{"id":5370,"date":"2019-06-24T17:46:53","date_gmt":"2019-06-24T17:46:53","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/20-7-functions-of-the-female-reproductive-system-3\/"},"modified":"2023-11-30T23:26:34","modified_gmt":"2023-11-30T23:26:34","slug":"20-7-functions-of-the-female-reproductive-system-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/20-7-functions-of-the-female-reproductive-system-3\/","title":{"raw":"18.7\u00a0Functions of the Female Reproductive System","rendered":"18.7\u00a0Functions of the Female Reproductive System"},"content":{"raw":" \r\n\r\n[caption id=\"attachment_5272\" align=\"aligncenter\" width=\"334\"]\"18.7.1 Figure 18.7.1 The miracle of new life.<\/em>[\/caption]\r\n\r\n
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Waiting Expectantly<\/h1>\r\n<\/div>\r\nA mother-to-be waits patiently for her baby to grow as her belly gradually swells. Reproduction is all about making babies, and the female reproductive system is specialized for this purpose. Its functions include producing female [pb_glossary id=\"6023\"]gametes<\/span>[\/pb_glossary]<\/span> called ova, secreting female sex hormones (such as [pb_glossary id=\"5995\"]estrogen[\/pb_glossary]), providing a site for [pb_glossary id=\"6009\"]fertilization[\/pb_glossary], gestating a [pb_glossary id=\"5086\"]fetus[\/pb_glossary] if fertilization occurs, giving birth to a baby, and breastfeeding a baby after birth. The only thing missing is [pb_glossary id=\"2506\"]sperm[\/pb_glossary].<\/span>\r\n
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Ova Production<\/h1>\r\n<\/div>\r\nAt birth, a female\u2019s [pb_glossary id=\"3487\"]ovaries[\/pb_glossary] contain all the [pb_glossary id=\"5099\"]ova[\/pb_glossary] she will ever produce, which may include a million or more ova. The ova don't start to mature, however, until she enters [pb_glossary id=\"5155\"]puberty[\/pb_glossary] and attains sexual maturity. After that, one ovum typically matures each month, and is released from an ovary. This continues until a woman reaches [pb_glossary id=\"5255\"]menopause[\/pb_glossary] (cessation of monthly periods), typically by age 52. By then, viable eggs may be almost depleted, and hormone levels can no longer support the monthly cycle. During the reproductive years, which of the two ovaries releases an egg in a given month seems to be a matter of chance. Occasionally, both ovaries will release an egg at the same time. If both eggs are fertilized, the offspring are fraternal twins (dizygotic, or \"two-zygote,\" twins), and they are no more alike genetically than non-twin siblings.\r\n

Oogenesis<\/h2>\r\nThe process of producing ova in the ovaries of a female fetus is called [pb_glossary id=\"2509\"]oogenesis[\/pb_glossary].<\/strong> Ova are [pb_glossary id=\"2484\"]haploid[\/pb_glossary] gametes, and their production occurs in several steps that involve different types of cells, as summarized in Figure 18.7.2. Oogenesis is completed long before birth. It occurs when diploid germ cells called [pb_glossary id=\"5273\"]oogonia[\/pb_glossary]<\/strong>\u00a0(singular, oogonium) undergo\u00a0[pb_glossary id=\"1987\"]mitosis[\/pb_glossary]. Each such\u00a0cell division\u00a0produces two [pb_glossary id=\"2103\"]diploid[\/pb_glossary] daughter\u00a0cells. One is called the primary oocyte, and the other is retained to help maintain a reserve of oogonia. The primary [pb_glossary id=\"5274\"]oocyte[\/pb_glossary], in turn, starts to go through the first cell division of [pb_glossary id=\"2486\"]meiosis[\/pb_glossary] (meiosis I). However, it does not complete meiosis I until much later. Instead, it remains in a resting state, nestled within a tiny, immature [pb_glossary id=\"5253\"]follicle[\/pb_glossary] in the ovary until the female goes through puberty.\r\n

