In the previous lecture we examined the sexual anatomical structures of human beings. In this lecture we will transition to the physiological and hormonal functions of males and females. Human physiology is the study of the mechanical, physical, and biochemical function of humans in good health, human organs, and the cells of which they are composed. In our examination of human physiology, we will focus on the functions of the systems of the body rather than the structure. Although complex, the physiological functions and sexual response of men and women play a critical role in human reproduction and satisfaction.
We will start by examining the female sexual physiology. Hormones are chemical substances that serve as messengers traveling within the body through the bloodstream. They are produced by the ovaries and the endocrine glands, such as the adrenals, pituitary, and hypothalamus. Hormones assist in a variety of tasks, including development of the reproductive organs and secondary sex characteristics during puberty. Secondary sex characteristics are changes to parts of the body other than the genitals that indicate sexual maturity. For women, these changes include a widening of the hips and development of pubic hair and underarm hair. Hormones are also responsible for the regulation of the menstrual cycle, maintenance of pregnancy, initiation of childbirth and lactation, and, to some degree, the regulation of the libido, or the sex drive. Among the most important of the female hormones are the estrogens, which affect the maturation of the reproductive organs, menstruation, and pregnancy. Progesterone is another important female hormone, which helps the maintenance of the uterine lining until menstruation occurs.
The female reproductive cycle can be viewed as having two components. The ovarian cycle is responsible for the development of the egg. The ovarian cycle averages 28 days in length, although there is considerable variation among women, ranging from 21 to 40 days. The ovarian cycle has three phases: the follicular phase, the ovulatory phase, and the luteal phase. The menstrual, or uterine cycle, is responsible for preparing the womb, or uterus, for a pregnancy. It is also divided into three phases: the menstrual phase, the proliferative phase, and the secretory phase. We will look at each cycle in more detail. The cycles repeat approximately every month for 35 to 40 years.
As discussed in the previous lecture, the development of female gametes is a process that begins even before a woman is born. In infancy and childhood the cells develop into ova, which is another term for eggs. During puberty hormones trigger the completion of the process known as oogenesis. Oogenesis results in the formation of both primary oocytes before birth and the secondary oocytes after it. The secondary oocyte, when mature, can be fertilized by a sperm cell. The first 10 days of the ovarian cycle are referred to as the follicular phase. During it, 10 to 20 ovarian follicles begin to grow, stimulated by follicle-stimulating hormone and luteinizing hormone. The maturation of more than one oocyte is a factor in multiple births. All the developing follicles begin secreting estrogen. Under the influence of follicle-stimulating hormone and estrogen, the oocyte matures, bulging from the surface of the ovary.
The ovulatory phase beings at about day 11 of the cycle and culminates with ovulation at about day 14. The primary oocyte undergoes cell division and becomes ready for ovulation. The follicular wall thins and ruptures, and the oocyte enters into the abdominal cavity near the fembria. Ovulation is now complete. Generally, ovulation occurs in only one ovary each month, alternating between right and left sides with each successive cycle. If a single ovary is removed, the remaining one begins to ovulate every month. The luteal phase typically lasts from immediately after ovulation through day 28 of the ovarian cycle. Even when cycles are more or less than 28 days, the duration of the luteal phase remains the same. This means that the time between ovulation and the end of the cycle is always 14 days. At this point the ovarian hormone levels are at their levels. The viable oocyte travels through the fallopian tubes towards the uterus for fertilization. As mentioned in the last lecture, the egg is viable for fertilization for about 24 hours.
