Sexual Health/Basic Reproductive Anatomy

Sexual reproduction requires genes from two different individuals, a male and a female. To produce healthy infants a these partners should not be closely related. Natural selection suggests that the greater diversity of parents, the better chance of adaptation and thus survival the infant will enjoy.

Wikipedia has related information at Female reproductive system (human)

Wikipedia has related information at Male reproductive system (human)

Puberty is the phase of development that begins the sexual processes in boys and girls. In girls, puberty triggers the beginning of the menstrual cycle, and the development of the breasts. In boys, puberty causes the penis and testicles to grow in size, and signals the beginning of nocturnal emissions and sperm production. In both girls and boys, puberty causes body hair (and facial hair, in boys) to begin appearing. During puberty, hair will start to grow under the armpits, in the groin, and on the chest and back. Leg and arm hair will also become thicker and darker.

In both sexes, nipples and the areas around them are frequently sensitive and responsive to sexual contact. Nipples may be soft or erect. Nipples become erect in response to changes in temperature, sexual contact, and other stimuli.

In the tissue surrounding the nipples, both males and females have milk ducts. In females, these glands are stimulated to produce milk by the hormones associated with pregnancy and delivery. In both sexes, certain kinds of nipple stimulation (such as from a breast pump), certain drugs, and certain hormonal disorders may cause non-pregnancy related milk production.

In females, fatty tissue is formed around the nipples during puberty. These are referred to as breasts, although the entire area may be referred to as a female's chest. In males, this area is simply referred to as the chest.

The male genitals consist of the penis and the testicles. The apparent size of a male genitalia depends on a number of factors, as they become pendulous (hanging down) when warm and relaxed but withdraw tight against the groin when experiencing cold or fear. This may have provided an evolutionary advantage to our ancestors.

The penis is a long, tube-like organ that is in the front of a man's body, in the groin area. The base of the penis connects in the groin (lower middle abdomen between the legs. The length of the penis is unimportant for reproduction, fertility or manliness, and can vary between 5cm and 20cm (2-8 inches) with an average of about 13cm (5 inches) for an erect adult over age 18. The middle area (the "shaft" of the penis) terminates in a pink gland where the urethra is exposed (the "head" of the penis). The base of the penis is frequently covered in pubic hair. The head of the penis is covered by the foreskin, which can be pulled back to expose the head itself. Men who are circumsized have had their foreskins removed (typically as infants).

Parts of a penis
	Diagram of the penis showing: a) Scrotum b) Shaft c) Glans d) Foreskin

The penis can be either "flaccid" (hanging down, soft, small) or "erect" (pointing upwards or outwards, hard, larger and longer). A penis is flaccid most of the time, and urination is difficult or impossible whilst the penis is erect. A penis becomes erect (tumescent or hard) in anticipation of sexual intercourse. Anticipation is not necessarily under conscious control, and erections can occur for no obvious reason. Orgasm (sexual climax or The Feeling) is difficult or impossible whilst the penis is flaccid. Ejaculation of semen usually but not necessarily occurs simultaneously with orgasm. Pre-emission refers to naturally produced fluid lubricant which may sometimes be excreted prior to ejaculation.

The testicles hang below the penis, and are covered by a patch of skin called the scrotum. The scrotum is typically covered in pubic hair, and is sensitive to sexual contact. Testicles produce sperm, and the hormone called "testosterone", among other things. Testosterone is a hormone that causes men to develop male secondary sex characters.

The testicles must remain several degrees below normal body temperature to produce healthy sperm. This is the reason why the testicles hang outside of the body in the scrotum. The muscles in the scrotum can expand and contract, to change the shape of the scrotum. If a man is hot, the muscles in the scrotum will lengthen, causing the testicles to move away from the body (thus cooling off the testicles). If a man is cold, the muscles in the scrotum will shorten, causing the testicles to move closer to the body (warming up the testicles).

