This question requires you to use your knowledge from A-Level biology on blood and the circulatory system, to come up with an educated guess on how this might come about.
As a quick overview, you could mention the anatomical features which mediate blood flow back to the heart – mentioning how the arterial system is responsible for blood flow away from the heart whilst the venous system is responsible for blood flow towards the heart. In this question it is talking about blood flow back to the heart, so you know that the blood would be getting from the feet back to the heart through the veins in the leg.
I think what is important here is to consider the problem and why they have asked you this question – in this case what the problem with blood is getting from the feet to the heart. Here you might begin by mentioning the ever-present force of gravity the venous system of the limbs is up against when in the upright position, causing blood which is trying to be shifted upwards towards the heart to go back down if there are no mechanisms in place to prevent this. This would be dangerous as there would be pooling of the blood in the limbs and it means that the blood could not get back to the heart to be oxygenated and supply organs/tissues again as required.
After addressing this, you would go onto talk about the different ways you think the body is adapted to allow blood from the feet to get back to the heart against the force of gravity. One reason you might come up with is the presence of valves in the veins, which you would have learnt in A-Level Biology when looking at the structure of veins and arteries in the circulatory system. These valves located within the veins close when the blood starts to flow in one direction, so that blood in the veins can only flow in one direction back to the heart and prevent the backflow of blood. You will have been told at A-Level that this mechanism is important to prevent the lower pressure venous blood backflow, but this is also used to get the blood back against gravity. Using your knowledge of how blood is pumped in other areas of the circulatory system e.g., pumping through muscular contractions of the heart, think about what could cause this squeezing motion in the legs to help pump blood up. When you are walking, running or standing, muscles in your leg are contracting to help you carry out these movements. As they contract, they squeeze the surrounding veins and help the blood to flow. This is called the skeletal muscle pump and is why it is important to get your legs moving.
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