Unit 4 - Diuretic therapy explained Flashcards
What are diuretics ?
Diuretics are commonly prescribed medicines used to treat oedema caused by
conditions such as heart failure, liver cirrhosis and kidney disease. Some diuretics
may be used to treat hypertension. However, adverse effects are common and it has
been estimated that diuretics contribute to around one in five of all medicinesrelated hospital admissions.
What is the anatomy and function of the nephron ?
Bowman’s capsule: water and electrolytes pass into the nephron from the
glomerulus.
Proximal convoluted tubule: reabsorbs two thirds of sodium and water, in
addition to bicarbonate and glucose.
Descending loop of Henle: passive reabsorption of water due to high
osmolarity in interstitium, resulting in concentration of tubular fluid.
Thick ascending loop of Henle: reabsorbs around 20% of sodium but is
impermeable to water.
Distal convoluted tubule: reabsorbs remaining sodium, water and
bicarbonate; makes bicarbonate; potassium secreted into the tubule.
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Collecting ducts: Fine adjustments of sodium and water; largely impermeable
to water without the presence of antiduretic hormone (ADH).
How do carbonic anhydrase inhibitors work ?
Acetazolamide is the only oral carbonic anhydrase inhibitor used in practice. It
decreases the amount of sodium that is reabsorbed from the proximal convoluted
tubule, increasing the volume of water excreted in the urine.
Carbonic anhydrase catalyses the formation of carbonic acid from water and carbon
dioxide in tubular cells. Carbonic acid then dissociates into bicarbonate ions (which
are reabsorbed into the blood) and hydrogen ions, which are exchanged for sodium
ions into the lumen of the tubule. Reversible inhibition of carbonic anhydrase results
in reduced sodium and hydrogen ion exchange and thus increases excretion of
sodium, bicarbonate and water
Why is acetazolamide known as a week diuretic ?
Acetazolamide is considered a weak diuretic because excess sodium delivered from
the proximal tubule can be reabsorbed further along the renal tubule.
What is acetazolamide usually used for ?
Acetazolamide is not commonly used as a diuretic, and is mainly prescribed for
glaucoma to reduce intraocular pressure by suppressing the secretion of bicarbonate
and water into the aqueous humour. It is also used unlicensed as prophylaxis for
altitude sickness.
Where is the main site of sodium reabsorption in the kidneys ?
The main site of sodium reabsorption is in the thick ascending limb of the loop of
Henle. Here, sodium ions are actively transported out of the tubule by the
membrane transport protein sodium-potassium-chloride (Na/K/2Cl) cotransporter.
