Vascular Resistance - Determinants

Vascular resistance

• The impediment to blood flow

Total peripheral resistance (aka, systemic vascular resistance)

• Describes the resistance to blood flow throughout the entire systemic vasculature (throughout the entire body)

Vascular resistance

• Resistance within a single organ; for example, resistance within in the kidney.

Three key determinants of resistance:

1. Blood viscosity
2. Vessel length
3. Vessel radius

Blood viscosity

• Is directly proportional to vascular resistance
• Hematocrit (the volume of red blood cells in the blood) is the primary determinant of blood viscosity.
• Clinical correlation: patients with abnormally elevated levels of blood products often manifest strokes from blood clots as a part of a broader hyperviscosity syndrome.

Vessel length

• Directly proportional to resistance
• Blood flow passing through a longer vessel will encounter greater friction, and, therefore, more resistance.

Vessel radius

• Indirectly proportional to resistance. Recall that the radius is the length of a line from the center of a circle to its perimeter; it is half the length of the diameter, which extends from one side to the other.
• The inverse relationship between vessel radius and resistance is NOT linear:
  When the radius decreases, resistance increases exponentially by the fourth power.

Poiseuille equation

• Describes how the determinants of blood resistance interact:
  Resistance = 8 * blood viscosity * vessel length / Pi * radius to the 4th power.

The arrangement of vessels affects resistance:

Series resistance:

• Illustrated by the blood vessels of a single organ
• Sum of the individual resistances that blood encounters as it flows through vasculature.
• Pressure decreases as blood moves through the series of vessels because of increasing resistance; it decreases most significantly in the arterioles.

Parallel resistance:

• Illustrated by the branching of the systemic circulation
• Each parallel artery receives a portion of the total blood flow
• Addition of parallel vessels decreases the total resistance
• If resistance within any one of the individual vessels increases, so will total vascular resistance.

Memory aid:

• If this is confusing, think of blowing through multiple straws: the more straws you add, the less resistance there is; but, if one of those straws becomes blocked, overall resistance increases.