URINARY SYSTEM

The urinary system removes wastes from the blood and forms it

into urine. The urine produced usually contains excess water, ions, urea and other toxic substances. The urine created by the kidneys is moved through the ureters and is stored in the bladder until it is released through the urethra. The nephron is the functional unit of the kidney. It consists of a renal corpuscle, a proximal tubule, a loop of Henle, and a distal tubule. The renal corpuscle is made up of Bowman’s capsule and the glomerulus. The Bowman’s capsule surrounds the glomerulus, which are capillaries that are packed together to form a ball-like structure. The Bowman’s capsule consists of podocytes, an epithelial cell, that attaches to the outer surface of the glomerular capillary. The glomerular capillaries and the podocytes form the filtration membrane. The descending limb is made up of a thin wall of simple squamous epithelium. The rest of the nephron consists of simple cuboidal epithelium, along with microvilli and many mitochondria. There are three processes that are critical for urine production which are filtration, reabsorption and secretion. Filtration is the movement of water and small molecules through the filtration membrane of the renal corpuscle. The portion entering the nephron is called the filtrate. The formation of filtrate depends highly on filtration pressure. The pressure in the nephron is higher than in any other capillary. This is due to the juxtaglomerular apparatus which releases renin. Renin causes the constriction of smooth muscle around the capillary producing an increase in pressure. The blood pressure is usually between 45-50 mmHg. Colloidal osmotic pressure moves water back through the filtration membrane. It is about 28 mmHg. Hydrostatic pressure also works against the high pressure coming in. It is the pressure of fluid in an enclosed space and it is 10 mmHg.