Peritubular capillaries are located next to the nephrons, the main working parts of the kidney. These capillaries are very small blood vessels and are an important part of the urinary tract. As blood leaves the kidneys, minerals and ions are reabsorbed through the capillaries back into the blood. Reabsorption occurs by transcytosis, passive transport or active transport. Ions that are not needed are excreted through the capillaries and sent to the bladder.
Blood travels into the kidneys to be filtered, and then the filtered blood exits the kidneys through the peritubular capillaries. These capillaries completely surround the tubules, which contain all of the fluid and particles that have been removed from the blood. It is through the peritubular capillaries that the important nutrients needed by the body are reabsorbed back into the blood.
Active transport of minerals through the capillaries utilizes ions. The collection of ions across the capillary membranes creates a concentration gradient. Ions are then transported across the membrane, moving from an area of high concentration to an area of low concentration. An electochemical gradient occurs when high concentrations of ions build up in the tubule. The ions pass through the membrane using ion channels or ion pumps.
Transcytosis is necessary for macromolecules to be reabsorbed into the blood. A macromolecule is a very large molecule or compound that cannot easily pass through cell membranes. Transcytosis uses vesicles for molecule transport. The vesicle containing the macromolecule forms on one side of the cell. It travels across the cell and expels the macromolecule into the adjacent cell.
The third method of reabsorption is passive transport. Passive transport functions based on the permeability of the membrane and does not use chemical energy. The mineral concentration in the tubules is high, and the concentration in capillaries is low. Through passive transport, the minerals move into the peritubular capillaries. Transport stops when the mineral concentration in the capillaries is the same as the concentration in the tubules.
Specifically, water and mineral ions are reabsorbed through the peritubular capillaries. About 80 percent of all of the ions and water that the body needs is absorbed in this location. This mineral-rich blood then moves into the veins and travels throughout the body.
Any item that is not absorbed through the peritubular capillaries moves into the Henle loop. Water is removed from the excess minerals and nutrients, forming concentrated urine. The concentrated urine moves into the bladder and eventually out of the body.