Extrinsic Control a. Cardiac center in medulla oblongata can alter heartbeat through autonomic nervous system. Parasympathetic system promotes normal responses including slowing down heartbeat. Sympathetic system promotes stress responses including speeding up heartbeat. Need for oxygen, increase in blood pressure, etc. Systolic blood pressure is highest arterial pressure when ventricles are contracting.
Diastolic pressure is lowest arterial pressure when ventricles are relaxing.
Blood pressure decreases with distance from left ventricle. Blood pressure is highest in aorta, drops sharply in arterioles and again in capillaries, correlating with an increase in total cross-sectional area of vessels. In capillaries, blood velocity is slowest due to greatest total cross-sectional area. Slow capillary velocity allows more time for exchange of nutrients, gases, and waste products with tissues. Reduction in cross-sectional area as small venules join to form veins.