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.