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Physiology

Endocrine

Question 29 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

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.

By OpenStax College [CC BY 3.0 , via Wikimedia Commons

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

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