Sex

Sex and Physiology

Sex is biologically determined based on chromosomes, hormones, gonads, internal reproductive anatomy, and external genitalia.

Learning Objectives

Characterize the physiological differences among male, female, and intersex individuals and the controversies surrounding “corrective” surgery

Key Takeaways

Key Points

  • In humans, biological sex is determined by five factors present at birth: chromosomes, gonads, hormones, internal reproductive anatomy, and external genitalia.
  • Sex is typically divided into male, female, or intersex (i.e., having some combination of both male and female sex characteristics).
  • Due to the existence of multiple forms of intersex conditions (which are more prevalent than researchers once thought), many view sex as existing along a spectrum, rather than simply two mutually exclusive categories.
  • Intersex infants with ambiguous outer genitalia are often surgically “corrected” to more easily fit into a socially accepted sex category. Human-rights defenders argue that this is an oppressive practice on par with genital mutilation.

Key Terms

  • intersex: A variation in biological sex characteristics including chromosomes, gonads, or genitals that do not allow an individual to be distinctly identified as male or female.
  • sex: The distinguishing biological properties by which organisms are classified as female, male, or intersex.
  • gonad: A sex organ that produces gametes; specifically, a testicle or an ovary.

The Biology of Sex

“Sex” refers to physiological differences between male, female, and intersex bodies. A person’s sex includes both primary sex characteristics (those that are related to the reproductive system) and secondary sex characteristics (those unrelated to the reproductive system, such as breasts and facial hair). In humans, biological sex is determined at birth, typically by doctors, through the observance of five factors:

  1. the presence or absence of a Y chromosome;
  2. the type of gonads;
  3. the sex hormones;
  4. the internal reproductive anatomy (such as the uterus in females); and
  5. the external genitalia.

In humans, sex is typically divided into male, female, or intersex. Intersex, in humans and other animals, is a variation in sex characteristics (including chromosomes, gonads, or genitals) that does not allow an individual to be distinctly identified as male or female. Such variation may involve genital ambiguity and/or combinations of chromosomes other than XY (typically found in males) and XX (typically found in females). In addition to the most common XX and XY chromosomal sexes, there are several other possible combinations commonly known as intersex, such as Turner syndrome (XO), Triple X syndrome (XXX), Klinefelter syndrome (XXY), de la Chapelle syndrome (XX male), and Swyer syndrome (XY female). Due to the existence of multiple forms of intersex conditions (which are more prevalent than researchers once thought), many view sex as existing along a spectrum, rather than as simply two mutually exclusive categories.

Intersex: The Debate Over “Corrective” Surgery

Intersex infants with ambiguous outer genitalia are often surgically “corrected” at birth so that they more easily conform to a socially accepted sex category. What is considered male, female, or even ambiguous is largely classified by society, and this kind of “corrective” surgery is a highly controversial topic.

Defenders of the surgery argue that it is necessary for individuals to be clearly identified as male or female in order for them to function socially. Many intersex individuals, however, argue that such a procedure is invasive and unnecessary. Some individuals may be raised as a certain gender (boy or girl) based on the sex (male or female) that was chosen for them at birth, but then identify with another gender later in life; some may even opt for sexual reassignment surgery later in life to align more truly with who they are.

Research done in the late 20th century has led to a growing medical consensus that diverse intersex bodies are normal—if relatively rare—forms of human biology, and up to 1.7% of live births exhibit some degree of sexual ambiguity. Opponents of the practice of “corrective” surgery maintain that the presumed social benefits of such “normalizing” surgery do not outweigh the potential costs; they argue that the effort to “normalize” or “correct” an intersex condition to fit what society deems acceptable is both problematic and oppressive. Intersex advocates such as Anne Fausto-Sterling label surgery without consent as a form of genital mutilation, and argue that surgery on intersex babies should wait until the child can make an informed decision for themselves. Studies have revealed that surgical intervention can have drastic psychological effects, impact well-being and quality of life, and does not ensure a successful psychological outcome for the child. Specialists at the Intersex Clinic at University College London began to publish evidence in 2001 that indicated the harm that can arise as a result of inappropriate interventions, and advised minimizing the use of childhood surgical procedures.

The Declaration of Montreal was the first to demand prohibition of unnecessary post-birth surgery to reinforce gender assignment until a child is old enough to understand and give informed consent. This was detailed in the context of existing UN declarations and conventions under Principle 18 of the Yogyakarta Principles, which called on states to “take all necessary legislative, administrative and other measures to ensure that no child’s body is irreversibly altered by medical procedures in an attempt to impose a gender identity without the full, free and informed consent of the child.”

