Guided Activity: Blood sugar and the constancy of the internal environment
1. Chemical analysis shows that sugar is present in the blood of mammals.
a. Where could this sugar have come from?
In food eaten Sugar and carbohydrates
b. What might be its eventual fate?
Sugars (glucose) are used in cellular respiration in all the tissues of the body
2. Very little was known about the transport of substances in the blood until last century. In 1843 the
French physiologist Claude Bernard, investigated the transport of digested foods. He started work
on sugar because its presence in the blood is easy to detect. In one of his experiments he fed a dog
on sugary milk. He tested the blood for sugar and found sugar present. At the same time as a
control, he tested the blood of a dog that had been fed meat. To his surprise he found the blood of
the control dog also contained sugar.
 Where might the sugar in the blood of the control dog have come from?
 Meat might contain some carbohydrates
 The body might be able to convert protein into sugars
 The sugar might be left over from a previous meal
3. There seem to be a number of possibilities worth exploring. The investigation of sugar in the blood
is much simpler these days and not only the presence of sugar but the concentration of sugar in the
blood can be quantified. What would you expect would happen to the amount of sugar in the blood
over a period of time? Suppose the dog that had eaten nothing for 12 hours was then given a meal
of sugary milk and then was not fed for the next 12 hours. Suppose also the dog was kept active
during this time.
 Draw a sketch graph to show what might happen to the concentration of Blood sugar during
the 12 hours following the meal given the assumption that the sugar comes from the meal
and is gradually used by respiring cells.
4. You have not been able to give exact concentrations for your graph because you don’t know how
much sugar was given to the dog or how rapidly it was used up in the tissues. Below is similar graph
plotted using information about the amount of absorbable sugar and the rate at which it is
absorbed into the blood and used by an active dog. This graph is a hypothetical or model graph,
showing predictions, based on our assumption
% Blood sugar v Hours
0.3
0.25
0.2
% BS 0.15
0.1
0.05
0
1
2
3
4
5
6
7
8
9
10
11
12
Hours
s.

How might the biologist obtain data to see if the actual situation fits this model?
Take hourly samples of blood from the dog and estimate the blood sugar.
5. When this kind of experiment was done the results look like the following graph.
% Blood sugar V Hours after a sugar meal
B
A
0.3
C
D
0.25
Model
0.2
% BS 0.15
0.1
Actual
0.05
0
1
2
3
4
5
6
7
Hours
8
9
10
11
12
6. Look first at the parts of the curve between B&C. Assume that the information used to draw up the
model graph was accurate. That is, assume that no mistake was made in estimating the amount of
sugar in the meal, the rate at which it was absorbed or the rate at which it was respired by active
tissue.
 How then could you account for the difference between the expected and the actual
concentrations of glucose in the first few hours after the meal?
Something more than use in respiration removing sugar from the blood.
Either sugar is stored or sugar is excreted.
7. In a dissected animal it can be seen that veins carry blood from the intestine to the heart pass
through the liver
Blood to the heart
Blood from the intestines

How could we test the hypothesis that the liver removes sugar from the blood? What data
would we need?
Data on the concentration of sugars in the portal vein v the hepatic vein during the
period after a sugar meal

If the liver is removing sugar from the blood, what should our data show? What would you
expect if the hypothesis is mistaken?
If the liver is removing sugars from the blood there should be a markedly lower concentration of sugars in
the hepatic vein compared to the portal vein.
If the liver is not removing sugars from the blood, there should be little or no difference between the hepatic
and portal vein.
8. When such blood samples were taken after a carbohydrate meal are analysed it is found that the
blood in the hepatic vein on its way back to the heart has markedly lower concentrations of glucose
than the blood in the portal vein. Two possible hypotheses about the action of the liver are that it
stores the sugar or that it destroys the sugar.
 Do our graphs suggest whether one of these hypotheses is more likely than the other?

Storage of sugars is most likely. In the graph between A-B and again C-D the level of blood sugars is higher
than expected indicating that the sugars may be temporarily stored and then released.

What further evidence might we look for as evidence for the hypotheses that glucose is
stored in the liver?
Any evidence of sugars in the liver
If sugars are present in the liver the level should increase as the level in the hepatic vein decreases.
Could have a subject fast so that blood sugar levels decrease and we would expect the level of
sugars in the hepatic vein to be higher than the portal vein.
9. On analysis, the liver is found to contain the polysaccharide glycogen. Also that the amount of
glycogen in the liver increases as the glucose concentration between the portal and hepatic vein
decreases. Moreover, in an active animal many hours after a meal the glycogen level in the liver
drops and the sugar level in the hepatic vein is higher than in the portal vein.
 How could you interpret these observations?
These observations support the hypothesis that sugar is stored in the liver after a meal (as glycogen) and
released later
Glucose is converted to glycogen
10. What advantage might there be to an animal in this storage and release of sugar by the liver?
Compare the situation represented by the model and actual graphs.
 What advantage might there be in the actual situation?
A constant supply of sugar to active cells between meals as well as just after meals
11. Claude Bernard, who discovered that sugar was stored and released from the liver, contrasted the
internal environment (the environment of the cells) where glucose is continuously available and
external environment of the animal where food is only sometimes available. He made the
generalisation that the activities of the intestine, liver, heart and other organs of the body work
together to maintain a constant internal environment despite fluctuations in the external
environment. We call this homeostasis.
 What are some other factors apart from the availability of sugar – in which our internal
environment is kept constant.