Physiology of renal medicine Flashcards
How many litres do the kidneys filter in 24 hours in health?
180L
How much urine is produced per day?
1.5-2L
How is urine formed?
Capillary hydrostatic pressure forcing the water and salts of plasma across the glomerular basement membrane.
What molecules are freely filtered into the nephron?
<12,000 Da
-Electrolytes, sugars, AAs and vitamins
What factors affect the filtration of a molecule across the glomerular basement membrane?
Molecular weight, surface charge (GBM -ve charge, albumin is -ve charged), hydrostatic pressure in the afferent arteriole
What is the capillary hydrostatic pressure in the glomerulus?
45mmHg
What is the hydrostatic pressure in Bowman’s space?
10mmHg
What is the plasma protein oncotic pressure in the Bowmans capsule?
25mmHg
What is GFR and how is it accurately measured?
An exact measure of kidney function. Accurate measurement requires the injection of technetium 99. Only really performed on live kidney donors.
What is the alternative to GFR?
Creatinine clearance. Creatinine is released from muscle at a relatively constant rate. Filtered by the kidneys - some secretion into the filtrate by the proximal tubule, not as accurate, rarely used in clinical practice.
What is serum creatinine?
Released by muscle and removed by kidneys. Accumulates in kidney disease. Normal range: 64-104.
What is eGFR?
Requires: age, sex, ethnicity and serum creatinine. Correlates with the % of kidney function eg eGFR of 50=50% kidney function.
At what eGFR will patients need dialysis?
10
At what level is plasma osmolarity maintained?
285 mOsm/L
What is normal urine osmolarity?
50-1400 mOsm/L
How does ADH work?
Inserts channels into collecting duct to allow reabsorption of water.
In a state of acute fluid loss there is increased secretion of:
ADH and renin
An acutely ill patient with kidney disease is at higher risk of:
Hyperkalaemia, acidaemia, uraemia and fluid imbalance
What is the Na/K ATPase pump?
Powerhouse. Lies on the apical surface of the tubule. Generates a potential energy gradient across the cell. Allows the recovery of many substance from the glomerular filtrate.
What are the features of the proximal convoluted tubule?
Recovers 70% of glomerular filtrate - water and electrolytes
Recovery and generation of bicarb.
Low energy cost - 5% of total energy.
What are the features of bicarb regeneration?
Dependent upon carbonic anhydrase (inhibited by acetazolamide).
Failure to regenerate in kidney disease results in acidaemia.
What is taken up in the ascending loop of Henle?
Na, K, Cl and water.
What is the site of action of loop diuretics?
Ascending loop of Henle (eg furosemide)
What are the features of the cortical collecting duct?
Site of reabsorption of: Na+water in exchange for K. Controlled by aldosterone.
Site of action of aldosterone anatagonists - spironalactone.
What is spironalactone?
K-sparing diuretic. High risk of hyperkalaemia.
What are the features of the medullary collecting duct?
Site of urinary concentration.
ADH acts to increase water reabsorption.
What is diabetes insipidus?
Central failure to secrete ADH or peripheral resistance to ADH, Polyuria, hypovolaemia and hyponatraemia.
What is the site of action of osmotic diuretics?
Glomerulus.
What is the site of action of carbonic anhydrase inhibitors?
DCT and PCT - inhibits carbonic anhydrase.
What is the site of action of Thaizides?
DCT - sodium pump.
What is the site of action of loop diuretics?
Ascending loop of Henle.
What is the site of action of Spironalactone and amiloride?
Cortical collecting duct.
What are possible treatments for hyperkalaemia?
Calcium gluconate
Insulin/glucose
dialysis
