Lecture 12:  Introduction to Animal Structure and Function
Countercurrent Exchange
Review of Specialized Exchange Surfaces
 Exchange surfaces are specialized to maximize the rate of exchange
Examples: Gas exchange, nutrient uptake and excretion of wastes
Properties of Specialized Exchange Surfaces
1. thin, moist and permeable
2. high SA (flat, sacs, folded, filamentous)
3. Some have countercurrent exchange to maximize efficiency
Definition of Countercurrent exchange
A system where exchange takes place between two fluids which  are flowing in opposite directions
Examples of countercurrent exchange systems
Fish Gill: to maximize O2 uptake
Water and blood flow in opposite directions
Note large SA of exchange surface (filaments, lamellae)
Kidney: to maximize water reabsorption
Large Fish: to minimize heat loss from muscles
Blood in veins  and arteries flow in opposite directions
Principle of countercurrent exchangers:
Countercurrent exchangers maintain a constant gradient along their length to ensure that exchange takes place along the entire length.
Detailed Explanation of Countercurrent Exchange
No exchange without gradient
The greater the gradient the greater the rate of exchange
Countercurrent have a small gradient over long distance
Therefore continuous exchange
Countercurrent vs concurrent exchange ( hypothetical)
Countercurrent
-->
<--
Concurrent (same direction)
-->
-->
Models of Countercurrent and Concurrent Exchangers
fish gill                hypothetical





Counter        Con
Countercurrent Model (Found in Fish Gills)
CountercurrentExchange
100      80      60     40       20
 -----------water------------------
 I           I          I         I          I   
V         V       V         V         V
 90         70        50     30        10        
<------------blood--------------------
Countercurrent Exchanger: blood is always encountering water of higher concentration
Countercurrent Exchanger: Gradient is constant (about 10)
Graph of Countercurrent Exchange (draw this)










From the graph how would you calculate how much O2 is gained by the blood?
Summary of Countercurrent Exchanger
Gradient constant therefore exchange is constant and continuous along length of exchanger
Concurrent Model (Not found in Fish Gills)

Concurrent Exchange

100      70      50     50       50
 -----------water------------------
 I           I          I         I          I   
V         V       V         V         V
 10         30        50     50        50        
<------------blood--------------------
Graph of Concurrent Exchange (draw this)







How much O2 is gained by the blood in Concurrent Exchange?
Summary of Concurrent Exchanger
Initially great exchange because of large gradient
As length increases gradient diminishes
Therefore rate of exchange diminishes
Stops when gradient dies
Max rate of exchange =50%
Comparison of Efficiencies of Concurrent & Countercurrent Exchangers
Does the length of the exchanger affect its efficiency?
Relationship between Length of Concurrent Exchanger and Efficiency
Length equals distance on gill (blood and water  travel equal distance)
Longer the exchanger the greater the O2 uptake
Stops when gradient dies
Max rate of exchange =50%
Shorter exchangers have faster uptake
Relationship between Length of Countercurrent Exchanger and Efficiency
Set of graphs (not just one)
Water and blood do not enter at the same end
Blood always enters with same concentration
Water always enters with same concentration
Since exchange is constant along length there is always two parallel lines
Longer exchangers are more efficient than short ones
Summary
    1. Countercurrent exchangers are more efficient than concurrent exchangers
    2. Concurrent exchangers do not exist in biological systems
    3.Longer exchangers are more efficient than short ones *(up to a point in concurrent exchangers)
    4. *Concurrent exchangers stop exchanging when gradient disappears