7.5 Formation of Urine Formation depends on three functions:   Formation depends on three functions: Filtration Reabsorption Secretion

FILTRATION Each nephron in the kidney has an independent blood supply. The glomerulus acts as a high pressure filter. Capillary pressure in the glomerulus is about 65 mm Hg where other capillaries have a pressure of about 25 mm Hg. Dissolved solutes pass through the walls of the glomerulus into the Bowman’s capsule. Not all materials pass through: Plasma Proteins Blood Cells Platelets

REABSORPTION Roughly 1mL of urine is formed for every 120mL that is filtered in the nephron. Remaining 119mL of solutes and fluid is reabsorbed. Reabsorption occurs via active and passive transport. Occurs until a threshold level of a substance is reached. The rest remains in the nephron and is excreted with urine. The solutes that are transported create an osmotic gradient that draws water from the nephron. As concentrations change, urea and uric acid will diffuse back into the nephron.

SECRETION Movement of wastes from blood into the nephron. Nitrogen-containing wastes Excess H+ ions K+ ions Some drugs

KIDNEY FUNCTION REVIEW   The nephron is a tiny filtering unit located in the kidneys of mammals between the cortex and the medulla. The nephron is the site at which the kidney rids the body of wastes such as urea and also regulation of water levels, pH, salt and ion concentrations. Blood first enters a tiny network of capillaries called the glomerulus that is surrounded by the Bowman’s capsule. The tiny, thin walls of the glomerulus allow wastes, salts, ions sugars and water to diffuse into the Bowman’s capsule to form a fluid called nephric filtrate. This nephric filtrate then enters the proximal tube where re-absorption of useful molecules such as glucose, amino acids and some water are recovered. This re-absorption is usually done with the help of active transport and some times diffusion.

KIDNEY FUNCTION REVIEW Nephric filtrate then enters the loop of Henle where more water is recovered by osmosis on the descending loop and salts are recovered from the ascending loop by active transport.   The distal tube adds more ions such as drugs, protons and creatinine to the nephric filtrate. Nephric filtrate then is collected from many nephrons by the collecting duct and transported to the renal pelvis where it can exit the kidney via the ureter on its way to the bladder. Urine should not contain sugars and amino acids in a healthy individual, as they should be re-absorbed.