User:BreckenTulloch/sandbox

The filtrate contains lots of important substances. In the proximal tubules of the pigeon kidney, substances that are needed, such as vitamins and glucose are reabsorbed into the blood. The pigeon kidney has a variety of ion channels involved in salt and water transport. Water is reabsorbed through aquaporins which are present in the lumen of proximal tubule, basolateral membrane, and blood vessel near proximal tubule. Water flows from the epithelial cells into the blood via osmosis. Since osmosis occurs, the osmolarity of the filtrate remains isotonic. Sodium/Potassium/ATPase transporter is located in the basolateral membrane of the epithelial cell, which is opposite of the lumen of proximal tubule, and actively pumps sodium out of the cell into the blood. This sets up a strong concentration gradient in the cell that provides a potential to allow sodium cotransporters in the apical membrane to reabsorb nutrients and electrolytes. The cotransporters present are a sodium/chloride and sodium/glucose in lumen of proximal tubule.

Pigeon kidneys, like mammalian kidneys, are capable of producing urine hyperosmotic to the plasma utilizing the processes of filtration, reabsorption and secretion, which will be discussed later and explained through the Starling-Landis Hypothesis. The medullary cones function as countercurrent units that achieve the production of hyperosmotic urine. Hyperosmotic urine can be understood in light of the law of diffusion and osmolarity. The danger of desiccation is a major threat to animals living on land. Water is lost in urine and feces, but evaporation is the principal route of water loss. Water lost must be replaced by drinking and water in food. Dehydration or salt-loading decreases the filtration rate primarily by the shut down of the nephrons, which is controlled by an antidiuretic hormone, arginine vasotocin. Pigeons adjust their drinking rates and food intake in parallel and when adequate water is unavailable for excretion, food intake is limited to maintain water balance. As Columbia Livia inhabit arid envinonments, research attributes this to their strong flying capabilities to reach the available water sources, not because of exceptional potential for water conservation.