Distribution of Cardiac Output at Rest and During Exercise
Use the table to fill out the task below
When we exercise there is an increased demand for ____________________ to be supplied to the working
________________. The ______________ is responsible for carrying the oxygen to the muscles.
____________________ can carry up to four molecules of oxygen and through the ____________________
(vessels) it is delivered to the muscles.
At rest the muscles are only supplied with _______% of the total cardiac output, which is ___________ml.
The main supply of blood is directed to the _____________ and ____________ at rest as they receive
_________% of the cardiac output in total. However, during light exercise these figures change
significantly. ___________% of the cardiac output is now delivered to the muscles, with the liver and
kidneys now only receiving ________% and _________%, respectively. During moderate exercise the
_______________ now receive 71% of the total cardiac output with the liver and kidneys only receiving
____% each. Finally, during maximal exercise, the liver and kidneys now only receive ___% of the blood
each, with the muscles receiving a massive ____%.
During exercise the brain still receives a _____________ supply of blood (even though it decreases slightly),
as the brain is always working and requiring ______________. It receives ____% at rest, _____% during
light exercise, ____% during moderate exercise and ___% during maximal exercise.
During light and moderate exercise the __________ receives 15% and 12% of blood, as opposed to 6% at
rest and 2% during maximal exercise. The blood is sent to the skin in order to regulate body temperature
(cool it down). However, during _____________ exercise the need of the muscles is so great that it
overrides the need of the skin (this is why when you work really hard during exercise, you sometimes go
pale).
Blood pooling
After exercise has stopped the large volumes of blood in the muscles has to be re-directed and returned to
the areas that use it at rest. In order for this to happen it is important that the venous return mechanisms
are all working. If the mechanisms are not working effectively then blood pooling will occur. This is where
blood sits in the muscles where there is insufficient pressure to return it to the heart.
Task
A cyclist completes an exhausting high intensity training programme and immediately stops, climbs off the
bike and stands against the wall whilst recovering. Feeling light headed or dizzy, they feint falling to the
floor. Once on the floor the cyclist starts to feel better.
Using your knowledge of blood pooling and venous return answer the following questions;
1. Why did the cyclist feel dizzy (be detailed in your answer)?
2. Why did the cyclist start to feel better once he was lying on the ground?
3. Suggest what the cyclist could have done to prevent the dizziness.
4. Why it is important that you don’t eat prior to exercising?
5. Why does blood flow to the brain remain constant during exercise and at rest?
Answers to questions
1. Why did the cyclist feel dizzy (be detailed in your answer)?
By stopping immediately the cyclist was not using the skeletal muscle pump to assist with the return of
blood to the heart. This meant that blood pooled in his leg muscles, therefore meaning that less blood
was going to his brain, making him feel dizzy.
2. Why did the cyclist start to feel better once he was lying on the ground?
Gravity would have assisted venous return, by lying down it is easier for the blood to return to the heart
from the muscles and to be redistributed to the brain
3. Suggest what the cyclist could have done to prevent the dizziness.
A cool down. This would have ensured that the skeletal muscle pump and respiratory muscle pump
were working to help the venous return mechanism. This would have aided the redistribution of blood
back to the brain.
4. Why it is important that you don’t eat prior to exercising?
A full stomach requires blood to digest the food – this will take priority. Blood will go to the stomach
denying active muscles from the large amount blood that they need. This will cause the performer to
fatigue more quickly. A full stomach will also make the performer feel sick whilst exercising.
5. Why does blood flow to the brain remain constant during exercise and at rest?
The brain is always working and requires a constant supply of blood to allow the brain to have the
oxygen that it will need