erection, enlargement, hardening, and elevation of the male reproductive organ, the penis. Internally, the penis has three long masses of cylindrical tissue, known as erectile tissue, that are bound together by fibrous tissue. The two identical areas running along the sides of the penis are termed corpora cavernosa; the third mass, known as the corpus spongiosum, lies below the corpora cavernosa, surrounds the urethra—(a tube that transports either urine or semen),—and extends forward to form the tip (or glans) of the penis. All three masses are spongelike; they contain large spaces between loose networks of tissue. When the penis is in a flaccid, or resting, state, the spaces are collapsed and the tissue is condensed. During erection, blood flows into the spaces, causing distention and elevation of the penis. The amount of blood entering the penis can be increased by physical or psychological stimulation. As blood enters, there is a temporary reduction in the rate and volume of blood leaving the penis. The arteries carrying blood to the penis dilate; this, in turn, causes tissue expansion. The veins leading from the penis have funnel-shaped valves that reduce the outflow of blood. As the erectile tissue begins to enlarge, the additional pressure causes the veins to be squeezed against the surrounding fibrous tissue, and this further diminishes the outflow of blood. Essentially, blood becomes temporarily trapped in the organ.
The corpus spongiosum does not become as erect as the corpora cavernosa. The veins are more peripherally located, so that there is a continual outflow of blood in this region. This constant circulation prevents the urethra from being collapsed by the adjacent tissue, which would prevent release of the semen.
The penis returns to its flaccid state when the arteries relax and begin to contract. Blood flow is once again reduced to its usual rate and volume. As blood drains from the erectile tissue spaces, pressure is reduced on the veins, and flow continues at its normal pace. See also ejaculation.