Reproductive system

The reproductive system is the structural and physiological network whose purpose is the creation of a new life to continue the species. It is the only body system which is not concerned with supporting the life of its host. Humanreproduction is sexual--meaning that both a male and a female are required toproduce a life. Gender is determined at conception by the sex chromosome inthe sperm that fertilizes an egg. The developing male or female has a reproductive system characteristic of its sex. However, boys and girls can not reproduce until sexual maturation occurs at puberty. The male reproductive systemis designed specifically to produce and deliver sperm to the egg in the female. The female reproductive system is designed to develop ova (eggs) and prepare for egg fertilization by a sperm. The male and female systems are both anatomically and biochemically designed to join and make a new life. However, the reproductive system is unique among body systems in that a person may choose not to use it to its full capacity--to procreate. Individuals can decide not to reproduce.

The main tasks of the male reproductive system are to provide sex hormones, to produce sperm, and to transport sperm from the male to a female. The firsttwo tasks are performed by the testes; while the third job is carried out bya series of ejaculatory ducts and the penis. The two testes are contained within the scrotum which hangs below the body between the legs. Each testis is attached at its top to an epididymis which contains numerous sperm ducts. Theepidiymides (plural) send sperm through the vas deferens to the penis. However, the seminal vesicles, prostate, and bulbo-urethral glands each contributeto the seminal fluid which carries the sperm to the penis. The epididymides and part of the vas deferens are within the scrotum, but the glands creating the seminal fluid are in the abdomen.

Each of the testes is divided into lobes, or septae, containing coiled seminiferous tubules lined with spermatozoa-producing cells. Between the tubules are hormone-producing cells called interstitial cells, or cells of Leydig. Testosterone is produced by the interstitial cells. Since the testes-containing scrotum hangs below the body, it has a temperature around 89°F (32°C)which is ideal for sperm production which requires a low temperature. When the scrotum is held too close to the body by restrictive clothing, sterility can result.

The full maturation of a single sperm takes about 70-80 days. Hence, substances a male is exposed to during that period of time may effect the health of his sperm at the end of that time period. Sperm are always available in healthy males after puberty, because spermatogenesis is an ongoing process with cells in all stages of development existing in different layers of the seminiferous tubules. As many as several hundred million sperm can be produced each day. And one man has approximately a quarter mile of coiled seminiferous tubules which produce all these sperm.

The vas deferens carries concentrated sperm from the scrotum into the abdominal cavity to the ejaculatory duct. Sperm that remain in the ejaculatory ductlonger than a couple of weeks degenerate and are disposed of. The prostate surrounds the ejaculatory duct and contains a sphincter that closes off the bladder during ejaculation. Seminal fluid from the seminal vesicles, the prostate, and the bulbo-urethral glands (or Cowper's glands) is added to the sperm.The seminal fluid plus the sperm is called semen.

Seminal fluid is designed to carry and nourish sperm. Seminal vesicles are located on either side of the bladder and contribute about 60% of the fluid. Seminal vesicle fluid is rich in essential sperm nutrients such as fructose which sustains sperm for up to 72 hours after ejaculation. Additional fluid is provided by the Cowper's glands (below the prostate) which secrete a pre-ejaculatory urethral lubricant that may contain some sperm. For this reason, withdrawal is not a foolproof contraceptive method. At ejaculation, additional Cowper secretions combine with the remaining seminal fluid and sperm. This semenis sent through the urethra in the penis.

The penis provides the route for transmitting sperm to an egg for reproduction. However, in its relaxed state, it can not effectively deliver sperm. In order for the sperm to have the best chance of fertilizing an egg, the penis must become erect and ejaculate semen close to an egg in the female reproductive tract.

The penis is part of the male's external reproductive system which becomes longer, thicker, and stiff during erection. It is comprised of a shaft region which is the cylindrical body of the penis and the glans, or head region. Theglans and the shaft are separated at the coronal ridge which is a rim of tissue that is very sensitive to touch. The skin covering the penis is loose andallows for expansion during erection. Some males have a prepuce or foreskin which is a movable skin that covers the penile glans. Circumcised males have had this foreskin removed. Uncircumcised males must carefully clean the foreskin daily to prevent bacteria and foul-smelling secretions (called smegma) from accumulating.

Three cylinders of spongy erectile tissue make up the internal portion of thepenis. Two cylinders run along the inner roof of the penis and are called the corpora cavernosa. The third cylinder runs along the lower side of the penis; it contains the urethra and is called the corpus spongiosum, or spongy body. The spongy body includes the penile tip and is more sensitive to touch than the rest of the penis. Several nerves and blood vessels run through the spongy body. An erection occurs when blood flow to the spongy tissue vessels increases. An average erect penis is 6.25 in (15.9 cm) long and 1.5 in (3.8 cm)wide at its base.

