Anatomy & Physiology

Anatomy & Physiology Study Guide Since their walls are thinner and have a lower proportion of smooth muscle, veins are much more distensible (expandable) than arteries. For a given rise in blood pressure, a typical vein will stretch about eight times as much as a corresponding artery. The capacitance of a blood vessel is the relationship between the volume of blood it contains and the blood pressure. If a vessel acts like a 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 Pressure 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 and maintain adequate cardiac output. Pressure 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.

©2018

Achieve Page 262

of 368