## Peripheral Resistance And Its Determinants

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Pressure, flow, and resistance are related most often through Poiseuille's equation, which was first formulated in 1842. Based on a series of careful observations of water flowing through rigid tubes, Poiseuille demonstrated that the resistance to flow R through a tube is proportional to tube length L and fluid viscosity h and inversely proportional to the tube radius to the fourth power (r4). These variables can be related to each other in the following way:

Poiseuille's equation applies to the behavior of Newtonian fluids flowing in a nonpulsatile, nonturbulent (laminar) manner through rigid tubes. Although the vascular system satisfies none of these parameters, the equation is useful because it predicts that flow Q is proportional to r4 (and inversely proportional to resistance: Qfl /'', or (JO. 1 /A>1, where r is radius and R is resistance), i.e., given the same initial pressure, doubling the inner radius of a tube will result in a sixteenfold (24) increase in flow. Estimates from intact vascular networks suggest that this may be an overestimation and that a third-power equation (Q « r3) may be more accurate.!73 Nevertheless, it is clear that relatively minor changes in arterial caliber can produce large changes in resistance and flow.

The relationship between vascular resistance and blood flow may be defined by an equation that is analogous to Ohm's law for the flow of electrons, where flow = perfusion pressure/resistance. The complete formulation includes the main determinants of resistance (labeled as above in Poiseuille's equation) and is called Poiseuille's law:

where Q is flow, PP is perfusion pressure, and the determinants of resistance (inverted during the simplification of the quotient) are signified as above. Although flow resistance also can be affected by the viscosity of the blood (h) and the length of the vessel (L), as predicted by the Poiseuille formula, these parameters are normally relatively invariant within the adult cardiovascular system. For this reason, lumen diameter is the single most powerful determinant of vascular resistance (and blood flow) under physiologic conditions. Its control is the primary end point for a variety of physiologic mechanisms.

Most of the pressure drop between large conduit arteries and capillaries occurs in vessels having lumen diameters of a few hundred microns or less. For this reason, small arteries and arterioles are considered to be of primary importance in regulating and determining peripheral resistance. These muscular vessels have a high wall-thickness to lumen diameter ratio and usually contain one to three layers of circumferentially oriented vascular smooth muscle cells. They also normally possess some degree of basal tone and are capable of diameter changes that range from fully open to virtually closed. Hence their potential to affect resistance is considerable. In contrast, large conduit arteries such as the aorta only constrict by 10 to 20 percent. Unless they become diseased, they are of minor importance to peripheral resistance and blood flow control. The relationship between blood pressure, blood flow velocity, and total cross-sectional area in various blood vessels of the systemic circulation is summarized in Fig. 3-23. Figure 3-23: Relations between total cross-sectional area of the vascular bed (cm2), velocity of blood flow (cm/s), and blood pressure (mmHg) in various vessels in systemic circulation. (From Marieb EN. Human Anatomy and Physiology. Redwood City, CA: Benjamin/Cummings; 1989:629. Reproduced with permission of the publisher.)

Figure 3-23: Relations between total cross-sectional area of the vascular bed (cm2), velocity of blood flow (cm/s), and blood pressure (mmHg) in various vessels in systemic circulation. (From Marieb EN. Human Anatomy and Physiology. Redwood City, CA: Benjamin/Cummings; 1989:629. Reproduced with permission of the publisher.)

Larger vessels do contribute significantly to resistance in some organs, such as the brain,174 primarily due the more linear geometry of the arterial vasculature. In a serial arrangement of tubes, total resistance Rt is the sum of individual resistance elements (Rt = Rj + R2 + R3, etc.). Conversely, in regional circulations in which vessels are highly and sequentially branched, resistance tends to be localized to smaller (<50 Mm) arteries. This occurs because a greater number of parallel elements lowers the overall resistance of the array. This makes sense because, given the same driving pressure, there are more tubes to conduct flow, and in this case, total resistance is defined by a reciprocal relationship (1/Rt = 1/Rj + I/R2 + I/R3, etc.). These concepts are shown in Fig. 3-24.

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### Responses

• JEANNE
What is the relationship between blood flow blood pressure and resistance?
8 years ago
• clarence
What is the relationship between peripheral resistance and blood pressure?
8 years ago
• michael
What is the principle determinant of coronary artery resistance?
6 years ago
• harri
Which blood vessels are the major determinant of peripheral resistance?
6 years ago
• Jordan
What is the primary determinant of peripheral resistance?
5 years ago
• Berhane
What are the determinants of peripheral resistance?
4 years ago
• Eyob
Which blood vessel is major determinant of peripheral resistance?
4 years ago
• belba
How is peripheral ressutance influence by velocity of blood flow and radius of blood vessel?
3 years ago
• fernando
Which blood vessel has high peripheral resistanve?
2 years ago
• Kristel
Which vessel is major deterninant of peripheral resistance?
2 years ago
• eglantine
Is peripheral resistance related to instant radius of arterioles vessels?
2 years ago
• zachary
What is the most important determinant of peripheral resistance?
8 months ago