The microcirculation consists of the vessels that make up the arterial and venous systems and is responsible for the exchange of nutrients and waste products between blood and tissues. The vasculature can be divided into three components: the arterial system, which distributes blood from the heart to the tissues; the venous system, which returns blood from the tissues to the heart; and the microcirculation, which separates the arterial and venous systems. This chapter describes these components and discusses the properties of blood flow to specific vascular beds and tissues.

Understanding the physics of blood flow through the vasculature is essential in comprehending the interplay between velocity of blood flow, blood pressure, and the dimensions of the various components of the systemic circulation. Although blood vessels are not rigid tubes and blood is not a homogeneous fluid, the principles of fluid flow through rigid tubes provide a foundation for understanding blood flow in blood vessels.

Velocity and flow are two important factors in fluid movement. Velocity refers to the distance traveled by a particle of fluid in a given time and is measured in units of distance per unit time. Flow, on the other hand, represents the rate of displacement of a volume of fluid and is measured in units of volume per unit time. In a rigid tube, velocity and flow are inversely related to the cross-sectional area of the tube. This means that as the cross-sectional area of the tube decreases, the velocity of the fluid increases and vice versa.

In the circulatory system, the velocity of blood flow varies at different points. As blood moves through the arterial system, its velocity progressively decreases. In the capillaries, the velocity reaches its lowest value. As blood flows centrally through the venous system towards the heart, its velocity gradually increases again. These variations in velocity are solely determined by the respective cross-sectional areas of the circulatory components.

The total energy in a hydraulic system consists of pressure, gravity, and velocity. The velocity of blood flow has a significant impact on the pressure within the tube. If we consider a tube with different cross-sectional areas, the total pressure within the tube equals the sum of the lateral pressure and the dynamic pressure. The gravitational component can be disregarded when the tube is horizontal. The total pressures in different segments of the tube are equal, assuming the total energy remains constant.
What vessels constitute the microcirculation? How is pulsatile blood flow in large arteries converted into steady flow in the microcirculation?
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What are the hydrostatic and osmotic factors that underlie Starling’s hypothesis for capillary function?
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How do intrinsic and extrinsic factors modulate peripheral circulation, and how do these factors affect blood flow in particular organs?
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How does the myogenic hypothesis account for autoregulation of blood flow? What is the effect of tissue metabolism on autoregulation?
he vasculature consists of a closed system of tubes or vessels that distributes blood from the heart to the tissues and returns blood from the tissues to the heart. It can be divided into three components: the arterial system, which takes blood from the heart and distributes it to the tissues; the venous system, which returns blood from the tissues to the heart; and the microcirculation, which separates the arterial and venous systems and is the site where nutrients and cellular waste products are exchanged between blood and tissues. These components of the vasculature are described in this chapter. In addition, the properties of blood flow to specific vascular beds and tissues are considered. As an introduction to this material, the physics of blood/fluid flow through the vasculature (i.e., hemodynamics) is reviewed.
The physics of fluid flow through rigid tubes provides a basis for understanding the flow of blood through blood vessels, even though the blood vessels are not rigid tubules (i.e., they are distensible) and blood is not a simple homogeneous fluid. Knowledge of these physical principles underlies understanding of the interrelationships among velocity of blood flow, blood pressure, and the dimensions of the various components of the systemic circulation.
Velocity of the Bloodstream
Velocity, as relates to fluid movement, is the distance that a particle of fluid travels with regard to time, and it is expressed in units of distance per unit time (e.g., centimeters per second). Flow, in contrast, is the rate of displacement of a volume of fluid, and it is expressed in units of volume per unit time (e.g., cubic centimeters per second). In a rigid tube, velocity (v) and flow (Q) are related to one another by the cross-sectional area (A) of the tube:
Equation 17.1
The interrelationships among velocity, flow, and area are shown in
Fig. 17.1 . Because conservation of mass requires that the fluid flowing through a rigid tube be constant, the velocity of the fluid varies inversely with the cross-sectional area. Thus fluid flow velocity is greatest in the section of the tube with the smallest cross-sectional area and slowest in the section of the tube with the greatest cross-sectional area.
Fig. 15.3 , velocity decreases progressively as blood traverses the arterial system. In the capillaries, velocity decreases to a minimal value. As the blood then passes centrally through the venous system toward the heart, velocity progressively increases again. The relative velocities in the various components of the circulatory system are related only to the respective cross-sectional areas.
The total energy in a hydraulic system consists of three components: pressure, gravity, and velocity. The velocity of blood flow can have an important effect on the pressure within the tube. Consider the effect of velocity on pressure in a tube with different cross-sectional areas (
Fig. 17.2 ). In this system, the total energy remains constant. The total pressure within the tube equals the lateral (static) pressure plus the dynamic pressure. The gravitational component can be neglected because the tube is horizontal. The total pressures in segments A, B, and C are equal, provided that •Fig. 17.1 Asfluidflowsthroughatubeofvariablecross-sectionalarea(A),thelinearvelocity(v)variesinverselywiththecross-sectionalarea.Q,flow.