Parathormone (PTH) is a hormone produced by the parathyroid glands, which are four small glands located behind the thyroid gland in the neck. The main function of PTH is to regulate the level of calcium in the blood. It does so by acting on the bones, kidneys, and intestines.



Bone:
- PTH stimulates the release of calcium from the bones into the bloodstream.
- It activates osteoclasts, which are cells responsible for breaking down bone tissue and releasing calcium ions into the blood.
- This process is known as bone resorption.

Kidneys:
- PTH increases the reabsorption of calcium in the kidneys, preventing its excretion in the urine.
- It also stimulates the conversion of vitamin D into its active form, calcitriol, in the kidneys.
- Calcitriol promotes the absorption of calcium from the intestines into the bloodstream.

Intestines:
- PTH indirectly increases the absorption of calcium from the intestines by stimulating the production of calcitriol in the kidneys.
- Calcitriol enhances the active transport of calcium across the intestinal lining into the bloodstream.



Maintaining the appropriate level of calcium in the blood is crucial for various physiological processes in the body, including:

1. Bone Health:
- Adequate calcium levels are necessary for the formation and maintenance of strong and healthy bones.
- Calcium is a critical component of the mineralized matrix in bones, which provides strength and structural support.

2. Nerve and Muscle Function:
- Calcium plays a vital role in nerve transmission and muscle contraction.
- It is required for the release of neurotransmitters from nerve endings and the initiation of muscle contraction.

3. Blood Clotting:
- Calcium is essential for the normal functioning of blood clotting mechanisms.
- It is involved in several steps of the clotting cascade, ensuring proper clot formation and preventing excessive bleeding.

4. Enzyme Activity:
- Many enzymes in the body require calcium as a cofactor for their proper activity.
- Calcium ions participate in various enzymatic reactions, influencing their efficiency and effectiveness.

Overall, parathormone plays a crucial role in maintaining the balance of blood calcium levels. It ensures that calcium is available for essential physiological processes while preventing its depletion from bones and excessive excretion through urine.