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Medicine 2.1 > Calcium and Phosphate Metabolism and Homeostasis > Flashcards

Calcium and Phosphate Metabolism and Homeostasis Flashcards

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1
Q

How is calcium distributed in the body?

A
  • 99% of total Ca is in bones

- Rest is intracellular and in extracellular fluid (eg. blood)

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2
Q

What is the total calcium in the body?

A

Around 25000 mmol

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3
Q

What are the biological roles of calcium?

A
  • Muscle contraction
  • Nerve excitation
  • Blood coagulation
  • Intracellular messenger
  • Enzymes of intermediary metabolism
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4
Q

How is calcium distributed in the blood ?

A
  • Around 47% free (ionised/bound)
  • Around 47% bound to albumin
  • Around 6% complexed
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5
Q

In blood what calcium distribution area is important physiologically and why?

A

-Free calcium is important as it regulates feedback mechanisms

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6
Q

What are the main organs involved in calcium homeostasis?

A
  • Kidney
  • Gut
  • Bone
  • Parathyroid glands
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7
Q

What are the main homones involved in metabolic control of calcium?

A
  • PTH

- 1.25 DHCC/ calcitriol/ active vit D

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8
Q

What is our dietary intake of Ca and how much of this is absorbed and where?

A
  • Around 25mmol/day
  • Around 6mmol/day absorbed
  • In duodenum and jejenum mainly
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9
Q

How much Ca does our kidneys filter and reabsorb and how much is lost in urine?

A
  • Filters around 240mmol/day
  • Around 234mmol/day reabsorbed.
  • Urine around 6mmol/day
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10
Q

How much Ca does the skeleton release and how?

A
  • Around 8 mmol/day, a similar amount is also laid back down in bone
  • Through resorption
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11
Q

Which of the major hormones control absorption of Ca?

A

-1.25 DHCC

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12
Q

Which of the major hormones control reabsorption of Ca?

A

-PTH

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13
Q

Which of the major hormones control ressorption of Ca?

A
  • 1.25 DHCC

- PTH

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14
Q

What measurements can we take for calcium in blood?

A

We can either measure free calcium of total calcium (bound and free).

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15
Q

If a patient has low albumin Ca conc then what implication does this have on results?

A

Total Ca will also be low but free Ca may still appear quite normal

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16
Q

If albumin conc is low can we still calculate what serum total would have been if it was normal and if so how?

A
  • Yes

- Ca (adj) = Ca (tot) + [0.02(45-alb)]

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17
Q

What is normal serum calcium usually and what do values above and below this indicate?

A
  • 2.2 - 2.6 mmol/L
  • Below is hypocalcaemia
  • Above is hypercalcaemia
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18
Q

Describe the pathways for calcium absorption?

Study These Flashcards
A
  • Cell mediated active transport controlled bhy 1.25DHCC.

- Passive diffusion which depends on luminal conc. of Ca and is unaffected by 1.25DHCC.

19
Q

How does 1.25 DHCC influence absorption?

Study These Flashcards
A

-It increases fractional absorption if:
Dietary intake falls
During growth, pregnancy and lactation

20
Q

What proportion of Ca is usually absorbed?

21
Q

How and where in the kidneys is Ca reabsorbed?

Study These Flashcards
A
  • 65% in PCT (coupled to bulk transport of molecules such as Na and water)
  • 20% in ascending loop of henle, this can be increased by effects of PTH
  • 15% in PCT, can be increased by effects of PTH.
22
Q

Where is PTH produced?

Study These Flashcards
A

-Parathyroid glands

23
Q

What are the actions of PTH?

Study These Flashcards
A
  • Promotes bone resorption
  • Stimulates renal tubular Ca reabsorption
  • Stimulates formation of 1.25DHCC in kidney, to enhance Ca absorption in gut.
24
Q

What regulates secretion of PTH?

Study These Flashcards
A

-Free Ca sensed by Ca sensing receptors

25
What type of receptors are Ca sensing receptors?
-G-protein coupled
26
Where can Ca sensing receptors be found?
- Parathyroid cells | - Renal tubule
27
What do Ca sensing receptors in parathyroid cells do?
Mediates effect of EC ionised Ca on PTH release.
28
What do Ca sensing receptors in renal tubule do?
Mediates effect of high peritubular ionised Ca to inhibit Ca reabsorption
29
Describe the action of PTH if there is a decrease in ionised Ca?
- Stimulates Ca reabsorption in renal tubule - Stimulates formation of 1.25DHCC in kidney to enhance Ca absorption from gut and bone resorption. - Promotes bone resorption
30
What are the possible causes for hypocalcaemia?
-PTH problem (not enough); maybe due to neck surgery, Mg deficiency, hypoparathyroidism -Vit D problem; deficiency (malabsorption, little sunlight exposure) Renal disease (kidneys fail to make active vit D)
31
What are the possible causes for hypercalcaemia?
-PTH problem (too much); maybe due to hyperparathyroidism (adenoma of parathyroid gland) This would cause both PTH and Ca levels to increase. -Malignancy; lung cancer, breast cancer, multiple myeloma (effect of PTH related peptide) This would increase Ca but PTH would be supressed. -Vit D problem; innapropriate dosage
32
What is phosphate important for?
- Skeletal development, bone mineralisation | - Composition of cell membranes, nucleotide structure, cell signalling
33
How is phosphate distributed in cells?
- 85% is mineralised matrix of bone - Rest mainly intracellular , bound to lipids and proteins eg. ATP, cell membranes - 1% in EC fluids
34
Does serum PO4 represent true PO4 stores?
No because phosphate is capable of shifing in and out of cells. From the organic and inorganic pool.
35
What factors impact the value of serum phosphate?
- It is nadir before noon and peaks after midnight - Dietary effects (rises post prandially then falls due to effect of insulin rise) - Age related changes (highest values in infancy when growth velocity high.
36
What are the main organs invilved in Phosphate homeostasis?
- Kidney - Gut - Bone
37
What are main hormones involved in phosphate metaboic control?
- PTH - 1.25 DHCC - FGF 23
38
What is our dietary intake of phosphate and how much of this is absorbed and where?
- Around 45mmol/day of which 32mmol/day is absorbed - However 7mmol/day of this is returned to gut - Less rigidly regulated than Ca absorption, occurs in whole small intestine
39
How much phosphate does our kidneys filter and reabsorb and how much is lost in urine?
- 15-20% phosphate is protein bound, rest filtered by kidney (thats not in bone) - Around 160mmol/day of which 135mmol/day is reabsorbed. - Urine about 25mmol/day
40
How much phosphate is released from resorption?
-Around 7mmol/day, similar amount also laid back down.
41
Where exactly in the kidney does phophate reabsorption occur?
- 75% in PCT | - 5-20% in DCT
42
What do PTH and FGF 23 do in phospahte reabsorption?
Inhibit reabsorption of phosphate by renal tubule.
43
What can disorders of Ca sensing receptors cause?
-Abnormalities in Ca serum concentrations.
44
How can PTH secretion be reduced clinically?
Activators for Ca sensing receptors given for patients with parathyroid cancer or hyperparathyroidism for example.

Medicine 2.1 (29 decks)

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