Biology of Sexual Behavior

The biology of human sexuality includes the reproductive system and the sexual response cycle, as well as the factors that affect them.

Learning Objectives

Characterize the four stages of the human sexual response cycle

Key Takeaways

Key Points

  • The biological aspects of human sexuality include the reproductive system, the sexual response cycle, and the neurological and hormonal factors that affect these processes.
  • Females have both external genitalia (known as the vulva) and internal reproductive organs (including the ovaries, uterus, fallopian tubes, and vagina).
  • Males also have both internal and external genitalia; the main sex organs are the penis and testicles.
  • The sexual response cycle is a model that describes the physiological responses that take place during sexual activity. The cycle consists of four phases: excitement, plateau, orgasm, and resolution.
  • The hypothalamus is the most important part of the brain for sexual functioning; it produces important sexual hormones that are then secreted by the pituitary gland.
  • Sex hormones that influence sexual behavior include oxytocin, prolactic, vasopressin, follicle-stimulating hormone (FSH), and luteinizing hormone (LH); others include testosterone in males and estrogen and progesterone in females.

Key Terms

  • libido: A person’s overall sexual drive or desire for sexual activity.
  • limbic system: Part of the human brain involved in emotion, motivation, and emotional association with memory.
  • hormone: Any substance produced by one tissue and conveyed by the bloodstream to another to effect physiological activity.

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.

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Biology of female reproductive system: The female reproductive system consists of both internal organs and external genitalia.

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.

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Biology of male reproductive system: Like females, males have both internal and external genitalia that are responsible for procreation and sexual intercourse.

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.

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.

Sexual Dysfunction and Disease

Biological, emotional, and sociocultural factors can influence various sexual issues, such as disease and dysfunction.

Learning Objectives

Discuss the psychological and physiological problems that can lead to sexual dysfunction

Key Takeaways

Key Points

  • Sexual disorders, according to the DSM-5, are disturbances in sexual desire and psycho-physiological changes that characterize the sexual response cycle and cause marked distress and interpersonal difficulty.
  • Sexual problems are often categorized in one of four ways: desire disorders, arousal disorders, orgasmic disorders, and sexual pain disorders.
  • Emotional factors that contribute to sexual disorders can include depression, sexual fears or guilt, anxiety, and past sexual trauma, while physical factors can include diseases, the use of drugs, failure of various organ systems, endocrine disorders, and hormonal deficiencies.
  • Sexually transmitted infections (STIs) are illnesses that have a significant probability of transmission by means of sexual behavior, including vaginal intercourse, anal sex, and oral sex. The most effective way to prevent STIs is to avoid contact of body parts or fluids.

Key Terms

  • dyspareunia: Painful or difficult sexual intercourse, especially in females.
  • vaginismus: A painful muscular contraction of the vagina.
  • sexually transmitted infection: Illnesses that have a significant probability of being spread by means of sexual behavior, including vaginal intercourse, anal sex, and oral sex.
  • asexual: Having a lack of sexual attraction to anyone, or low or absent interest in sexual activity; it may also be considered the lack of a sexual orientation, or one of the variations thereof, alongside heterosexuality, homosexuality, and bisexuality.

“Human sexuality ” refers to people’s sexual interest in and attraction to others, and the capacity to have erotic or sexual feelings and experiences. Sexuality has biological, emotional, and sociocultural aspects, all of which can influence various sexual disorders and diseases.

Sexual Disorders

The World Health Organization ‘s International Classifications of Diseases defines sexual problems as “the various ways in which an individual is unable to participate in a sexual relationship as he or she would wish.” Sexual disorders, according to the 5th edition of the American Psychiatric Association’s Diagnostic and Statistical Manual (DSM-5), are disturbances in sexual desire and psycho-physiological changes that characterize the sexual response cycle and cause marked distress and interpersonal difficulty. The DSM-5 includes sex-specific sexual dysfunctions, and subtypes for all disorders include “lifelong versus acquired” and “generalized versus situational.” Sexual dysfunctions in the DSM-5 (except those that are substance- or medication-induced) now require a duration of at least 6 months and must meet more exact criteria in order to be diagnosed.

Sexual problems are often categorized in one of four ways: desire disorders, arousal disorders, orgasmic disorders, and sexual pain disorders.

