How do parathyroid hormone and calcitonin regulate calcium

TL;DR

PTH is released when blood calcium is low to raise calcium levels by bone resorption, kidney reabsorption, and activating vitamin D. Calcitonin is released when calcium is high to lower calcium by inhibiting bone resorption. Calcium sensing and cellular steps control the release and action of both hormones.

Transcript

All right, engineers. So, what we're going to do in this video is we're going to talk about the parathyroid gland. All right. So, where would you find the parathyroid gland? So, you would find it actually located in the posterior aspect of the thyroid gland. Okay. Where's the thyroid gland found? The thyroid gland is actually found kind of like in ... Read More

Key Insights

• PTH is secreted when calcium is low to raise calcium levels.
• Calcitonin is secreted when calcium is high to lower calcium levels.
• Calcium sensing receptors regulate PTH release.
• Parafollicular C cells respond to high calcium to secrete calcitonin.
• PTH increases bone resorption and renal calcium reabsorption.
• Vitamin D activation is promoted by PTH to boost intestinal calcium absorption.
• Calcitonin inhibits osteoclastic bone resorption.
• The body uses humeral stimuli to regulate endocrine calcium control.
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Questions & Answers

Q: What triggers the release of parathyroid hormone (PTH)?

PTH release is triggered by low blood calcium levels, a condition called hypocalcemia. When calcium drops, fewer inhibitory signals reach the parathyroid chief cells, leading to PTH secretion. The hormone then acts on bone, kidney, and vitamin D pathways to increase serum calcium. This feedback loop helps restore normal calcium levels.

Q: What is the primary action of PTH on bones?

PTH stimulates bone resorption, which releases calcium and phosphate into the bloodstream. This process involves osteoclast activity and is part of the body's mechanism to raise blood calcium when levels are low. The net effect is an increase in circulating calcium available for essential physiological functions.

Q: How does PTH affect the kidneys?

PTH enhances renal calcium reabsorption, reducing urinary calcium loss. It also decreases phosphate reabsorption, increasing phosphate excretion in urine. Additionally, PTH promotes the activation of vitamin D, which increases intestinal calcium absorption, contributing to higher serum calcium levels.

Q: What role does vitamin D play in this system?

Vitamin D activation is promoted by PTH. Active vitamin D (calcitriol) increases intestinal absorption of calcium, helping raise blood calcium levels when PTH is secreted. This creates a coordinated response across bone, kidney, and gut to maintain calcium homeostasis.

Q: When is calcitonin released and what does it do?

Calcitonin is released in response to high blood calcium levels, a condition called hypercalcemia. It helps lower calcium by inhibiting osteoclastic bone resorption, reducing calcium release from bone into blood, and thus supporting a decrease in serum calcium.

Q: What cells secrete calcitonin and where are they located?

Calcitonin is secreted by parafollicular cells, also known as C cells, located in the thyroid gland between thyroid follicles. These cells respond to elevated calcium levels by releasing calcitonin to inhibit bone resorption and promote calcium lowering.

Q: How do high calcium levels influence the calcium sensing apparatus?

High calcium levels activate calcium sensing receptors on parathyroid chief cells and possibly other calcium-responsive pathways. This signaling inhibits PTH release, reducing bone resorption and calcium reabsorption, helping to prevent further increases in blood calcium and maintain homeostasis.

Q: What is the overall goal of the parathyroid and calcitonin system?

The overall goal is to maintain calcium homeostasis across bone, kidney, and gut. PTH raises calcium when it is low, calcitonin lowers calcium when it is high, and vitamin D activation augments calcium absorption in the gut. Together they create a balanced endocrine system to keep calcium within a healthy range.

Summary & Key Takeaways

• PTH is released in response to hypocalcemia and acts to raise blood calcium through multiple organs and processes. Calcitonin is released in response to hypercalcemia and acts to lower blood calcium by reducing bone resorption. The balance between these hormones maintains calcium levels within a narrow range.

• Bone, kidney, and intestinal actions of PTH and calcitonin are coordinated through feedback mechanisms involving calcium levels and vitamin D activation to sustain calcium homeostasis.

• Understanding the parathyroid and parafollicular C cells clarifies how the body detects calcium changes and translates them into hormonal responses that regulate bone metabolism and calcium absorption.