Text 7. The endocrine system

The endocrine system is composed of glands located in many different regions of the body, all of which release specific chemical substances directly into the bloodstream. These chemical substances, called hormones, can regulate the many and varied functions of an organism. For example, one hormone stimulates the growth of bones, another causes the maturation of sex organs and reproductive cells, and another controls the metabolic rate (metabolism) within all the individual cells of the body. In addition, one powerful endocrine gland in the brain secretes a wide variety of different hormones which travel through the bloodstream and regulate the activities of other endocrine glands.

All the endocrine glands, no matter which hormones they produce, secrete their hormones directly into the bloodstream rather than into ducts leading to the exterior of the body. Those glands which send their chemical substances into ducts and out of the body are called exocrine glands. Examples of exocrine glands are sweat, mammary, mucous, salivary, and lacrimal (tear) glands.

The ductless, internally secreting endocrine glands are the following: thyroid gland, parathyroid glands (4), adrenal glands (1 pair), pancreas, pituitary gland, ovaries in female (1 pair), testes in male (1 pair), pineal gland, thymus (immune) gland.

The last two glands on this list, the pineal and thymus glands, are included as endocrine glands because they are ductless, although little is known about their endocrine function in the human body. The pineal gland, located in the central portion of the brain, is believed to secrete a substance called melatonin. Melatonin contributes to the process of skin pigmentation. The pineal is also known to regulate the estrus (reproductive) cycle in lower animals. The thymus gland, located behind the sternum and extending into the neck, is large in childhood but shrinks in size in adults. Its structure, although ductless, resembles a lymph gland (contains lymphatic tissue and antibody-producing lymphocytes), and it is thought that the thymus might play a role in the immune process (antibody-antigen reactions) in the body. Although the exact functions of the thymus are not known, it may also be involved in various disease processes; for example, removal of the thymus gland is found to be helpful in treating a muscular-neurological disorder called myasthenia gravis.