Chief Cells and Parathormone
Chief cells produce and secrete parathormone.
Parahormone (PTH) is a polypeptide.
Functions of Parathormone (PTH)
Parathormone (PTH), , is the key hormone regulating calcium and phosphate balance. Its actions can be grouped into effects o
n bone, kidney, and intestine.
1. Increases Serum Calcium (Main Function)
PTH raises blood calcium by acting on three organs:
A. Bone – Mobilizes Calcium
Stimulates osteoclast activity indirectly (via osteoblasts).
Leads to bone resorption, releasing calcium and phosphate into the bloodstream.
2. Decreases Serum Phosphate
Even though bone resorption releases phosphate, PTH lowers phosphate because it:
B. Kidney – Promotes Phosphate Excretion
Inhibits proximal tubular phosphate reabsorption → phosphaturia.
Net effect: ↓ serum phosphate.
3. Increases Renal Calcium Reabsorption
Increases calcium reabsorption in the distal convoluted tubule.
Result: ↑ serum calcium.
4. Activates Vitamin D (Calcitriol)
PTH stimulates the kidney to convert 25-OH vitamin D to 1,25-(OH)₂ vitamin D, the active form.
Enhances intestinal calcium absorption.
Enhances intestinal phosphate absorption.
5. Stimulates Urinary cAMP
PTH increases cAMP in urine—a classic marker of PTH activity.
Summary Table
Target Organ PTH Effect Bone ↑ Osteoclast activity → ↑ Ca²⁺ & PO₄³⁻ release Kidney ↑ Ca²⁺ reabsorption (DCT) ↓ PO₄³⁻ reabsorption (proximal tubule) ↑ 1,25-(OH)₂ vitamin D activation Intestine Indirect ↑ Ca²⁺ and ↑ PO₄³⁻ absorption via activated vitamin D
Net Effects of PTH
↑ Serum Calcium
↓ Serum Phosphate
↑ Vitamin D Activation
If you want, I can also provide:
A Mermaid diagram of PTH actions
A clinical correlation, such as how PTH explains the lab pattern in hyperparathyroidism
A Dreamweaver-ready embed block summarizing the functions
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<!-- Mermaid.js Embed for Dreamweaver --> <!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8" /> <title>PTH Functions Diagram</title> <!-- Load Mermaid --> <script src="https://cdn.jsdelivr.net/npm/mermaid/dist/mermaid.min.js"></script> <script> mermaid.initialize({ startOnLoad: true }); </script> <style> body { font-family: Arial, sans-serif; margin: 20px; } .mermaid { background: #f8f8f8; padding: 20px; border-radius: 10px; } </style> </head> <body> <h2>Parathormone (PTH) – Functions Flowchart</h2> <div class="mermaid"> flowchart TD A[PTH Secretion<br>from Parathyroid Glands] --> B[Bone] A --> C[Kidney] A --> D[Intestine (Indirect via Vitamin D)] %% Bone B --> B1[Stimulates Osteoblasts → Activates Osteoclasts] B1 --> B2[Bone Resorption] B2 --> B3[↑ Serum Calcium] B2 --> B4[↑ Serum Phosphate] %% Kidney C --> C1[↑ Calcium Reabsorption (Distal Tubule)] C --> C2[↓ Phosphate Reabsorption (Proximal Tubule)] C --> C3[↑ 1,25-(OH)₂ Vitamin D Activation] C1 --> C1a[↑ Serum Calcium] C2 --> C2a[↓ Serum Phosphate] C3 --> C3a[↑ Ca²⁺ & PO₄³⁻ Absorption in Gut] %% Intestine D --> D1[↑ Calcium Absorption] D --> D2[↑ Phosphate Absorption] %% Net Effects B3 --> E[Net: ↑ Serum Calcium] C1a --> E D1 --> E B4 --> F[Net: ↓ Serum Phosphate] C2a --> F </div> </body> </html>
PTH is a potent regulator of plasma-ionized calcium levels in response to low extracellular calcium levels detected by receptors in the parathyroid cell membrane.
PTH increases the production of 1,25-dihydroxycholecalciferol.
1,25-dihydroxycholecalciferol is the active form of vitamin D, normally made in the kidney.
It is a hormone which binds to and activates the vitamin D receptor in the nucleus of the cell.
Vitamin D (or Calcitriol) increases blood calcium (Ca2+) mainly by increasing the uptake of calcium from the intestines.
PTH stimulates bone resorption, resulting in increased serum calcium (and consequently filtered calcium),
PTH promotes Phosphatic (Pi) excretion resulting in low serum inorganic phosphate (Pi)
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[The urinary calcium excretion likely reflects a reabsorption process that has been overwhelmed by the excess calcium filtered. The increase in bone resorption and turnover would be expected to be associated with increased serum alkaline phosphatase.]
- PTH increases renal tubular calcium reabsorption,
- PTH ncreases inorganic phosphate excretion from the kidney.
- PTH stimulates the hydroxylation of 25-hydroxyvitamin D at the 1-position, leading to the formation of the active form of vitamin D (calcitriol).
- vitamin D increses intestinal calcium absorption.
Inhibits renal phosphate reabsorption in proximal tubule, resulting in phosphate excretion.
Three factors that increase 1 α-hydroxylase activity: Increased PTH, decreased serum calcium and phosphate levels.
In the presence of low PTH levels, these effects are reversed, resulting in a lowering of plasma calcium levels.