From Guyton and Hall Textbook of Medical Physiology:

Every time a person inspires, the pressure in the thoracic cavity becomes more negative than usual, causing the blood vessels in the chest to expand. This reduces the quantity of blood returning to the left side of the heart and thereby momentarily decreases the cardiac output and arterial pressure.

Why does vasodilation increase blood flow in the capillaries, but decrease it here?

As I understand it, vasodilation results in decreased resistance. According to Ohm's law, decreased resistance should increase blood flow.


1 Answer 1


The passage isn't talking about a magical diameter increase in a physics problem. If negative outside pressure causes blood vessels to increase in diameter, something has to fill the vacated space inside: blood.

That means that some of the blood that would be flowing through the vessels into the heart is instead filling the extra space in the vessels themselves.

You could most directly model this in terms of analogy to electricity by thinking about the vessel walls as a capacitor, and it takes some time to charge that capacitor when the outside pressure is released. If you measure flow at the input, you'll see a higher flow because a good portion is charging the capacitor/filling the vessels, but if you measure flow at the output you'll find a reduction.

I'm not strongly familiar with the effect described in Guyton and Hall but I'm guessing that the "momentarily" word in there is doing a lot of work, as this effect is going to be brief relative to the respiratory cycle.

  • $\begingroup$ ...if you measure flow at the output you'll find a reduction. This doesn't seem to happen in the capillaries (any capillaries, not necessarily in the chest) during vasodilation. Why? $\endgroup$
    – athlonusm
    Dec 11, 2021 at 8:31
  • $\begingroup$ @athlonusm It would momentarily if arterioles suddenly expanded. Once they are expanded and "full", though, there's nowhere to go but through. I don't know that it's possible for arterioles to expand via vasodilation as rapidly as you get in the chest from inspiration, though. $\endgroup$
    – Bryan Krause
    Dec 11, 2021 at 15:21

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