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Lab # 1

Exchange principles con- and counter-current systems

Michael Axelsson University of Gothenburg

Department of Zoophysiology

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INTRODUCTION

Life depends on the exchange of mass and energy so a constant and

permanent transfer of molecules such as gases (oxygen and carbon dioxide), ions

(protons, cations like sodium, potassium and calcium or anions like chloride and

bicarbonate) and energy (heat) is taking place between organisms and their

environment.

Mass transfer occurs by molecular diffusion while heat transfer occurs by

conduction, convection and radiation.

The flow of either mass or heat depends on the properties of the media

where exchange is taking place, the difference in temperature or concentration

and the distance. Because of this, mass and thermal exchange between an animal

and its environment will always occur. Such ubiquity constitutes an important

selection pressure towards the appearance of specialized exchange structures

designed to avoid or improve the exchange of heat or molecules with the

environment.

The following are examples of exchange structures designed to avoid loss of heat

or molecules:

• Vascular design of blood flow in the appendages of aquatic animals or animals

living in cold climates to avoid heat loss

• Vasa recta in kidneys to supply nutrients to the inner medulla without removing

the osmotic gradient

• Vascular heat traps that allow the maintenance of specific organ systems at

temperatures above the temperature of the rest of the body of the animal

• Airway structure in desert animals to avoid excessive evaporative water loss

The following are examples of exchange structures designed to facilitate heat or

mass transfer:

• Salt glands in birds and reptiles that aid in the excretion of excess salts (mostly

NaCl)

• Rete mirabile in fish aided to filling the swim bladder with oxygen at high

pressures

• Fish gills that uptake oxygen from water and download carbon dioxide

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• Brain cooling mechanisms associated with respiratory passages in some birds

and mammals

• Extensively vascularized and expanded respiratory epithelia in tetrapods to

allow the exchange of respiratory gases

With the exception of the last example, all others are based on a common

scheme that has appeared recurrently in the evolution of many animal groups: the

countercurrent exchanger.

The purpose of this laboratory exercise is to study the basic function of the

countercurrent exchanger and to compare it with its alternative arrangement: the

concurrent exchanger. The system used for this lab is an obvious simplification of

the anatomical arrangements encountered in nature and it is based on two copper

tubes soldered together along its entire length (1 m). When the water inside the

tubes flows in opposite directions, we talk about countercurrent exchange. When

water flows in the same direction we talk about concurrent exchange.

DESCRIPTION OF THE SYSTEM The actual system is located in the Department of Zoology at the University

of Göteborg but it can be controlled remotely from any location in the world. A

camera placed over the system allows direct visualization of the actual lab setting.

Below is a schematic drawing of the setup with numbers that will be referred

to in the text. Thermostated baths (1) are used to simulate two different

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temperatures; in this case one bath is set to 5°C and the other to 37°C. Two

peristaltic pumps (2) are used to pump water from the two baths through the heat-

exchanger model (3, see below for more details). The temperature in the

exchanger is measured at 4 locations along each of the two tubes in the heat

exchanger model (at 4 cm, 36 cm, 65 cm and 96 cm respectively) plus at the very

entrance at three locations. The temperature is measured by an interface box (4,

PICO TC08 thermocouple interface) and the information is entered into a

computer (6). The computer controls the two pumps via a control box (5) and also

sends an alarm signal to an external alarm box (7) every time a person logs on

remotely.

The computer records the temperature along the exchanger and controls the

two peristaltic pumps (see below for more information about the program).

OPERATION OF THE SYSTEM If you are going to run the lab from a remote location (any location outside

the lab were the equipment is setup) you need a computer with access to the

Internet and a minimum connection speed of 126 kbits/second (ISDN or higher).

Log on to the following address

http://vivaldi.zool.gu.se/Labview_WWW/Index.htm.

A window looking like

the picture to the right will

show. On this page there

are links to different things

that you will need during

the lab. “Heat exchanger” is the link to the program

that runs the lab. If you

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click on this link another window will open. If this is the first time you log on it will

take some time before the program-window is shown (up to several minutes). The

first time you log on the program automatically downloads two files from National

Instruments, first the run-time module that let you run the program through the web

browser, the second is a plug-in that let your web browser show the program.

During this process a window will be shown asking if you would like to install the

run-time module, just click the “Yes” button. When the run-time module and plug-in

are installed the program will be shown (as in the figure below) and you can see

the temperature in the heat exchanger.

In the window that opened (and also in the initial window), there is a link to a

web-camera that will allow you to see the lab. You can control the camera via you

web browser. The are several preset positions that you can use to see different

areas of the lab, and you can also

use the PAN, TILT and ZOOM

functions to navigate the lab

yourself.

There is also a link to a chat

that will help you to communicate

with other students that will be

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logged on at the same time (if you are doing the lab alone or if all people in the

lab-group is present in front of the same computer you will not need this). When

you activate the chat you will be asked for a login name and then the window with

the chat will open.

OPERATION OF THE PROGRAM To run the lab you need control of the setup, one person at the time can be in

control and to gain control you “right click” on the program-window and select

“Request control of VI” (VI stands for Virtual Instrument) in the window that pops

up. You will then be granted control unless somebody else has control of the

program at the moment. If so, you will be placed in a queue. When you have

control you can use the mouse to click on the various control buttons (see

description of the program below).

