Anatomy & Physiology I and II

Anatomy & Physiology Study Guide balloon, easily expanding at low pressures, it has high capacitance. If it acts more like a truck tire, expanding only at high pressures, it has low capacitance. Veins, which expand easily, are called capacitance vessels. Thus, veins can accommodate large fluctuations in blood volume because the elastic walls stretch or recoil as necessary to withstand changes. 20.8 Blood P ssure Under normal circumstances, blood flow equals cardiac output. When cardiac output goes up, the blood flow through capillary beds does too; when cardiac output declines, capillary blood flow is reduced. Capillary blood flow is decided by the interplay between pressure (P) and resistance (R) in the cardiovascular network. There is a resistance to blood flow in the pulmonary and systemic circuits. To keep blood moving, the heart must overcome this resistance with sufficient pressure. In general terms, flow (F) is directly proportional to the pressure (increased P → increased F) and inversely proportional to resistance (increased R → decreased F). However, the absolute pressure is less important than the pressure gradient—the difference in pressure from one end of the vessel to the other. Blood leaving the peripheral capillaries enters the venous system. Although the pressure gradient across the venous system is relatively small, venous resistance is very low. This low venous blood pressure—aided by valves, skeletal muscle contraction, gravity, and other factors—is sufficient to return the blood to the heart. When necessary, cardiovascular control centers can elevate venous pressure (through venoconstriction ) to enhance venous return andmaintain adequate cardiac output. P ssure When talking about cardiovascular pressures, three values are usually reported: • Blood pressure: The term blood pressure (BP) refers to arterial pressure, usually reported in millimeters of mercury (mmHg). Average systemic arterial pressures range from an average of 100 mm Hg at the entrance to the aorta to roughly 35 mmHg at the start of a capillary network. • Capillary hydrostatic pressure: The pressure within capillaries is the capillary hydrostatic pressure (CHP), or capillary pressure. Along the length of a typical capillary, pressures decline from roughly 35 mmHg to about 18 mmHg. • Venous pressure: Venous pressure is the pressure within the venous system. Venous pressure is quite low. The pressure gradient from the venules to the right atrium is only about 18 mmHg. Total Peripheral Resistance Total peripheral resistance of the cardiovascular system indicates several factors: vascular resistance, blood viscosity, and turbulence. ©2018 Achieve Test Prep Page 261 of 367