The menstrual cycle prepares the uterus, or womb, for pregnancy each month by building up the endometrial lining. When a pregnancy does not occur, the lining is shed and the process begins again. The shedding of the endometrial tissue and the bleeding that accompanies it are a monthly event in the lives of women from puberty through menopause. Our feelings about this phenomenon are influenced by cultural and religious attitudes, as well as our own personal experiences. The first phase of the menstrual cycle is the menstrual phase and begins with the shedding of the endometrium. This endometrial tissue, along with mucus, other cervical and vaginal secretions, and a small amount of blood, are expelled through the vagina. The menstrual flow, or menses, generally occurs over a period of 3 to 5 days. The proliferative phase lasts about 9 days. During this phase the endometrium thickens in response to increased estrogen. The proliferative phase ends with ovulation. During the first part of the secretory phase, the endometrium begins to prepare for the arrival of a fertilized ovum. The secretory phase lasts 14 days, corresponding with the luteal phase of the ovarian cycle. If no fertilization occurs, the endometrium is shed, and the cycle begins again.
The menstrual cycle consists of processes that occur in the uterus. The ovarian cycle consists of activities with the ovaries and the development of the oocytes. It is important to note that the menstrual and ovarian cycles occur concurrently. What occurs in the uterus is linked to what occurs in the ovaries, but only the final phases of each of these cycles—the luteal and the secretory phases—actually coincide.
This is a beautiful figure depicting the changes in the linings of the uterus, the oocyte, and hormones during the menstrual and ovarian cycles. Notice at the bottom how the lining of the endometrium builds after the menstrual phase, during the menstrual cycle, in preparation for a fertilized egg. Above that are images of the ovarian follicle maturing and then rupturing with the mature ovum, or oocyte, during the ovarian cycle. After the follicle ruptures, the structure turns into a corpus luteum, which plays an important role in progesterone secretion.
A variety of physical, emotional, and psychological changes may occur 7 to 14 days before a woman's menstrual period. This is often referred to as premenstrual syndrome, or PMS. These symptoms disappear soon after the start of menstrual bleeding. Though no one knows for sure what causes PMS, it seems to be linked to alterations in the levels of sex hormones and brain chemicals. Mild to moderate symptoms of PMS fall into two categories: physical symptoms, which may include bloating, breast tenderness, swelling, weight gain, headaches, and cramping; and physiological and emotional symptoms, which include fatigue, depression, irritability, and changes in libido. There is great variation in our culture, as in many others, regarding sexual intercourse during menstruation. Young adults who are comfortable with menstrual sex saw it as just another part of a committed intimate relationship. However, it is important to note that although it is unusual, conception can occur during menstruation, and regular contraceptive methods should be used.
Now we are going to examine male sexual physiology. The reproductive processes of the male body include the manufacture of hormones and the production and delivery of sperm. The most important male sex hormone is testosterone. Testosterone triggers sperm production and regulates the sex drive. Other important hormones in male reproductive physiology are follicle stimulation hormone and luteinizing hormone. Testosterone is a steroid hormone synthesized from cholesterol. Testosterone is made by both sexes—by women mostly in the adrenal glands and ovaries, and by men primarily in the testes. During puberty, testosterone acts on the seminiferous tubules to produce sperm. It causes the penis, testicles, and other reproductive organs to grow, and is responsible for the development of secondary sex characteristics. In men these changes include the growth of pubic, facial, underarm, and other body hair, and a deepening of the voice. Testosterone also influences the growth of bones and increase of muscle mass and causes the skin to thicken and become oilier. Although men do not have a monthly reproductive cycle comparable to women, they do experience regular fluctuations of hormone levels. On a daily basis men's testosterone levels appear to be lowest in the evening and highest in the morning. Additionally, their overall levels appear to be relatively lower in the spring and higher in the fall.
Within the testes from puberty on, spermatogenesis is the production of sperm and is an ongoing process. Every day a healthy fertile male produces several hundred million sperm within the seminiferous tubules of his testicles. The sex of the zygote produced by the union of the egg and the sperm is determined by the chromosomes of the sperm. Sperm may contribute either a female or male sex chromosome. The egg always contributes a female sex chromosome. The combination of two X chromosomes means the zygote will develop as a female. With an X and a Y chromosome means it will develop as a male. In some cases, combinations of sex chromosomes other than XX or XY occur, causing sexual development to proceed differently.