Testicles may swell uncomfortably if a male has extended sexual stimulation without ejaculation. This is colloquially known as "blue balls" and is not a harmful condition.

Testicular cancer is one of the most common cancers in young men. It is important for men to examine their size and shape is constant on a regular basis, and do not produce pain when handled gently. If there is any change, an early consultation without delay is essential for effective treatment.

Each testicle has a tiny duct connecting it to the penis. These ducts are known as the vas deferens, and they are the path sperm take from the testicles into the penis during ejaculation. It is these ducts that are severed in a vasectomy (male sterilization). If conducted before puberty, vasectomy can arrest sexual development, as in medieval Castrati (male singers) in whom cutting the vas deferens was said to make a vast difference (to the voice). Their testicles were not usually removed (actual castration) Such interventions are now strictly prohibited as a serious affront to the boy's human rights, since he can not give informed consent for life-changing operations at such a young age.

Semen is a combination of fluids from multiple glands, and sperm from the testicles. During ejaculation, semen comes out of the erect penis. In some cases, semen squirts or shoots out vigorously, while at other times the semen oozes out slowly and does not travel any distance. Both situations are normal, and both force and volume of ejaculation is unimportant.

Semen can carry STDs and other germs into a partner, so contact with semen should be minimized, and condom protection should always be used during any overtly sexual activity.

Some people choose to take semen into their mouth, and may swallow semen as well. Oral ingestion of semen is not likely to be harmful, but there are no health benefits. There is some risk of transmission of STDs, especially if there are open wounds or sores in the mouth or on the lips. For this reason, it is recommended that people should use protection such as a condom when performing oral sex (described later), and get tested for STDs regularly.

The prostate is one of the glands that contributes fluid to semen. The prostate can be stimulated directly through the perineum (skin between the testicles and anus) or from inside the anus, a practice that some men find pleasurable. Prostates are at risk for developing cancer, especially in older men. For this reason, men should get prostate exams regularly to check for cancer and other disorders.

Boys of any age may naturally become aroused during sleep and may experience orgasm voluntarily or involuntarily. Those who have passed puberty can experience a phenomenon known as "nocturnal emission". Nocturnal emissions occur at night, while sleeping, and are involuntary releases semen while sleeping. Typically, the emission of semen is the first sign that a young man has reached puberty and is producing sperm normally.

Nocturnal emissions may occur more frequently when ejaculation is not achieved consciously on a regular basis.

Unlike the male penis and testicles, female reproductive anatomy is almost entirely internal. The components that are externally visible are known as the vulva.

There is a small structure called the clitoris at the front of the vulva. The clitoris is very sensitive to stimulation. During fetal development, the cells that in males develop into the penis, in females develop into the clitoris. The clitoris has more nerve endings than a male's penis, and even light stimulation can cause sensations of pain or discomfort. The clitoris is surrounded by folds of flesh known as the clitoral hood. The clitoral hood can be pulled back to expose the tip of the clitoris underneath.

Immediately below the clitoris is the urethra. This opening is connected to the bladder, and is where urine is expelled.

The largest feature of the vulva is the vaginal canal, or vagina. The vaginal canal is a fleshy tube, between 3-5 inches deep (but made of stretchable membranes) that opens between the legs, and connects to the uterus.

On each side of the vaginal opening is a Bartholin's gland. The vaginal canal and Bartholin's glands produce lubricating fluid in response to sexual stimulus. Sometimes these glands become clogged, and develop into a Bartholin's cyst. Usually these cysts resolve on their own, but if they are painful, unusually large, or persist for a long period of time, they should be evaluated by a doctor.

Inside the vagina, toward the front of the body, is an area of spongy tissue known as the Gräfenberg spot, or G-spot. This tissue may be related to Skene's glands (the female equivalent of the prostate), or may connect to the interior portion of the clitoris. Some women find stimulation of this area enjoyable.

