Hormones are chemical compounds which are synthesized and secreted to the circulation by specialized organs in the body and which are called endocrine glands. Hormones can be secreted to the blood circulation in response to neuronal stimulation or due to hormonal stimulation by an organ which is higher in priority and significance.
An example of a hormone which is secreted due to neuronal stimulation in the hormone epinephrine and norepinephrine which are secreted by the adrenal medulla in response to a stimulation by the sympathetic nervous system. In addition, hormones of the posterior pituitary gland are also secreted to the blood using neuronal stimulation.
These hormones are antidiuretic hormone and oxytocin. Examples of hormones that are secreted due to hormonal stimulation are hormones of the thyorid gland and hormones of the gonads which are secreted in response to hormones which are in turn secreted in response to hormones from the pituitary gland.
Hormones in the human body usually make their effect on a specific target organ or tissue inside the body. Exception to this rule are the two hormones growth hormone and thyroid hormones. These teo hormones are secreted by the anterior pituitary gland and the thyroid gland respectively. These two hormones have effect which is universal on all the tissues in the body increasing the metabolic rate and increasing cellular proliferation and body growth.
Hormones are usually under negative feedback control from higher endocrine glands. Hormones of the pituitary gland are controlled by a positive feedback from hormones secreted by the hypothalamus. This is in general true except for one hormone that is called prolactin that is controlled by a negative feedback control from a hormone that is called dopamine and which is secreted by the posterior pituitary gland.
Usually, hormones function on a remote target from their source of secretion. It is secreted to the blood and is carried a certain distance before the target organ. Function of hormones is mediated through receptors on the cellular membrane of the target organ. Each hormone has a unique function that depends on its structure.
Hormones function is completely dependent on its structure. There are lipid soluble hormones and water soluble hormones. This will eventually determine the manner in which the hormones will interact with the cell. Lipid soluble hormones include the steroid hormones such as estrogen and testosterone. In addition, thyroid hormones are also lipid soluble due to the existence of a benzene ring in its structure which is nonpolar and hydrophobic. Thyroid hormones are derived from the amino acid tyrosine. In addition, nitric oxide which is also a hormone is lipid soluble and functions in the same way that lipid soluble hormones function.
Water soluble hormones are usually polypeptides and proteins which are water soluble due to the existence of amino acids charged residues in its structure. Most hormones in the body are of this type. Examples of this type of hormones are hormones of the pituitary gland and the pancreatic hormones such as insulin and glucagon.
In addition, hormones which are derived from amino acids are also water soluble except for thyroid hormones that are lipid soluble. Examples are epinephrine and norepinephrine which are synthesized and secreted by the adrenal medulla from the amino acid tyrosine by a decarboxylation and hydroxylation processes.
These two hormones are water soluble and their manner of function is different than the steroid hormones. These two hormones are also neurotransmitters in the nervous system. Therefore they have dual function as hormones and as neurotransmitters. Other type of water soluble hormones are eicosanoids such as prostaglandins and leukotrienes.
Water soluble hormones cannot penetrate the phospholipid bilayer of the cellular membrane due to their hydrophilic nature. Therefore they exert their effect by binding to their receptors on the cellular membrane. Thus activating other messengers which lead eventually to specific effects such as protein synthesis. Lipid soluble hormones are able to penetrate the phospholipid bilayer. Therefore, their receptors are located inside the cell such as the nuclear membrane exerting their effect by increasing transcription rate which leads to protein synthesis that is accelerated or decelerated.