Vasoconstriction Resistance Increase

Vasoconstriction increases resistance by narrowing blood vessels, affecting blood flow and pressure in the cardiovascular system.

Vasoconstriction Resistance Increase is the rise in vascular resistance that results from active contraction of vascular smooth muscle, narrowing the luminal diameter of the affected vessel beyond its baseline, resting level and thereby increasing the resistance that vessel offers to blood flow according to the fourth power relationship between radius and resistance. As the physiological counterpart to vasodilation, vasoconstriction represents one of the two directions of active adjustment available to the vasculature for regulating local and systemic hemodynamics, achieved through the contractile response of smooth muscle cells within the vessel wall to a range of neural, hormonal, and local chemical stimuli.

Cellular Mechanism of Vasoconstriction

Calcium Mediated Smooth Muscle Contraction

Vasoconstriction at the cellular level results from an increase in intracellular calcium concentration within vascular smooth muscle cells, which activates the calcium binding protein calmodulin, which in turn activates myosin light chain kinase, an enzyme that phosphorylates myosin light chains and permits cross bridge cycling between actin and myosin filaments, generating the contractile force responsible for narrowing the vessel lumen.

Sources of the Constrictor Stimulus

The initiating stimulus that triggers this calcium dependent contractile cascade can arise from several distinct sources, including binding of norepinephrine released from sympathetic nerve terminals to alpha adrenergic receptors on the smooth muscle cell membrane, binding of circulating vasoconstrictor hormones such as angiotensin II or vasopressin to their respective receptors, or direct depolarization of the smooth muscle cell membrane in response to increased intraluminal pressure, reflecting the myogenic component of vascular tone.

Quantitative Effect on Resistance

Direct Application of the Fourth Power Relationship

Because resistance is inversely proportional to the fourth power of vessel radius, even a modest reduction in luminal radius produced by vasoconstriction results in a disproportionately large increase in vascular resistance, following directly from the Hagen-Poiseuille relationship.

R = \frac{8 η L}{π r^{4}}

Magnitude of Resistance Increase for a Given Constriction

A reduction of vessel radius to half its original value, a physiologically plausible degree of vasoconstriction in a resistance vessel, produces a sixteen fold increase in resistance, illustrating the powerful and disproportionate effect that modest degrees of smooth muscle shortening can exert on the resistance encountered by flowing blood.

\frac{R_{constricted}}{R_{baseline}} = {(\frac{r_{baseline}}{r_{constricted}})}^{4}

Consequences of Vasoconstriction for Flow and Pressure

Local Reduction of Flow

At the level of an individual vascular bed, vasoconstriction of the supplying arterioles increases the local resistance of that bed, and, assuming the driving pressure gradient remains unchanged, produces a corresponding decrease in blood flow delivered to the tissue supplied by that bed, a local effect exploited physiologically to reduce perfusion to tissues of lower momentary metabolic priority.

Q = \frac{Δ P}{R}

Systemic Elevation of Arterial Pressure

When vasoconstriction occurs simultaneously across a large proportion of the systemic arteriolar bed, the resulting increase in total peripheral resistance produces a corresponding rise in mean arterial pressure, assuming cardiac output remains relatively constant, illustrating how widespread vasoconstriction functions as a systemic pressure raising mechanism in addition to its local flow reducing effect within any single vascular bed.

Visual Representation of Vasoconstriction and Resistance Increase

Physiological Roles of Vasoconstriction

Redistribution of Blood Flow

Selective vasoconstriction within specific vascular beds, most notably the splanchnic and cutaneous circulations, allows the circulatory system to redirect a limited total cardiac output away from tissues of lower momentary priority toward tissues with greater immediate metabolic demand, such as active skeletal muscle during exercise or vital organs during circulatory stress.

Thermoregulatory Function

Vasoconstriction of cutaneous blood vessels reduces heat loss from the skin surface by decreasing blood flow through the superficial dermal circulation, a thermoregulatory mechanism activated in response to cold exposure that illustrates a role for vasoconstriction extending beyond pressure and flow regulation into the maintenance of core body temperature.

Compensatory Response to Hemorrhage and Hypotension

In response to blood loss or a fall in arterial pressure, baroreceptor mediated reflex activation of sympathetic vasoconstrictor outflow produces widespread arteriolar constriction, raising total peripheral resistance and helping to defend arterial pressure despite a reduced circulating blood volume or reduced cardiac output, representing one of the body's primary rapid compensatory mechanisms for maintaining adequate perfusion pressure during circulatory stress.