Page 56 - 86 human physiology part-2

The Endocrine System

of the calcium present in bone to extracellular fluid. PTH does this by activating the production of
osteoblasts, special cells of the body involved in the production of bone and slowing down osteoclasts,
other specialized cells involved in the removal of bone.

There are two major types of cells that make up parathyroid tissue:

• One of the major cells is called oxyphil cells. Their function is basically unknown.
• The second type are called chief cells. Chief cells produce parathyroid hormone.

The structure of a parathyroid gland is very different from that of a thyroid gland. The chief cells
that produce parathyroid hormone are arranged in tightly-packed nests around small blood vessels,
quite unlike the thyroid cells that produce thyroid hormones, which are arranged in spheres called the
thyroid follicles.

PTH or Parathyroid Hormone is secreted from these four glands. It is released directly into the
bloodstream and travels to its target cells which are often quite aways away. It then binds to a structure
called a receptor, that is found either inside or on the surface of the target cells.

Receptors bind a specific hormone and the result is a specific physiologic response, meaning a
normal response of the body. The activity of all the hormones or growth factors secreted by endocrine
glands and circulating in blood is controlled by the exocrine system of the body.

PTH finds its major target cells in bone, kidneys, and the gastrointestinal system.

Calcitonin, a hormone produced by the thyroid gland that also regulates ECF calcium levels and
serves to counteract the calcium-producing effects of PTH.

The adult body contains as much as 1 kg of calcium. Most of this calcium is found in bone and
teeth.

The four parathyroids secrete the parathyroid hormone (PTH). It opposes the effect of
thyrocalcitonin. It does this by removing calcium from its storage sites in bones, releasing it into the
bloodstream. It also signals the kidneys to reabsorb more of this mineral, transporting it into the blood.
It also signals the small intestine to absorb more of this mineral, transporting it from the diet into the
blood.

Calcium is important for steps of body metabolism. Blood cannot clot without sufficient calcium.
Skeletal muscles require this mineral in order to contract. A deficiency of PTH can lead to tetany,
muscle weakness due to lack of available calcium in the blood.

The parathyroids were long thought to be part of the thyroid or to be functionally associated with
it. We now know that their close proximity to the thyroid is misleading: both developmentally and
functionally, they are totally distinct from the thyroid.

The parathyroid hormone, called parathormone, regulates the calcium-phosphate balance between
the blood and other tissues. Production of this hormone is directly controlled by the calcium
concentration of the extracellular fluid bathing the cells of these glands. Parathormne exerts at least the
following four effects: (1)it increases gastrointestinal absorption of calcium by stimulating the active
transport system and moves calcium from the gut lumen into the blood; (2) it increases the movement

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