Sexual intercourse does not necessarily lead to reproduction, but the physiology of reproductive versus non-reproductive sexual arousal is indistinguishable. Sexual arousal has been divided into four stages by Masters and Johnson.These stages are the same whether the arousal results from physical stimulation (such as touch) or mental stimulation (such as reading an arousing book).Hence, arousal can be influenced by personal beliefs, desires, or values. Thestages of arousal are: excitement, plateau, orgasm, and resolution.

The male stage of sexual excitement is marked by increased blood flow to thepelvic area and penis. Increased parasympathetic nerve activity causes the blood vessels in the penis to dilate, allowing for vasocongestion which leads to an erection. This may happen in a matter of seconds. Testes size also increases, and nipples become erect in some men.

The amount of time spent in the plateau phase varies considerably. In this stage, the head of the penis enlarges and darkens from blood pooling. Testes darken, enlarge from vasocongestion, and are lifted back away from the penis. At this point, pre-ejaculatory secretion from the bulbo-urethral gland occurs,and respiration, heart rate, and blood pressure increase.

Male orgasm results from both emission and ejaculation. Emission is the release of the ejaculatory fluid into the urethra. Emission is caused by increasedsympathetic nerve stimulation in the ejaculatory ducts and glands which leads to rhythmic contractions that force the fluid out. For ejaculation, rhythmic contractions of the urethra expel the semen (usually 3-5 ml) while the prostate gland closes off the bladder.

In the resolution phase, blood exits the penis and testes, and the penis relaxes. Respiration, blood pressure, and heart rate return to normal, and sexualarousal enters a refractory period. During the refractory period, erection can not occur while the system "reloads." The length of refractory period varies from a couple of minutes to several hours and increases with fatigue and age.

The main tasks of the female reproductive system are to produce hormones, develop ova, receive sperm, and promote fertilization and the growth of a newlyconceived life. These events occur internally. Ova mature in the ovaries. Sperm are received in the vagina and cervix. Fertilization takes place usually in the fallopian tubes and less often in the uterus, with the newly formed life developing in the endometrial lining of the uterus. The female reproductivetract can be pictured as a capital Y with the upper arms forming the fallopian tubes. The ovaries would be at the end of these arms. The uterus would bethe upper half of the supporting stalk, and the vagina would be the lower half. External female genitals are involved in female sexual arousal.

The ovaries are oval-shaped and about 1-1.5 in (2.5-3.8 cm) long. They are connected to the body of the uterus by an ovarian ligament which tethers the ovaries in place. The ovaries parallel the testes in that they release sex hormones and develop gametes (ova or sperm). However, the job of the ovaries differs from that of the testes: while sperm are created daily through a man's life after puberty, all of a female fetus's eggs have been created by the sixthgestational month. Several million primordial follicles capable of forming ova are formed. About 1 million primordial follicles mature into primary follicles that still exist at birth. (The rest have degenerated.) When puberty begins, about 400,000 follicles remain. Mature eggs leave alternating ovaries monthly beginning in puberty in a process called ovulation. Unfertilized eggs are lost through menstruation, when the uterine lining is shed. Women typically menstruate for 30-40 years losing 360-480 eggs in a lifetime. Ovulation ishormonally suppressed during pregnancy and shortly after childbirth.

The formation of mature ova in the ovaries is called oogenesis. Unlike spermatogenesis, which occurs daily, oogenesis is on an average 28 day (or monthly)cycle. During embryonic development, primordial follicles are formed, each of which contains an oocyte surrounded by a layer of spindle-shaped cells. These spindle cells multiply during the mid-fetal stage of development and become granulosa cells which surround the egg. Granulosa cells function much likethe Sertoli cells in men: they prevent destructive drugs from getting to theegg while also providing essential nutrients for its development. Granulosa cells also secrete a rich substance that forms a follicular coating called thezona pellucida. Before birth, the cellular layers surrounding the follicle differentiate into a layer of cells called the theca interna. At birth, a babygirl's ova are suspended at the first meiotic division inside the primary follicles. After the onset of puberty, a new follicle enters the next phase offollicular growth monthly.

The first two weeks of the menstrual cycle are called the follicular phase because of the follicular development that occurs during that time. Around day14 of the cycle, LH and FSH surge to initiate ovulation. Ovulation entails the release of the mature oocyte from the ovarian follicle as it ruptures fromthe surface of the ovary into the abdominal cavity. Once released, the ovum is caught by the fimbria, which are finger-like projections off the ends of the fallopian tubes. The follicle which housed the growing egg remains in the ovary and is transformed into the corpus luteum. The corpus luteum secretes high levels of progesterone and some estrogen. The corpus luteum secures a position near the ovarian blood vessels to supply these hormones which prevent another follicle from beginning maturation. If the ovum is fertilized, then these hormone levels continue into pregnancy to prevent another cycle from beginning. However, if fertilization does not occur, then the corpus luteum degenerates allowing the next cycle to start. The second 14 days of the menstrual cycle are called the luteal phase because of the corpus luteum's hormonal control over this half of the cycle.

