The kidney is built up of one million small kidney units known as nephron. Nephrons are involved in the process of blood cleansing and urine formation through the three step process of filtration, reabsorption and secretion.
Filtration: when blood arrives into the kidneys via the renal artery it firstly travels through a network of capillaries known as the glomerulus where the high pressure experienced forces the blood into the Bowmans capsule. In the Bowmans capsule filtration allows small molecules to continue through and larger molecules (proteins) to remain behind. Some of the small molecules are waste such as urea and some are valuable and reusable such as amino acids and glucose. After the small and large molecules are separated the small molecules are forced again by high pressure up the proximal tube where the solution is adjusted until only the unwanted substances are left, the remains are the urine.
Reabsorption: all essential substances the body requires are reabsorbed into the body/blood through active transport. These nutrients include vitamins, minerals, amino acids, glucose, bicarbonate ions and hormones.
Secretion: The job of the loop of Henle is to make the tissue fluid in the medulla hypertonic compared to the filtrate in the nephron. The purpose of this “salt bath” is to reabsorb water. The loop of Henle does this by pumping sodium and chloride ions out of the filtrate into the tissue fluid. The first part of the loop (the descending limb) is impermeable to ions, but some water leaves by osmosis. This makes the filtrate more concentrated as it descends.
The second part of the loop (the ascending limb) salt passes out passively across a thin-walled section and then actively across a thick-walled portion. The thick-walled portion contains a Na+ and a Cl− pump, so these ions are actively transported out of the filtrate into the surrounding tissue fluid. Water would follow by osmosis, but it can’t, because the ascending limb is impermeable to water. So the tissue fluid becomes more salty (hypertonic) and the filtrate becomes less salty (hypotonic). Since the filtrate is most concentrated at the base of the loop, the tissue fluid is also more concentrated at the base of the medulla, where it is three times more concentrated than seawater.
In the distal convoluted tubule certain substances are actively transported from the blood into the filtrate, in other words they are secreted. It is relatively short and has a brush border (i.e. microvilli) with numerous membrane pumps for active transport. The important point about this secretion is that it is regulated by hormones, so this is the homeostatic part of the kidney. Substances secreted include H+ (for pH homeostasis), K+ (for salt homeostasis), ethanol, toxins, drugs and other “foreign” substances.
As the collecting duct passes through the hypertonic…