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!