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    I do not understand why there is a drop in pressure as the blood flows into tiny capillaries in the lungs. Surely as they are small the pressure should increase?
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    (Original post by SophieL1996)
    I do not understand why there is a drop in pressure as the blood flows into tiny capillaries in the lungs. Surely as they are small the pressure should increase?
    Pressure drops in the capillaries as they are a considerable distance away from the heart, where the blood is pumped from.
    The pressure HAS to be low, else the thin walls of the capillary would burst, causing internal bleeding.
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    (Original post by SophieL1996)
    I do not understand why there is a drop in pressure as the blood flows into tiny capillaries in the lungs. Surely as they are small the pressure should increase?
    Unless I am very much mistaken - it doesn't have two pumps - it is one large muscle which acts as a pump.
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    The reason the pressure drops is because the volume of all the capillaries is much larger than the arterioles. Although the capillaries have a very small diameter there are so many of them that the total volume is huge. Any fluid passing from a smaller to a larger volume will experience a drop in pressure
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    When we talk about blood pressure it is not at the capillaries, but of the whole system.

    If that blood is being distributed into additional channels then the overall pressure is reduced. If those capillaries constrict and force the blood back to the core then the pressure increases.
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    (Original post by squeakysquirrel)
    Unless I am very much mistaken - it doesn't have two pumps - it is one large muscle which acts as a pump.
    Technically it is all one muscle yes, though one could argue that given that the circulatory system is known as a double pump system and that the sides of the heart pump blood off to separate locations that it does have 2 pumps. This is also unrelated to the original post
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    (Original post by SophieL1996)
    I do not understand why there is a drop in pressure as the blood flows into tiny capillaries in the lungs. Surely as they are small the pressure should increase?
    The total cross-sectional area of blood vessels rises as the blood goes from the heart to the lungs (because of all the little capillaries), meaning that there is a fall in the velocity of blood flow and also in the pressure. This is important as it means that there is enough time for exchange to take place.

    On the "return" journey back to the heart, the total cross-sectional area reduces so pressure gradually increases.

    I don't really understand what you mean by "two pumps", but the heart as two circulations: pulmonary and systemic.
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    (Original post by Democracy)

    I don't really understand what you mean by "two pumps", but the heart as two circulations: pulmonary and systemic.
    By 2 pumps I mean that each side of the heart pumps blood to a different place. The right side to the lungs and the left to the body. The book states it as 2 separate pumps.
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    (Original post by SophieL1996)
    By 2 pumps I mean that each side of the heart pumps blood to a different place. The right side to the lungs and the left to the body. The book states it as 2 separate pumps.
    Oh I see, yes, that's a weird way of putting it imho, but I get where you're coming from
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    (Original post by SophieL1996)
    By 2 pumps I mean that each side of the heart pumps blood to a different place. The right side to the lungs and the left to the body. The book states it as 2 separate pumps.
    Because blood loses pressure when it passes through the capillaries in the lungs to be oxygenated. If it were to go directly to body tissues, the blood would be travelling far too slowly in circulation - tissues wouldnt be perfused as regularly. With two pumps, the left ventricle can 'boost' (if you like) blood pressure when it returns to the heart from the lungs. Therefore the oxygenated blood travels fast in circulation. Think of blood as the delivery man, the left ventricle/second pump is like a rest stop so he can then continue on his round faster after he leaves the depot/letter centre? :lol:
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    Also when you're doing a lot of physical activity the double circulatory system comes into play. Without the double system deoxygenated blood would have to travel to the lungs before the rest of the body which means you'd "run out of breath" quicker and also the double pump helps keep the pressure higher. If it was one closed system the heart would have about 1/3 more work to do. Your life span would be lower due to an increased heartbeat to compensate.
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    (Original post by SophieL1996)
    I do not understand why there is a drop in pressure as the blood flows into tiny capillaries in the lungs. Surely as they are small the pressure should increase?
    Pressure can't increase as you go through the system anyway. Liquid always flows from high pressure to low pressure. If pressure were highest in the capillaries then blood would be flowing from the capillaries back to the heart!
 
 
 
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