Unit 8-Endocrine System
The Endocrine System in General
1. General Principles:
All the organs of the body are linked to each other by the blood stream, as well as by the central nervous system. This linking by way of the blood stream makes it theoretically possible for any organ to influence the activity of any other organ by way of chemical substances released into the blood by the first organ. Although it is theoretically possible that any organ can send chemical messages to any other, the number which do so is quite small, and the number of specific substances which carry chemical information is also limited.
The organs which produce chemical messengers are called the endocrine glands. Emptying their secretions directly into the blood stream, they require no duct and they are also called the ductless glands. The secretion which enters the blood stream is called a hormone, which in the blood to the organ which is to be activated. The target organ may be a single organ or a group of organs, but each target organ responds to a hormone in its own characteristic way.
Some examples of hormone action mentioned earlier will be given here to show the nature of the endocrine response.
(1) The first hormone discovered was secretin. This hormone is produced in the duodenal mucosa when acid enters from the stomach. It is a polypeptide. Entering the blood draining the duodenum, secretin passes into the hepatic portal circulation, then to the hepatic vein and the heart. A small amount of the total secreted passes by way of the pancreatic arteries to the portions of the pancreas concerned with secretion. These are stimulated; the pancreas secretes a watery enzyme, alkaline juice, which neutralizes the gastric acid in the duodenum.
(2) When the body fluids become hyperosmolar, hypothalamic centers respond by stimulating the posterior lobe of the pituitary to release antidiuretic hormone into the blood. The hormone is to the heart by the venous drainage of the posterior lobe and a small part is directed to the distal tubules of the kidney. These become permeable to water in the area where they pass through the hyperosmolar medulla, and water is reclaimed by the kidneys, tending to lower the osmolarity of the body fluids. The antidiurctic hormone is also a polypeptide.
(3) Sodium deficiency in the body leads to the secretion of aldosterone by the adrenal cortex. Some have speculated that this is brought about by the kidney through the renin-angiotensin system, but this is not altogether proved (See (4) below). However brought about, the aldosterone, as a steroid, appears to reach the kidney and favors the reabsorption of sodium. This leads to correction of the initiatial disturbance; the sodium deficiency is compensated by increased sodium reabsorption.
(4) A fall in arterial pressure, such as after a hemorrhage, stimulates the kidney to release renin into the blood. Renin acts on renin substrate, producing angiotensin. Angiotensin is a powerful vasoconstrictor and tends to raise the blood pressure, the target organs being arterioles throughout the body. As just noted, it seems possible that the adrenal cortex is also a target organ for angiotensin and that its response is the production of aldosterone. Note that both effects of angiotensin are such as to bring about correction of the initial defects: the loss of volume is compensated by retention, while the loss of pressure by arteriolar vasoconstriction.
Using these examples, we can discuss some general principles which apply quite regularly in the endocrine system:
(1) The endocrine gland is not continuously active. It is provoked to produce its secretion by a characteristic stimulus. The stimulus may be a chemical one, a nervous one, or both.
(2) The hormone released into the blood through the activity of the endocrine gland is usually delivered to the target organ by way of the arterial blood, usually only a fraction of the hormone produced reaches the target organ, the remainder being wastefully distributed to other organs.
(3) The response of the target organ is almost invariably such as to abolish the initial stimulus. In the examples quoted, acid in the duodenum leads to its own neutralization by way of the secretion of an alkaline pancreatic fluid, and secretin formation is inhibited until new gastric acid comes into the duodenum. Hyperosmolarity leads to water retention, which corrects it; sodium losses, to sodium retention; and hemorrhage, to vasoconstriction and volume retention.
(4) Almost any type of molecule may serve as a hormone. In the examples used above, renin is a protein, secretin and antidiuretic hormone are polypeptid, and aldosterone is a steroid. It will be seen later that many types of organic molecules may have endocrine significance. This diversity suggests that there is at least as great a diversity in the mechanism of hormone actions.
Many of the properties of hormones are illustrated by a toy with which some readers may be familiar. This toy is a black box with only a switch in front--that is all.
If the switch is turned on, the top of the box opens and a little hand comes out. The hand reaches for the switch, turns it off, and withdraws. The lid closes and the box remains as before. The student may find the analogy depressing, since the toy seems to serve no useful purpose at all. On the other hand, if one considers that the off portion of the switch is the best one; and that many such "off" switches exist in the body, the purpose becomes evident. Most parts of the organism seem to function best when they begin from the inactive state. Stimuli-producing changes cause deviation from the inactive state. The endocrine system, among other systems, restores the initial condition.
2. Classification of Hormones:
Consideration of hormones will be assisted by the following system of functional classification.
(1) Hormones concerned with the digestive processes.
(2) Hormones concerned with regulation of extracellular ions and water.
(3) Hormones acting on intermediary metabolism.
(4) Hormones acting like the autonomic nervous system.
(5) Hormones controlling other endocrine glands.
(6) Sex hormones.
The first five groups will be considered in Chapter 25, and the sex hormones will be considered in Chapter 26.
3. The Principal Endocrine Glands:
Most hormones are derived from a few organs which will be described briefly.
(1) At the base of the brain, just below the hypothalamus, is the pituitary gland, or hypophysis. This structure has two major subdivisions. One, directly related to the hypothalamus, is called the posterior lobe. To this is attached the anterior lobe. The connection between the anterior lobe and the hypothalamus is a secondary one. No nerve fibers enter the anterior lobe, and its blood supply is derived from hypo thalamic and posterior lobe capillaries.
(2) The thyroid gland is closely related to the trachea in the neck. Just behind the thyroid gland are two parathyroid glands on each side.
(3) The duodenum is the site of production of a number of endocrines related to the digestive processes.
(4) The pancreas is a combination of endocrine and exocrine glands. The exocrine function is concerned with digestion, while the endocrine function with metabolism of carbohydrate.
(5) The adrenal glands, divided into an outer cortex and an inner medulla, are the sites of production of a number of important hormones.
(6) The kidneys, which are the site of renin formation, are like the pancreas organs, which are both exocrine and endocrine.
(7) The testicles and ovaries secrete important endocrine substances as well as the germ cells.
(8) The placenta is a rich source of a number of hormones.
The location of these glands is shown in Figure 338. Figures 339 to 347 show the gross and microscopic structures of each gland.
4. Types of Endocrine Disorders:
In general, the regulation of hormonal activity is so good that the hormone is produced in precisely the right quantities. This is partly due to the property of most endocrine systems of being "turned on" when needed and "turned off" when they have done the job.
This regulating system is not always perfect, however, and sometimes the signal for hormone production is not well detected; sometimes it is too well detected. The same applies to the signals for secretion of hormone production. Thus various types of under-production and over-production of hormones may be observed. When the hormone in question is under produced, the prefix hypo- is used together with the name of the hormone (or sometimes the endocrine gland) concerned. Thus we may speak of hypothyroidism or hypo-insulinism. Conversely, an overactive endocrine system is described with the prefix hyper-. Too much thyroid activity is hyperthyroidism, while too much much insulin production is hyperinsulinism. Examples of these types of disorders will be given in the next two chapters.
Continue to Chapter 25.