There are a number of endocrine glands that secrete hormones with direct effects on target tissues
I have not gone into great detail on these glands. Instead, you have the PowerPoint presentation, the table I drew, your textbook, and even your lab book to supplement this information.
The Thyroid Gland
The thyroid gland is located below the larynx, around the trachea. It is rather large, and has two major halves to it, like two wings of a bat. Each of the wings is called a lobe. Three hormones are produced here. Two of them are thyroxines (the "thyroid hormones"), and one is calcitonin. The thyroxines, T3 & T4, are influenced by the thyroid tropic hormone TSH. The third, calcitonin, is only influenced by blood calcium levels.
Why are only T3 & T4 influenced by TSH? Well, these hormones are involved in your metabolism. Your metabolic rate is judged by blood levels of specific chemicals, and there are many. Also, there are times in our days when an increased metabolic rate might hinder our activities. The ultimate judge over all these factors is your hypothalamus. When it gets information that more T3 & T4 are necessary, it secretes TRH which then causes the anterior pituitary to secrete TSH, and then, finally, T3 & T4 are secreted.
The need for calcitonin is directly related to blood calcium levels. Calcium levels in the blood can be determined by the cells that make calcitonin, and don't have to be determined through the brain. Calcitonin acts directly to cause calcium deposition in bone.
Please note that there are disorders, like cretinism and goiters, that are the result of improper thyroid hormone (not calcitonin) levels.
The Parathyroid Glands
These glands are tiny. They are each around the size of a small green pea. There are four of these glands, one stuck to each corner of the thyroid gland. This is shown in this figure... the thyroid gland is wrapped around the trachea, and has two main lobes. If you were to unwrap the thyroid gland so that it was laid out flat on the monitor, you'd see the image at the far right (or something a bit more realistic looking!).
You should notice that once it is laid out, the parathyroid glands, in orange, are noticeable. These parathyroid glands lie on the posterior side of the thyroid gland.
PTH is the hormone made by the parathyroid glands. This hormone opposes the action of calcitonin. It is only by the joint contribution of calcitonin (to increase calcium deposition in bone) and PTH (to extract calcium from bone) that blood calcium levels remain pretty constant.
Please note: if the parathyroid glands get hyperactive, as may happen if a tumor grows within one of them, too much calcium will be continually extracted from bone. This can lead to a form of osteoporosis.
Each person has two adrenal glands. Each one sits on top of a kidney within the body, and the entire kidney/adrenal gland assembly is wrapped in adipose tissue for protection. If you were to cut open an adrenal gland, you would realize that its outer rind has a different composition and consistency than the inner tissue. It turns out that there are different types of cells in the outer portion than in the middle, and the different cell types secrete different hormones. The outer rind is called the cortex of the adrenal gland; you learned this term already, when we learned about the brain and that the outermost region of the telencephalon was the cortex. The middle portion of the adrenal gland is called the medulla, which means middle; the medulla oblongata can be considered the middle portion of the CNS, separating the brain and spinal cord. The adrenal cortex and adrenal medulla have such different endocrine functions that they can almost be considered as separate endocrine organs!
This is the part of the adrenal gland that secretes aldosterone, cortisol, and sex hormones. Aldosterone is involved in maintaining homeostasis of blood chemical and osmotic concentrations. Cortisol has many effects, all of which help our cells to obtain enough glucose for energy production. And the sex hormones have many functions, which we will discuss later in the semester.
This region is like a glorified portion of the autonomic nervous system! Its cells are under control of preganglionic neurons, and its cells secrete epinephrine and norepinephrine.
Why do we need this extra epinephrine and norepinephrine from the adrenal medulla? Because when they are secreted by the adrenal medulla, they enter the blood stream and have widespread, longer lasting effects. The postganglionic neurons that secrete these chemicals (that you learned about in Unit 14) only secrete them directly onto their target tissue as localized neurotransmitter. The adrenal medulla is able to kick the entire effect of these chemicals into action throughout the body and sustain them longer.
This organ has both exocrine and endocrine functions. We will only discuss the endocrine functions here... the exocrine functions will be covered during our digestion unit.
Insulin and glucagon are produced here. Insulin causes our livers to store up our nutrients into macromolecule form, while glucagon causes our livers to break down our macromolecules into more usable nutrients. Therefore, if you just ate a meal, insulin kicks into action. All the nutrients you ingested cannot be used immediately, so it causes storage of those nutrients (in macromolecule form) for later. Then, after a while without eating more, you need to begin to break down those macromolecules so that you can continue to obtain nutrients until your next meal.
Insulin and glucagon work in opposition. They normally work with such beautiful orchestration (based on our blood glucose levels) that we can maintain our normal activities regardless of when our last meal was ingested. However, sometimes there are problems in this orchestration. Hypoglycemia and diabetes are problems that occur when the insulin and glucagon in a person's body cannot be secreted or regulated properly.
Read Clinical Application 13.4 on page 509 to gain an understanding of diabetes mellitus. You can also visit this pharmaceutical site on it, this page from the American Heart Association, or this site from a doctor at Temple University.
The function of the pineal gland has only been discovered more recently. This gland secretes melatonin, which helps to regulate our overall activity. The location of the pineal gland,
as part of the diencephalon, puts it in a good position to be involved in understanding light-dark cycles. Therefore, it is regulated by our circadian rhythm (which is actually generated and kept in check via the suprachiasmatic nucleus in the hypothalamus) and secretes different amounts of melatonin at different times of day. Its peak melatonin secretion occurs in the night.
Many people, upon hearing about a brain chemical produced at night, have gone crazy and overboard with their focus on the pineal gland. If you do a search for "pineal gland" on the web, you will find many sites that are trying to sell melatonin as a wonderdrug for sleep and all sorts of things. This other, odd, bad website, keeps a couple of the facts straight, and goes way overboard with other facts. For example, the person that wrote that page says that the pineal gland "regulates the function of all the endocrine organs through the hormone melatonin." I hope you have learned enough so far to realize that is really a ridiculous statement.
That doesn't mean that all the hoopla is without merit. It's just that it is more important to control our blood glucose and osmotic balance than it is to play with anything like melatonin!
A pretty web site that shows microscopy of human pineal cells has much more detail anatomically than you need, but I wanted to you remember that all of these glands are just compilations of regular cells that secrete chemicals!
Your book notes that the thymus gland, reproductive glands, digestive glands, and others, produce hormones, too. There is no possible way to mention them all here! We'll just focus on the major ones!
© 2011 STCC Foundation Press