The endocrine system is composed of numerous endocrine glands situated in different parts of the body whose primary role is to secrete hormones in the blood stream. A gland can be described as a set of specialized cells that have the ability to release chemical substances (hormones in this case) after stimulation by some kind of signals. These glands neither require nor use these chemical substances they secrete. It is worth noting that endocrine glands release hormones directly into blood circulatory system. Hormones can be defined as chemical regulators whose primary role is to integrate and coordinate all activities of the body. They can either slowdown or speed up entire body systems or organs. These hormones secreted by the endocrine glands play a vital role in controlling homeostasis. These hormones are transported by the blood circulatory system into their target tissues or cells, where they result in various physiological and biochemical changes. E.g. hormones promoting growth and development; they lower or raise levels of glucose in blood, promote sodium and water retention, they stimulate male sex characteristics development. On reaching target tissue or cells, the hormones may either binds receptors present within the target cells or specific receptors present on the plasma membranes of these target cells.
Glands of the endocrine system secrete two primary hormones: nonsteroid and steroid hormones. Steroids hormones are basically lipids manufactured from cholesterol. Examples of these hormones include progesterone, cortisone, testosterone, estrogen and cortisol. Nonsteroid hormones are composed of or include amines, peptides and proteins. Examples of protein hormones are calcitonin produced by the thyroid gland, insulin produced by the pancreas and the pituitary gland hormones. Examples of peptide hormones are oxytocin and ADH. Examples of Amine hormones include epinephrine produced by the adrenal gland and thyroxine produced by the thyroid gland.
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Most of the amine, peptide and protein hormones function as first messenger chemicals or molecules and bind to the target membrane’s receptor sites. They stimulate or increase, regulate the activity of specific proteins in the target cell membrane. Of such proteins/enzymes affected by most nonsteroid hormones is the adenyl cyclase. Following its activation, adenyl cyclase catalyses the transformation of molecules of ATP to cAMP molecules. cAMP, commonly referred to as secondary messenger, stimulates various changes in the cell. Some of these changes include enzyme activations, changes in the permeability of the cell membrane and rise in the rate of protein synthesis.
Some glands of the endocrine system display the paracrine or autocrine effect. In paracrine effect, the hormone secreted by an endocrine gland influences or stimulates adjacent cells. However, in autocrine effect, the secreted hormones act on the endocrine gland secreting it.
The endocrine system offers an essential mechanism for the moderation or control and integration of each and every body tissue, organ, cell and systems. The primary functions of the endocrine system include regulation of metabolism, maturation, growth and reproduction. Endocrinal system glands include parathyroid, pineal, thyroid, adrenal and pituitary. Various organs that are partially endocrine glands also have cells that release hormones. Some of those organs include thymus, hypothalamus, pancreas and gonads.
The pituitary gland
It is a pea-sized gland located in the selle turcica, a sphenoid bone’s saddle located in the inferior region of the brain. The pituitary gland is composed of two lobes; the neurobypophysis constituting the posterior lobe and the adenohypophysis constituting the anterior lobe. The posterior lobe should not be looked at as a hormone, but rather as a reservoir or storage facility for hormones emanating from the hypothalamus, located just above it. The pituitary gland is connected to base of the hypothalamus via an infundibulum. Neurohormones synthesized by the hypothalamus’ neurosecretory cells are transported for temporal storage in the neurobypophysis. The release is of these hormones is effected following stimuli produced by neurons of the hypothalamus (Rosdahi, 2008).
It is worth noting that the two parts of the pituitary gland are closely related in anatomical basis, but differ functionally and embryologically. The posterior pituitary originates from the neurons, whereas the anterior pituitary is basically a glandular tissue. The pituitary gland is located very to the hypothalamus, at the base of the brain, which plays in controlling the functioning of the pituitary. The anterior pituitary produces numerous hormones. The peptide hormones produced by the anterior pituitary play a central role in vital functions of reproduction, protein and fat metabolism, linear growth and secretion of the thyroid hormones essential in regulating energy metabolism and growth. Some of these hormones are said to be trophic, i.e. have a stimulating effect on other endocrine glands. The production of hormones by the anterior pituitary is under the control of hormones produced by the hypothalamus. The hormones produced by the hypothalamus get their way to the pituitary via portal blood vessel system. Hypothalamic hormones secretion is under the influence of the brain’s high centers. Both pituitary and hypothalamic hormone secretion are regulated via feedback mechanisms by the hormones they produce. A large section of the blood supply to the anterior is from the hypothalamus, this arrangement ensures that it is always subjected to increased concentrations of the hormones of the hypothalamus. The pituitary gland’s anterior lobe is connected to the hypothalamus via hypothalamic-hypophysial portal system (Alcamo and Barbara, 2004). Thus the anterior pituitary receives a direct blood supply from the hypothalamus ensuring that large concentrations of hypothalamic hormones reach the gland. Hormones originating from the hypothalamus, for example GHRF (growth hormone releasing factor) can either inhibit or stimulate secretion of hormones of the anterior pituitary gland like growth hormone.