In the top part of the program, information about the pumps are shown, in the

case below both pumps are on and pumping and the direction of warm and cold

water through the exchanger will be indicated by arrows (blue for the cold and red

for the warm water). In the middle the temperature data are shown. The thick

lines/dots indicate the temperature at each measuring point along the heat

exchanger while the thin dotted lines indicate the temperature of the water

entering the heat exchanger.

There are four

different tabs that

you can select,

“Time diagram”,

“Temperature diagram” “Pump settings” and

“Logg”, this will be

explained later. In

the lower part there

is one button “Save data”, it is used to

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start or stop saving data to the hard disk. At the right information about the time to

the next measurement is shown; this is set to 5 seconds between each measuring

point.

In the next tab

“Temperature diagram” the

temperature data

is shown inside 4

circles that

represent the 4

measuring points

along each tube

and the flow

direction is

indicated by the

large arrows or in

the case when one or both pumps are off a white area and the text “Pump X is off”

The temperature of the water going in to the heat exchanger is also shown outside

of the

corresponding

arrow.

When you

click on the button

“Pump settings” a

window will show

with the controls

for the pumps as

shown in the figure

to the right. You

can turn on and off

the pumps

individually,

change the speed of each pump and change the flow set-up between counter

current and con current. The speed of the pumps can be set to 1, 2 or 3

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corresponding to low medium and high flow. Once you finished setting the pumps

you close the window by clicking “Close window”. Below is s schematic drawing

showing how the

water (cold in blue and

warm in red) is

pumped through the

exchanger during

counter-current and

con-current setup The

fourth tab “Log”

shows a list of the

systems that are logged on to the computer. Every time someone logs in an alarm

in the lab is set of and a green small windows saying “New user logged on”

shows for 5 seconds

in the lower white

field next to the

“Save data” button“.

In the “Log” tab

there is also a button

called “Get attention” that when

pressed sounds an

audio/visual alarm in

the lab to get

attention from the

person that is locally

responsible for the

lab. This alarm is to inform the person responsible for the lab that somebody has

logged in so that we can make sure that everything is working in the lab and we

can also join the chat to help you with any problem.

You can then follow the development of the temperature either by looking at

the “Time diagram” or “Temperature diagram”.

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Saving data

When the temperature readings have stabilized i.e. there is not further changes

you should save a short period of data that will be used for the calculations

afterwards. To start saving data to the hard disk click on the “Save data” button

now every 5 seconds one set of temperature data will be saved. After 1-2 minutes

you can stop saving data by clicking on the same button that now reads “Stop

saving”.

You have to download the data to you local hard disk (or floppy) for later

processing and to do this you “right click” on the diskette symbol to the left of the

program and select “Save Target As” in the pop-up window, this will open up a

window and you can now select were on your local machine you want to save the

file and what you would like to call it. By default this file is called DATAFILE.TXT

and you should rename it (give it a name that indicate what type of experiment you

were running and speed of pumps and so on)

The data files that you have saved are plain ASCII files and can be open in Excel

or any other program that can open ASCII (text) files. To open the files in Excel go

to “Files” > “Open” and then to the sub directory were you have saved the file and

then in the tab “File of types” select “All files *.*”, this is important otherwise

Excel only looks for its own files with the extensions “.exl”.

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DESCRIPTION OF THE LAB EXERCISE When reading about countercurrent exchangers in textbooks the first concept

that usually appears is that countercurrent exchangers are more efficient than

concurrent exchangers.

Think about this and come up with a way to quantify the exchanger

efficiency. Efficiency is usually expressed as a percentage. Try to calculate the

efficiency using the temperature data you obtain after running the heat exchanger

in the countercurrent and the concurrent modes.

When running the heat exchanger, wait until the temperature values stabilize.

You can keep track of this using the Time diagram. Once the temperatures

stabilize, write the numbers down in your notebook, plot them in an Excel graph

and compare both exchange modes. Or use the Save data function and download

the data file to your computer

Now that you have done this exercise, try to answer the following questions:

1. Did you achieve a 100% efficient exchange in any mode?

2. Can countercurrent exchange have an efficiency of 100%? How?

3. Can concurrent exchange have a 100% efficiency? How?

Flow through the exchanger tubes can be changed as specified in the

previous section. Do you think that flow can affect efficiency? Try to make a

prediction before running the exchanger at lower or higher flows and verify if you

prediction is right.

Describe how efficiency varies with alterations of flow. Plot your data in

graphs to demonstrate your results and discuss the possibility that efficiency in

counter current exchangers is similar to the efficiency in concurrent exchangers.

Let’s consider a real scenario, the fish gill

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The two panels below shows one single gill lamella inserted between to Y-axis

with oxygen partial pressure. The task is to draw the lines for the partial pressure

of oxygen in the water and in the blood in the two different situations (counter-

current to the left and con-current to the right). The water and venous blood

oxygen tension is given in the top of the figure.

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References

Illustration for this manual is from the following sources, some have been redrawn

while some have just been scanned.

• Animal Physiology, Mechanisms and adaptions, R. Eckert & D. Randall,

W.H. Freeman and Company, San Fransisco, ISBN 0 7167 1423 X

• Biology of fishes.2 nd edition. Q Bone, NB Marshall and JHS Blaxter. ISBN

0 412 741140 7

• Comparative Animal physiology, P. C. Withers, Suanders College

Publishing, Forth Worth, Philadelphia, ISBN 0-03-012847-1