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

a. Veins are about 8X more distensibleb. Therefore, an increase in pressure causes about

8X as much increase in a vein than in an arteryc. Increase about 6X in the pulmonary circulation

Distensibility (cont.)

• Vascular Compliance (Vascular Capacitance)

a. Compliance is equal to distensibility timesvolume

b. Usually more important to know the totalquantity 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 eachpulse

• Diastolic Pressure- the pressure at the bottom ofeach pulse

• Pulse Pressure- the difference between the systolicand diastolic pressures

Arterial Pressure Pulsations (cont.)

• Pulse Pressure- depends on two major factors

a. Stroke volume output

b. Compliance (total distensibility)

Abnormal Pressure Pulse Contours

Fig. 15.4

Changes in Pressure Pulse Contours

Fig. 15.6

Veins and Their Functions

• General Functions

a. By constricting and enlarging they can storeeither large or small amounts of blood and makeit available to the rest of the circulation whenneeded

b. Propel blood forward by the “venous pump”

Veins and Their Functions (cont.)

• Venous Pressures

a. Central venous pressure (pressure of the rightatrum. Regulated by a balance between

1. The ability of the heart to pump blood to the lungs, and

2. The tendency of blood to flow from the peripheral veins into the right atrium

Veins and Their Functions (cont.)

b. Increase in venous return is influenced by

1. Increased blood volume2. Increased large vessel tone resulting in

increased peripheral venous pressure3. Dilation of the arterioles

c. 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

a. Valves are arranged so that so that the directionof blood flow can only be toward the heart

b. Every time someone contracts a muscle or tensesa muscle, a certain amount of venous blood ispropelled toward the heart

Veins and Their Functions (cont.)

• Blood Reservoir Function

a. Specific blood reservoirs

1. Spleen (red pulp)2. Liver3. Large abdominal veins4. Venous plexus beneath the skin