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Vascular Resistance

Vascular Resistance
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).
Resistance within an organ for example, resistance within in the kidney.
Determinants
Three key determinants of resistance: 1. Blood viscosity 2. Vessel length 3. Vessel radius
Blood viscosity
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.
Series vs Parallel Resistance
The arrangement of vessels affects resistance.
Series resistance: Illustrated by the blood vessels of a single organ.
Equal to the 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.

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