Although there are different body parts associated with different reproductive systems, the evolutionary goal for these systems is to produce children. Reproductive systems are responsible for producing and transporting gametes (the sperm or egg), synthesizing hormones, and nurturing the offspring.
The ovaries and the testes release different concentrations of hormones, estrogen and testosterone, that drive puberty and the reproductive cycle. Sperm is stored in the testes, and eggs are stored in the ovaries. Every ovulation cycle, the eggs migrate through the fallopian tubes towards the uterus. Sperm travels from the testes through the vas deferens to the urethra and out through the opening at the tip of the penis. If an egg is fertilized with sperm, usually in the fallopian tubes, the combination of the two cells will develop in the uterus.
When sperm enters the uterus, it can move around to find and fertilize the released egg. Only one sperm can fertilize an egg. So how do twins happen? Fraternal twins are created when two eggs are released at once, and identical twins are created when one egg splits into two parts! The combination of an egg and a sperm in the uterus creates a cluster of cells that embeds itself in the lining of the uterus. Here, the cluster of cells, initially called a zygote, slowly divides and over many weeks, become more specialized and eventually begin to look like body parts.
While new sperm cells are created in the testes every day, a person with ovaries never creates more eggs after birth. The sperm and the egg each contain one half of the genetic material of the parent. This way, when the sperm and egg combine in the uterus, the resulting cell will have a full set of genetic material!
Sperm is found in the fluid that is ejaculated from the penis during orgasm, so sex that involves a penis and a vagina can result in pregnancy, even if the other partner doesn’t orgasm. So, if two people with these body parts engaging in sex do not want a child, precautions must be taken to prevent an unwanted pregnancy, like using a condom.
When the penis becomes aroused, a greater quantity of blood flows to the tissue, making it larger and more rigid. More blood also flows to the vulva when aroused, and certain tissues also secrete fluid to prepare the vulva and vagina for sex.
Did you know that some parts of the genitalia used in sex are actually not directly used for reproduction? The clitoris, part of the vulva, and the glans, part of the penis, have high concentrations of sensitive nerve endings and are associated with sexual pleasure, not reproduction.
Over time, a person’s number of eggs slowly decreases through a process called menstruation. Every 20-40 days, the uterus prepares a lining that would help sustain a fertilized egg. When an egg isn’t fertilized, the uterus sheds its lining out the vaginal canal as blood. Menstruation is different for different people – some people get symptoms like cramps, headaches, and irritability, and others don’t. The blood flow can last from 2 to 7+ days. Menstruation cannot be controlled like urine.
While some people describe these systems as the male and female reproductive systems, it’s important to remember that transgender people will not always have genitals that align with their gender. Therefore, it’s more accurate to talk about the specific parts in question, rather than generalizing the systems. Some people are actually born with genitals and reproductive systems that are be a blend of these two systems – this is called intersex.