The endocrine system is formed by the set of endocrine glands, which are responsible for the secretion of substances generally known as hormones.
The endocrine glands (from the Greek endos, In and krinossecretion) are so-called because they release their secretion (hormones) directly into the blood or hemolymph exocrine glands (from Greek exos, outside), which throw their secretions out of the body or into the hollows of hollow organs.
How do hormones act
Endocrine glands always release hormones in the blood (or hemolymph) because they reach every cell in the body. Each hormone acts only on some cell types called target cells. Target cells of a given hormone have, in the membrane or cytoplasm, proteins called hormone receptors capable of combining specifically with hormone molecules. It is only when the right combination occurs that the target cells exhibit the characteristic response of hormonal action.
Hormonal regulation by FEED-BACK
How does the endocrine gland “know” how much hormone to release into the blood?
This question has already been answered in time by scientists. Regulation of the secretion of various hormones is done by a mechanism known as negative feedback.
The English expression feed-back (translated as "feedback") is used to indicate the regulation of a gland by its own end product. The feedback is negative because the increase in the end product inhibits gland activity.
Feedback thyrotropin regulation
An example of negative feedback is pituitary control over thyroid gland. The pituitary produces a trophic hormone, thyrotropin, which stimulates the thyroid to release hormones. thyroxine and triiodothyronine. When these hormones reach a certain concentration in the blood, they inhibit the production of thyrotropin by the pituitary gland.
When the rate of thyrotropin in the blood decreases, so do the rates of thyroxine and triiodothyronine in the blood. This undoes the inhibitory effect on the pituitary gland, which increases thyrotropin production and restarts the regulatory cycle.
Hormonal regulation of blood calcium level
Another example of negative endocrine feedback is the regulation of hormone production calcitonin and parathyroid hormonerespectively by the thyroid and parathyroid glands. These two hormones are responsible for maintaining normal circulating calcium levels around 9 to 11 mg per ml of blood.
Raising the level of calcium in the blood stimulates the thyroid to secrete calcitonin. This hormone promotes the deposition of calcium in the bones and the elimination of calcium in the urine, besides inhibiting the absorption of this material in the intestine. With this, the rate of calcium in the blood decreases.
When the calcium rate becomes less than 10 mg per 100 ml of blood, calcitonin secretion is inhibited and the parathyroid glands are stimulated to secrete the parathyroid hormone. This hormone has the opposite effect of calcitonin: it releases calcium from the bones to the blood, stimulates the absorption of calcium from the intestine and decreases its elimination through the kidneys.
Thus, calcitonin and parathyroid hormone maintain an adequate level of calcium in the blood, an essential condition for the proper functioning of cells.