Amanda Clark, RN.
Deputy Editor (Professional Nurse)
Diuretic drugs act by turning excess water in the body into urine, which is then excreted. Renal disease, liver cirrhosis and oedema and congestion caused by heart failure are all indications of the presence of excess water in the body tissues (Hopkins, 1999). Diuretics inhibit the re-absorption of filtrate in the kidney tubules, leading to an increase in the volume of urine produced. There are several types of diuretics, as listed in Box 1, which act on the different parts of the tubules.
Anatomy and physiology
There are approximately one million nephrons in each kidney. Each nephron has a Bowman’s capsule consisting of two tubules connected by the loop of Henle and a knot of blood vessels that form the glomerulus. As blood passes through the glomerulus, blood cells, plasma proteins and lipids are absorbed and passed into the circulation, leaving a filtrate that consists of water, sodium, potassium salts and waste products. As the filtrate passes through the remainder of the nephron, most of the water and filtered electrolytes are reabsorbed back into the circulation. The remaining filtrate is excreted as urine. Diuretics work by blocking the transfer of sodium back into the bloodstream, leading to a reduction in the amount of water that can be reabsorbed. The reduced volume of water in the blood leads to excess water in the tissues being drawn into the bloodstream, where it is filtered by the kidneys and excreted as urine.
Indications for using diuretics
Diuretics are indicated when it is apparent that the kidneys’ ability to maintain electrolyte balance in the blood has been compromised. If there is a reduced blood supply to the kidneys caused by, for example, heart failure or hypotension, the glomerular filtration rate slows down, although the tubular re-absorption rate is unaffected. This leads to the accumulation of excess water and electrolytes in the body tissues. The main conditions for which diuretics may be needed include:
- Heart failure
- Nephrotic syndrome
- Cirrhosis of the liver
- Pre-menstrual syndrome.
Thiazides act on the distal tubules of the nephron, where they cause a reduction in the reabsorption of sodium and chloride. This class of diuretics alters the balance of potassium and sodium, resulting in a reduction in the amount of potassium reabsorbed into the circulation. The increase in potassium excreted may cause hypokalaemia, which requires the administration of a potassium supplement, such as Slow K or Sando-K. This type of diuretic can also be given in conjunction with a potassium-sparing diuretic.
Loop diuretics are the most powerful types of diuretic used. They are so called because they act directly on the thick section of the loop of Henle, where they prevent the movement of sodium chloride back into the circulation. This leads to a marked increase in the amount of urine excreted over a few hours. Because they are fast acting, loop diuretics are often used in emergencies, such as pulmonary oedema. As with thiazides, loop diuretics may cause an excessive loss of potassium, which will need to be addressed with the use of supplements.
Potassium-sparing diuretics work in a selective way by reducing the amount of potassium lost by the distal tubules. They have only a mild action and are often used alongside a loop diuretic or a thiazide to prevent excessive potassium loss. The aldosterone antagonist spironolactone is a diuretic that inhibits the loss of potassium. This synthetic steroid blocks the aldosterone receptors involved in the re-absorption of sodium and the excretion of potassium, resulting in a potassium-sparing effect. This treatment may be given alongside another diuretic.
Osmotic diuretics act by increasing the osmotic pressure of the filtrate passing through the tubules, reducing the amount of water the tubules re-absorb. They are often used to maintain the flow of urine through the kidneys following surgery or trauma. They are also used to reduce pressure rapidly within fluid-filled cavities.
Hopkins, S.J. (1999) Drugs and Pharmacology for Nurses. London: Churchill Livingstone