A body part is said to be erythematous when it exhibits erythema or redness of the skin, and any part of the body that has excess flow of blood is said to be hyperemic, states The Patient's Guide. Erythema is defined as abnormal redness of the skin due to capillary congestion, and hyperemia is defined as excess of blood in a body part, according to Merriam-Webster.
When there is an increase in blood flow in the small blood vessels called capillaries in the lower layers of the skin, this is called hyperemia. This excess blood flow causes redness of the skin, which is called erythema, explains The Patient's Guide. Erythema can occur with inflammation, such as with sunburn and allergic reactions to drugs, states MedicineNet. Erythema is a symptom of hyperemia and usually disappears with finger pressure, reports The Patient's Guide.
Erythema can be caused by infection, medications, allergies, exercise or sunburn. These conditions cause the capillaries in the skin to dilate, resulting in redness, notes The Patient's Guide. Hyperemia causes a portion of skin to become red and warm to touch. This is because the blood collects in the small blood vessels. Hyperemia is most commonly caused by any obstruction or inflammation that prevents normal blood flow, explains SteadyHealth.com.