Mechanism of erection

Mechanism of erection: An erection starts in the brain. Something you saw, felt, smelled, heard, or thought makes your nerves send a chemical message to the blood vessels in your penis. The arteries relax and open to let more blood flow in. At the same time, the veins close. Once the blood is in penis pressure is exerted in corpora cavernosa. So, penis expands and holds the erection. When the inflow of blood stops and the veins open, your penis becomes soft. 

Copulation: When you are aroused vas deferens squeeze sperm from the testes towards the urethra. The seminal vesicles also release fluid there. Then at the height of sexual excitement, it sends signals to your spinal cord which in turn sends signals to muscles at the base of your penis. These contract powerfully and quickly every 0.8 seconds. This forces the semen out of the penis as you climax. This is called ejaculation. At the peak of sexual pleasurable sensation occurs which is called orgasm. It lasts only a few seconds.