10 Questions
What happens when serum calcium concentration decreases?
Parathyroid hormone secretion increases
What is the effect of growth hormone on calcium?
It increases calcium excretion in the urine and also increases intestinal absorption of calcium
What is the effect of thyroid hormones on calcium?
They cause hypercalcemia, hypercalciuria, and osteoporosis
What is the effect of oestrogens on osteoporosis?
They prevent osteoporosis
What is the effect of insulin on bone formation?
It increases bone formation
What are the three hormones that regulate calcium homeostasis?
Parathyroid hormone, vitamin D, and calcitonin
What is the effect of parathyroid hormone on plasma calcium?
It increases plasma calcium
What is the effect of 1,25 dihydroxycholecalciferol on calcium?
It increases calcium absorption from the intestine
What is the effect of calcitonin on plasma calcium?
It decreases plasma calcium
What is the role of the calcium-sensing receptor (CASR) in calcium regulation?
It regulates extracellular calcium
Study Notes
Calcium Homeostasis
- The adult human body contains approximately 1100 g (27.5 mol) of calcium, with 99% found in bones and 1% distributed in soft tissue and extracellular fluid.
Calcium Distribution in Blood
- Blood calcium levels are normally 9-10.2 mg/dL (2.25-2.55 mmol/L), with 50% as free ionized calcium, 40% bound to serum proteins (mainly albumin), and 10% complexed with anions.
Intestinal Absorption of Calcium
- 30-80% of ingested calcium is absorbed, primarily in the upper small intestine.
- Active transcellular calcium transport occurs in the duodenum, dependent on vitamin D and calbindin protein.
- Passive absorption occurs in the jejunum and ileum, predominating when dietary calcium intake is adequate or high.
Renal Handling of Calcium
- 99% of filtered calcium is reabsorbed in the kidneys, with 60% reabsorbed in the proximal tubules and the remainder in the ascending limb of the loop of Henle and the distal tubule.
Regulation of Calcium Homeostasis
- Plasma Ca2+ levels are regulated by interactions among parathyroid hormone (PTH), dietary vitamin D, and calcitonin from the thyroid gland.
Parathyroid Hormone (PTH)
- PTH is secreted by the parathyroid glands, regulating calcium and phosphorus concentrations in extracellular fluid.
- PTH finds its major target cells in bone and kidney.
- Biosynthesis involves pre-Pro PTH, Pro-PTH, and finally PTH, which is stored in secretary vesicles.
Control of Parathyroid Hormone Secretion
- PTH is released in response to low extracellular concentrations of free calcium.
- Low levels of PTH are secreted even when blood calcium levels are high.
Physiologic Effects of Parathyroid Hormone
- PTH mobilizes calcium from bone by stimulating osteoclastic activity.
- PTH enhances absorption of calcium from the small intestine by stimulating production of the active form of vitamin D.
- PTH suppresses calcium loss in urine by stimulating tubular reabsorption of calcium.
Key Points in Calcium Homeostasis
- Calcium homeostasis is regulated by three hormones: parathyroid hormone, vitamin D, and calcitonin.
- Free, ionized calcium concentration is physiologically important for excitable tissues like nerve and muscle.
- Parathyroid hormone increases plasma calcium by mobilizing it from bone, increasing reabsorption from the kidney, and forming 1,25 dihydroxycholecalciferol.
This quiz covers the biochemistry of parathyroid glands and calcium homeostasis, including the daily intake and role of calcium in the human body.
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