The penis is an external male reproductive organ. The urethra passes through the length of the penis and allows urine and semen to exit the body. The penis has two distinct parts: The head (glans) and the shaft. In uncircumsized males, foreskin covers the head of the penis.
When a man becomes sexually aroused, the spongy erectile tissue in the penis becomes engorged with blood. When this happens, the penis becomes erect, or hard. This makes it possible for the penis to be inserted into the vagina during sexual intercourse. The friction during intercourse then stimulates the sensitive nerve endings in the penis, which can eventually lead to ejaculation and orgasm.