Physiology
Endocrine
Regarding vitamin D, which of the following statements is CORRECT:
Answer:
Vitamin D3 can be manufactured from cholesterol within the body, via a reaction that is enabled by ultraviolet irradiation of the skin (therefore a lack of sun exposure can result in vitamin D deficiency). A lack of vitamin D in children leads to inadequate calcification of bones, which become malformed (rickets). Vitamin D deficiency in adults leads to bone wasting resulting in soft malleable bone (osteomalacia). Vitamin D activation is stimulated by parathyroid hormone not calcitonin. Activated vitamin D acts to enhance activity of PTH to promote net bone resorption with calcium and phosphate release. Activated vitamin D acts to increase calcium and phosphate absorption in the small intestine, and increase calcium and phosphate reabsorption in the kidneys.Vitamin D
Physiology / Endocrine / Calcium and Bone
Last Updated: 21st April 2019
Vitamin D2 (ergocalciferol), derived from plants and yeasts and Vitamin D3 (cholecalciferol), derived from animal (particularly dairy) products, are obtained in the diet and absorbed in the small intestine. Vitamin D3 can also be manufactured from cholesterol within the body, via a reaction that is enabled by ultraviolet irradiation of the skin.
Production of Activated Vitamin D
The D vitamins are converted to calcifediol (25-hydroxycholecalciferol) by vitamin D 25-hydroxylase in the liver.
This is further converted to activated vitamin D (calcitriol or 1, 25 - dihydroxycholecalciferol) by 1-alpha-hydroxylase in the cells of the renal proximal tubule. This final reaction is the slowest step in the process and therefore regulates the speed of the entire chain of reactions.
The enzyme 1-alpha-hydroxylase is stimulated by parathyroid hormone (PTH) and thus this final step is regulated by PTH.
Activation of Vitamin D. (Image modified by FRCEM Success. Original by SportEx Medicine via www.flickr.com)
Effects of Activated Vitamin D
Activated vitamin D acts to:
- GUT:
- increase calcium and phosphate absorption in the small intestine (the main action)
- KIDNEYS:
- increase renal calcium reabsorption (in the distal tubule via activation of a basolateral Ca2+ ATPase pump)
- increase renal phosphate reabsorption
- inhibit 1-alpha-hydroxylase activity in the kidneys (negative feedback)
- PARATHYROID GLANDS:
- inhibit PTH secretion from the parathyroid glands
Effects of Activated Vitamin D. (Image by SportEx Medicine via www.flickr.com)
Report A Problem
Is there something wrong with this question? Let us know and we’ll fix it as soon as possible.
Loading Form...
- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
