Synonyms in a broader sense
Medical: Glandula parathyroidea
- Beischilddrüsen
- Epithelial corpuscles
Anatomy
The parathyroid glands represent four lenticular-sized glands weighing about 40 mg. They are located behind the thyroid gland. Usually two of them are located at the upper end (pole) of the thyroid lobe, while the other two are located at the lower pole. Rarely, the lower thyroid glands are also found on the thymus or even in the middle chest space between the lungs (this space is also called the mediastinum). Sometimes additional parathyroid glands are found.
Function
In contrast to many other glands (e.g. the pancreas), the parathyroid gland (Glandula parathyroidea) does not have its own ducts for the secretion it produces, the hormone parathyroid hormone (PTH, also known as parathyrin). Therefore, the messenger substance is released (secreted) directly into the blood and thus reaches its destination. This secretion mechanism is also known as endocrine secretion.
For this reason, the parathyroid glands are criss-crossed by a dense capillary network whose capillaries have a special structure. Capillaries are the smallest human vessels through which exactly one red blood cell (erythrocyte) still fits. In the parathyroid gland there are special, so-called fenestrated capillaries, whose cells do not form a tightly enclosed vessel, but have small gaps (so-called 70 nm “windows”) and thus allow the hormone to pass through into the bloodstream without leaking blood components.
The target tissue, i.e. the site of action of the parathyroid hormone, is bone and kidney. There the peptide hormone (i.e. it is composed of 10 to 100 amino acids) intervenes in the calcium metabolism in a regulating manner. The amount of hormone is controlled by a simple feedback mechanism: the amount of hormone released depends on the calcium concentration in the blood.
The parathyroid glands have their own “calcium sensor” for this purpose. If there is a lack of calcium-calcium, more parathormone is released into the blood; if there is sufficient calcium in the blood, secretion (release) is inhibited. The hormone promotes the supply of calcium via two mechanisms: calcium is released from the bones by the osteoclasts, the cells that break down the bone.
The osteoclasts are stimulated by parathyroid hormone. In the kidney, the hormone prevents too much calcium from being excreted with the urine: (It causes calcium to be reabsorbed from the primary urine produced in the kidney and supplied to the organism). It reduces the excretion of calcium in the urine.
This effect is reinforced indirectly by promoting the formation of vitamin D, which also reduces the excretion of calcium with the kidneys and also promotes absorption from food in the intestines. Both hormones thus counteract osteoporosis (bone decalcification). The concentration of blood calcium is thus kept constant within narrow limits at 2.5 mmol per liter. Furthermore, PTH (parathyroid hormone) promotes the excretion of phosphate via the kidneys.
