The biological aspects of human sexuality include the reproductive system, the sexual response cycle, and the neurological and hormonal factors that affect these processes.
Physical Anatomy and Reproduction
Different sexes are anatomically very similar; however, they each have different physical mechanisms that enable them to perform sexual acts and procreate. For the purposes of this discussion, we will be examining male and female anatomy; however, it is important to keep in mind the wide variety of intersex anatomy that exists, and that much of the biology below corresponds to different intersex bodies in different ways.
Female Anatomy
Female external genitalia is collectively known as the vulva, which includes the mons veneris, labia majora, labia minora, clitoris, vaginal opening, and urethral opening. Female internal reproductive organs consist of the vagina, uterus, fallopian tubes, and ovaries. The uterus hosts the developing fetus, produces vaginal and uterine secretions, and passes the male's sperm through to the fallopian tubes; the ovaries release the eggs. A female is born with all her eggs already produced. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the fallopian tubes. Females have a monthly reproductive cycle; at certain intervals the ovaries release an egg, which passes through the fallopian tube into the uterus. If, in this transit, it meets with sperm, the sperm might penetrate and merge with the egg, fertilizing it. If not fertilized, the egg is flushed out of the system through menstruation.
Biology of female reproductive system
The female reproductive system consists of both internal organs and external genitalia.
Male Anatomy
Males also have both internal and external genitalia that are responsible for procreation and sexual intercourse. Males produce their sperm on a cycle, and unlike the female's ovulation cycle, the male sperm production cycle is constantly producing millions of sperm daily. The main male sex organs are the penis and the testicles, the latter of which produce semen and sperm. The semen and sperm, as a result of sexual intercourse, can fertilize an ovum in the female's body; the fertilized ovum (zygote) develops into a fetus, which is later born as a child.
Biology of male reproductive system
Like females, males have both internal and external genitalia that are responsible for procreation and sexual intercourse.
The Sexual Response Cycle
Sexual motivation, often referred to as libido, is a person's overall sexual drive or desire for sexual activity. This motivation is determined by biological, psychological, and social factors. In most mammalian species, sex hormones control the ability to engage in sexual behaviors. However, sex hormones do not directly regulate the ability to copulate in primates (including humans); rather, they are only one influence on the motivation to engage in sexual behaviors. Social factors such as work and family also have an impact, as do internal psychological factors like personality and stress. Sex drive may also be affected by medical conditions, medications, lifestyle stress, pregnancy, and relationship issues.
The sexual response cycle is a model that describes the physiological responses that take place during sexual activity. According to William Masters and Virginia Johnson, the cycle consists of four phases: excitement, plateau, orgasm, and resolution. The excitement phase is the phase in which the intrinsic (inner) motivation to pursue sex arises. The plateau phase sets the stage for orgasm. Orgasm is the release of tension, and the resolution period is the unaroused state before the cycle begins again.
The Brain and Sex
The brain is the structure that translates the nerve impulses from the skin into pleasurable sensations. It controls nerves and muscles used during sexual behavior. The brain regulates the release of hormones, which are believed to be the physiological origin of sexual desire. The cerebral cortex, which is the outer layer of the brain that allows for thinking and reasoning, is believed to be the origin of sexual thoughts and fantasies. Beneath the cortex is the limbic system, which consists of the amygdala, hippocampus, cingulate gyrus, and septal area. These structures are where emotions and feelings are believed to originate, and are important for sexual behavior.
The hypothalamus is the most important part of the brain for sexual functioning. This is the small area at the base of the brain consisting of several groups of nerve-cell bodies that receives input from the limbic system. Studies with lab animals have shown that destruction of certain areas of the hypothalamus causes complete elimination of sexual behavior. One of the reasons for the importance of the hypothalamus is its relation to the pituitary gland, which secretes the hormones that are produced in the hypothalamus.
Hormones
Several important sexual hormones are secreted by the pituitary gland. Oxytocin, also known as the "hormone of love," is released during sexual intercourse when an orgasm is achieved. Oxytocin is also released in females when they give birth or are breast feeding; it is believed that oxytocin is involved with maintaining close relationships. Both prolactic and oxytocin stimulate milk production in females. Follicle-stimulating hormone (FSH) is responsible for ovulation in females by triggering egg maturity; it also stimulates sperm production in males. Luteinizing hormone (LH) triggers the release of a mature egg in females during the process of ovulation.
In males, testosterone appears to be a major contributing factor to sexual motivation. Vasopressin is involved in the male arousal phase, and the increase of vasopressin during erectile response may be directly associated with increased motivation to engage in sexual behavior.
The relationship between hormones and female sexual motivation is not as well understood, largely due to the overemphasis on male sexuality in Western research. Estrogen and progesterone typically regulate motivation to engage in sexual behavior for females, with estrogen increasing motivation and progesterone decreasing it. The levels of these hormones rise and fall throughout a woman's menstrual cycle. Research suggests that testosterone, oxytocin, and vasopressin are also implicated in female sexual motivation in similar ways as they are in males, but more research is needed to understand these relationships.