An erection occurs when the soft spongy tissue in the shaft of a man's penis fills with blood, causing the penis to enlarge and stiffen. Spongy spaces (technically known as corpora cavernosa and corpora spongiosa) along the length of the penis fill with blood in response to physical stimulation, psychological stimulation, or both. This process requires that the blood supply and the nerve connections to the penis are working properly.

Dilation of the arteries that feed blood to the penis results in engorgement of the spongy tissue. Simultaneous contraction of the muscles at the base of the penis prevents the blood from draining out through the veins, thus maintaining the erection.

Nerves in the spinal cord also control erection, which receive input from physical contact to the penis and/or surrounding areas, sexual thoughts, dreams, or images, and sex hormones.

Barring an erectile disease, and provided there is sufficient blood flow and nerve impulses, a man is capable of getting an erection when sexually stimulated. It is important to know that erections come and go. The ability of a man to get an erection is an automatic, normal function similar to his ability to breathe and blink his eyes.

A Normal Part of Aging
An erection can take place in as little as several seconds or it can occur gradually over a longer period of time. In the later years of a man's life, beginning in the 50's and increasingly in the 60's and 70's, it can sometimes take longer to achieve an erection even with direct stimulation and a man may notice that his erection is not as firm as when he was a teenager. This is a normal part of aging, but causes some men distress because they measure their maleness or ability to please a partner by the firmness and speed with which they become erect.

However, the older man has some advantages over the younger one because his ejaculatory control is usually greater, therefore he can maintain an erection for a considerably longer period of time without feeling the ejaculatory urgency common in younger men. This advantage may be lost in men who have prostate problems because they often experience leakage of the blood supply required to maintain an erection, and can have weaker ejaculations.

Penis Size Not Related to Pleasure
Men of all ages occasionally have concerns about the size of their erect penis and whether it is sexually adequate. Although a common worry, the size of a man's erection is not related to his ability to please a partner or enjoy sex himself.

In fact, continually thinking about penis size can interfere with achieving an erection, and with the giving and receiving of pleasure.