FAINTING – FLOW OF BLOOD
However, blood flow back to the heart is by means of the veins, and there is no pump to assist in the venous return … in the case of the lower limbs the flow is all uphill, against gravity.
The veins contain valves which allow oneway flow only, towards the heart and also break the blood up into smaller columns.
Flow of blood in the veins is mainly due to the action of the muscles, compressing the blood and forcing it onwards, towards the heart.
Negative pressure in the chest, on breathing, does tend to “suck up” the blood and aid its return.
The veins and arteries have nerves supplying the muscle in their walls, which maintain contraction of these vessels, so that there is a certain natural tone.
Sometimes an emotional cause, such as a fright, or bad news or the sight of blood, acting through the nervous system, causes the blood vessels to lose tone. They dilate, and the blood then tends to stagnate or pool in the abdomen and lower limbs.
The venous return to the heart is impaired, the blood pressure falls, an inadequate volume is pumped to the brain, the person feels giddy, nauseated, the vision goes grey and then black, the person is pale and sweaty and then consciousness is lost.
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