What else happens in your cardiovascular system in response to your sympathetic nervous system? why might your blood vessels need to expand to accommodate this?

When arteries become so tiny their walls are only a cell or two thick, they become capillaries. Capillaries are where the real action happens. Across those thin capillary walls, oxygen, nutrients, hormones, and other substances diffuse from the blood into the fluid that surrounds cells, and waste products diffuse from that fluid into the blood. This process is called capillary exchange, and it is the source of supply for every one of the trillions of cells in your body.Capillaries connect the arteries to veins in structures called capillary beds. At the end of an artery, the blood vessel branches into a netting of tiny capillaries that rejoin again on the otherside. Capillary beds are all over your body, and they account for over half of your 60,000 miles of blood vessels.