The endocrine system is a biological system of glands that secretes hormones into the circulatory system. It is involved in keeping homeostasis (regulating the bodies internal environment) as well as carrying out many applications controlled by the sympathetic and parasympathetic nervous system. The endocrine system is controlled by the nervous system as well as by independent mechanisms, such as the renin-angiotensin-aldosterone system. With nerve impulses, the reaction is usually quick, but with hormone signals, the effects can be variable in their time frame and be short lived or long lasting. All the chemical signals of the endocrine system are known as ‘hormones’, which are released by many different glands. Each hormone has a unique function and can work differently in various species. In humans, the endocrine system and the nervous system work together very efficiently.
- Main Article: Hormone
In multicellular animals, most of information that is required for body control and regulation is transmitted as electric and chemical signals. Hormones, which are secreted by endocrine cells are these chemical signals.
Hormones are divided into three chemical groups:
- Peptides or proteins
Peptides or proteins are water-soluble hormones. Therefore they are easily transported through the blood. Despite this they cannot pass through lipid-rich cell membranes well. These hormones are packaged in vesicles and released into the blood through exocytosis. When they arrive at target tissues, they are brought in through endocytosis. Most hormones are of this type.
- Steroid hormones
Steroid hormones, which are lipid-soluble, can easily dissolve through the cell membranes. For this reason, instead of being packaged in vesicles, they are sent out and taken in through diffusion. Steroids are the fastest acting hormones and are transported by sticking to carrier proteins in the blood.
Amine hormones include both water-soluble and lipid-soluble hormones, so they are released in different ways dependent on their type.
According to the distance over which hormonal messages are transmitted, they can also be classified into two groups – circulating and local hormones. When the target tissues are close to the site of release, the are considered to be local acting hormones. These local hormones include autocrine and paracrine messages. When a hormone binds to a receptor which is located on the releasing cell, it acts as an autocrine message usually as a negative feedback mechanism.
The pituitary gland is located at bottom of the skull. The pituitary has two different different regions that are formed seperately during development and have different functions. One is the anterior pituitary, which originates as an outpocketing of the embryonic mouth cavity. The other is the posterior pituitary, which originates as an outpocketing of the developing brain.
- The posterior pituitary: the posterior pituitary is the gland that secretes antidiuretic hormone and oxytocin. Both are peptides and called neurohormones because they are synthesized in the hypothalamus. When they are produced, they are packaged in vesicles as normal peptides. Then, vesicles are transported down through axons and stored in the axon terminal until released by a nerve impulse stimulation. Antidiuretic hormone (ADH) increases the amount of water conserved by the kidneys. If ADH is increased, the kidneys reabsorb water and produce highly concentrated urine. If ADH is decreased, the kidneys make lots of dilute urine. Oxytocin is produced in quantity in women only during childbirth and nursing. It stimulates the contractions of the uterus during deliver and is also responsible for release of milk from the mammary glands.
- The anterior pituitary: the anterior pituitary releases one peptide, two protein, and two glycoprotein hormones. These are thyrotropin, adrenocorticotropin, luteinizing hormone, and follicle-stimulating hormone, which are tropic hormones, and their function is to control the activities of other endocrine glands. In different types of pituitary cells, each tropic hormone is made.
- The anterior pituitary influence tissues, which are not endocrine glands, also produce hormones. These tissues produce the other peptide-and protein hormones such as growth hormones, prolactin, melancocyte-stimulating hormone, endorphin, and enkephalins.
- Main Article: Thyroid
The Thyroid gland is located around the front of the trachea and a lobe on either. This gland produces two important hormones, thyroxine and calcitonin. Thyroxine is produced by follicles, which are round structures and belong to thyroid gland. Calcitonin is made the cells that are between the follicles.
- Thyroxine regulates cell metabolism in mammals by stimulating the transcription of a huge number of genes in most body cells. To produce thyroxine, the follicles cells need to be activated by the tropic hormone thyrotropin. When the hypothalamus produces TRH(thyrotropin-regulating hormone) and transport it to the anterior pituitary through blood, TRH activates the thyrotropin-producing pituitary cells.
- Calcitonin acts when it is required to reduce the concentration of calcium in blood. There are cells called osteoclasts that break down bone and release calcium. By decreasing their activity, calcitonin lowers the concentration of calcium.
