You've probably noticed that your penis often does its own thing. You may remember times when it was completely inappropriate to have an erection; and yet you couldn't wish it away. It's true that you have less command over your penis than body parts like your arms and legs. That's because the penis answers to a part of the nervous system that's not always under your conscious control. This is called the autonomic nervous system, which also regulates heart rate and blood pressure.
Flaccid Penis: Average Size, How You Hang, and Other Facts
An erection clinically: penile erection or penile tumescence is a physiological phenomenon in which the penis becomes firm, engorged, and enlarged. Penile erection is the result of a complex interaction of psychological, neural, vascular, and endocrine factors, and is often associated with sexual arousal or sexual attraction , although erections can also be spontaneous. The shape, angle, and direction of an erection varies considerably in humans. Physiologically, erection is triggered by the parasympathetic division of the autonomic nervous system , causing the levels of nitric oxide a vasodilator to rise in the trabecular arteries and smooth muscle of the penis. The arteries dilate causing the corpora cavernosa of the penis and to a lesser extent the corpus spongiosum to fill with blood ; simultaneously the ischiocavernosus and bulbospongiosus muscles compress the veins of the corpora cavernosa restricting the egress and circulation of this blood.
Whether you have one or not, you probably think you know everything there is to know about penises. Older men are more likely to suffer from erectile dissatisfaction than erectile dysfunction. Penises are fragile. Yes, you can break your penis.