Unit Four: The Circulation Chapter 15: Vascular Distensibility and Functions of the Arterial and Venous Systems Guyton and Hall, Textbook of Medical Physiology, 12 edition

Vascular Distensibility Units of Vascular Distensibility Differences in Distensibility of Arteries and Veins Veins are about 8X more distensible Therefore, an increase in pressure causes about 8X as much increase in a vein than in an artery c. Increase about 6X in the pulmonary circulation

Distensibility (cont.) Vascular Compliance (Vascular Capacitance) Compliance is equal to distensibility times volume Usually more important to know the total quantity of blood

Volume-Pressure Curves Fig. 15.1 “Volume-pressure curves” of the systemic arterial and venous systems, showing the effects of stimulation or inhibition of the sympathetic nerves to the CV system

Delayed Compliance (Stress-Relaxation) of Vessels Fig. 15.2 Effect of the intravascular pressure of injecting a volume of blood into a venous segment and later removing the excess blood, demonstrating the principle of delayed compliance

Arterial Pressure Pulsations Fig. 15.3 Pressure pulse contour in the ascending aorta

Arterial Pressure Pulsations (cont.) Systolic Pressure- the pressure at the top of each pulse Diastolic Pressure- the pressure at the bottom of each pulse Pulse Pressure- the difference between the systolic and diastolic pressures

Arterial Pressure Pulsations (cont.) Pulse Pressure- depends on two major factors Stroke volume output Compliance (total distensibility)

Abnormal Pressure Pulse Contours Fig. 15.4

Changes in Pressure Pulse Contours Fig. 15.6

Veins and Their Functions General Functions By constricting and enlarging they can store either large or small amounts of blood and make it available to the rest of the circulation when needed Propel blood forward by the “venous pump”

Veins and Their Functions (cont.) Venous Pressures Central venous pressure (pressure of the right atrum. Regulated by a balance between The ability of the heart to pump blood to the lungs, and The tendency of blood to flow from the peripheral veins into the right atrium

Veins and Their Functions (cont.) Increase in venous return is influenced by Increased blood volume Increased large vessel tone resulting in increased peripheral venous pressure Dilation of the arterioles Venous resistance and peripheral venous pressure

Fig. 15.9 Compression points that tend to collapse the veins entering the thorax

Veins and Their Functions (cont.) Effect of Gravitational Pressure on Venous Pressure Fig. 15.10 Effect of gravitational pressure on the venous pressures throughout the body

Veins and Their Functions (cont.) Venous Valves and the Venous Pump Valves are arranged so that so that the direction of blood flow can only be toward the heart Every time someone contracts a muscle or tenses a muscle, a certain amount of venous blood is propelled toward the heart

Veins and Their Functions (cont.) Blood Reservoir Function Specific blood reservoirs Spleen (red pulp) Liver Large abdominal veins Venous plexus beneath the skin