5.8 Calcium and Phosphate Homeostasis

Question 1

Give some physiological processes that calcium ions are important in

Answer 1

-muscle contraction
-bone/teeth health
-intracellular signal transduction
-blood clotting
-energy metabolism

Question 2

Give some physiological processes that phosphate ions are important in

Answer 2

-bone/teeth health
-forming nucleotides
-forming cell membranes
-acid base balance
-energy balance

Question 3

Why is free intracellular calcium concentration low

Answer 3

Most calcium in cells is kept in the endoplasmic reticulum and the mitochondria. The calcium is not free, it is bound to proteins (e.g calsequestrin). This calcium concentration is maintained by a calcium pump

Question 4

Where is calcium absorbed into the bloodstream

Answer 4

In the duodenum and jejunum of the small intestine

Question 5

What is the blood plasma calcium concentration

Answer 5

2.5 mM

Question 6

What are the 3 hormones that regulate plasma calcium concentration

Answer 6

-calcitonin
-calcitriol
-parathyroid hormone (PTH)

Question 7

Describe calcitonin

Answer 7

It is an enzyme secreted by the parafollicular C cells of the thyroid gland

Question 8

Describe the secretion of PTH

Answer 8

PTH is secreted by chief cells of the parathyroid gland. The secretion is regulated by the extracellular calcium concentration as the chief cells have calcium sensing receptors

Question 9

What is the result of PTH

Answer 9

To increase calcium concentration in the blood

Question 10

What does PTH result in on different types of cells

Answer 10

PTH targets GPCRs on different types of cells. In bone, PTH stimulates osteoclast activity to release calcium and phosphate ions from the bone. In the kidneys, PTH increases calcium ion reabsorption in the nephron. PTH also activates enzymes in the kidney that form vitamin D, this vitamin D then increases calcium absorption from food in the small intestines

Question 11

What is calcitriol and how is it formed

Answer 11

Calcitriol is a steroid hormone that is the biologically active form of vitamin D3. The vitamin D is first converted to calcidiol in the liver before the calcidiol is converted to calcitriol in the kidneys

Question 12

What are the actions of calcitriol and what stimulates the production and release of calcitriol

Answer 12

Calcitriol increases calcium and phosphate absorption in the small intestine. Calcitriol also regulates PTH levels providing negative feedback to the parathyroid gland. Calcitriol production and release is stimulated by hypocalcaemia (not enough calcium) and hypophosphataemia (not enough phosphate)

Question 13

How are plasma calcium levels detected in the kidney

Answer 13

There are calcium sensing receptors in the thick ascending loop of henle. In times of hypercalcaemia theu inhinit calcium reabsorption and in times of hypocalcaemia they stimulate calcium reabsorption.

Question 14

What is the difference between transcellular and paracellular pathways.

Answer 14

Transcellular pathways is when the substance moves through the interior of the epithelial cell, crossing both the apical and basal membrane. Paracellular pathways is when substances move between adjacent cells, bypassing the interior of the epithelial cells.

Question 15

Which enzyme has the greatest effect of urinary calcium excretion from the kidneys

Answer 15

PTH

Question 16

What are the 3 components of the collecting ductr

Answer 16

-cortical collecting duct
-outer medullary collecting duct
-inner medullary collecting duct

Question 17

Briefly describe the reabsorption of calcium in the kidneys

Answer 17

70% of the calcium is reabsorbed in the PCT, mostly via paracellular reabsorption. 20% of the calcium is reabsorbed in the thick ascending loop in the loop of henle, mostly via paracellular reabsorption. 9% is reabsorbed in the DCT, mostly via transcellular reabsorption

Question 18

What are the 4 hormones that regulate phosphate homeostasis

Answer 18

-calcitonin
-calcitriol
-PTH
-phosphatonin

Question 19

Briefly describe the reabsorption of phosphate in the kidneys

Answer 19

80% of the phosphate is reabsorbed in the PCT, 10% in the DCT and 10% in the inner medullary collecting duct. In the PCT phosphate is reabsorbed by 3Na/P symporters and then the phosphate is taken into the blood stream by an Na/K-ATPase

Question 20

Why must phosphate and calcium concentrations be strictly regulated

Answer 20

Hypercalcaemia can cause decreased neuromuscluar excitability resulting in conditions such as muscle weakness and cardiac arrhytmias. Hypocalcaemia can cause increased neuromuscular excitability resulting in conditions such as tremors and cramps. Hyperphophataemia and hypophosphataemia dont have direct effects, but will cause reciprocal changes in the calcium levels causing the previously described problems