Copulation is also known as coitus, sexual intercourse or the reproductive act wherein the reproductive organ of a male (in human beings and other superior animals) enters into the reproductive zone of a female.
In case the reproductive act gets completed, the sperm cells pass on from a male body into the female body. If the female body has ovulated an egg during that period of time, one of the sperm cells may penetrate and fertilize the egg beginning formation of a new organism.
In many vertebrates, like fish, their eggs are laid out of their body and the fertilization takes place externally.
For accomplishing internal intercourse, some body and natural variations are essential. In a human male, the penis does both the reproductive and excretory tasks.
During copulation, the blood flow temporarily surges and gets trapped in the penis. And with this, the penis swells to become large and takes an elevated position. This condition of the penis is called erection.
Erection transforms the usually flexible, soft and drooping organ into the one of ample proportions and acquires stiffness to allow effortless penetration into the female reproductive tract.
The intercourse terminates and culminates in orgasm. This is a process wherein the copulating male releases semen having sperm cells. One of the sperm may get united with the egg and fertilize it. The semen, thus expelled into the woman’s vaginal canal, also contains metabolites, salts, and water, besides other necessary cell nutrients.
In the reproductive system of a woman, the outer opening leading to the vagina, in turn, communes with the uterus, a pear-shaped and thick-walled organ where the zygote formation, after the egg fertilization, takes place. The zygote, thus formed, develops into an embryo and later as a fetus to, eventually, take birth as a baby.
In living humans, the physiological events happen in a series during sexual arousal and copulation. This series of events may be recognized as happenings, one after the other, in four phases:
- Orgasm, and
The fundamental pattern is the same in both the sexes, in spite of having precise sexual motivations.
During the excitement phase, the body prepares itself for performing sexual activity and in the process your pelvic muscles get tensed and heart rate also increases.
In males, the blood gushes into the penis, making it to swell and become large and erect; in females, the vaginal walls get wet, the internal part of vagina gets widened, and the clitoris becomes bigger.
During the plateau phase, breathing gains impetus and the muscles remain tensed. The penis glans, also known as the penis head, swells and the testicles becomes large in the man; in the woman, the clitoris draws back and the external vagina constricts.
During the orgasm stage, the neuromuscular anxiety that was built up during the preceding phases gets released in a few seconds. In the female, the vagina starts a series of continuous contractions; in the male, the penis begins contracting rhythmically to release the sperm and semen. The condition is better known as ejaculation.
During the subsequent resolution phase comes a gradual return of the resting phase, which may take many hours. In the man, the penis contracts back to its usual size; in the woman, the vagina and her other genital formations also come back to the pre-excitement state.
The resolution stage among men includes a refractory period rendering the man incompetent to handle any more sexual arousal. This refractory period may last from several to a small number of hours.
Women do not need or have such refractory period. They quickly get aroused again at any point of their resolution stage.
Some of the animals, like migratory birds and sheep, indulge in copulation only seasonally. During the reproduction season, hormones get generated produced in both the female and male species. These hormones help in preparing their reproductive systems for intercourse.
During the non-breeding period, the hormonal levels drop rendering the animals incapable of fertilized reproduction. They, thus, have no desire to indulge in copulation during this period.