Phosphate And Calcium Homeostasis

Question 1

Why is it important the calcium concentration remains in homeostasis?

Answer 1

Because calcium is a major component of structures such as teeth and connective tissue, it plays a central role in blood clotting and is important in the excitation-contraction coupling mechanism of muscle movements

Question 2

What are the main organs responsible for calcium homeostasis?

Answer 2

Small intestine
Kidney
Bone

Question 3

Where does the body’s supply of calcium come from?

Answer 3

The diet - particularly dairy products

Question 4

Approximately 99% of the body’s calcium is stored in bone. T/F?

Answer 4

True

Question 5

In what form do calcium and phosphate exist within bone?

Answer 5

Hydroxyapatite crystals

Question 6

PTH hormone controls calcium homeostasis. Where does it come from and what is its secretion governed by?

Answer 6

PTH is secreted by chief cells in the four parathyroid glands which exist on the posterior aspect of the thyroid gland in the neck. These glands have calcium receptors and will release more PTH in response to low serum calcium concentrations.

Question 7

What is the name of the active form of vitamin D which is important in calcium an phosphate homeostasis?

Answer 7

1,25-dihydroxyvitamin D

Question 8

What are the ways in which calcium can be absorbed into the small intestine and how are these controlled?

Answer 8

The most efficient way is by active transport and this is controlled by 1,25-dihydroxyvitamin D.
Calcium can also be absorbed by passive diffusion and this is determined by intracellular calcium concentration (i.e. the calcium concentration gradient)

Question 9

How does PTH affect calcium homeostasis?

Answer 9

PTH increases the reabsorption of calcium in the loop of Henle and distal convoluted tubules of the kidney.
PTH promotes the resorption of bone which causes calcium to move from the bone to the extracellular fluid

Question 10

What affect does PTH have on phosphate homeostasis?

Answer 10

PTH promoted bone resorption so this releases phosphate from bone to the extracellular fluid.
PTH inhibits the reabsorption of phosphate in the kidneys and so promotes its excretion from the body in urine

Question 11

How does PTH affect vitamin D synthesis?

Answer 11

In the final stage of making active vitamin D in the kidneys. PTH promotes the conversion of 25-hydroxyvitamin D to 1,25-dinydroxyvitamin D by 1-hydroxylase.

Question 12

Where does the body get vitamin D from?

Answer 12

A very small amount of vitamin D comes from plant matter in the diet. Most vitamin D is formed in the body using energy from UV radiation from su light exposure.regardless of source, it must be converted to its active form in the body.

Question 13

How does vitamin D influence calcium homeostasis?

Answer 13

Vitamin D promotes calcium absorption in the small intestines and also promotes bone resorption

Question 14

How does vitamin D affect phosphate homeostasis?

Answer 14

Vitamin D promotes the absorption of phosphate ions in the small intestine and also promotes bone resorption

Question 15

How is the peptide hormone calcitonin involved in calcium and phosphate homeostasis?

Answer 15

It decreases calcium and phosphate serum concentration by inhibiting osteoclasts to reduce bone resorption.
However it does not have a profound effect in humans

Question 16

Describe the synthesis of active vitamin D by the body.

Answer 16

7-dehydrocholesterol is acted upon by Uv radiation from sunlight to produce vitamin D3. This is converted to its active form by two rounds of hydroxylation - in the liver 25-hydroxylase converts vitamin D3 to 25-hydroxyviamin D and in the kidney 25-hydroxyvitamin D is converted to 1,25-dinysroxyvitamin D by 1-hydroxylase

Question 17

Around 47% of serum calcium exists freely, a further 47% of serum calcium exists bound to which protein?

Answer 17

Albumin

Question 18

What are the normal levels of calcium in serum?

Answer 18

2.20 and 2.60mmol/L

Question 19

Phosphate levels change throughout the day due to what hormone?

Answer 19

Insulin

Question 20

How much sunlight exposure is recommended for adequate vitamin D synthesis?

Answer 20

25 mins 2-3 times a week for lighter skins amd up to 120 mins 2-3 times a week for darker skins

Question 21

Where is calcitonin secreted?

Answer 21

Calcitonin is secreted by parafollicular cells in the thyroid glands

Question 22

How does calcitonin affect calcium levels?

Answer 22

It decreases calcium ion concentration in the plasma by osteoclast inhibition

Question 23

Some malignant tumours release PTH related peptide. What effect does this have on PTH and calcium levels?

Answer 23

Gives hypercalcaemia with suppressed PTH secretion

Question 24

What effects will either an inappropriate dose of vitamin D or hyperparathyroidism have on calcium levels?

Answer 24

Hypercalcaemia

Question 25

Hypocalcaemia could be due to a vitamin D deficiency. What are the possible causes of this?

Answer 25

Malabsorption
Lack of exposure to sunlight
Renal disease leading to failure to make active vitamin D

Question 26

Hypoparathyroidism can cause hypocalcaemia. How can hypoparathyroidism be caused?

Answer 26

Idiopathic
Mistake in neck surgery
Magnesium deficiency