The parathyroid glands are four tiny structures located in the posterior surface of the thyroid gland. These glands produce only one type of hormone, parathyroid hormone(PTH, parathormone). This hormone is the critical hormone that increases the concentration of calcium in blood opposed to calcitonin. Receptors in the plasma membrane of the parathyroid senses levels of calcium in blood to determine that they need to synthesize and release PTH. When they need, PTH works in many different ways to raise the concentration of calcium. Although PTH cannot act on osteoclasts directly, it can act on osteoblasts, the cells which take up circulating calcium and deposit new bone. PTH activates bone turnover by activating osteoblasts, which in turn release cytokines that activate the osteoclasts.
- Main Article: Pancreas
The pancreas is a very important endocrine gland that makes insulin and glucagons to regulate blood glucose levels. The reason pancreas is very important is its close relationship with diabetes mellitus, which was a fatal disease before the 1920s. The main fact that causes diabetes mellitus is a lack of insulin or insulin receptors on the target tissues. This insulin is produced in cluster of endocrine cells. Insulin transforms glucose to glycogen, and glucagon transforms glycogen to glucose. These two hormones work as antagonism.
Table of glands
The following table lists the major endocrine glands, hormones, and functions 
|Endocrine Gland||Hormone Released||Major functions|
|Hypothalamus||Hypothalamic-releasing and hypothalamic-inhibiting hormones||Regulation of anterior pituitary hormones|
|Posterior pituitary||Antidiuretic Hormone||Stimulation of kidneys to trigger water-reabsorption|
|Oxytocin||Contraction of uterus muscles during child birth/release of milk from the mammary glands during nursing|
|Anterior pituitary||Thyroid-stimulating hormone||Stimulation of thyroid hormone secretion|
|Adrenocorticotropic hormone||Stimulation of adrenal cortex hormone secretion|
|Gonadotropic hormone (FSH and LH)||Stimulation of gonads for gamete production/sex hormone production|
|Prolactin||Stimulation of mammary glands for milk production|
|Growth hormone||Stimulation of soft tissues and bones for cellular division, proteins synthesis, bone growth|
|Melanocyte-stimulating hormone||Function is largely unknown in humans, but is responsible for regulation of skin color in lower vertebrates|
|Thyroid||Thyroxine and triiodothyronine||Stimulation of all tissues - increases the metabolic rate and regulates growth and development|
|Calcitonin||Stimulation of bones, kidneys, and intestine primarily to lower blood calcium levels|
|Parathyroids||Parathyroid hormone||Stimulation of bones, [kidneys]], and intestine primarily to increase blood calcium levels|
|Adrenal cortex||Glucocorticoids (cortisol)||Stimulates the breakdown of protein and raises blood glucose levels|
|Mineralocorticoids (aldosterone)||Stimulation of the kidneys to reabsorb sodium and excrete potassium|
|Sex hormones||Stimulation of the reproductive organs and the expression of sex characteristics|
|Pancreas||Insulin||Stimulation of the liver to convert glucose to glycogen, lowering the blood glucose level|
|Glucagon||Stimulation of the [[liver] to convert glycogen to glucose, raising the blood glucose level|
|Gonads||Androgens (testosterone)||Stimulation of body to express male sex characteristics|
|Estrogen and progesterone||Stimulation of body to express female sex characteristics|
|Thymus||Thymosins||Stimulation of T lymphocytes to increase maturation and production|
|Pineal gland||Melatonin||Controls circadian and circannual rhythms by stimulation of the brain - possibly involved in the maturation of sexual organs|
- Main Article: Molting
Organisms, such as Arthropods, have exoskeletons and cannot grow bigger until they shed their shell. Between each molts, they have a growth stage called an instar. There are two kinds of hormones working in sequence that regulate molting – brain hormone and ecdysone. Brain hormone is transported to and sorted in the corpora cardiaca, a pair of structures attached to the brain. When corpora cardiaca releases brain hormone, it diffuses to the prothoracic gland. Then, the prothoracic gland is stimulated and releases ecdysone. This hormone diffuses to target tissues and stimulates molting.
- Inquiry into Life 10th edition, Mader, McGraw Hill, copyright 2003, p393 t.20.1
- Cliffs AP Biology 3rd Edition by Phillip E. Pack. Wiley Publishing, Inc. NJ
- Life: The Science of Biology. Purves, Sadava, Orians, Heller. 2004. Sinauer Associates, Inc. W.H. Freeman and Company