A nephron is the filtration-reabsorption device in the kidney. Bowman's capsule is located at the front of the nephron. The glomerulus is a fine network of capillaries located within Bowman's capsule. Filtration occurs as blood pressure forces fluid through the capillary walls, retaining proteins, cells, and large molecules and allowing water and small molecules to pass through to the interior of the capsule. The kidney first evolved in freshwater fishes.
Cartilaginous fishes use urea to elevate the osmotic concentration of their tissues.
Mammals and birds produce urine hyperosmotic to their tissues. Most salt and water is absorbed across the membranes of the proximal convoluted tubule. The remaining filtrate is isosmotic to blood plasma. The descending arm of the Loop of Henle is most freely permeable to water. The base of the loop is freely permeable to salt, and the thick region of the ascending loop is where active transport of sodium ions occurs.