Porth's Essentials of Pathophysiology, 4e

380

Circulatory Function

U N I T 5

U N D E R S T A N D I N G Hemodynamics of The term hemodynamics is used to describe factors such as (1) pressure and resistance, (2) vessel radius, (3) cross-sectional area and velocity of flow, and (4) laminar versus turbulent flow that affect blood flow through the blood vessels in the body.

Pressure difference

1

P 1

P 2

Pressure, Resistance, and Flow. The flow of fluid through a tube, such as blood through a blood vessel, is directly related to a pres- sure difference (P 1 − P 2 ) between the two ends of the tube and inversely proportional to the resistance (R) that the fluid encounters as it moves through the tube. The resistance to flow, in peripheral resistance units (PRUs), is determined by the blood viscosity, vessel radius, and whether the vessels are aligned in series or in parallel. In vessels aligned in series, blood travels sequentially from one vessel to another such that the resistance becomes additive (e.g., 2 + 2 + 2 = 6 PRU). In vessels aligned in parallel, such as capillaries, the blood is not confined to a single channel but can travel through each of several parallel channels such that the resistance becomes the reciprocal of the total resistance (i.e., 1/R). As a result, there is no loss of pressure, and the total resistance (e.g., 1/2 + 1/2 + 1/2 = 3/2 PRU) is less than the resis- tance of any of the channels. Vessel Radius. In addition to pressure and resistance, the rate of blood flow through a vessel is affected by the fourth power of its radius (the radius multiplied by itself four times). Thus, blood flow in vessel B with a radius of 2 mm will be 16 times greater than in ves- sel A with a radius of 1 mm. 2

Flow

Resistance

P 1

P 2

P i

P o

Flow

R 1

R 2

R 3

Series

R 1

P i

P o

R 2

Flow

R 3

Parallel

Pi, pressure in; Po, pressure out.

1 mm

1 mL/min

Vessel A

2 mm

16 mL/min

Vessel B