How does our body control its release of hormones?
By using tropic hormones. Tropic hormones are hormones that control the release of other hormones. You see, if you might want a lot of hormone released quickly, you need to signal many cells to do that. How can you signal many cells quickly? Well, release a tropic hormone. That hormone will travel through the blood to the appropriate gland and cause the cells of that gland to all start releasing hormone. This system is especially advanced when controlled by the brain, like it is when our hypothalamus begins the process.
Therefore, use of tropic hormones allows for rapid hormone control, amplification of original signal (just a bit of tropic hormone can evoke a large release of the next hormone), and control of hormone release by the brain.
An example of tropic hormone use is that the hypothalamus secretes TRH (Thyrotropin Releasing Hormone) into the blood. This hormone causes the anterior pituitary to secrete TSH (Thyroid Stimulating Hormone) into the blood. This tropic hormone causes the thyroid gland to get going in releasing a thyroid hormone. Another example is that PRH (Prolactin Releasing Hormone) is a tropic hormone released by the hypothalamus that causes the the anterior pituitary to release its hormone, PRL (PRoLactin).
Tropic hormones can cause release of hormones (like the releasing hormones described above) or they can also inhibit release. PIH (Prolactin release-Inhibiting Hormone) can be released by the hypothalamus and decrease the amount of PRL released by the anterior pituitary.
Glands can sense the composition of the blood and respond to it. By this, I mean sensing materials in the blood other than tropic hormones. For instance, levels of calcium in the blood can influence whether the parathyroid gland will secrete PTH (ParaThyroid Hormone) and whether the thyroid gland will secrete calcitonin. Also, levels of glucose in the blood will affect whether the pancrease secretes insulin or glucagon into the blood.
Negative feedback is used to regulate hormones. The simplest example of this is one you already learned a bit about back in the bone chapter. Low levels of calcium in the blood can cause the parathyroid gland to secrete PTH. PTH causes calcium to be extracted from bone, increasing blood calcium levels. However, as soon as the blood calcium levels reach their norms, it makes sense to stop secreting PTH so that we don't have too much calcium extracted from our bones.
I have tried to diagram this here. You see, as soon as PTH does its job, PTH needs to be taken out of commission to prevent overdoing it. The only way our body can know if PTH has done its job is by evaluating calcium levels in the blood. When the calcium levels increase, no more PTH should be made.
This is an example of negative feedback. In all cases of negative feedback, the end result affects the pathway that got it there.
Your book describes this concept with a slightly more intricate diagram, shown below.
In this image, the specific hormones are not identified. Instead, the diagram describes a scenario where two glands affect one another. In this example, only the hormone released by gland B (hormone B) has an effect on the target organ. But gland A releases a tropic hormone (hormone A) that influences the secretion of hormone B. Here, they show you that a tropic hormone no longer needs to be released once the hormone it triggers gets going.
Therefore, hormone B has two effects... the effect we consider primarily is its effect on its target organ. But the other effect is back on gland A to prevent any more secretion of the tropic hormone, "hormone A."
An example of a real situation that could work like this is the release of PRH by the hypothalamus... If it works to cause PRL release in the anterior pituitary, then the hypothalamus should stop secreting PRH as soon as enough PRL enters the blood. Right?
Now consider this diagram a bit more. I ask a question in your assignment this week about it. I want you to think about where receptors for each hormone would be found, and what their effects might be. Then, you will be ready to answer the assignment questions.
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