Waste products/osmosis

Animals use an excretory system to remove any waste products such as urea. The role of one key organ, the kidney, is to form a more concentrated urine and reabsorb glucose, sodium ions and water while excluding the urea. The membranes of the kidney tubules are adapted to allow this function. The narrowing of the afferent arteriole generates a hydrostatic pressure at the glomerulus which forces blood against the capillary network. Water and small molecules pass through the pores while proteins and cells are excluded by the process of ultrafiltration. These smaller molecules enter the Bowman’s capsule and the proximal convoluted tubule, which has many sodium and glucose channels. These allow the selective reabsorption of these materials into the surrounding tissues. This lowers the water potential so water moves out of the tubule by osmosis and is reabsorbed with the ions into the capillaries that surround the tubules. As the membrane does not have channels for urea, urea remains in the tubule increasing in concentration. The ascending limb of the loop of Henle is impermeable to water. Sodium and chloride ions are actively transported out onto the surrounding tissues through a specific channel using ATP. This lowers water potential creating a water potential gradient that draws water from the descending limb by osmosis. This counter current multiplier further contributes to the reabsorption of water, one of the key functions of the kidney. A protein hormone, ADH is released by the pituitary gland and binds to specific receptors on the collecting ducts of the kidney in situations when the blood water potential is too low. This increases the membranes permeability to water effectively increasing the volume reabsorbed at the same time decreasing the volume of urine produced.