The clitoris, urethra, and the vaginal opening are surrounded on both sides by the labial lips. The labia is divided into an inner and an outer set of lips, "the labia minora" and "the labia majora", respectively. The labia serve as protective coverings for the clitoris and the vaginal canal, and are also responsive to sexual contact. It is more common for the labia majora to be larger and completely enclose the labia minora. However, it is also normal for the labia minora to be larger and protrude somewhat from the labia majora.

The cervix is the muscle connecting the uterus and the vaginal canal. The opening in the cervix is called the cervical os. The size of the os changes throughout a woman's menstrual cycle, being more open during menstruation, and also near ovulation, and more closed at other times. The rigidity of the cervix also varies, from being soft like a person's lips, to being more firm like a person's nose.

Glands inside the cervix, known as cervical crypts, produce a substance known as cervical mucus or cervical fluid. Cervical mucus is usually thick, sticky, and acidic (to help kill any infectious microorganisms, and to physically block the entrance into the uterus). In the several days before ovulation, however, cervical mucus increases in water content, becomes less acidic, and develops a texture that helps guide sperm into the uterus. At this time, cervical mucus may resemble hand lotion, or raw egg white.

Cervical cancer is caused by certain strains of HPV, a sexually transmitted disease. It is important to use barrier protection during intercourse to reduce the risk of STDs, and for sexually active women to receive regular pap smears to screen for precancerous changes.

The uterus is a pear-shaped organ about the size of a fist. Two small conduits, called fallopian tubes extend from each side of the uterus. Each fallopian tube ends with a structure called a fimbria, which catches eggs after ovulation and guides them into the fallopian tube.

The lining of the uterus is called the endometrium. Several days before ovulation, the endometrium begins to become thicker and grows new blood vessels. If pregnancy does not occur, the endometrium is shed approximately two weeks after ovulation. This shedding of the endometrium is known as menstruation.

The ovaries are the location of a female's gametes (eggs), and are analogous to the male testicles (which contain the male gametes, sperm). Ovaries release eggs, also called ova (or ovum if it is just one) into the fallopian tubes. Ovaries also produce estrogen, which is the dominant female sex hormone. Estrogen causes the body to feminize, and is the reason why women grow breasts, and undergo other changes at puberty.

Human females begin menstruating, or having periods, as part of puberty. A girl's first menstruation is called menarche, and usually happens between the ages of eight and sixteen. After menarche, menstruation typically continues to occur at regular intervals - this is called a menstrual cycle. Women stop having menstrual cycles when they enter menopause, most commonly between the ages of forty-five and fifty-eight.

Wikipedia has related information at Menstrual cycle

The bleeding of menstruation is the most obvious part of the menstrual cycle. However, there is one other important feature: ovulation. The time between menstruation and ovulation is called the follicular phase, because this is when follicles on the ovaries begin growing. On average, the follicular phase lasts fourteen days, however this varies widely from woman to woman and also from cycle to cycle.

When one of the growing follicles reaches maturity, the ovum (egg) bursts through the ovarian wall. This event is known as ovulation. (The remaining follicles atrophy, or die). Many women experience pain associated with ovulation - this is called mittelschmerz.

The time between ovulation and menstruation is on average fourteen days, however this may vary from woman to woman. Some women have post-ovulatory, or luteal phases of ten days, while others may have luteal phases sixteen days long.

Prior to menstruation, a woman can experience certain hormonal and chemical changes in her body that can lead to a period of physical and psychological stress known as premenstrual syndrome, or PMS. PMS is usually characterized by cramping, bloating and water retention, moodiness and discomfort. PMS is not always experienced, and it is not experienced in all women, but it is considered a normal part of the menstrual cycle.

However, some women experience an extreme form of PMS known as Premenstral dysphoric disorder, or PMDD. PMDD is a more serious condition characterized by extreme moodiness, mood swings, and depression. PMDD can frequently be treated medically with success.

• Human Physiology/The female reproductive system
• Human Physiology/The male reproductive system