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Thread: Cross-sectional area of arterioles and capillaries?

  1. #1 Cross-sectional area of arterioles and capillaries? 
    j.r
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    Hi there,

    I'd be grateful if anyone might help me to understand something I'm confused about regarding the cross-sectional area of blood vessels. I've read three facts about this topic in three different books.
    First I read that the total cross-sectional area of the vessels increases between the aorta and the capillaries and that this causes an increased frictional resistance between the blood and the vessel wall, decreasing the rate of blood flow. However, in one of the other books, I read that capillaries are very narrow so that they can permeate tissues and so that red blood cells are squeezed flat against their sides to reduce the diffusion path for the exchange of materials. Finally, I read, in the third book, that hydrostatic pressure occurs at the arterial end of the capillaries due to blood that is pumped from the heart having passed through "arteries, even narrower arterioles, and even narrower capillaries" (I've written this in quote marks as it is exactly what the book says).
    What I can't understand is how the total cross-sectional area can increase between the arteries and the capillaries (as stated in book 1) and yet arterioles and capillaries are said to be increasingly narrower than arteries, in order to create hydrostatic pressure of blood that passes through them, and so that the capillaries can permeate tissue and also be narrow enough for red blood cells to be squeezed flat against their sides?
    How does this work? When book 1 describes the total cross-sectional area of vessels as increasing, does it perhaps not refer to the lumens of these vessels, which in fact actually get narrower?
    I'm really confused!

    If anyone could clear this up for me I'd be very appreciative!

    Thank you!


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  3. #2  
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    Quote Originally Posted by j.r View Post
    Hi there,

    I'd be grateful if anyone might help me to understand something I'm confused about regarding the cross-sectional area of blood vessels. I've read three facts about this topic in three different books.
    First I read that the total cross-sectional area of the vessels increases between the aorta and the capillaries and that this causes an increased frictional resistance between the blood and the vessel wall, decreasing the rate of blood flow.
    If the cross-sectional area of the vessels increases the flow velocity will decrease. The "rate of blood flow" as measured in liters per minute or whatever, is a constant. This is true just because of conservation of mass. All blood leaving the heart is returning at the same rate (unless there is a cut from which blood is being lost). No new blood is being created along the path, so the flow rate is the same. The velocity decreases with increasing cross-sectional area just because of the math (volume=velocity*cross-sectional area*time), not anything to do with the frictional resistance.
    However, in one of the other books, I read that capillaries are very narrow so that they can permeate tissues and so that red blood cells are squeezed flat against their sides to reduce the diffusion path for the exchange of materials. Finally, I read, in the third book, that hydrostatic pressure occurs at the arterial end of the capillaries due to blood that is pumped from the heart having passed through "arteries, even narrower arterioles, and even narrower capillaries" (I've written this in quote marks as it is exactly what the book says).
    What I can't understand is how the total cross-sectional area can increase between the arteries and the capillaries (as stated in book 1) and yet arterioles and capillaries are said to be increasingly narrower than arteries, in order to create hydrostatic pressure of blood that passes through them, and so that the capillaries can permeate tissue and also be narrow enough for red blood cells to be squeezed flat against their sides?
    How does this work? When book 1 describes the total cross-sectional area of vessels as increasing, does it perhaps not refer to the lumens of these vessels, which in fact actually get narrower?
    I'm really confused!

    If anyone could clear this up for me I'd be very appreciative!

    Thank you!
    Remember also that the hydrostatic pressure is decreasing all around the loop. This must be true because the blood flows from a high pressure part of the loop to a low pressure part of the loop. If the arterioles are more restrictive than the arteries, the pressure drop across them will be greater than across the arteries, but the arteries will still have a higher absolute hydrostatic pressure, because they are closer to the heart.

    The total cross-sectional area can increase even if each individual capillary is smaller, if there are many, many more capillaries than arteries or arterioles.


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  4. #3  
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    I'm getting the feeling that j.r means something totally different. I understood the same thing with that then harold answered with. I'm into blood coagulation right now, so capillaries is really my thing. But the red blood cells are never squeezed flat between the cappilaries, as the thinnest cappilary will support the entire size of a normal (isotonic) red blood cell. Even some hypertonic red blood cells, as a little variation would not immediately kill a human. But it is true, that the blood flow in the middle of the capillary is faster, then on the sides. There is a true pressure difference in there. (not strong enough to rip red blood cells into pieces though). Though see it as smooth, but bobbed sides, and small turbulences will occur on the sides.

    Compare capillaries to a sponge, and arteries and arterioles like a hose. That's kinda all there is.

    If you want some actual answers, rephrase your question easier, so it's not open to interpretation.
    The past teaches, the present watches and the future learns.

    Though religion is a concept that simply can not be ignored. The fact that a deity could stand idly by when one part of his creation slaughters another part, simply for his namesake, is a mystery i doubt theologist would dare touch.

    ~Zwolver...
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    I should correct something I wrote. The rate of flow is constant throughout the flow path. Of course, it will change in time depending on the heart rate and such and could be affected by the flow resistance. But that would affect the flow throughout the system.
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