Calcium Homeostasis and Bone-Endocrine Links - Anatomy and Physiology
- After thyroid surgery, the parathyroid glands are accidentally damaged. Which electrolyte disorder and symptom are expected?
- Hypocalcemia, causing neuromuscular excitability such as tingling, muscle cramps, and tetany.
- In which two organs, in order, is vitamin D converted to its active form, calcitriol?
- First the liver (25-hydroxylation), then the kidney (1-alpha-hydroxylation, stimulated by PTH).
- What is the main action of calcitriol (active vitamin D)?
- It increases intestinal absorption of calcium and phosphate.
- Which tissue stores about 99 percent of the body's calcium?
- Bone (and teeth), as hydroxyapatite in the mineralized matrix.
- What are the effects of PTH on the kidney?
- It increases Ca2+ reabsorption in the distal tubule, decreases phosphate reabsorption in the proximal tubule, and activates vitamin D.
- Which cells secrete calcitonin, and what does it do to blood calcium?
- Parafollicular (C) cells of the thyroid. It lowers blood Ca2+, mainly by inhibiting osteoclasts.
- Which cells secrete parathyroid hormone (PTH), and what stimulates its release?
- Chief cells of the parathyroid glands, in response to low blood Ca2+ detected by the calcium-sensing receptor.
- Excess PTH raises filtered calcium in the urine. Which kidney complication can result?
- Calcium kidney stones. Hypercalcemia raises the filtered calcium load, increasing urinary calcium despite more reabsorption.
- Why does low extracellular calcium increase neuromuscular excitability?
- Low Ca2+ lets voltage-gated Na+ channels open more easily, so nerves and muscles fire with less stimulus.
- Chronic kidney disease reduces calcitriol production. Which hormone rises in response?
- PTH (secondary hyperparathyroidism), because low calcitriol lowers calcium absorption and blood Ca2+.
- A patient has primary hyperparathyroidism. What happens to blood calcium and phosphate?
- Calcium rises and phosphate falls, because PTH releases bone calcium and increases renal phosphate excretion.
- How does PTH increase bone resorption, given that osteoclasts lack PTH receptors?
- Indirectly: PTH acts on osteoblasts, which increase RANKL and stimulate osteoclasts to release calcium from bone.