Tutorial eBooks PDF File Download

The walls of the blood vessels contain concentric layers of smooth muscle.These muscles and the muscles of the heart are controlled by autonomicnervous system nerves that regulate blood vessel size. A decrease in the diameters of the blood vessels makes itmore difficult for blood to flow throughthe vessels. When this decrease in sizeoccurs, greater pressure is required toforce the blood through the vessels. Thisincreased pressure is exerted by the heartas it forces blood into the aorta. In con-trast, when the smooth muscles of theblood vessels relax, increasing the diam-eters of the vessels, resistance to bloodflow is reduced and less blood pressureis required to maintain a given flow rate.Not only does the autonomic nervoussystem control the diameter of bloodvessels, it also modulates the rate atwhich the heart pumps—either byspeeding it up, or slowing it down.Through these functions, the autonomicnervous system regulates blood pressure.