Question

describe the process of erection

Answer 1

The blood then gets trapped under high pressure, creating an erection. ... Impulses from the brain and local nerves cause the muscles of the corpora cavernosa to relax, allowing blood to flow in and fill the open spaces. The blood creates pressure in the corpora cavernosa, making the penis expand and creating an erection

Answer 2

Under the influence of sexual stimulation blood flows in a large amount into  the penis and enters into the vascular spaces or sinuses which makes it rigid and erect. Expansion of these spaces compresses the veins and thus blood cannot drain out. Such condition of penis is called erection