Resistance Opposition to Flow

Resistance Opposition to Flow is the physical property of the vasculature that quantifies the degree to which a given vessel or network of vessels impedes the movement of blood for a given driving pressure gradient, arising primarily from frictional interaction between flowing blood and the vessel wall as well as internal friction between adjacent layers of blood moving at different velocities. Resistance functions as the denominator term in the fundamental hemodynamic relationship linking pressure and flow, meaning that for any fixed pressure gradient, a higher resistance necessarily produces a lower flow, and a lower resistance necessarily produces a higher flow.

Physical Origin of Resistance

Frictional Interaction Between Blood and the Vessel Wall

As blood moves through a vessel, the layer of fluid immediately adjacent to the vessel wall experiences drag from that wall and moves more slowly than fluid nearer the center of the lumen, a phenomenon that gives rise to a velocity profile across the vessel cross section and that requires continuous expenditure of energy, supplied by the pressure gradient, to overcome. This wall associated friction is the dominant source of resistance in vessels of small to moderate diameter, where the surface area of the wall is large relative to the volume of fluid contained within the lumen.

Internal Friction Between Fluid Layers

In addition to friction at the vessel wall, adjacent layers of blood moving at different velocities exert friction on one another as they slide past each other, a property directly related to the viscosity of the fluid itself, so that a more viscous fluid generates greater internal friction and therefore greater resistance for a given vessel geometry and flow rate than a less viscous fluid would generate under the same conditions.

Quantitative Determinants of Resistance

The Hagen-Poiseuille Relationship

For steady, laminar flow of a Newtonian fluid through a rigid cylindrical vessel, resistance is described by the Hagen-Poiseuille relationship, which expresses resistance as a function of fluid viscosity, vessel length, and vessel radius.

Dominance of Radius Among Resistance Determinants

Among the three variables determining resistance, radius exerts by far the greatest influence, since resistance depends on radius raised to the fourth power, while it depends only linearly on length and viscosity, meaning that a given proportional change in radius produces a far larger proportional change in resistance than an equal proportional change in either length or viscosity. This extreme sensitivity to radius is the physical basis for why vasoconstriction and vasodilation, which alter radius, serve as the primary physiological mechanism for adjusting resistance, rather than mechanisms that would alter vessel length or blood viscosity.

Viscosity as a Secondary but Physiologically Relevant Determinant

Blood viscosity, while exerting only a linear influence on resistance, is nonetheless physiologically relevant because it can vary meaningfully under both normal and pathological conditions, rising with increased hematocrit, as occurs in polycythemia, and falling with decreased hematocrit, as occurs in anemia, so that changes in blood composition can alter resistance and therefore flow even in the absence of any change in vessel geometry.

Resistance in Networks of Multiple Vessels

Resistances Arranged in Series

When vessels are arranged in series, such as the successive artery, arteriole, capillary, venule, and vein supplying a single organ, the total resistance of the pathway is equal to the simple sum of the individual resistances of each segment.

Resistances Arranged in Parallel

When vessels are arranged in parallel, such as the many individual capillaries within a single organ's capillary bed, the total resistance is always less than the resistance of any individual parallel branch and is calculated according to the reciprocal summation rule.

This distinction between series and parallel summation has substantial physiological significance, since recruiting additional parallel capillaries within an actively metabolizing tissue lowers the total resistance of that tissue's vascular bed and increases flow to it, whereas the sequential arrangement of successive vessel types within a single pathway means that increased resistance at any single series segment raises the total resistance experienced along that entire pathway.

Visual Representation of Resistance Opposition to Flow

Physiological Significance of Resistance Regulation

Basis for Local and Systemic Flow Control

Because resistance is so sensitively determined by vessel radius, the circulatory system relies on active regulation of arteriolar smooth muscle tone as its primary mechanism for controlling both local tissue flow and systemic arterial pressure, exploiting the strong radius dependence of resistance to achieve substantial regulatory effects from comparatively modest changes in vessel diameter, a strategy far more efficient than attempting to regulate flow through changes in blood viscosity or vessel length, both of which are far less readily or rapidly adjustable by the body.