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Hemodynamics

CARDIOVASCULAR PHYSIOLOCY

Pressure Difference (AP)

= Resistance to flow

Resistance to Flow (R)

Poiseuille's Law	Resistance (R)	100
itxLx8 ^R	per unit length	10
	( ^mmHg \	1
T) = viscosity L - vessel length		0.1
r “ vessel radius		0.01

292521 1713 9 3 9 13 17212529 Vcs$d Radiu* (|un)

J. Perkins

rv2
L*v2	g f»00
Ł* V,	a 3 500
m+v2	< * *X)
	| 300-
Lv	% 200-
Łv, > ^V2	0 100-
-Large are	a (A2)
lowvełodty (V2)

Turbulent Flow

Pressure Difference (AP)

Figurę 4.13 Hemodynamics

Panel A: The tlow of blood (Iow through a vessel (Q) depends on the pressure difference (AP) and the resistance to tlow (R). In the systemie circulation, AP = aortic pressure — right atrial pressure, and R = the total peripheral resistance (TPR). Panel B: As described by Poiseuille's law, the most important factor determining resistance to blood flow i$ the radius of the vessel. The smali arterides and capil* laries have the highest resistance. Because of their ability to regulate their tonę, the smali arterioles are the most important vessels involved in regulating the TPR. Panel C: As blood flows from the aorta it courses through an increasingly branched arterial system.

This branching inereases the total cross-sectional area through which the blood flows and reduces the velocity of this flow (V = Q/A). The reduced flow velocity in the capillaries facilitates the exchange of fluid* and nutrients across the capillary wali by allowing sufficient time for diffusion to occur. Panel D: Normally, blood flow through most of the vascular system is laminar. The exception is at the root of the aorta. However, in pathological conditions <e.g., lesions of the heart valves, narrowing or partial blockage of vessels), turbulent flow occurs and can be heard with the stethoscope as murmurs (in the heart) or bruits (in yessels). Laminar flow reduces the pressure gradient needed to pro|>el the blood through the vessel.

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