1. Blood Transport System
Learning Objectives:
To understand how blood pressure and velocity changes during exercise.
To know the mechanisms that aid venous return.
To be able to explain the transport of O2 and CO2 at rest and during exercise.

2. Circulation
Blood travels through a series of blood vessels when going to and from the muscles (or any other body part).
Arteries
Arterioles
Capillaries
Body part (e.g. muscle)
Venules
Veins

3. Direction of blood flow Pressure and velocity of blood
Keep all facts on the same row but rearrange them into their correct position
Arteries
Veins
Capillaries
Direction of blood flow
Towards heart
Away from heart
Type of blood
Deoxygenated (usually)
Oxygenated and deoxygenated
Oxygenated (usually)
Thickness of walls
Semi-permeable
Medium
Thick
Pressure and velocity of blood
Low
High
Contains valves? No
Yes
Size of lumen
Larger
Small

4. Direction of blood flow Pressure and velocity of blood
Arteries
Veins
Capillaries
Direction of blood flow
Away from heart
Towards heart
Type of blood
Oxygenated (usually)
Deoxygenated (usually)
Oxygenated and deoxygenated
Thickness of walls
Thick
Medium
Semi-permeable
Pressure and velocity of blood
High
Low
Contains valves? No
Yes
Size of lumen
Small
Larger

5. For each point explain the effect in enabling the blood vessel to work more effectively
Arteries have a small lumen
Maintains a high blood pressure
Arteries have thick, elastic walls
Allows them to carry blood at high pressure/velocity without damage
Veins contain valves
Helps to return blood to the heart against the effects of gravity
Capillaries have very thin walls
Allows for gas exchange at alveoli and muscle cells
Blood flow at capillaries is very slow
Allows time for gas exchange to take place

6. Redistribution of Blood During Exercise
Blood is shunted to the working muscles (and away from other organs) during exercise due to their greater demand for oxygen.
Blood supply to the skin also increases.
This is done through the vasodilation (widening) of arterioles supplying muscles and the vasoconstriction (narrowing) of arterioles supplying other organs.
This is controlled by the sympathetic nervous system as factors such as blood acidity, O2 levels etc are detected.

7. Blood Pressure and Velocity
Systolic pressure: blood pressure when heart contracts.
Diastolic pressure: blood pressure when heart relaxes.
Blood pressure reduces as blood moves further from the heart due to friction and an increase in surface area (blood gets more spread out).
Blood velocity also reduces as blood moves further from the heart, however it increases when blood moves from capillaries to venules and veins as surface area decreases.

9. Venous Return
Blood pressure in large veins is so low that mechanisms are required to allow the blood to return to the heart against the effects of gravity. These include:
Valves (found only in veins)
Skeletal muscle pump – when muscles contract they squeeze veins forcing blood back to the heart. Exercise increases this. A sudden stop will result in ‘pooling’.
Respiratory pump – breathing movements forcing blood back to the heart.

10. Transport of Respiratory Gases
Oxygen is transported in haemoglobin in red blood cells (oxyhaemoglobin).
Where O2 concentration is high (e.g. the lungs), haemoglobin becomes fully (100%) saturated.
Where O2 concentration is lower (e.g. working muscles), the percentage saturation of haemoglobin is lower.

11. Bohr Shift
The amount of 02 released from haemoglobin is affected by the partial pressure concentration of oxygen.
It is also affected by blood acidity levels. As more CO2 and lactic acid are produced, oxygen splits more readily from haemoglobin.
This is known as the Bohr Shift.
Increases in temperature have a similar effect.
Myoglobin in the muscle tissues takes up oxygen even more readily than haemoglobin and acts as an oxygen store in muscles.
Bohr shift video clip

13. Carbon Dioxide Transport
The carbon dioxide that enters the body via the alveoli is transported in three ways:
7% in blood plasma
23% in haemoglobin
70% as bicarbonate ion.

14. Gas Exchange at Tissues
Gas exchange at the muscles is mainly effected by the partial pressure of gases involved.
Other factors such as an increase in temperature and decrease in pH also encourage O2 to diffuse from blood into muscles.
The aterio-venous difference (a-vO2) is the difference between the O2 content of arterial blood and venous blood (i.e. the amount of O2 that the muscle uses).
At rest a-vO2 is low, during exercise it is high.
Training increases a-vO2 as individuals can extract more O2 from blood.