Pulsatile arterial blood flow, resulting from the intermittent ejection of blood by the heart, is suppressed (Fig. 15.3). The systemic circulation is made up of a network of vessels, including major arteries, small arteries, arterioles, capillaries, venules, and veins (Fig. 15.1). The pressure drop across the small arteries and arterioles is connected to the relationship between blood flow velocity and cross-sectional area (Fig. 15.1). The arteries are represented by thick lines, the inverse relationship between maximal cross-sectional area and minimal flow rate in the capillaries is depicted by thin lines connecting the arteries to the capillary beds (Fig. 15.1). The veins are shown on the left side, while the arteries are on the right side of the diagram (Fig. 15.1). The crescent-shaped thickenings proximal to the capillary beds represent the arterioles, which serve as resistance vessels (Fig. 15.1). The internal diameter, wall thickness, and relative amounts of the vessel walls of various blood vessels are not drawn to scale due to the vast size range (Fig. 15.2). The combination of arterial distensibility and frictional resistance in the small arteries and arterioles contributes to the damping of blood flow velocity at the capillary level (Fig. 15.3). Despite the smaller cross-sectional area of each capillary compared to each arteriole, the total cross-sectional area of the capillary bed is large, resulting in a decrease in blood flow velocity in the capillaries, similar to the wide regions of a river (Fig. 15.3). The slow blood flow velocity in the capillaries, combined with their thin walls and short tubes, creates an ideal environment for the exchange of diffusible substances between blood and tissue (Fig. 15.3). As blood returns to the heart from the capillaries, it passes through venules and veins of increasing size (Fig. 15.3). The pressure within these vessels progressively decreases until the blood reaches the right atrium (Fig. 15.3). In proximity to the heart, the number of veins decreases, the thickness and composition of vein walls change, the total cross-sectional area of the venous channels diminishes, and the velocity of blood flow increases (Fig. 15.3). The velocity of blood flow and the cross-sectional area at each level of the vasculature exhibit a mirror image relationship (Fig. 15.3).
Fig. 15.3 ). Pulsatile arterial blood flow, caused by the intermittent ejection of blood from the heart, is damped • Fig. 15.3 Phasic Pressure, Velocity of Flow, and Cross-Sectional • Fig. 15.1 Schematic Diagram of the Parallel and Series Arrange-Area of the Systemic Circulation. The important features are the major ment of the Vessels That Constitute the Circulatory System. The pressure drop across the small arteries and arterioles, the inverse capillary beds are represented by thin lines connecting the arteries relationship between blood flow velocity and cross-sectional area, and (on the right) with the veins (on the left). The crescent-shaped thicken-the maximal cross-sectional area and minimal flow rate in the capilings proximal to the capillary beds represent the arterioles (resistance laries. (From Levick JR. An Introduction to Cardiovascular Physiology. vessels). (Redrawn from Green HD. In: Glasser O [ed]. Medical Physics. 5th ed. London: Hodder Arnold; 2010.)Vol 1. Chicago: Year Book; 1944.) • Fig. 15.2 Internal Diameter, Wall Thickness, and Relative Amounts of the Principal Components of the Vessel Walls of the Various Blood Vessels That Constitute the Circulatory System. Cross-sections of the vessels are not drawn to scale because of the huge range in size from aorta and venae cavae to capillary. (Redrawn from Burton AC. Relation of structure to function of the tissues of the wall of blood vessels. Physiol Rev. 1954;34:619.) at the capillary level by a combination of two factors: distensibility of the large arteries and frictional resistance in the small arteries and arterioles.
Many capillaries arise from each arteriole. The total cross-sectional area of the capillary bed is very large despite the fact that the cross-sectional area of each capillary is less than that of each arteriole. As a result, blood flow velocity becomes quite slow in the capillaries (see
Fig. 15.3 ), which is analogous to the decrease in velocity of flow in the wide regions of a river. Conditions in the capillaries are ideal for the exchange of diffusible substances between blood and tissue because capillaries consist of short tubes with walls that are only one cell thick and in which flow velocity is low.
On its return to the heart from the capillaries, blood passes through venules and then through veins of increasing size. Pressure within these vessels progressively decreases until the blood reaches the right atrium (see
Fig. 15.3 ). Near the heart, the number of veins decreases, the thickness and composition of the vein walls change (see
Fig. 15.2), the total cross-sectional area of the venous channels diminishes, and the velocity of blood flow increases (see
Fig. 15.3 ). The velocity of blood flow and the cross-sectional area at each level of the vasculature are essentially mirror images (see
Fig. 15.3