Diuresis

Diuresis (/ˌdaɪəˈriːsɪs/) is increased urination and the physiologic process that produces such an increase. It involves extra urine production in the kidneys as part of the body's homeostatic maintenance of fluid balance. Diuresis (/ˌdaɪəˈriːsɪs/) is increased urination and the physiologic process that produces such an increase. It involves extra urine production in the kidneys as part of the body's homeostatic maintenance of fluid balance. In healthy people, the drinking of extra water produces mild diuresis to maintain the body water balance. Many people with health problems such as heart failure and kidney failure need diuretic medications to help their kidneys deal with the fluid overload of edema. These drugs help the body rid itself of extra water via the extra urine. The concentrations of electrolytes in the blood are closely linked to fluid balance, so any action or problem involving fluid intake or output (such as polydipsia, polyuria, diarrhea, heat exhaustion, starting or changing doses of diuretics, and others) can require management of electrolytes, whether through self-care in mild cases or with help from health professionals in moderate or severe cases. Sometimes a connotative difference is felt between diuresis in the sense of appropriate increase (as in successful diuretic therapy that is controlling symptoms well) and polyuria in the sense of inappropriate increase, that is, excess (as in failed oral antihyperglycemic therapy that must be stepped up to achieve control). However, sometimes the words are simply synonymous. Osmotic diuresis is the increase of urination rate caused by the presence of certain substances in the small tubes of the kidneys. The excretion occurs when substances such as glucose enter the kidney tubules and cannot be reabsorbed (due to a pathological state or the normal nature of the substance). The substances cause an increase in the osmotic pressure within the tubule, causing retention of water within the lumen, and thus reduces the reabsorption of water, increasing urine output (i.e. diuresis). The same effect can be seen in therapeutics such as mannitol, which is used to increase urine output and decrease extracellular fluid volume. Substances in the circulation can also increase the amount of circulating fluid by increasing the osmolarity of the blood. This has the effect of pulling water from the interstitial space, making more water available in the blood and causing the kidney to compensate by removing it as urine. In hypotension, often colloids are used intravenously to increase circulating volume in themselves, but as they exert a certain amount of osmotic pressure, water is therefore also moved, further increasing circulating volume. As blood pressure increases, the kidney removes the excess fluid as urine. Sodium, chloride and potassium are excreted in osmotic diuresis, originating from diabetes mellitus (DM). Osmotic diuresis results in dehydration from polyuria and the classic polydipsia (excessive thirst) associated with DM. Forced diuresis (increased urine formation by diuretics and fluid) may enhance the excretion of certain drugs in urine and is used to treat drug overdose or poisoning of these drugs and hemorrhagic cystitis. Most diuretic drugs are either weak acids or weak bases. When urine is made alkaline, elimination of acidic drugs in the urine is increased. The converse applies for alkaline drugs. This method is only of therapeutic significance where the drug is excreted in active form in urine and where the pH of urine can be adjusted to levels above or below the pK value of the active form of drug. For acidic drugs, urine pH should be above the pK value of that drug, and converse for the basic drugs. It is because the ionization of acidic drug is increased in alkaline urine and ionized drugs cannot easily cross a plasma membrane so cannot re-enter blood from kidney tubules. This method is ineffective for drugs that are strongly protein bound (e.g. tricyclic antidepressants) or which have a large apparent volume of distribution (e.g. paracetamol, tricyclic antidepressants). For forced alkaline diuresis, sodium bicarbonate is added to the infusion fluid to make blood and, in turn, urine alkaline. Potassium replacement becomes of utmost importance in this setting because potassium is usually lost in urine. If blood levels of potassium are depleted below normal levels, then hypokalemia occurs, which promotes bicarbonate ion retention and prevents bicarbonate excretion, thus interfering with alkalinization of the urine. Forced alkaline diuresis has been used to increase the excretion of acidic drugs like salicylates and phenobarbitone, and is recommended for rhabdomyolysis. For forced acid diuresis, ascorbic acid (vitamin C) is sometimes used. Ammonium chloride has also been used for forced acid diuresis but it is a toxic compound. Usually however, this technique only produces a slight increase in the renal clearance of the drug. Forced acid diuresis is rarely done in practice, but can be used to enhance the elimination of cocaine, amphetamine, quinine, quinidine, and strychnine when poisoning by these drugs has occurred.