1.17.1 Vascular Resistance Definition

Vascular resistance is the force that opposes blood flow, determined by vessel diameter, length, and blood viscosity, crucial in regulating blood pressure and circulation.

Vascular Resistance Definition is the opposition to blood flow generated by the vessels of the circulatory system, quantifying the relationship between the pressure gradient driving blood forward and the resulting flow achieved through a given vascular segment. Vascular resistance arises primarily from friction between moving blood and the vessel wall, and it is determined chiefly by vessel radius, with vessel length and blood viscosity contributing additional, though generally less variable, influence.

Mathematical Definition

Vascular resistance is derived directly from the relationship between pressure gradient and flow.

Resistance as Pressure Divided by Flow

Vascular resistance equals the pressure gradient across a vascular segment divided by the flow passing through it.

Resistance = \frac{Δ}{Flow} Pressure

Poiseuille's Law

More fundamentally, resistance depends on the physical properties of the vessel and the fluid moving through it, expressed by Poiseuille's law as a function of viscosity, vessel length, and vessel radius.

Resistance \propto \frac{Viscosity \times Length}{r^{4}}

Determinants of Vascular Resistance

Each physical variable within Poiseuille's law contributes to overall resistance, though not with equal physiological importance.

Vessel Radius

Because resistance varies inversely with the fourth power of radius, vessel radius exerts by far the most powerful influence on resistance, making it the primary variable physiologically regulated to control blood flow.

Vessel Length

Longer vessels present greater total resistance than shorter vessels of the same radius, though vessel length is generally fixed anatomically rather than being subject to acute physiological regulation.

Blood Viscosity

More viscous blood generates greater resistance for a given vessel geometry, with viscosity influenced by factors such as hematocrit, though it typically changes more slowly than vessel radius.

Sites of Resistance Within the Circulation

Resistance is not distributed evenly throughout the vascular system but is concentrated predominantly at specific locations.

Arteriolar Dominance

Arterioles, owing to their small radius and substantial smooth muscle content, contribute the largest share of total vascular resistance and serve as the principal site through which resistance is actively regulated.

Relationship to Blood Pressure

Vascular resistance, together with cardiac output, determines the level of arterial blood pressure maintained within the systemic circulation.

Mean Arterial Pressure \approx Cardiac Output \times Systemic Vascular Resistance

Diagrammatic Summary

Clinical Relevance

Because vascular resistance so strongly influences both blood pressure and the distribution of blood flow, its regulation is a central target of cardiovascular therapy, with medications that alter arteriolar tone used widely to manage hypertension, heart failure, and conditions of abnormal regional perfusion.