Final answer:
Penile erections are caused by vasocongestion, or an increase in blood flow to the tissues of the pen is. Sexual arousal triggers the release of nitric oxide, which relaxes the smooth muscles surrounding the penile arteries and allows them to dilate. This dilation allows more blood to enter the pen is and leads to an erection.
Step-by-step explanation:
Penile erections are the result of vasocongestion, or engorgement of the tissues because of more arterial blood flowing into the pen is than is leaving in the veins. During sexual arousal, nitric oxide (NO) is released from nerve endings near blood vessels within the corpora cavernosa and spongiosum. Release of NO activates a signaling pathway that results in relaxation of the smooth muscles that surround the penile arteries, causing them to dilate. This dilation increases the amount of blood that can enter the pen is and induces the endothelial cells in the penile arterial walls to also secrete NO and perpetuate the vasodilation. The rapid increase in blood volume fills the erectile chambers, and the increased pressure of the filled chambers compresses the thin-walled penile venules, preventing venous drainage of the pe nis. The result of this increased blood flow to the pen is and reduced blood return from the pen is is erection.