Male sex organ, which also provides the channel for urine to leave the body. Three long columns of tissue extend through its length, covered by elastic tissue and a thin layer of skin. One expands at the tip into a mushroom-shaped structure (glans penis) and contains the urethra (see urinary system), which ends in a slitlike opening. In sexual arousal, blood fills spaces in the tissue, and blood vessels constrict to hold it there, enlarging and hardening the penis in an erection. The foreskin, a circular fold of skin covering the glans, is often removed (see circumcision). See also reproductive system.
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