Lecture 12
Introduction to Animal Structure and Function
Countercurrent Exchange

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

Kidney: to maximize water reabsorption

Tetrapod Limbs: to maximize heat exchange

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

greater the gradient the greater the rate of exchange

countercurrent have small gradient over long distance

therefore continuous exchange

Countercurrent vs concurrent exchange ( hypothetical)

Countercurrent
-------------------->
<-------------------

Concurrent (same direction)
------------------->
------------------->

Countercurrent
Exchange

20    40       60        80      100
 <-----------water--------------------

 I           I          I         I          I  
V          V         V       V         V

10        30            50       70     90
------------blood-------------------->

Countercurrent Exchange

(figure not available: see lab 6 in lab book)

Summary of Countercurrent Exchanger

Gradient constant therefore exchange is constant and continuous along length of exchanger

Concurrent Exchange

(figure not available: see lab 6 in lab book)

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%

Short vs Long Countercurrent Exchangers

The effect of length on efficiency of countercurrent exchangers

increase in efficiency

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

Regulating the internal environment

Homeostasis is the process of regulating a constant internal set of conditions

Steady state

Dynamic equilibrium: constantly fluctuating around set point 

Why is homeostasis necessary?

Homeostasis is necessary for enzyme function

Enzymes only operate in a narrow range

Components of system controlled by homeostasis

Receptor: detects change in internal environment (e.g. sense organ)

Control center: processes information; relays message to effector (e.g. brain)

Effector: responds to message; carries out an action (e.g. muscles, glands)

Homeostasis is regulated by negative feedback

Definition: Negative feedback occurs when an output from a system is fed back into a system as a signal to shift the system in the opposite direction

Thermostat: an example of negative feedback

Thermostat is both receptor (detects temperature) and control center (switches on heater)

Thermostat has a set point (20 degrees)

Temp falls below 20 ->

Thermostat detects drop ->

Turns on heater->

Temp rises (output) ->

Thermostat detects increase in temp (feedback) ->

Turns off heater (negative feedback)

Homeostasis is a dynamic equilibrium

Temp not constant

Fluctuates around set point

Stops big swings

Remains in the range

Homeostasis in mammals

Conditions kept constant by keeping interstitial fluid constant

Interstitial fluid surrounds cells

Interstitial fluid kept constant by blood

E.g. temp, pH, glucose, ions, hormones, CO2, O2

Body Temperature

Complex

Increased by metabolic rate, blood flow, shivering

Decreased by metabolic rate, blood flow, sweating

Homeostasis of Body Temperature

Receptor in brain

E.g. temp too high->

Signal to effector (sweat glands) ->

Evaporative cooling->

Temp drops->

Detected by receptor ->

Signal to stop sweating->

Temp rises