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Which tissues fill with blood during an erection?

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Front view anatomical diagrams of the male and female reproductive systems, with labeled organs including penis, testis, prostate gland, uterus, ovary, and vagina. — human reproductive system Organs and structures of the male and female reproductive systems.

During an erection, blood flows into three long, cylindrical masses of spongelike erectile tissue within the penis. These tissues are:1

Corpora cavernosa: Two identical bodies running along the sides of the penis. These contain large spaces that fill with blood, causing the organ to enlarge and harden.12
Corpus spongiosum: A third mass located below the corpora cavernosa that surrounds the urethra. While also considered erectile tissue, it does not become as rigid as the corpora cavernosa because it maintains a more constant blood flow, which prevents the urethra from collapsing.12

When stimulated, the arteries dilate to increase blood flow into these spaces, while the veins are compressed, temporarily trapping the blood and causing the penis to become erect.12