Physiology
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This was previously featured in an exam
Endocrine
Question 33 of 180
You are reviewing a patient with hypocalcaemia secondary to hypoparathyroidism. Parathyroid hormone (PTH) acts to increased calcium reabsorption at which of the following sites in the nephron:
Answer:
Parathyroid hormone (PTH) acts to increase calcium reabsorption in the distal tubule of the nephron (by activating Ca2+ entry channels in the apical membrane and the Ca2+ ATPase pump in the basolateral membrane) and increase phosphate excretion by inhibiting reabsorption in the proximal tubule of the nephron.Parathyroid Hormone
Physiology / Endocrine / Calcium and Bone
Last Updated: 26th July 2024
Parathyroid hormone (PTH) is a peptide hormone synthesised by the chief cells of the parathyroid glands, located immediately behind the thyroid gland.
Release of PTH
PTH is released in response to:
- decreasing plasma [Ca2+] concentration
- increasing blood [PO43-] concentration (indirectly by its binding to ionised calcium and thereby effective reduction of plasma calcium levels).
PTH release is thus inhibited by normal blood calcium levels and also by hypomagnesaemia.
Effects of PTH
PTH acts to:
- BONE:
- increase calcium and phosphate resorption from bone (via immediate stimulation of osteocytic osteolysis and later, by upregulation of osteoclast activity)
- KIDNEYS:
- increase calcium reabsorption in the distal tubule of the nephron (by activating Ca2+ entry channels in the apical membrane and the Ca2+ ATPase pump in the basolateral membrane)
- increase phosphate excretion by inhibiting reabsorption in the proximal tubule of the nephron
- inhibit renal bicarbonate reabsorption stimulating a metabolic acidosis which favours dissociation of calcium from plasma proteins
- stimulate 1-alpha-hydroxylase in the kidneys to produce more activated vitamin D
- GUT:
- indirectly increase calcium and phosphate absorption in the small intestine (via activated vitamin D)
Overall PTH acts to increase plasma calcium levels and decrease plasma phosphate levels.
Calcium and Bone Physiology. (Image by OpenStax College [CC BY 3.0 , via Wikimedia Commons)
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- 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 |
