Erection
What causes an Erection? Well, whatever turns you on basically but the hard science is this: erections occur when the small muscles in your penis, which are usually tightly contracted, relax and let blood start flowing in. The spongy tissue in the penis fills with blood and expands, pushing against the veins and closing them so the blood cannot drain out again. Well, you did ask.
enlargement, hardening, and elevation of the male reproductive organ, the penis is called an Erection. 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 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 the Erection, 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.
Erection can occur throughout life, happening before birth and into the 90's in healthy men. Nocturnal Erection occur during all male dreams (regardless of what the dream is about), unless the man has physical problems (this is the easiest way to determine if impotence is physical or emotional in nature). The so called "morning Erection" is the result of being wakened during, or just after a dream; and it can be a very persistent Erection. While a morning erection is not a sign of arousal, it's presents and the pleasurable sensations it can create may result in arousal.
Men have only very limited control over their Erection. During puberty the young man is often embarrassed by Erection in public settings, but he gradually becomes able to suppress erections when the stimulation is mild. Likewise, it is impossible to "will" an Erection, although sexual thoughts can cause erection. During prolonged foreplay a man's Erection may go away; this is normal, and is not a sign of lessening interest.