Semen, or seminal fluid, is the ejaculated liquid that contains sperm. The function of semen is to nourish sperm and provide them with a hospitable environment and means of transport if they are deposited within the vagina. Semen is mainly made up of secretions from the seminal vesicles and prostate gland, which mix together in the urethra during ejaculation. Normally, about 1 teaspoonful of semen are ejaculated at one time. This amount of semen generally contains between 100 to 600 million sperm. However, fewer than 1,000 sperm will reach the fallopian tubes. In spite of their significance, sperm occupy only about 1% of the total volume of semen. The remainder comes primarily from the seminal vesicles and the prostate gland.
Several sex researchers have outlined the various physiological changes that both men and women undergo when they are sexually stimulated. The sequence of changes and patterns that take place in the body during sexual arousal is referred to as the sexual response cycle. We will discuss two of the most prominent models. William Masters and Virginia Johnson are two sexologists that we discussed previously. Their sex response cycle is the most widely cited, and is one of their most important contributions to the field. Masters and Johnson's 4-phase model of sexual response identifies the significant stages of response. The excitement stage starts when physical and/or psychological stimulation produces characteristic physical changes. The plateau stage is described as when sexual tension levels off. Orgasm is when increased tension peaks and discharges, affecting the whole body. The last stage, resolution, is when the body returns to its unaroused state. Helen Singer Kaplan created a sexual response cycle known as Kaplan's triphasic model of sexual response. In it she collapses the sexual and plateau phases into one stage—excitement. She also eliminates the resolution phase, ending the cycle with the orgasm stage. She also added a desire phase to the beginning of the process. The desire stage is when some form of thought, fantasy, or erotic feeling causes an individual to seek sexual gratification. Kaplan argues that one must first desire to be sexually aroused to seek sexual pleasure. Masters and Johnson's and Kaplan's are prominent models used to describe the phases of the sexual response cycle. However, they do not acknowledge the affective, or emotional, parts of the human response.
The brain is crucial to sexual response. It receives stimuli from five senses, plus one through the neural system, including sight, smell, touch, hearing, taste, and imagination. Research has found that increases in brain activity occur during sexual arousal. Orgasms elicit activity in various parts of the brain producing strong physical and emotional sensations. The relationship between thoughts and feelings and our actual behavior is not well understood. Relational factors, cultural influences, expectations, fantasies, hopes, and fears combine with sensory inputs and neurotransmitter to bring us to where we are ready, willing, and able to be sexual.
The senses of sight, smell, touch, taste, hearing, and imagination are very important to sexual response and stimuli. Seeing an attractive person or smelling their body fragrance, tasting through lick or kiss, touching their skin with a loving caress, and hearing erotic whispers are all senses that are capable of sending signals to the brain. Preferences of stimuli are largely determined by culture and are very individualized. The limbic system is the structures of the brain associated with emotions and feelings and involved in sexual arousal. It produces connections between sensory data and emotional response. Erogenous zones are highly sensitive to touch. Examples of the erogenous zones include the genitals, breasts, mouth, ears, neck, inner thigh, and buttocks. These vary between cultures and individuals. The olfactory sense, or smell, may bring sexual messages below the level of conscious awareness, and pheromones appear to increase the libido.
Image: http://www.deceptology.com/2011/11/amazingly-his-brain-told-stupid-lie.html
. For both males and females, physiological changes during sexual excitement depend on two processes. Vasocongestion is the concentration of blood in body tissues. An example of vasocongestion is when blood fills the genital regions for both males and females, causing the penis to become erect and the clitoris to swell. Myotonia is increased muscle tension accompanying the approach of an orgasm. When orgasm occurs, the body undergoes involuntary muscle contractions and then relaxes.
