In the normal situation, red blood (with oxygen)
passes through capillaries. The oxygen leaves the
blood which without oxygen is blue in colour.
The capillaries are very narrow and the pressure
driving the blood round falls as the blood passes
through them. Thus the pressure in the veins is
There are no capillaries so that the red blood
(with oxygen) loses very little of its oxygen and
remains much more red in colour in the veins.
There are no capillaries to reduce the pressure in
the blood. Thus the pressure in the veins is
higher than normal. Veins are not made to resist
high pressures. Thus bleeding may occur.