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Functional hyperemia, or active hyperemia, is the increased blood flow that occurs when tissue is active.

When cells within the body are active in one way or another, they use more oxygen and fuel, such as glucose or fatty acids, than when they are not. The blood vessels compensate for this metabolism by dilatation, allowing more blood to reach the tissue. This prevents deprivation of the tissue.

Since most of the common nutrients in the body are converted to carbon dioxide when they are metabolized, smooth muscle around blood vessels relax in response to increased concentrations of carbon dioxide within the blood and surrounding interstitial fluid. The relaxation of this smooth muscle results in vascular dilation and increased blood flow.

Some tissues require oxygen and fuel more quickly or in greater quantities. Examples of tissues and organs that are known to have specialized mechanisms for functional hyperemia include:

• The brain through the neuron-dependent hemodynamic response.
• Penile erection tissue by release of nitric oxide.

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