Just wondering i heard people talk about it.
-
Counter-current flow between two fluids allows a greater amount of exchange than parallel flow.
Think about the flow of blood through a fish's gills. The blood has a lower concentration of oxygen and higher concentration of CO2 relative to the water, so O2 diffuses into the blood and CO2 out. If neither the blood nor water was moving, they would reach equilibrium. Fortunately, the blood is continually refreshed and the fish keeps swimming into new water.
Picture a single capillary in the gill. Water flows across it in one direction, its oxygen content decreasing as it goes. If the blood flows in the same direction, by the end the oxygen is evenly distributed between water and blood.
But if the blood flows in the opposite direction, what happens? Newly arrived oxygen-poor blood meets water that has already given up some oxygen but still has some to give. As the blood travels through the gill it picks up oxygen, but the water it encounters also has an increasing supply of oxygen. There is no equilibrium.
There is a similar effect for heat exchange in a duck's legs. Blood flowing in arteries from inside a duck's warm, feathery body out into its exposed legs tends to lose a lot of heat. By placing the veins adjacent to those arteries, the duck is able to transfer much of that heat to the blood that is returning. The feet get very cold, but this minimizes the heat that is actually lost to the surroundings.
Think about the flow of blood through a fish's gills. The blood has a lower concentration of oxygen and higher concentration of CO2 relative to the water, so O2 diffuses into the blood and CO2 out. If neither the blood nor water was moving, they would reach equilibrium. Fortunately, the blood is continually refreshed and the fish keeps swimming into new water.
Picture a single capillary in the gill. Water flows across it in one direction, its oxygen content decreasing as it goes. If the blood flows in the same direction, by the end the oxygen is evenly distributed between water and blood.
But if the blood flows in the opposite direction, what happens? Newly arrived oxygen-poor blood meets water that has already given up some oxygen but still has some to give. As the blood travels through the gill it picks up oxygen, but the water it encounters also has an increasing supply of oxygen. There is no equilibrium.
There is a similar effect for heat exchange in a duck's legs. Blood flowing in arteries from inside a duck's warm, feathery body out into its exposed legs tends to lose a lot of heat. By placing the veins adjacent to those arteries, the duck is able to transfer much of that heat to the blood that is returning. The feet get very cold, but this minimizes the heat that is actually lost to the surroundings.