Negative Feedback Control of Calcium and Phosphate Balance

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The secretion of parathyroid hormone is controlled by the plasma calcium concentrations. Its secretion is stimulated by low calcium concentrations and inhibited by high calcium concentrations. Since parathyroid hormone stimulates the final hy-droxylation step in the formation of 1,25-dihydroxyvitamin D3, a rise in parathyroid hormone results in an increase in production of 1,25-dihydroxyvitamin D3. Low blood calcium can thus be corrected by the effects of increased parathyroid hormone and 1,25-dihydroxyvitamin D3 (fig. 19.22).

It is possible for plasma calcium levels to fall while phosphate levels remain normal. In this case, the increased secretion of parathyroid hormone and the production of 1,25-dihydroxyvitamin D3 that result could abnormally raise phosphate levels while acting to restore normal calcium levels. This is prevented by the inhibition of phosphate reabsorption in the kidneys by parathyroid hormone, so that more phosphate is excreted in the urine (fig. 19.20). In this way, blood calcium can be raised to normal levels without excessively raising blood phosphate concentrations.

Experiments in the 1960s revealed that high blood calcium in dogs may be lowered by a hormone secreted from the thyroid gland. This hormone thus has an effect opposite to that of

Decreased plasma

Low Plasma Ca2 Feedback

Stimulates reabsorption of Ca2+ Inhibits reabsorption

Stimulates intestinal absorption of

CaPO4 crystals

Increased plasma Ca2+

■ Figure 19.22 Homeostasis of plasma Ca2+ concentrations. A

negative feedback loop returns low blood Ca2+ concentrations to normal without simultaneously raising blood phosphate levels above normal.

Increased plasma Ca2+

Calcitonin Pth Balance

Stimulates excretion of Ca2+ and PO43-

Inhibits dissolution of CaPO4 crystals

Negative feedback

Decreased plasma Ca2+

■ Figure 19.23 The negative feedback control of calcitonin secretion. The action of calcitonin is antagonistic to that of parathyroid hormone.

parathyroid hormone and 1,25-dihydroxyvitamin D3. The calcium-lowering hormone, called calcitonin, was found to be secreted by the parafollicular cells, or C cells, of the thyroid. These cells are scattered among the follicular cells that secrete thyroxine.

of PO43

The secretion of calcitonin is stimulated by high plasma calcium levels and acts to lower calcium levels by (1) inhibiting the activity of osteoclasts, thus reducing bone resorption, and (2) stimulating the urinary excretion of calcium and phosphate by inhibiting their reabsorption in the kidneys (fig. 19.23).

Although it is attractive to think that calcium balance is regulated by the effects of antagonistic hormones, the significance of calcitonin in human physiology remains unclear. Patients who have had their thyroid gland surgically removed (as for thyroid cancer) are not hypercalcemic, as one might expect them to be if calcitonin were needed to lower blood calcium levels. The ability of very large pharmacological doses of calci-tonin to inhibit osteoclast activity and bone resorption, however, is clinically useful in the treatment of Paget's disease, in which osteoclast activity causes softening of bone. It is sometimes also used to treat osteoporosis, as previously described.

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Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • dina sackville-baggins
    What is the negative feedback for calcium?
    8 years ago
  • Amanda Gupta
    Which hormones affects calcium and phosphate balance?
    8 years ago
  • vala
    Which homones affect calcium and phospate balance?
    8 years ago
  • Ursula
    How is calcitonin negative feedback?
    8 years ago
  • licia
    How is calcitonin and pth hormones like negative feedback control?
    8 years ago
  • salla
    What is the negative feedback of calcium resorption.?
    7 years ago
  • robert
    What is the negative feedback loop for calcium concentration?
    6 years ago
  • ashleigh king
    Is calcitonin involved.in negative feedback control?
    4 years ago
  • J
    Is calcitonin blood calcium positive negative feedback?
    3 years ago
  • Meghan
    What effect does calcitonin negative feedback?
    3 years ago
  • ESSI
    Is phosphate homeostasis positive or negative feedback?
    2 years ago
  • dina lothran
    Is blood calcium regulation an example of.negative feedback control or positive feedback control?
    2 years ago
  • norma
    Are the effects of calcitonin considered positive feedback?
    2 years ago
  • Robel Yonas
    What doesthe blood calcium feedback loop do for yu?
    2 years ago
  • claudia
    Is production of calcitonin a positive feedback hormone?
    2 years ago
  • brigitte hoffmann
    How the regulation of blood calcium concentration is an example of negative feedback and homeostasis?
    2 years ago
  • anja
    Is maintaining calcium level in blood positive feedback?
    1 year ago
  • uwe
    Is blood calcium regulation a negative or positive mechanism?
    1 year ago
  • Assunta Davide
    What is the integrator in the negative feedback system for blood calcium?
    1 year ago
  • robyn
    How does the body use negative feedback to prevent low blood calcium levels?
    12 months ago
  • Abela Piazza
    Why is the regulation of calcium levels in the blood is a negative feedback loop?
    10 months ago
  • Semhar
    How is calcuim in blood a negative?
    10 months ago
  • sara koivula
    Why is calcium regeneration a negative feedback loop?
    10 months ago
  • Jordan
    How negative feedback system high calcium concentration?
    10 months ago
  • dennis
    What controls calcium levels in humans during homeostasis?
    9 months ago
  • Maya
    Are blood calcium levels regulated by positive or negative feedback?
    9 months ago
  • hayden
    What acts as a negative feedback for calcitonin?
    8 months ago
  • Christin
    How does negative and positive feedback loops regulate the blood calcium levels in humans?
    6 months ago
  • Joseph
    What is the negative feedback loop for 25 hydroxy d3?
    6 months ago
  • YORDA
    What is the feedback loop in the instance of low calcium in the blood?
    5 months ago
  • Tauno
    What type of feedback system controls calcitonin?
    4 months ago
  • Esko
    How to explain to a 5th grader how bone maintains normal concentrartions of calcium and phosphate?
    1 month ago
  • Maik
    Is acalcium and phosphate a negative feedback?
    22 days ago

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