Reproductive System

Human Male Reproductive Anatomy

The human male's external reproductive organs are the scrotum and penis. The internal reproductive organs consist of gonads that produce both sperm and reproductive hormones, accessory glands that secrete products essential to sperm movement, and ducts that carry the sperm and glandular secretions (Figure 1).

  

Figure 1

Testes

The male gonads, or testes (singular, testis), produce sperm in highly coiled tubes called seminiferous tubules. Most mammals produce sperm properly only when the testes are cooler than the rest of the body. In humans and many other mammals, testis temperature is maintained about 2°C below the core body temperature by the scrotum, a fold of the body wall. The testes develop in the abdominal cavity and descend into the scrotum just before birth (a testis within a scrotum is a testicle). In many rodents, the testes are drawn back into the cavity between breeding seasons, interrupting sperm maturation. Some mammals whose body temperature is low enough to allow sperm maturation such as whales and elephants retain the testes in the abdominal cavity at all times.

Ducts

From the seminiferous tubules of a testis, the sperm pass into the coiled duct of an epididymis. In humans, it takes 3 weeks for sperm to travel the 6-m length of this duct, during which time the sperm complete maturation and become motile. During ejaculation, the sperm are propelled from each epididymis through a muscular duct, the vas deferens. Each vas deferens (one from each epididymis) extends around and behind the urinary bladder, where it joins a duct from the seminal vesicle, forming a short ejaculatory duct. The ejaculatory ducts open into the urethra, the outlet tube for both the excretory system and the reproductive system. The urethra runs through the penis and opens to the outside at the tip of the penis.

Accessory Glands

Three sets of accessory glands-the seminal vesicles, the prostate gland, and the bulbourethral glands-produce cretions that combine with sperm to form semen, the fluid that is ejaculated. Two seminal vesicles contribute about 60% of the volume of semen. The fluid from the seminal vesicles is thick, yellowish, and alkaline. It contains mucus, the sugar fructose (which provides most of the sperm's energy a coagulating enzyme, ascorbic acid, and local regulator called prostaglandins.

The prostate gland secretes its products directly into the urethra through small ducts. Thin and milky, the fluid from this gland contains anticoagulant enzymes and citrate sperm nutrient). The prostate undergoes benign (noncancerous) enlargement in more than half of all men over age 40 and in almost all men over 70. In addition, prostate cancer, which most often afflicts men 65 and older, is one of the most common human cancers.

The bulbourethral glands are a pair of small glands along the urethra below the prostate. Before ejaculation, they secrete clear mucus that neutralizes any acidic urine remaining in the urethra. There is evidence that bulbourethral fluid carries some sperm released before ejaculation, which contribute to the high failure rate of the withdrawal method of birth control (coitus interruptus).

Penis

The human penis contains the urethra as well as three cylinders of spongy erectile tissue. During sexual arousal, the erectile tissue fills with blood from the arteries. As this tissue fills, the increasing pressure seals off the veins that drain the penis, causing it to engorge with blood. The resulting erection enables the penis to be inserted into the vagina. Alcohol consumption, certain drugs, emotional issues, and aging all can cause an inability to achieve an erection (erectile dysfunction). For individuals with long-term erectile dysfunction, drugs such as Viagra promote the vasodilating action of the local regulator nitric oxide; the resulting relaxation of smooth muscles in the blood vessels of the penis enhances blood flow into the erectile tissues Although all mammals rely on penile erection for mating, the penis of raccoons, walruses, whales, and several other mammals also contains a bone, the baculum, which is thought to further stiffen the penis for mating The main shaft of the penis is covered by relatively thick skin. The head, or glans, of the penis has a much thinner outer layer and is consequently more sensitive to stimulation. The human glans is surrounded by a fold of skin called the prepuce or foreskin, which is removed if a male is circumcised.

Function of Male Reproductive System

The purpose of the organs of the male reproductive system is to perform the following functions:

• To produce, maintain, and transport sperm (the male reproductive cells) and protective fluid (semen).
• To discharge sperm within the female reproductive tract during sex.
• To produce and secrete male sex hormones responsible for maintaining the male reproductive system.

Human Female Reproductive Anatomy

The human female's external reproductive structures are the clitoris and two sets of labia, which surround the clitoris and vaginal opening. The internal organs consist of gonads which produce eggs and reproductive hormones, and a sys- tem of ducts and chambers, which receive and carry gametes and house the embryo and fetus (Figure 2).

Figure 2

Ovaries

The female gonads are a pair of ovaries that flank the uterus and are held in place in the abdominal cavity by ligaments. The outer layer of each ovary is packed with follicles, each consisting of an oocyte, a partially developed egg, surrounded by support cells. The surrounding cells nourish and protect the oocyte during much of its formation and development.

Oviducts and Uterus

An oviduct, or fallopian tube, extends from the uterus to The dimensions of ward a funnel-like opening at each ovary this tube vary along its length, with the inside diameter near the uterus being as narrow as a human hair. Upon ovulation cilia on the epithelial lining of the oviduct help collect the egg by drawing fluid from the body cavity into the oviduct. Together with wavelike contractions of the oviduct, the cilia convey the egg down the duct to the uterus, also known as the womb. The uterus is a thick, muscular organ that can expand during pregnancy to accommodate a 4-kg fetus The inner lining of the uterus, the endometrium, is richly sup- plied with blood vessels. The neck of the uterus, called the cervix, opens into the vagina.