The optimal time for an oocyte to be fertilized is when it enters a fallopiantube. The fallopian tubes are fluid-filled, cilia-lined channels about 4-6 in (10-15 cm) long that carry the oocyte to the uterus. At ovulation, the primary oocyte completes its suspended meiosis and divides in two. A secondary oocyte and a small polar body result. If the secondary oocyte is fertilized, then it will go through another division which forms another polar body.

As the ripening egg travels along the fallopian tube, it is washed along by cilia which knock away residual nutrient cells on the outside of the egg. Thisarray of cells leaving the cell forms a radiant cluster called the corona radiata. If sperm have made their way to the fallopian tube, then they have already been capacitated. Capacitation is the modification of a sperm's acrosomal tip which enables it to burrow into the egg. Fertilization blocks the ability of additional sperm to enter the egg. Once the nuclei of the egg and spermcells have fused, the new cell is called a zygote. The zygote contains all the genetic information required to become a complete human being. This new life signifies the beginning of successful reproduction. As the zygotic cell divides into more cells, it travels from the fallopian tube to the uterus.

The uterus, or womb, is a muscular, inverted pear-shaped organ in the femalepelvis which is specifically designed to protect and nurture a growing baby.It averages 3 in (7.6 cm) long by 2 in (5 cm) wide. However, during pregnancy, it expands with the growing embryo and fetus. Embryo is a term used to describe a human in the first eight weeks of development. After that, the human is called a fetus.

During the follicular phase of the menstrual cycle, the lining (or endometrium) of the uterus becomes thick and filled with many blood vessels in preparation for supporting an embryo. If fertilization does not occur within about eight days of ovulation, then this lining is shed in menstrual blood through the cervix. This cycle continues until menopause, when menstruation becomes less frequent and eventually stops altogether.

The cervix is the base of the uterus which extends into the vagina. The narrow passageway of the cervix is just large enough to allow sperm to enter and menstrual blood to exit. During childbirth, it becomes dilated (open) to allowthe baby to move into the vagina, or birth canal. However, for most of the pregnancy, the cervix becomes plugged with thick mucous to isolate the developing baby from vaginal events. For this reason, non-reproductive, sexual intercourse is usually safe during pregnancy.

The vagina is a muscular tube about 5 in (12.7 cm) long. A thin layer of tissue called the hymen may cover the vaginal opening, but is usually gone in physically or sexually active females. A mucous membrane lines and moistens thevagina. During sexual intercourse, the vagina is lubricated further and functions to direct the penis toward the cervix to optimize fertilization. Duringchildbirth, the vagina stretches to accommodate the passage of the baby. Boththe uterus and the vagina contract to relatively original sizes some time after delivery.

External female genitals include the mons veneris, labia majora, labia minora, clitoris, and vestibule. They differ in size and color from female to female, but their location and function are consistent. The mons is a pad of fattytissue filled with many nerve endings which becomes covered with pubic hairin puberty. The labia majora are two folds of skin which protect the openingto the urethra and internal genitals. Pubic hair grows on their outer surfacein puberty. These fat padded folds of skin contain sweat glands, nerve endings, and numerous blood vessels. Inside these outer skin folds are the labia minora which are hairless. The labia minora form a spongy covering for the vaginal entrance. These smaller skin folds meet at the top of the genitals to form the clitoral hood. The hood houses the clitoris, a very sensitive organ which has a spongy shaft and a nerve-rich glans (tip). Between the labia minoraand the vagina is the area called the vestibule. Within the vestibule are the two Bartholin's glands which lubricate the vagina.

Sexual arousal in females parallels the arousal stages in males. Female sexual arousal is not required to reproduce, but it does facilitate reproduction.In the excitement phase, blood flow to the vagina increases which, in turn, pushes fluid into the vaginal canal. This lubricating process is called transudation and allows for comfortable penile insertion. During this phase, bloodinfiltrates the spongy clitoris and labia, and the cervix and uterus are lifted up away from the vagina. Nipples often become erect, and respiration, heart rate, and blood pressure increase.

During the plateau stage, the vagina expands, forming a pocket near the cervix which is an ideal deposit site for sperm; this is called "tenting." The increased sensitivity of the clitoris causes it to retract in the clitoral hood,and breasts sometimes become flushed. In the orgasmic phase, the vaginal opening contracts rhythmically for about 15 seconds. Unlike the lengthy refractory period which males experience in the resolution stage, females are more likely to be multi-orgasmic and capable of more closely spaced orgasms. In theresolution stage, genital blood flow returns to normal. Respiration, heart rate, and blood pressure also return to normal. Within 72 hours of sexual intercourse reproduction will either have successfully begun or not succeeded.

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