The SRC is composed of four phases:
Excitement: Activation of the sympathetic branch of the autonomic nervous system defines the excitement phase; heart rate and breathing accelerates, along with increased blood flow to the penis, vaginal walls, clitoris, and nipples. Involuntary muscular movements (myotonia), such as facial grimaces, also occur during this phase.
Plateau: Blood flow, heart rate, and breathing intensify during the plateau phase. During this phase, often referred to as “foreplay,” females experience an orgasmic platform—the outer third of the vaginal walls tightening—and males experience a release of pre-seminal fluid containing healthy sperm cells (Killick et al., 2011). This early release of fluid makes penile withdrawal a relatively ineffective form of birth control (Aisch & Marsh, 2014). (Question: What do you call a couple who use the withdrawal method of birth control? Answer: Parents.)
Orgasm: The shortest but most pleasurable phase is the orgasm phase. After reaching its climax, neuromuscular tension is released and the hormone oxytocin floods the bloodstream—facilitating emotional bonding. Although the rhythmic muscular contractions of an orgasm are temporally associated with ejaculation, this association is not necessary because orgasm and ejaculation are two separate physiological processes.
Resolution: The body returns to a pre-aroused state in the resolution phase. Males enter a refractory period of being unresponsive to sexual stimuli. The length of this period depends on age, frequency of recent sexual relations, level of intimacy with a partner, and novelty. Because females do not have a refractory period, they have a greater potential— physiologically—of having multiple orgasms. Ironically, females are also more likely to “fake” having orgasms (Opperman et al., 2014).
Further, a partner or environmental object is sufficient, but not necessary, for the SRC to occur.
Pregnancy
One of the potential outcomes of the SRC is pregnancy—the time a female carries a developing human within her uterus. How does this happen?
Human male, sex chromosomes highlighted [Image: National Human Genome Research Institute, https://goo.gl/8UWQwa, Public Domain]
The process begins during vaginal intercourse when the male ejaculates, or releases semen. Each ejaculate contains about 300 million sperm cells. These sperm compete to make their way through the cervix and into the uterus. Conception typically occurs within a fallopian tube when a single sperm cell comes into contact with an ovum (egg). The sperm carries either an X- or Y- chromosome to fertilize the ovum— which, itself, usually carries an X- chromosome. These chromosomes, in combination with one another, are what determine a person’s sex. The combination of two X chromosomes produces a female zygote (fertilized ovum). The combination of an X and Y chromosome produces a male zygote.
Interestingly, at least 1 in every 1,000 conceptions results in a variation of chromosomal sex