Sexual Desire

Sexual desire disorders, or decreased libido, are characterized by a lack or absence of desire for sexual activity or of sexual fantasies. The condition may have started after a period of normal sexual functioning or the person may always have had low or no sexual desire. The causes vary considerably, but include a possible decrease in the production of normal estrogen in women or testosterone in both men and women. Other causes may include aging, fatigue, hormone imbalance, pregnancy, postpartum depression, medications (such as SSRIs ), or psychiatric conditions such as depression and anxiety. Classifying the lack of sexual desire as a “disorder” is considered by some to be controversial because it pathologizes those who are asexual.

Sexual Arousal

Sexual arousal disorders were previously known as frigidity in women and impotence in men, though these have now been replaced with less judgmental terms, such as erectile dysfunction. In the revisions to the DSM-5, sexual desire and arousal disorders in females were combined into female sexual interest/arousal disorder. These conditions can manifest themselves as an aversion to and avoidance of sexual contact with a partner. In males, there may be partial or complete failure to attain or maintain an erection or a lack of sexual excitement and pleasure in sexual activity. There may be medical causes to these disorders, such as decreased blood flow or lack of vaginal lubrication. Chronic disease can also contribute, as can a history of trauma and the nature of the relationship between partners.

Orgasm Disorders

Orgasm disorders are persistent delays or absence of orgasm following a normal sexual excitement phase. The disorder can have physical, psychological, or pharmacological origins. SSRI antidepressants are a common pharmaceutical culprit, as they can delay orgasm or eliminate it entirely.

Sexual Pain

Sexual pain disorders affect women almost exclusively, raising the question of possible societal influences. In the DSM-5, the conditions of dyspareunia and vaginismus were combined into the new diagnosis of genito-pelvic pain/penetration disorder. Dyspareunia, or painful intercourse, may be caused by insufficient lubrication (vaginal dryness) in females. Poor lubrication may result from insufficient excitement and stimulation; hormonal changes caused by menopause, pregnancy, or breast-feeding; irritation from contraceptive creams and foams; or fear, anxiety, or past sexual trauma. Vaginismus is an involuntary spasm of the muscles of the vaginal wall that interferes with intercourse. It is unclear exactly what causes it, but it is thought that past sexual trauma may play a role. Another female sexual-pain disorder is called vulvodynia or vulvar vestibulitis; in this condition, women experience burning pain during sex, which seems to be related to problems with the skin in the vulvar and vaginal areas. The cause, again, is unclear.

Examining the Causes of Sexual Disorders

There are many factors that may result in a person experiencing a sexual dysfunction. Emotional factors include interpersonal or psychological problems and can result from depression, anxiety, past sexual trauma, sexual fears, or guilt. Ordinary anxiousness can cause erectile dysfunction without psychiatric problems, but clinically diagnosable disorders such as panic disorder commonly cause avoidance of intercourse and premature ejaculation. Pain during intercourse is often correlated with anxiety disorders among females.

Physical factors include the use of drugs, such as alcohol, nicotine, narcotics, stimulants, antihypertensives, antihistamines, and some psychotherapeutic drugs. Other factors include back injuries, enlarged prostate gland, problems with blood supply, and nerve damage (as in spinal cord injuries). Diseases such as diabetic neuropathy, multiple sclerosis, tumors, and tertiary syphilis may also have an impact, as can failure of various organ systems (such as the heart and lungs), endocrine disorders (thyroid, pituitary, or adrenal gland problems), and hormonal deficiencies (low testosterone, other androgens, or estrogen).

Sexually Transmitted Infections

Sexually transmitted infections (STIs), also referred to as sexually transmitted diseases (STDs) or venereal diseases (VDs), are illnesses that have a significant probability of transmission by means of sexual behavior, including vaginal intercourse, anal sex, and oral sex. Some STIs can also be contracted by sharing intravenous drug needles with an infected person, as well as through childbirth or breastfeeding. Common STIs include:

  • chlamydia;
  • herpes (HSV-1 and HSV-2);
  • human papillomavirus (HPV);
  • gonorrhea;
  • syphilis;
  • trichomoniasis;
  • HIV (human immunodeficiency virus) and AIDS (acquired immunodeficiency syndrome).

The most effective way to prevent transmission of STIs is to practice safe sex and avoid direct contact of skin or fluids which can lead to transfer with an infected partner. Proper use of safe-sex supplies (such as male condoms, female condoms, gloves, or dental dams) reduces contact and risk and can be effective in limiting exposure; however, some disease transmission may occur even with these barriers.

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Safe sex practices: The use of barriers, such as dental dams for oral sex, can help prevent the spread of STIs.