Reabsorption of Water
- The role of the loop of Henle is to create a low water potential in the tissue of the medulla.
- This ensures that even more water can be reabsorbed from the fluid in the collecting duct.
The Loop of Henle
- The loop of Henle consists of a descending limb that descends into the medulla and an ascending limb that ascends back out to the cortex.
- The arrangement of the loop of Henle allows salts (sodium and chloride ions) to be transferred from the ascending limb to the descending limb.
- The overall effect is to increase the concentration of salts in the tubule fluid and consequently they diffuse out from the thin-walled ascending limb into the surrounding medulla tissue, giving the tissue fluid in the medulla a very low water potential.
- The ascending limb actively transports Na+ ions and Cl- ions out into the intercellular space, producing a concentration gradient in the medulla.
- Water cannot flow because the ascending limb is impermeable to water.
- Water moves out of the permeable descending limb via osmosis, increasing the filtrate concentration.
- Filtrate at the base of the loop is very concentrated. When it enters the ascending limb, Na+ ions are pumped out.
- This makes the filtrate at the top of the limb dilute again, allowing more water diffusion in the distal convoluted tubule.
- The filtrate drains into the collecting duct which flows back through the concentrated medulla.
- The hormone ADH causes the walls to become permeable to water, meaning water moves via osmosis out into the surrounding blood capillaries
- The arrangement of the loop of Henle is known as a hairpin countercurrent multiplier system, meaning that one part of the tubule passes close to another part of the tubule with the fluid flowing in opposite directions.
- This allows exchange between the contents and can be used to create a very high concentration of solutes.
- The overall effect is to increase the efficiency of salt transfer from the ascending limb to the descending limb.
- This causes a build-up of salt concentration in the surrounding tissue fluid.
- It is beneficial for mammals living in arid regions to have higher salt concentration in their medullas and a longer loop of Henle because the greater the gradient from the top to the bottom of the tubule, the more water can be reabsorbed, which is important when there is little water available in the environment.
The Collecting Duct
- The distal convoluted tubule uses active transport to adjust the concentrations of various salts.
- From here fluid flows into the collecting duct.
- At this stage the tubule fluid has a high water potential.
- The collecting duct carries the fluid back down through the medulla to the pelvis.
- As the tubule fluid passes down the collecting duct, water moves by osmosis from the tubule fluid into the surrounding tissue.
- It then enters the blood capillaries, by osmosis, and is carried away.
- The amount of water that is reabsorbed depends on the permeability of the collecting duct.
- Only about 1.5-2.0 dm3 of fluid (urine) reaches the pelvis each day.
- By the time the urine reaches the pelvis it has a low water potential and the concentration of urea is higher than that of the blood plasma.
- The tissue fluid in the medulla has a low water potential, but the arrangement of blood vessels creates blood plasma with an even lower water potential than the tissue fluid due to the presence of proteins in the blood plasma as they could not be ultrafiltrated into the renal filtrate.
- The collecting duct must pass back through a region of low water potential so that the water can diffuse out of the collecting duct to be reabsorbed back into the blood.
- Terrestrial mammals must reabsorb as much water as possible because they lose water in other ways such as sweating and through water vapour when exhaling.
- Animals such as beavers have small loops of Henle because they live in aquatic environments where water is readily available, therefore they do not need to conserve water as much as an animal that lives in arid conditions.
That's the end of the topic!
Drafted by Bonnie (Biology)