Endocrine Glands and Hormones

Endocrine Glands and Hormones 
Other glands in the body also produce hormones, and we will discuss two of these. The pineal (pin' e-al) gland is a cone-shaped gland located in the roof of the brain's third ventricle. It is smaller than the pituitary gland and decreases in size as a person ages. In the adult, it becomes a thickened strand of fibrous tissue. 
The pineal gland secretes the hormone melatonin, par­ticularly at night, which is believed to regulate daily rhythms, such as a person's sleep pattern. An injection of melatonin can induce sleep. It also inhibits the secretion of the go­nadotropic hormones FSH and LH; therefore, excessive 
amounts of melatonin inhibit the ovarian and uterine cycles. 
The thymus is a lobular gland in the upper thoracic cavity. This organ reaches its largest size and is most active during childhood. With aging, the organ gets smaller and becomes fatty. Certain white blood cells, called T (for thymus) lymphocytes, originate in the bone marrow but must pass through the thymus to reach maturity. The thymus produces various hormones called thymosins, which aid the differentiation of T lymphocytes and may stimulate immune cells in general. There is hope that thy­mosins will prove helpful in patients suffering from AIDS (acquired immune deficiency syndrome). 
The pineal gland secretes melatonin. which is believed to regulate daily rhythms. The thymus gland secretes thymosins necessary to immunity. 

Hormones Not Associated with Glands 
Even organs that are not usually considered to be endocrine glands have been found to secrete hormones. As discussed earlier, the heart produces atrial natriuretic hormone (ANH), which helps regulate blood volume and pressure by promoting renal excretion of sodium and water. ANH is a peptide that is released not only by the atria but also by the aortic arch, the ventricles, the lungs, and the pituitary gland in response to increases in blood pressure. The stomach and the small intestine produce pep­tide hormones that help regulate digestive secretions.
A number of different types of organs and cells secrete peptide growth factors, which cause an increase in certain cells. Peptide growth factors are like hormones in that they act on cells that have specific receptors to receive them. Some, including lymphokines and blood cell growth fac­tors, are released into blood; others diffuse to nearby cells. Other growth factors include:
Platelet-derived growth factor, which is released from platelets and many other cell types. It helps in wound healing and causes an increase in the number of fibro­blasts, smooth muscle cells, and certain cells of the nervous system.
Epidermal growth factor and nerve growth factor, which stimulate the cells indicated by their names as well as many others.
Tumor angiogenesis factor, which stimulates the forma­tion of capillary networks and is released by tumor cells. One treatment for cancer is to prevent the activity of this growth factor.
Prostaglandins (PG) are produced by cells and act on tissues or cells in the immediate vicinity. They are active in very small quantities and have diverse actions that affect such processes as nervous system function, blood flow in the kidneys, pregnancy, and the inflammation of arthritis.
Sometimes, prostaglandins have contrary effects. For exam­ple, one type helps prevent the formation of blood clots, while another helps bring about the formation of blood clots. Also, a large dose of PG may have an effect opposite that of a small dose. Therefore, standardizing PG therapy is difficult, and in most instances, the therapy is still consid­ered experimental.
Prostaglandins stimulate the inflammatory response.
Drugs such as ibuprofen, aspirin, and acetaminophen, which block the synthesis of prostaglandins, are therefore anti-inflammatory drugs useful in relieving bursitis, arthri­tis, tennis elbow, and similar conditions.
Various grovvth factors stimulate cell production. Prostaglandins are only active locally and have many varied effects.