The kidney is a little bean-shaped organ around the size of a fist. In a healthy kidney, half a cup of blood is filtered in about a minute. Filtration is the first step in urine formation, which occurs in the glomerular of the kidney. In this process, the excess fluid and wastes are filtered out of the blood into the collecting tubules of the kidney. In connection to this, there are four basic steps for the formation of urine.
The first step of urine formation is reabsorption, where excess water and solutes from the tubule are absorbed back into the plasma. In the proximal tubule, reabsorption is not regulated by the hormones, and therefore, there is massive reabsorption of more than 70 % of the total filtrate. In addition, active transportation of essential solutes such as amino acids, glucose, and bicarbonate occurs in the proximal tubule leading to lower quantities of the residual fluid. However, in the loop of Henle, reabsorption of large quantities of sodium takes place.
In the second stage, hormone-regulated reabsorption of sodium and water occurs in the collecting tubule and the collecting duct, depending on the systematic condition. However, secretion is the third stage in which tubules continually produce extra chemicals into the tubular fluid even after filtration. Thus, the ability of the kidney to eliminate wastes and poisons is enhanced, as well as keeping plasma potassium levels and pH at the required levels. Finally, excretion occurs in urine, which is termed the final product of the above three processes. The concentration of the plasma may be close to that of the tubule fluid substances. Nevertheless, subsequent reabsorption and secretion can alter the highest concentration in the urine.
When diuretic medication is ingested, the kidneys automatically sense that they need to excrete more sodium. The flow of blood through the blood vessels is slackened due to the reduced blood volume, which aids in lowering blood pressure. Thus, the part of the kidney which is affected is glomerular by lowering the filtration rate, and as a result, sodium is eliminated while the blood pressure is lowered.