Vasopressin also referred to as ADH (antidiuretic hormone) is the first hormone secreted from the neurobypophysis. Antidiuretic hormone affects blood pressure by acting on kidney tubules resulting in water absorption regulation i.e. regulating water absorption. Hyposecretion of ADH results in a medical complication referred to as diabetes insipidus, primary characterized by excessive thirst and urine production. Oxytocin is the second hormone of the neurobypophysis of the pituitary gland. The mammary glands and the uteral smooth muscles are the primary target of oxytocin. This hormone results in the secretion of milk as well as contraction of the uterus respectively. Suckling regulates oxytocin secretion partially (Costanzo, 2007).
The adenohypophysis also referred to as the anterior lobe of the pituitary secrets several essential hormones. The hypothalamus regulates adenohypophysis by producing inhibiting and releasing hormones to the blood vessels (portal blood vessels) supplying to the pituitary. Releasing hormones primary play a central role in the stimulation/ promotion of the biosynthesis and secretion of adenohypophysis hormones, whereas the inhibiting hormones act to counter/ prevent these processes. Hormones of the adenohypophysis / anterior pituitary regulate several other glands of the endocrine system, hence commonly referred to as master gland.
The first hormone of the anterior pituitary is the HGH (human growth hormone), also referred to as somatotropin. The primary role of this hormone is promotion of body growth through the stimulation of protein and amino acid uptake cells, by encouraging of fat mobilization and protein biosynthesis. HGH is a protein composed of one hundred and ninety one amino acid residues. Secretion of this hormone is inhibited by feedback inhibition due its high concentration in the blood. Hyper production of this hormone during the earlier stages of development results in gigantism, while hypo production leads to pituitary dwarfism. In adults, overproduction of human growth hormone leads to acromegaly, characterized with change in an individual’s appearance due soft tissue growth and bone thickening (Rosdahi, 2008).
Thyroid stimulating hormone, abbreviated as TSH, is the second hormone of the adenohypophysis. It primary role is thyroid gland growth regulation as well as its iodine uptake. It regulates biosynthesis and secretion of the hormones of the thyroid glands.
In the third position is the adrenocorticotropic hormone commonly denoted as ACTH. The primary target for this hormone is the adrenal gland cortex. ACTH not only stimulates the production of glucocorticoid hormones by the adrenal glands but also the growth of the tissue itself.
The fourth position of the adenohypophysis hormones is taken by a hormone called prolactin. The primary targets for this hormone are the mammary glands. In the mammary glands, prolactin initiates milk production for newborn nourishment.
Follicle stimulating hormone, commonly abbreviated as FSH is the next anterior pituitary hormone in line. The primary targets for this hormone are tests and ovaries. In males, FSH promotes sperm cell production, while in females it promotes follicle development which ultimately leads to egg production. Follicle stimulating hormones is also part of hormones playing a central role in regulation of sex organs (gonads), there is part of gonadotropins. LH (luteinizing hormone) is another example of adenohypophysis’gonadotropin. LH promotes follicle maturation and hence stimulates ovulation. Luteinizing hormones has also been shown to play a vital role in stimulation of corpus luteum to produce progesterone and estrogen in females. LH promotes testosterone production by tests in males.
The pituitary gland’s posterior lobe is formed from neural tissue. The bodies of the nerve cells are situated in the hypothalamic nuclei. In simple terms, the posterior lobe can be taken as a hypothalamic growth and it is has an embryonic origin in the nervous system. The hormones of the posterior pituitary gland are made in the bodies of nerve cells, enclosed in the secretory granules and transferred via the nerve axon, down to the posterior pituitary lobe for secretion into blood circulation.
Some researchers have indicated that presence of a third lobe, the middle lobe. MSH (melanocyte stimulating hormone) is the major hormone in this lobe.
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