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Organ Systems Involved

All organs systems in the body are affected by peripheral vascular resistance. The resistance of the blood
vessels is a significant component of what dictates blood pressure and perfusion of the tissues.
Mechanism

In the human body there is very little change in blood pressure as it travels in the aorta and large arteries,
but when the flow reaches the arterioles, there is a large drop in pressure, and the arterioles are the main
regulators of SVR.

The basis for the mechanism of peripheral vascular resistance is expressed by the Hagen-Poiseuille
equation:

R = 8Ln/(pi*r^4)

• R is the resistance of blood flow [change in pressure between the starting point and end
point]
• L is the length of the vessel
• n is the viscosity of blood
• r is the radius of the blood vessel

This equation shows that modifying the radius of the vessel has drastic effects on the resistance to blood
flow. As the vessel dilates (radius increases), the resistance is divided by the change to the fourth power;
this goes for a decrease in radius as well, such as during an adrenergic state (e.g., exercise) when blood
pressure must increase.

Related Testing

The calculation used to determine resistance in blood vessels (and all other liquid flow) is

R = (change in pressure across the circulatory loop) / flow.

Concerning systemic vascular resistance, this would be: (pressure immediately leaving the left ventricle –
pressure immediately upon entering the right atrium)/cardiac output.

Blood pressure is calculated by multiplying the cardiac output by the systemic vascular resistance.
The mean arterial pressure (MAP) can be calculated by the following:

[(2/3) x (diastolic blood pressure)] + [(1/3) x (systolic blood pressure)]

Mean arterial pressure is normally between 65 and 110 mmHg, with a MAP of over 70mmHg needed for
basic organ function.[6]
In septic shock, a MAP of 65 mmHg is considered sufficient for end-organ perfusion.[7]

Pathophysiology

Blood pressure mediation is by a balance of the cardiac output and the peripheral vascular resistance. In
idiopathic hypertension, most patients will have a near normal cardiac output, but their peripheral
resistance is elevated. As mentioned earlier, mediation of this resistance is at the level of the arteriole. As
with other tissues in the body, if there is prolonged constriction of the smooth muscle within the arterioles,
this will lead to hypertrophy and thickening of the vessel. There are several mechanisms by which the
systemic vascular resistance may be altered.[2][3]

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