The process of male erection begins with stimulation - visual, psychological, or physical. When the...
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The process of male erection begins with stimulation - visual, psychological, or physical. When the brain perceives stimulation and excitement, it sends signals down the spinal cord. The signals travel to nerve endings in two spongy tubes in the penis called the corpora cavernosa. The nerve endings release the chemical compound nitric oxide into blood vessels of the corpora cavernosa and surrounding tissues. The nitric oxide stimulates production of cyclic guanosine monosulphate, or cGMP. cGMP causes smooth muscles in the walls of arteries bringing blood into the penis to relax. When the walls of the arteries relax, blood flow into the corpora cavernosa increases. At the same time, the walls of the veins carrying blood out of the penis contract, so blood cannot flow out. As the arteries become full of blood, the penis becomes longer and harder, producing an erection. The erection is maintained as long as the nerve endings in the corpora cavernosa receive signals from the brain. When the brain is no longer stimulated, an enzyme called phosphodiesterase or PDE5 is released. PDE5 causes the breakdown of cGMP. The smooth muscles in the artery walls contract, stopping the flow of blood into the penis. And the walls of the veins relax, so blood can flow out.

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