Penile erection occurs when the soft spongy tissue in the shaft of a man's penis fills with blood, causing the penis to enlarge, stiffen and becoming erect.
Spongy spaces (technically known as corpora cavernosa and corpora spongiosa) along the length of the penis fill with blood in response to physical stimulation, mental stimulation, or both. Blood pooling in these spaces produces an erection.
Penile erections are dependent on opening of the arteries that feed blood to your penis resulting in engorgement of the spongy tissue. At the same time muscles at the base of the penis contract to stop the blood from draining out through the veins, thus maintaining the erection. Nerves in your spinal cord also control your erection, which receive input from physical contact to the penis and/or surrounding areas. Sexual thoughts, dreams, or images, and testosterone hormones can also stimulate an erection. If everything is working properly and there is sufficient blood flow and nerve impulses, a man is capable of getting an erection when sexually stimulated.
Penile erections come and go
The ability to get an erection is an automatic, normal function similar to breathing or blinking. An erection can take place in as little as several seconds or it can happen gradually over a longer period of time. As we age, 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 you may notice that his erection is not as firm as when you were 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 attain an erection.
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 and erections.