Peripheral Resistance: Vessel Radius, Viscosity, Length + Blood Flow

Peripheral Resistance

Most resistance to blood flow is encountered away from the heart, in the body’s periphery

Blood Vessel Radius

Resistance varies inversely with the vessel’s radius (one-half of the vessel’s diameter); As a vessel dilates, the radius increases and resistance decreases

Blood Viscosity

Inherent resistance that all liquids have to flow; The more viscous a liquid, the more its molecules resist being put into motion and staying in motion

Changes in Blood Viscosity

Blood viscosity is relatively constant, but can be altered by states that change the number of cells, proteins, or the amount of water, such as dehydration

Blood Vessel Length

The longer the blood vessel, the greater the resistance; More pressure is needed to propel blood through a long vessel than a short one; Resistance is lower in the pulmonary circuit, which is shorter; Resistance is higher in people who are overweight

Obstructions

Obstructions, such as tumors, fatty plaques, or blood clots will also affect blood flow

Laminar Flow

Blood in the center of the vessel flows freely; Produces no audible sounds

Turbulent Flow

Blood is obstructed and produces sounds called Bruits, as it moves through the vessels

Cardiac Output (CO)

Product of stroke volume (the amount of blood pumped with each beat) times heart rate (the number of beats per minute)

Pressure Gradient, Cardiac Output, and Peripheral Resistance

ΔP = CO × PR

Cardiac Output and Blood Pressure

When cardiac output increases, blood pressure increases

Autonomic Effects on Cardiac Output

CO increases in response to sympathetic innervation and decreases in response to parasympathetic innervation