There are specific physiological changes for females during sexual excitement. One of the first signs of sexual excitement is the seeping of moisture through the vaginal walls through a process called sweating. The upper two-thirds of the vagina expand in a process called tenting. During tenting, the vagina expands about an inch in length and doubles in width. Additionally, the uterus will elevate during sexual excitement to provide the best possibility for sperm to reach the fallopian tubes. Many women experience a sex flush, which is a darkening of the skin that temporarily appears as the result of blood rushing to the skin's surface during sexual excitement. The labia minora swell, and if the excitement continues they can double and even triple in size. The clitoris engorges with blood and swells during sexual excitement. Then, immediately before orgasm, the clitoris will retract under the clitoral hood.
Continued stimulation brings orgasm, a peak sensation of intense pleasure that creates an altered state of consciousness. The orgasm is accompanied by involuntary rhythmic uterine and anal contractions, myotonia, and a state of well-being. Heart and respiratory rates and blood pressure reach their peak during orgasm. Women are often physiologically able to be orgasmic immediately following the previous orgasm. Therefore, women can have repeated orgasms, often called multiple orgasms, if they continue to be stimulated. A biological fact is that male orgasm and ejaculation are necessary for reproduction to occur. However, reproduction can occur without the female orgasm. The evolutionary purpose of the female orgasm has been debated. One possible explanation is that the female orgasm helps create a suction to pull the semen upwards towards the uterus to increase the chances of fertilization.
Even though their sexual anatomy is quite different, women and men follow roughly the same pattern of excitement and orgasm. There are, however, two exceptions. Generally, but of course not always, men tend to be fully aroused and ready for penetration in a shorter amount of time than women do. Additionally, once men experience ejaculation, they usually cannot do so again for some time, whereas women may experience repeated orgasms. As mentioned earlier, sexual arousal in men include the processes of myotonia and vasocongestion. Vasocongestion in men is most apparent in the erection of the penis.
When a male becomes aroused, the blood circulation within the penis dramatically changes. During the process of erection the blood vessels expand. When the blood vessels expand, the volume of the blood increases, especially within the corpora cavernosa. At the same time, expansion of the penis compresses the veins that normally carry blood out, so the penis further becomes engorged. It should be noted that there are no muscles in the penis that make it erect, nor is there a bone in it. The male erection is caused solely by the engorgement of blood in the erectile tissue. Furthermore, during sexual excitement, the skin of the scrotum wrinkles and the testes elevate, bringing them closer to the body. The Cowper's gland is activated, and a secretion will appear at the tip of the erect penis. The prostate will enlarge, and the color of the glans penis will deepen as a response to the increased blood flow.
Increasing simulation of the penis generally leads to ejaculation. Ejaculation then occurs in two stages: emission and expulsion. In the first stage, emission, rhythmic contractions move the sperm towards the urethra. Additionally, the bladder sphincter muscle closes to prevent urine from mixing with the semen and semen from entering the bladder. At this point the man feels a distinct sensation of ejaculatory inevitability. This is the point at which ejaculation must occur, even if stimulation ceases. These events are accompanied by increased heart rate, respiratory rate, elevated blood pressure, and general muscular tension. In the second stage of ejaculation, expulsion, there are rapid rhythmic contractions from the urethra, the prostate, and the muscles at the base of the penis. The first few contractions are the most forceful, causing semen to spurt from the urethral opening. Breathing rate and heart rate may reach their peak at expulsion.
Orgasm occurs when the impulses that cause erection reach a critical level. The intensely pleasurable physical sensations and general release of tension that typically accompany ejaculation constitute the experience of orgasm. However, orgasm does not always occur with ejaculation. It is possible to ejaculate without having an orgasm, and to experience orgasm without ejaculating. Additionally, ejaculation and orgasm don't necessarily require an erection. Following ejaculation men experience a refractory period during which they are not capable of having an ejaculation again. During this time nerves cannot respond to any additional stimulation. Refractory periods vary in length, ranging from a few minutes to many hours. Or, in some older men, the refractory period may be measured in days.
Although complex, we have learned the physiological functions and sexual response of men and women play a critical role in human reproduction, and can be briefly summarized into two aspects: egg and sperm production as a result of hormones and vasocongestion and myotonia.