Question

Detailed description of just what "counter-current exchange" is, how it differs from "con-current", which organisms it can be found in, and what function does it serve in these various organisms.

Answer 1

Counter current exchange mechanism involves the  exchange of gases in certain organisms from outside environment to inside body, mainly the transfer of oxygen into the blood. Fish gills use a design called ‘countercurrent oxygen exchange’ to maximize the amount of oxygen that their blood can pick up. They achieve this by maximizing the amount of time their blood is exposed to water that has a higher oxygen level.  The blood flows through the gills in the opposite direction as the water flowing over the gills. This flow pattern ensures that as the blood progresses through the gills and gains oxygen from the water, it encounters increasingly fresh water with a higher oxygen concentration that is able to continuously offload oxygen into the blood. The low-oxygen blood, which is just entering the gill, meets low-oxygen water. Since there is more oxygen in the water, the oxygen can flow from water to blood. Likewise, the high-oxygen blood, which has nearly passed the entire length of the gill, meets fresh, high-oxygen water, and oxygen continues to flow from water to blood. Since there is more oxygen in the water, the oxygen can flow from water to blood. In fish less than 80% of the oxygen in the water flowing over the gills is generally transferred to the blood through this mechanism.

In co current mechanism the flow of biood and flow of water is in same direction. Here the exchange of gas will eventually stop when an equilibrium has been reached .If fish instead had blood flowing in the same direction as water through their gills (called ‘concurrent flow’), the low-oxygen blood entering the gill would first meet the high-oxygen water also entering the gill. Oxygen would quickly pass from the water into the blood, until the oxygen levels of the blood and water rapidly became the same, and oxygen diffusion into the blood would stop.

The birds use a countercurrent heat exchange system during cooler weather. Cool blood com­ing back from the foot travels through veins grouped around arteries that are sending warm blood from the body to the foot. Heat is transferred from the warm arteries to the cool veins. This countercurrent heat exchange system is very efficient at maintaining heat in its foot. Periodic increases in blood flow allow a little heat to reach the foot and prevent it from freezing.