BIOLOGY Volume 2 - A Guide to General Biology - 2004

17. COORDINATION AND REGULATION IN ANIMALS

17.6. The Endocrine System

17.6.3. The Parathyroid Glands

Humans have four small Parathyroid glands embedded in the tissue of The Thyroid Gland. These glands produce only one hormone—parathyroid hormone (PTH, or parathormone), which is a polypeptide consisting of 84 amino acid residues. Parathyroid hormone and the thyroid hormone Calcitonin (see below) act antagonistically to regulate calcium and phosphate concentrations in the plasma. Parathyroid hormone maintains normal plasma calcium levels and lowers phosphate concentration. The activity of the parathyroid glands is regulated by negative feedback (Fig. 17.51).

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Fig. 17.51. The Role of parathyroid hormone and calcitonin in regulating Blood calcium levels.

Underactivity of the parathyroid glands (hypoparathyroidism) leads to a decrease in calcium levels in the blood (hypocalcaemia) and other Tissues due to its increased excretion in the urine. This can lead to tetany—sustained Muscle contraction. At the same time, phosphate excretion in the urine is reduced, and its plasma levels rise.



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