Hormones, blood vessels, nerves, and muscles must all work together to make an erection. Your brain starts an erection by sending nerve signals to the penis when it senses sexual stimulation. Touching may cause this arousal. Another trigger may be something you see or hear. It may be a sexual thought or dream.
Your brain starts an erection by sending nerve signals to the penis.
The nerve signals cause the muscles within the penis to relax and let blood flow into the spongy tissue within the penis. Blood collects in this tissue like water filling a sponge. The penis becomes larger and firmer, like an inflated balloon. The veins then get shut off to keep blood from flowing out.
After climax, or after the sexual arousal has passed, the veins open back up and blood flows back into the body.
Healthy blood vessels are needed for an erection.
Source: National Kidney and Urologic Diseases Information Clearinghouse
The penis contains two chambers called the corpora cavernosa, which run the length of the organ (see figure 1). A spongy tissue fills the chambers. The corpora cavernosa are surrounded by a membrane, called the tunica albuginea. The spongy tissue contains smooth muscles, fibrous tissues, spaces, veins, and arteries. The urethra, which is the channel for urine and ejaculate, runs along the underside of the corpora cavernosa and is surrounded by the corpus spongiosum.
Erection begins with sensory or mental stimulation, or both. Impulses from the brain and local nerves cause the muscles of the corpora cavernosa to relax, allowing blood to flow in and fill the spaces. The blood creates pressure in the corpora cavernosa, making the penis expand. The tunica albuginea helps trap the blood in the corpora cavernosa, thereby sustaining erection. When muscles in the penis contract to stop the inflow of blood and open outflow channels, erection is reversed.
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Figure 1. Arteries (top) and veins (bottom) penetrate the long, filled cavities running the length of the penis—the corpora cavernosa and the corpus spongiosum. Erection occurs when relaxed muscles allow the corpora cavernosa to fill with excess blood fed by the arteries, while drainage of blood through the veins is blocked.
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Source: National Kidney and Urologic Diseases Information Clearinghouse
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