<P> Low pressure in the arteries, causes the opposite reflex of constriction of the arterioles, and a speeding up of the heart rate (called tachycardia). If the drop in blood pressure is very rapid or excessive, the medulla oblongata stimulates the adrenal medulla, via "preganglionic" sympathetic nerves, to secrete epinephrine (adrenaline) into the blood . This hormone enhances the tachycardia and causes severe vasoconstriction of the arterioles to all but the essential organ in the body (especially the heart, lungs and brain). These reactions usually correct the low arterial blood pressure (hypotension) very effectively . </P> <P> The plasma ionized calcium (Ca) concentration is very tightly controlled by a pair of homeostatic mechanisms . The sensor for the first one is situated in the parathyroid glands, where the chief cells sense the Ca level by means of specialized calcium receptors in their membranes . The sensors for the second are the parafollicular cells in the thyroid gland . The parathyroid chief cells secrete parathyroid hormone (PTH) in response to a fall in the plasma ionized calcium level; the parafollicular cells of the thyroid gland secrete calcitonin in response to a rise in the plasma ionized calcium level . </P> <P> The effector organs of the first homeostatic mechanism are the bones, the kidney, and, via a hormone released into the blood by the kidney in response to high PTH levels in the blood, the duodenum and jejunum . Parathyroid hormone (in high concentrations in the blood) causes bone resorption, releasing calcium into the plasma . This is a very rapid action which can correct a threatening hypocalcemia within minutes . High PTH concentrations cause the excretion of phosphate ions via the urine . Since phosphates combine with calcium ions to form insoluble salts, a decrease in the level of phosphates in the blood, releases free calcium ions into the plasma ionized calcium pool . PTH has a second action on the kidneys . It stimulates the manufacture and release, by the kidneys, of calcitriol into the blood . This steroid hormone acts on the epithelial cells of the upper small intestine, increasing their capacity to absorb calcium from the gut contents into the blood . </P> <P> The second homeostatic mechanism, with its sensors in the thyroid gland, releases calcitonin into the blood when the blood ionized calcium rises . This hormone acts primarily on bone, causing the rapid removal of calcium from the blood and depositing it, in insoluble form, in the bones . </P>

What is the difference between homeostasis and adaptation