Vagina and Vulva

The vagina is a muscular but elastic chamber that is the site for insertion of the penis and deposition of sperm during copulation. The vagina, which also serves as the birth canal through which a baby is born, opens to the outside at the vulva, the collective term for the external female genitalia. A pair of thick, fatty ridges, the labia maj encloses and protects the rest of the vulva. The vaginal opening and the separate opening of the urethra are located within a cavity bordered by a pair of slender skin folds, the labia minora. A thin piece of tissue called the hymen partly covers the vaginal opening in humans at birth and usually until sexual intercourse or vigorous physical activity ruptures it. Located at the top of the labia minora, the clitoris consists of erectile tissue supporting a rounded glans, or head, covered by a small hood of skin, the prepuce. During sexual arousal, the clitoris, vagina, and labia minora all engorge with blood and enlarge. Richly supplied with nerve endings, the clitoris is one of the most sensitive points of sexual stimulation. Sexual arousal also induces the vestibular glands near the vaginal opening to secrete lubricating mucus, thereby facilitating intercourse.

Mammary Glands

The mammary glands are present in both sexes, but they normally produce milk only in females. Though not part of the reproductive system, the female mammary glands are important to reproduction. Within the glands, small sacs of epithelial tissue secrete milk, which drains into a series of ducts that open at the nipple. The breasts contain connective and fatty (adipose) tissue in addition to the mammary glands. Because the low level of estradiol in males limits the development of the fat deposits, male breasts usually remain small.

Function of Female Reproductive System

The female reproductive system (or female genital system) serves multiple functions. Firstly, it is responsible for producing the eggs necessary for reproduction, thus facilitating the occurrence of reproduction. In addition, it produces the female sex hormones that maintain the reproductive cycle and that play a direct or indirect role elsewhere in the body.

Menstrual Cycle

Phases of the Menstrual Cycle
The four main phases of the menstrual cycle are:

• menstruation
• the follicular phase
• ovulation
• the luteal phase.
• Menstruation

Menstruation is the elimination of the thickened lining of the uterus (endometrium) from the body through the vagina. Menstrual fluid contains blood, cells from the lining of the uterus (endometrial cells) and mucus. The average length of a period is between three days and one week.

Sanitary pads or tampons are used to absorb the menstrual flow. Both pads and tampons need to be changed regularly (at least every four hours). Using tampons has been associated with an increased risk of a rare illness called toxic shock syndrome (TSS).

Follicular Phase

The follicular phase starts on the first day of menstruation and ends with ovulation. Prompted by the hypothalamus, the pituitary gland releases follicle stimulating hormone (FSH). This hormone stimulates the ovary to produce around five to 20 follicles (tiny nodules or cysts), which bead on the surface.

Each follicle houses an immature egg. Usually, only one follicle will mature into an egg, while the others die. This can occur around day 10 of a 28-day cycle. The growth of the follicles stimulates the lining of the uterus to thicken in preparation for possible pregnancy.

Ovulation

Ovulation is the release of a mature egg from the surface of the ovary. This generally occurs mid-cycle, around two weeks or so before menstruation starts. During the follicular phase, the developing follicle causes a rise in the level of oestrogen. The hypothalamus in the brain recognises these rising levels and releases a chemical called gonadotrophin-releasing hormone (GnRH). This hormone prompts the pituitary gland to produce raised levels of luteinising hormone (LH) and FSH.

Within two days, ovulation is triggered by the high levels of LH. The egg is funnelled into the fallopian tube and towards the uterus by waves of small, hair-like projections. The life span of the typical egg is only around 24 hours. Unless it meets a sperm during this time, it will die.

Luteal Phase

During ovulation, the egg bursts from its follicle, but the ruptured follicle stays on the surface of the ovary. For the next two weeks or so, the follicle transforms into a structure known as the corpus luteum. This structure starts releasing progesterone, along with small amounts of oestrogen. This combination of hormones maintains the thickened lining of the uterus, waiting for a fertilised egg to stick (implant).

If a fertilised egg implants in the lining of the uterus, it produces the hormones that are necessary to maintain the corpus luteum. This includes human chorionic gonadotrophin (HCG), the hormone that is detected in a urine test for pregnancy. The corpus luteum keeps producing the raised levels of progesterone that are needed to maintain the thickened lining of the uterus.

If pregnancy does not occur, the corpus luteum withers and dies, usually around day 22 in a 28-day cycle. The drop in progesterone levels causes the lining of the uterus to fall away. This is known as menstruation. The cycle then repeats.

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Campbell, Reece, Urry, Cain, Wasserman, Minorsky, Jackson. Biology A Global Approach. 10th ed. Pearson: Pearson Education Limited, 2015.

Lmwoodward. 2012. Male Reproductive Tract. https://www.flickr.com/photos/79474535@N02/6968445060. Accessed 24 February

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Wordpress. May25, 2015. Female Reproductive System. https://scienceeasylearning.wordpress.com/2015/05/25/female-reproductive-system/. Accessed 24 February 2018.

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