Nephrons: How they work and their relation to counter currents or hydrostatic skeletons.

The nephron is the basic unit of the kidney that serves a role in filtering blood. It's structure is comprised of a long, thin tube that is closed at one end, has two twisted regions interspaced with a long hairpin loop, ends with a long straight portion, and is surrounded by capillaries. The parts of a nephron are broken down into the following: the Bowman's capsule, the proximal convoluted tubule or proximal tubule, the Loop of Henle, the distal convoluted tubule or distal tubule, and the collecting duct. Each have different types of cells with different properties, which is important in understanding how the kidney regulates the composition of blood. Along with these parts, the nephron includes a variety of arteries, capillaries, and veins that connect the parts of the nephron, giving it a unique blood supply compared to other organs.
In the nephron, about 20% of the blood is filtered under pressure through the cell walls of the glomerular capillaries and Bowman's capsule. Filtration is carried out at a rate of approximately 125 mL/min. or 45 gallons (180 L) each day. The amount of any substance that gets filtered is the product of the concentration of that substance in the blood and rate of filtration. Small molecules, such as ions, glucose, and amino acid, are reabsorbed from the filtrate inside the lumen of the nephron by specialized proteins called transporters. Any excess is goes through secretion, which means they will be excreted into urine and eliminated from the body.
The movement of fluid and substances through pressure that is involved in the work of nephrons is similar to the role of hydrostatic skeletons in many cold-blooded organisms. In a hydrostatic skeleton, the pressure of the fluid and action of the surrounding circular and longitudinal muscles are used to change an organism's shape and cause movement, like burrowing or swimming.