Before understanding how the male reproductive organ gets erect when stimulated sexually, it is imperative that we know a few terms.

Corpora cavernosa: The 2 tissues cylindrical in shape and running parallel on both side of the male organ. Its fibrous muscles are contracted when the organ is limp. Due to its porous nature they get fill with the blood.

Tunica: It is an outer sheath that is tough and covers the corpora and dictates the organ size after expansion. When blood flows within, it becomes rigid and hence is responsible for the hardness.

Corpus spongeosum: There exists a third tissue (also cylindrical) in midst of the corpora where the urethra is located. Urethra is from where the seminal and bodily fluids flow out. It thickens and tapers off to the end and looks like a helmet and is protected by the foreskin.

Step by step description of the male reproductive organ hard on:

1. A man gets sexually stimulated either by touch or images or sounds or reading or fantasies.
2. The signals from the para ventricular nucleus (a part of the brain) get increased.
3. The path of the signal is as follows: special autonomic nerves in spine → pelvic nerves → cavernous nerves (along the prostate glands) → the corpora cavernosa. It also reaches the arteries that will eventually supply the blood.
4. These signals cause the muscles in the corpora to relax which allows for a free flow of the blood (nearly 8 fold increase) both inside and outside of it.
5. This excess blood increases the size of the sinusoidal areas within the corpora that then expands the tunica.
6. This expansion of tunica consequently blocks the veins responsible for the ebbing of blood. This action traps the blood and the male organ gets erect.
7. During climax the signals from the brain modifies abruptly and the development of noradrenaline increases in the nerves of the genitalia. This results in further climaxing and also in the contraction of the muscle fibers in the corpora and surrounding arteries, thereby reducing the blood supply or flow.
8. The build up pressure drops and the tunica starts softening which opens up the veins and the blood ebbs out and the male reproductive organ becomes soft again.

If any one of this process breaks down then the organ does not get erect and this is the disorder termed as erectile dysfunction.