Explain the process of ultrafiltration in the kidney.

Ultrafiltration begins in the nephron in the kidney. Blood travels through a coiled structure of capillaries called the glomerulus surrounded by the Bowman's capsule. The hydrostatic pressure of the flow of bloods allows small molecules within the blood to pass through the capillary membrane and through the gaps in the podocytes surrounding the Bowman's capsule. These molecules include small ions such as sodium and potassium, glucose, water, urea and amino acids. Large molecules such as erythrocytes (red blood cells) and plasma proteins cannot cross the plasma membrane due to their size. The fluid in the capsule is now referred to as the FILTRATE. The filtrate then enters the proximal consulted tubule, where the majority of glucose and amino acids are selectively reabsorbed. The filtrate then passes to the loop of Henlé, which is responsible for the transport of salts and creating an osmotic gradient in the surrounding tissue to aid the absorption of water. Sodium ions and potassium ions are pumped out of the loop to decrease the water potential of the surrounding fluid, creating an osmotic gradient that drives water out of the fluid and into the tissue. The longer the length of the tubule, the more salt is pumped out and the more water is reabsorbed into the body. Water reenters the filtrate in the distal convoluted tubule and collecting ducts due to the negative osmotic gradient created by the remaining urea in the filtrate (urea decreases the water potential of the filtrate, so some water moves into the filtrate due to osmosis.) The volume of water absorbed in the collecting ducts is controlled by a hormone called anti-diuretic hormone (ADH) which increases the expression of aquaporins in the cell membrane of the collecting ducts causing more water to be reabsorbed. More ADH means more water is absorbed and less, concentrated urine is produced, less ADH means less water is absorbed and more, dilute urine is produced.