Sexual excitement, which stimulates the nerves of the autonomic system, causes the veins in the penis to constrict and the arteries to dilate. The arteries carry a great volume of blood under significant pressure into the penis. The blood fills the spaces within the penis's spongy, erectile tissue of which the penis is largely composed. The engorged tissue causes the penis to become greatly increased in size. As a result, the penis becomes hard and erect, prepared for insertion into the female vagina.
During copulation, the penis is stimulated by friction. When sufficiently stimulated, waves of contraction sweep through the smooth muscles of the epididymis, vasa deferentia, seminal glands, and urethra. The contractions are caused by nervous reflexes involving pathways of the sympathetic system. The contractions expel the mixture of seminal fluids and sperm called semen from the urethra into the vagina. One human male ejaculation contains an average of around 100 million sperm in a volume of about 3 to 4 ml of semen.
In addition to serving as a vehicle of transport for the sperm, the seminal fluid serves three other important functions. The fluid lubricates the passages through which the sperm must travel, the fluid acts as a buffer protecting the sperm from the acid environment of the female genital tract, and the fluid which contains a great amount of sugar acts as the food source for the active sperm. Not able to store food themselves, the sperm absorb energy from the external sugar source. The mitochondria packed base of the sperm flagellum converts the nutrients in the semen
