Hormones play a crucial role in maintaining homeostasis. They interact with the nervous system to coordinate cellular metabolism. The majority of hormones are produced by endocrine glands. Endocrine glands are categorized into three groups; amino acid based, steroids, and eicosanoids.
Amino acid based hormones include amines and proteins. Since these hormones are not lipid soluble, they rely on a second messenger system to enter cells. In cyclic AMP system, the amino based hormone (primary messenger) binds to a receptor on a plasma membrane. This action causes the receptor to change shape, thus forming a bind with a G protein and activating it. The G protein is the signal transducer and binds to the efffector enzyme, adenylate cyclase (AMP), which is able to enter the cell via ATP. Once the cAMP is in the cell, a chemical action is triggered.
Steroid hormones are classified by their four carbon ring structure and are synthesized from cholesterol. They are a lipid and can pass through the cell membrane. The hormone will enter the cell and bind to a receptor protein, which activates a gene on the protein by transcription via mRNA.
Finally, eicosanoids are lipid based and synthesized from arachidonic acid. They are considered paracrines, which effect tissues within the same area. They are classified into two groups: 1) leukotrienes- mediate inflammation and allergic reaction. 2) prostaglandin- can increase blood pressure and blood clotting. It is also found in semen, resulting in unnoticeable contractions to force semen up the vaginal canal.
The main glands that produce hormones include:
1. Hypothalamus: It is considered the master gland by dictating the actions of the pituitary gland. It is connected to the anterior lobe of the pituitary gland by the infundibulum and hypophyseal portal system. The hypothalamus also connects to the posterior lobe of the pituitary gland by the hypothalamus/hypophyseal tract, which is a nerve bundle running through the infundibulum. Nerve stimulation results in the pituitary releasing hormones from its posterior lobe
2. The pituitary gland: Originally thought to be the master gland due to the vast array of hormones released, until scientists discovered the release of these hormones were due to the dictation of the hypothalamus. The anterior lobe produces six hormones, all of which are amino acid based and require the cAMP messenger system to enter a cell.
Growth Hormone (GH) is made by somatotroph cells of the pituitary gland. This hormone stimulates cells to increase in size and divide, speed up protein synthesis, and the breakdown of glycogen to glucose in the liver ( fat as fuel). GH levels are highest during sleep and decrease with age. GH is regulated by hormones GHRH and GHIH, both released by the hypothalamus. Growth hormone releasing hormone (GHRH) is released by the hypothalamus into the blood via the hypophyseal portal system, which then stimulates the release of GH from the pituitary gland.
Thyroid stimulating hormone (TSH), also known as Thyrotropin, is produced by the thyrotrope cells of the pituitary gland. They stimulate the thyroid gland to release Thyroid hormone (TH). Thyroid Releasing Hormone (TRH) is released by the hypothalamus, which stimulates the release of TSH from the pituitary gland, and eventually the release of TH from the thyroid. As one can see, the levels of some hormones directly affect the release or inhibition of other hormones.
Adrenocorticotirotropic hormone (ACTH), stimulates the adrenal glands to release corticosteroid hormone. ACTH is produced by the corticotrope cells of the pituitary gland. Once again the hypothalamus regulates the release of ACTH based on the levels of CRH, cortico releasing hormone, are in the blood stream. The end result of the adrenal gland releasing corticosteroid hormone is used in situations of "fight or flight". These hormone levels rises is stressful situations of survival.
Prolactin, PRL, is produced by lactotropic cells of the pituitary gland and stimulates milk production.
Both follicle stimulating hormone (FSH) and lutenizing hormone (LH) are produced from the gonadotropic cells of the pituitary gland. FSH stimulates the production of gametes; eggs/sperm, while LH produces gonadal hormones. In females it would be estrogen and progestergen, resulting in follicle maturation. For males, LH is also called ICSH. Interstitial cell stimulating hormone, found in the testis and results in testosterone.
Now onto the hormones associated with the posterior side of the pituitary gland.
Both oxytocin and ADH are released due to nerve impulses from the hypothalamus. Oxytocin stimulates uterine contractions and the "let down" reflex for milk. In other words, it aids in squeezing milk out of the breast, but does not produce the milk. This hormone enters cells through a secondary messenger system called PIP/Ca
Antidiaretic hormone (ADH) inhibits urine formation. Osmoreceptors of the hypothalamus monitor solute concentrations. When the concentration is too high, nerve impulses result in the release of ADH, which target the kidneys and water is reabsorbed back into the blood stream, resulting in a higher blood volume and blood pressure. Alcohol inhibits the release of ADH as does a high intake of water. Both these situations lead to a high urine output.
Thymus gland: This gland is located anterior of the trachea. It is made of follicular and extra follicular cells. These cells produce T3 and T4 (assoc. with iodine) and aid in metabolism. T4 is usually converted to T3. T3 is more active and affects all cells except for those of the brain, spleen, testis/uterus, and thyroid. T3 stimulates enzymes associated with glucose. The thymus also releases calcitonin, produced by the extra follicular cells. It decreases blood calcium levels. It inhibits the breakdown of bone and leads to calcium being reabsorbed into the bone.
Parathyroid gland: Consists of four tiny yellow glands on the posterior aspect of the thyroid. PTH is involved with increase calcium in the blood. It leads to the breakdown of bone by calcium being released into the blood stream from the bones. It is essentially the opposite of calcitonin.
Adrenal glands: located on top of the kidneys. Consists of an inner medulla of nervous tissue and an outer cortex consisting of three layers, each which release a different hormone.
The zona glomerulosa (outer layer), produces mineral corticoids, which help maintain electrolyte balance. The most potent of these is aldosterone, which stimulates the kidneys to reabsorb sodium and release potassium. The end result is that water follows the sodium, and leads to a higher blood volume and blood pressure. If blood pressure or sodium levels are low, the kidneys release renin, which converts angiotensinogen into angiotensin II. The presence of angiotensin II stimulates the release of the aldosterone, whose effects include vasoconstriction of the blood vessels.The inner layer of the cortex, the zona reticularis, produce adrenal androgens such as testosterone, who's primary function is secondary sex characteristics in males. Testosterone also aids in the female sex drive. High levels in females can lead to an enlarged clitoris and facial hair growth.
The zona fasciculate (middle layer), produces glucocoticoids, which effect cell metabolism and help resist stressors. The most abundant is cortisol, which stimulates the formation of glucose from non carbs such as fats and proteins. The hypothalamus release CRH, stimulating the release of ACTH from the pituitary gland, then finally the release of cortisol. The lowest levels occur at night, and levels are interrupted in fight or flight situations. Energy is stored.
The pancreas is located in the abdomen and acts as an endocrine and exocrine gland. The endocrine aspect of the pancreatic organ releases alpha and beta cells, both located in the islets of Langerhans. The Alpha secretes glucagon, which increase blood glucose levels, targeting the liver. Glucagon is released when glucose levels are too low. The liver breaks down glycogen into glucose. The beta cells secrete insulin, which decreases blood glucose levels. It inhibits the breakdown of glycogen and the conversion of fats into sugars. Its role is to convert glucose into glycogen.
Finally, the pineal gland located in the epithalamus, produces melatonin, which is a hormone that regulates sleep/wake cycles. Light receptors from the eye trigger the release of the hormone when it senses dark. The circulation of the hormone in the blood results in sleepiness.
It is evident that hormones play a crucial role in homeostatic balances such as blood pressure and electrolye balances. Some hormones regulate the release of other hormones. Although many hormones and their roles have been mentioned, there is still a huge library of chemicals not mentioned and their roles of homeostasis and effects on other hormones.