Ūplangung bið þæs stīfianung and rīsanung þæs pinteles, þe gelimpð āmang gemānalic āwreccanung, þēah hit cann ēac limpan in ungemānalic forcumungs. The primary physiological mechanism that brings about erection is the autonomic dilation of arteries supplying blood to the penis, which allows more blood to fill the three spongy erectile tissue chambers in the penis, causing it to lengthen and stiffen. The now-engorged erectile tissue presses against and constricts the veins that carry blood away from the penis. More blood enters than leaves the penis until an equilibrium is reached where an equal volume of blood flows into the dilated arteries and out of the constricted veins; a constant erectile size is achieved at this equilibrium.