands. One is called the adrenal medulla and the other is called the adrenal cortex. These two endocrine glands have absolutely nothing to do with each other with reference to what they do. They just happened physically to be part of one structure. 

Adrenal Medulla :
The adrenal medulla secretes two hormones. They are epinephrine and norepinephrine. These two hormones increase the heart rate and blood pressure and they also increase alertness. 

Adrenal Cortex :
The adrenal cortex secretes a lot of hormones. Remember that many of the hormones it secretes are steroids. In fact, a lot of the adrenal cortex hormones are called corticosteroids. Remember this; some of the corticosteroids are called glucocorticoids. Glucocorticoids have a lot of effect on a lot of organs. One of them is similar to glucagon’s effect. These hormones increase the blood’s concentration of glucose and help the body to adapt to stress. Remember that cortisone is a glucocorticoid. Some corticosteroids are called mineral corticoids. Mineral corticoids help the kidney regulate sodium and phosphate balance. One mineral corticoid is called aldosterone. 

Thyroid and Parathyroid :
The thyroid gland is located in your neck. It secretes thyroid hormone. Thyroid hormone acts on most body cells and increases the rate of metabolism. The thyroid also secretes calcitonin, which decreases the blood’s concentration of calcium. The parathyroid gland is also in the neck. It secretes parathyroid hormones. Parathyroid hormone increases the blood’s concentration of calcium. Important that thyroid gland itself is under the influence of another hormone secreted by another gland. When this other gland secretes its hormone and that hormone reaches the thyroid gland, the thyroid is stimulated to secrete thyroid hormone. So, the thyroid is an endocrine gland and is also the target organ of another endocrine gland. 

Anterior Pituitary Gland :
The anterior pituitary gland is in the brain. It secretes a hormone called thyroid stimulating hormone. That hormone is secreted into the bloodstream and when it reaches the thyroid gland, it causes the thyroid to secrete thyroid hormone. The anterior pituitary gland also secretes adrenocorticotropic hormone. This hormone is called ACTH for short. Its effect is to stimulate the adrenal cortex to secrete glucocorticoids. 

Other than the two hormones mentioned before, the anterior pituitary gland secretes two other hormones. They are the growth hormone and prolactin. Growth hormone targets the organs, bone, and muscle and it stimulates them to grow. Children, who are deficient in growth hormone don’t grow normally. Prolactin’s target organ is the breast. It stimulates milk production. In addition to secreting the four hormones we have just discussed, the anterior pituitary gland is a key player in the female menstrual cycle and releases FSH and LH. We will discuss these hormones in just a little while. 

Posterior Pituitary Gland :
The posterior pituitary secretes two hormones. One hormone secreted by the posterior pituitary gland is called vasopressin. Vasopressin causes the kidney to retain water. Vasopressin is also known as ADH (Antidiuretic hormone). So, Vasopressin is similar in its effect to the mineralocorticoids. The other hormone secreted by the posterior pituitary is called oxytocin. When it is time for a pregnant woman to give birth, oxytocin causes the uterus to contract and pushes the baby through the birth canal. Both vasopressin and oxytocin are first made in the hypothalamus and then transported to the pituitary for storage. Therefore, the posterior pituitary gland does not produce any hormones, it stores them. 

Hypothalamus :
The hypothalamus is also in the brain and releases hormones that have the anterior pituitary gland as their target organ. These hormones inhibit or stimulate the release of anterior pituitary hormones. 

Female Menstrual Cycle :
The female menstrual cycle is a cycle of hormone secretion. The key players in the female menstrual cycle are the ovaries and the anterior pituitary gland. The female menstrual cycle has three phases. In phase one, the anterior pituitary secretes two hormones. They are called follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These two hormones, FSH and LH, stimulate one of the follicles in the ovary to grow. So, during phase one, a follicle in the ovary is growing. Phase one is actually called the follicular phase. 

The growing follicle secretes a hormone called estrogen. Therefore, during the follicular phase, a total of three hormones are being secreted; FSH, LH, and estrogen. The follicular phase lasts about 10 days and just before it is over, the anterior pituitary suddenly secretes a very large amount of LH, this is called the surge. The sudden increase in LH secretion by the anterior pituitary is called the luteal surge. Do not forget, at the end of the follicular phase, the anterior pituitary suddenly secretes a lot of luteinizing hormone (LH) that is called the luteal surge. The luteal surge causes the follicle to release the ovum into the fallopian tube. 

So, the luteal surge produces ovulation. When the follicular phase ends, an ovum is sitting in the fallopian tube and a broken follicle is left behind in the ovary. The broken follicle is called the corpus luteum. The corpus luteum, which is still sitting in the ovary, starts to secrete two hormones. These two hormones are estrogen and progesterone. The progesterone gets the uterus ready for pregnancy. The uterus develops some new glands and blood vessels. Usually, of course, there is no pregnancy, but the progesterone gets the uterus ready just the same. 

After about 13 days, the corpus luteum stops secreting estrogen and progesterone. Once the corpus luteum stops secreting estrogen and progesterone, the uterus sheds its new glands and blood vessels, that what causes menstrual bleeding. The bleeding last about five days and that completes the cycle. If the ovum is fertilized, there is no flow and the developing placenta secretes human chorionic gonadotropin (HCG). HCG is a hormone that stimulates the continuous release of progesterone by the corpus luteum. 

Testes :
Testes are endocrine organs. Each testis has seminiferous tubules in it and within the seminiferous tubules, spermatozoa are formed. Lying around in the testes, there are some cells that are not part of the seminiferous tubules. These cells are called interstitial cells. These cells secrete a hormone called testosterone. Testosterone is the male sex hormone. In children, the testes secretes only a little bit of testosterone. At puberty, the testes secret a lot of testosterone into the bloodstream and it reaches every part of the body. Testosterone causes the cells inside the seminiferous tubules to start undergoing meiosis and spermatogenesis. It also promotes the secondary sexual characteristics such as deepening of the voice, muscle development, and facial hair development.