S11 L1 part 1 - Parathyroid gland and calcium control

Question 1

• *Parathyroid gland**
• Name and role of the 3 main cells in this gland
• Histology
• *Source of calcium**
• Food sources
• Recommended amount

Answer 1

Parathyroid gland:
Name and role of the 3 main cells in this gland:
- Chief cells/Principal cells - Produce PTH
- Oxyphil cells - not sure what they do
- Adipose
Histology:
pic

Source of calcium:
Food sources:Dairy is the main source of dietary calcium.​
Recommended amount:
Recommended dietary calcium: 1000mg a day – if unused, it is excreted

Question 2

• *Calcium stores in the bones**
• Calcium is stored as what?
• Skeleton’s importance in calcium metabolism
• How does calcium enter this store? Exit this store?

Answer 2

Calcium is stored as what?
The majority of calcium found in adults is stored in bone as hydroxyapatite crystals (i.e Calcium and phosphate in this form). This means that we liberate phosphate when we liberate calcium ions.

Skeleton’s important role in calcium metabolism:
The skeleton is a major reserve** of calcium, and acts to **buffer serum calcium levels** through releasing **calcium phosphate into the interstitium when required and uptaking calcium phosphate when required (serum calcium levels are normally between 2.2-2.6mM).

How does calcium enter this store? Exit this store?
Bones: Ca²⁺ + P_i = hydroxyapatite crystals
- Bone deposition: Osteoblasts produce collagen matrix which is mineralised by hydroxyapatite
- Bone reabsorption: Osteoclasts produce acid micro-environment hydroxyapatite dissolves
– Bone is dynamic

Question 3

Forms of plasma calcium

Answer 3

In plasma, calcium exists in one of three forms;

• As a free ionised species (biologically active calcium)
• Associated with serum proteins at their anionic sites (particularly albumin)
• Complexed with low-molecular-weight organic anions

Question 4

Role of calcium ions include…

Role of calcium ions in blood clotting…

Answer 4

Role of calcium ions include:
Assists in blood clotting, regulates heart rhythm, nerve transmission at NMJ, intracellular signalling pathways, nerve function, passage of nutrients in and out of cell walls, builds and maintains bones and teeth etc…

Role of calcium ions in blood clotting: (refer to Path Pro)
Calcium ions are Factor IV in the clotting cascade
- EDTA: Chelates calcium, by ‘gathering the calcium up’ and making it biologically inactive, this stops the blood clotting (EDTA is put in tubes with blood in e.g. tubes in blood tests)
- Citrate: Works in the same way as EDTA, citrate chelates calcium ions. If giving someone a large blood transfusion, will also need to give them intravenous calcium, otherwise this transfused blood will have trouble clotting!

Question 5

• *3 Hormones involved in calcium regulation**
• Names
• Brief role

Answer 5

Three hormones are involved in the regulation of calcium:

• Parathyroid hormone: Role is to increase serum calcium levels acutely
• Calcitriol: Role is to increase serum calcium levels over a longer period of time
• Calcitonin: Role is to reduce serum calcium levels

Question 6

• *1. Parathyroid hormone (PTH)**
• Role
• Produced by…
• Type of hormone
• Synthesis regulation
• Half-life
• Storage
• 3 major ways does PTH increase serum calcium levels

Answer 6

Question 7

How does the PTH help to stop formation of calcium phosphate crystals in the kidney or urine?

Answer 7

PTH signals to kidneys to increase the amount of calcium they reabsorb, they also signal to the kidney’s to increase the amount of phosphate they excrete.
This is a PREVENTATIVE measure to prevent both elevated Ca²⁺ ions and elevated phosphorous occurring at the same time – this reduces the likelihood of calcium phosphate crystals from forming

Question 8

• *2. Calcitriol**
• Role
• How is it synthesised?

Answer 8

Question 9

Where does vitamin D3 come from?

Answer 9

Food sources with vitamin D3: Cheese, butter, margarine, fortified milk, fish, fortified cereals
Body can make vitamin D3 when it is exposed to the sun

Question 10

• *3. Calcitonin**
• Where is it found?
• Role

Answer 10

Where is it found:
Calcitoninis found in theparafollicular cells (also called C cells)of thethyroid gland
Role:
To oppose PTH in order to lower calcium levels when serum calcium levels are increased.
There is also some suggestion that, during pregnancy, calcitonin may help preserve the maternal skeleton.