Ggdf
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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 and explain I'd be so appreciative!

Thank you!
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Turkishee
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(Original post by Ggdf)
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 and explain I'd be so appreciative!

Thank you!
Well, think of it like this...

Thick --> Narrower --> Narrowest.

Arteries--> Arterioles --> Capillaries. [Note: There are different types of Arteries with different functions]

So you'd want the arteries to be the largest in diameter. Arterioles main role is regulating resistance, the friction between walls and the blood causes resistance and this resistance can increase and decrease dependant on various factors. For instance, contraction of smooth muscle causes vasoconstriction and restricts blood flow, similarly decrease in constriction decreases resistance and allows blood flow.

Capillaries are very very thin, and often haem- group has to fold it self to fit through the lumen, so you can imagine how thin it is, it's got a huge branch of network and hence a huge surface area.

Does that clear a few things up? I didn't want to write too much, as well, honestly, I can't be arsed. :-D
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thegodofgod
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(Original post by Ggdf)
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Think about it in terms of the numbers of each blood vessel:

Artery --> Arteriole --> Capillary

Few --> More --> Most

Therefore, even though the cross sectional area decreases, the total surface area increases dramatically as the number of each blood vessel increases.
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Ggdf
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(Original post by thegodofgod)
Think about it in terms of the numbers of each blood vessel:

Artery --> Arteriole --> Capillary

Few --> More --> Most

Therefore, even though the cross sectional area decreases, the total surface area increases dramatically as the number of each blood vessel increases.
Thank you very much for your reply!
So, do you mean that the diameter of the lumens decrease but the surface area of the vessels increase, and that it is this increase in surface area that accounts for the increased frictional resistance (stated by the first book) and hence a reduced rate of blood flow?
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thegodofgod
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(Original post by Ggdf)
Thank you very much for your reply!
So, do you mean that the diameter of the lumens decrease but the surface area of the vessels increase, and that it is this increase in surface area that accounts for the increased frictional resistance (stated by the first book) and hence a reduced rate of blood flow?
Yes, but it's the reduced diameter of the blood vessel's lumen that accounts for the increased frictional resistance, and hence a reduced rate of blood flow
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Pulkit Dandebaj
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It is right that Artery to Arteriole to Capillaries narrowing occurs But we consider Total Cross sectional areaIt means if i put these vessels of same length than suppose I have One aorta consider its cross section area suppose 2Then i have suppose 20 arteries since one artery is narrower than aorta so its cross section area is suppose 1 less than aorta but they are 20 so total cross sectional area is 1 multiply 20 equal 20 so this happens☺️☺️☺️
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