Lab 4

Question 1

Countercurrent Exchange Systems

Answer 1

Exchange in many biological systems has been maximized by having the exchange take place between two streams of fluids travelling in opposite directions. Such systems are called countercurrent exchange systems and are important in gas exchange in fish gills and in heat conservation in the extremities of birds, mammals and even some fish. 2 different streams flowing in opposite directions. Constant gradient along length.

Question 2

Countercurrent exchange in fish gills

Answer 2

In fish gills oxygen-loaded water flows over the gills in the opposite direction to a stream of deoxygenated blood flowing within blood vessels of the gills.

Question 3

Countercurrent exchange in a bird’s leg

Answer 3

In a bird’s leg (as well as many other documented examples) cooler blood returning from the leg within in vein passes in close proximity to warmer blood flowing within an artery coming from the main body.

Question 4

In both fish gills and bird’s leg

Answer 4

In both cases uptake (oxygen in the gill, heat in the bird’s leg) is maximized because streams of fluids different in some exchangeable quantity are travelling in opposite directions in close proximity to each other.

Question 5

Exchange

Answer 5

In biological systems, exchange usually involves the diffusion of materials (i.e. ions, gases, water, etc.) from an area of higher concentration (of that material) to an area of lower concentration (of that material) across a permeable barrier. The amount of exchange that takes place within a given time is proportional to the size of the gradient. In the bird’s leg, however, heat is conducted from the artery to the vein (and not diffused). But the same principles apply: the amount of exchange is dependent on the size of the gradient (called a temperature gradient) and heat must be able to be conducted through the barrier.

Question 6

Concurrent exchange

Answer 6

streams flowing in the same direction. Diminishing gradient along length

Question 7

The importance of contrasting countercurrent exchange with concurrent exchange

Answer 7

This is done to demonstrate the importance of direction of flow and that systems selected for biological systems (countercurrent exchange systems) are superior to their theoretical opposites (countercurrent exchange systems). Copper tubes are a good model for heat exchange between arteries and veins because they readily exchange heat.

Question 8

Comparison of countercurrent and current exchangers

Answer 8

Countercurrent exchangers are more efficient than concurrent exchangers because countercurrent exchangers maintain a constant gradient along their entire length whereas concurrent exchangers have a gradually diminishing gradient along their length. Longer countercurrent exchangers are more efficient than shorter ones because the exchange takes place over a longer length. Regardless of length, countercurrent exchangers are always more efficient than concurrent exchangers because their gradient remains constant over the entire length whereas gradients decrease over the length of a concurrent exchanger.