Vasodilation Resistance Decrease

Vasodilation Resistance Decrease is the fall in vascular resistance that results from relaxation of vascular smooth muscle, widening the luminal diameter of the affected vessel beyond its baseline, resting level and thereby reducing the resistance that vessel offers to blood flow according to the fourth power relationship between radius and resistance. As the physiological counterpart to vasoconstriction, vasodilation represents the second of the two directions of active adjustment available to the vasculature for regulating local and systemic hemodynamics, achieved through relaxation of smooth muscle cells within the vessel wall in response to a range of neural withdrawal, hormonal, and local chemical signals.

Cellular Mechanism of Vasodilation

Reduction of Intracellular Calcium and Myosin Light Chain Dephosphorylation

Vasodilation at the cellular level results from a decrease in intracellular calcium concentration within vascular smooth muscle cells, reducing calmodulin activation of myosin light chain kinase, while myosin light chain phosphatase continues to dephosphorylate myosin light chains, together shifting the balance away from actin myosin cross bridge cycling and permitting the smooth muscle cell to relax and the vessel lumen to widen.

Cyclic Nucleotide Mediated Relaxation Pathways

A major mechanism underlying active vasodilation involves elevation of intracellular cyclic guanosine monophosphate, produced in response to nitric oxide binding to soluble guanylate cyclase within the smooth muscle cell, or elevation of intracellular cyclic adenosine monophosphate, produced in response to activation of certain vasodilator hormone receptors, both of which activate protein kinases that reduce intracellular calcium availability and promote myosin light chain dephosphorylation, converging on the same fundamental relaxation pathway despite arising from distinct upstream signals.

Sources of the Dilator Stimulus

The initiating signal that triggers vasodilation can arise from reduced sympathetic vasoconstrictor discharge, allowing basal tone to fall toward its unopposed, more relaxed state, from binding of vasodilator substances such as nitric oxide, prostacyclin, or adenosine to their respective receptors on the smooth muscle cell, or from accumulation of local metabolic byproducts, including carbon dioxide, hydrogen ion, and potassium ion, within actively metabolizing tissue.

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 increase in luminal radius produced by vasodilation results in a disproportionately large decrease in vascular resistance, following directly from the Hagen-Poiseuille relationship.

Magnitude of Resistance Decrease for a Given Dilation

A doubling of vessel radius, a physiologically plausible degree of vasodilation in a resistance vessel, reduces resistance to one sixteenth of its original value, illustrating the powerful and disproportionate effect that modest degrees of smooth muscle relaxation can exert on the resistance encountered by flowing blood.

Consequences of Vasodilation for Flow and Pressure

Local Increase of Flow

At the level of an individual vascular bed, vasodilation of the supplying arterioles decreases local resistance, and, assuming the driving pressure gradient remains unchanged, produces a corresponding increase in blood flow delivered to the tissue supplied by that bed, a local effect exploited physiologically to increase perfusion to tissues with elevated metabolic demand.

Systemic Reduction of Arterial Pressure

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

Visual Representation of Vasodilation and Resistance Decrease

Physiological Roles of Vasodilation

Metabolic Flow Matching

Local vasodilation in response to accumulated metabolic byproducts, a mechanism referred to as active or metabolic hyperemia, allows individual tissues to autonomously increase their own blood supply in direct proportion to their momentary metabolic activity, a fundamental physiological mechanism through which actively contracting skeletal muscle, for example, secures a substantially increased blood supply during exercise.

Flow Mediated and Reactive Vasodilation

In addition to metabolically driven vasodilation, increased wall shear stress stimulates endothelial nitric oxide release and produces flow mediated dilation, while a period of vascular occlusion followed by restoration of flow produces an exaggerated, transient vasodilation referred to as reactive hyperemia, both mechanisms serving to match vascular caliber to prevailing or recently altered flow conditions within the vessel.

Compensatory and Thermoregulatory Roles

Vasodilation of cutaneous vessels increases heat loss from the skin surface in response to elevated core body temperature, serving a thermoregulatory function complementary to the heat conserving role of cutaneous vasoconstriction, while vasodilation of coronary and cerebral vessels helps preserve flow to these critical organs even under circulatory conditions that provoke widespread vasoconstriction elsewhere in the body.