hyperemia

Hyperemia is an increase in blood flow to a region or tissue, produced by arteriolar dilation. It manifests as redness (erythema) and warmth in the affected area and can be physiological or pathological.

Functional (active) hyperemia occurs when blood flow rises to meet increased metabolic demand or to dissipate heat, such as in exercising muscle or inflamed skin. It results from local release of vasodilators, including nitric oxide, prostaglandins, and histamine, which expand the arterioles and capillary beds. Reactive hyperemia follows a period of brief ischemia and represents a transient surge of blood flow after reperfusion. Inflammatory hyperemia is part of the vascular response to injury, contributing to edema and leukocyte delivery.

Passive hyperemia, or venous congestion, arises from impaired venous outflow and increased venous pressure. It can occur with heart failure, venous thrombosis, or portal/systemic congestion and may produce tissue swelling and a blue-red coloration (cyanosis) with prolonged duration. Chronic passive hyperemia can lead to tissue hypoxia and structural changes.

Clinically, hyperemia is distinguished from edema by its origin in increased inflow rather than fluid accumulation in the interstitium; however, both can co-occur in inflammatory conditions. Diagnosis is usually clinical, supported by history and, if needed, imaging or functional tests to assess blood flow.

In summary, hyperemia describes enhanced regional blood flow due to arteriolar dilation, with functional and inflammatory forms representing increased inflow, and passive hyperemia reflecting venous congestion and impaired outflow.