Maturation of a Follicle<\/h2>\r\nBeginning in puberty, about once a month, one of the follicles in an ovary undergoes maturation, and an egg is released. As the follicle matures, it goes through changes in the numbers and types of its cells, as shown in Figure 18.7.2. The primary oocyte within the follicle also resumes meiosis. It completes meiosis I, which began long before birth, to form a secondary oocyte and a smaller cell, called the first polar body. Both the secondary oocyte and the first polar body are haploid cells. The secondary oocyte has most of the cytoplasm from the primary oocyte and is much larger than the first polar body, which soon disintegrates and disappears. The secondary oocyte begins meiosis II, but only completes it if the egg is fertilized.\r\n\r\n[caption id=\"attachment_5276\" align=\"aligncenter\" width=\"1024\"]\"18.7.2 Figure 18.7.2 Formation of a secondary oocyte that may become a zygote begins with mitosis of an oogonium. This is followed by two meiotic cell divisions. In humans, the first polar body does not undergo the second meiotic division illustrated here. Instead of producing four gametes, as in spermatogenesis, this process results in one ovum and three polar bodies.<\/em>[\/caption]\r\n

Release of an Egg<\/h2>\r\nIt typically takes 12 to 14 days for a [pb_glossary id=\"5253\"]follicle[\/pb_glossary] to mature in an ovary, and for the secondary oocyte to form. Then, the follicle bursts open and the ovary ruptures, releasing the secondary oocyte from the ovary. This event is called\u00a0[pb_glossary id=\"5239\"]ovulation[\/pb_glossary].<\/strong>\u00a0The now-empty follicle starts to change into a structure called a\u00a0[pb_glossary id=\"5277\"]corpus luteum[\/pb_glossary].<\/strong> The expelled secondary oocyte is usually swept into the nearby oviduct by its waving, fringe-like [pb_glossary id=\"5247\"]fimbriae[\/pb_glossary].\r\n

Uterine Changes<\/h2>\r\nWhile the follicle is maturing in the ovary, the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] is also undergoing changes to prepare it for an embryo if\u00a0fertilization\u00a0occurs. For example, the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] gets thicker and becomes more vascular. Around the time of [pb_glossary id=\"5239\"]ovulation[\/pb_glossary], the [pb_glossary id=\"5235\"]cervix[\/pb_glossary] undergoes changes that help sperm reach the ovum to fertilize it. The cervical canal widens, and cervical mucus becomes thinner and more alkaline. These changes help promote the passage of sperm from the [pb_glossary id=\"5143\"]vagina[\/pb_glossary] into the uterus and make the environment more hospitable to sperm.\r\n
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Fertilization\u00a0\u2014\u00a0or Not<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"6009\"]Fertilization[\/pb_glossary] of an ovum by a sperm normally occurs in an oviduct, most often in the part of the tube that passes above the ovary (see Figure 18.7.3). In order for fertilization to occur, sperm must \u201cswim\u201d from the vagina where they are deposited, through the cervical canal to the uterus, and then through the body of the uterus to one of the oviducts. Once sperm enter a oviduct, tubular fluids help carry them through the tube toward the secondary oocyte at the other end. The secondary oocyte also functions to promote fertilization. It releases molecules that guide the sperm and allow the surface of the ovum to attach to the surface of the sperm. The ovum can then absorb the sperm, allowing fertilization to occur.\r\n\r\n[caption id=\"attachment_5278\" align=\"alignnone\" width=\"1024\"]\"18.7.3 Figure 18.7.3 This diagram shows the structures through which sperm must pass if fertilization of an egg is to occur. It also shows the event of fertilization, and where fertilization usually occurs.<\/em>[\/caption]\r\n

If Fertilization Occurs<\/h2>\r\nIf the secondary oocyte is fertilized by a sperm as it passes through the oviduct, the secondary oocyte quickly completes meiosis II, forming a [pb_glossary id=\"2103\"]diploid[\/pb_glossary] [pb_glossary id=\"2471\"]zygote[\/pb_glossary] and another polar body. This second polar body, like the first, normally breaks down and disappears. The zygote then continues the journey through the oviduct to the [pb_glossary id=\"5102\"]uterus[\/pb_glossary], during which it undergoes several mitotic cell divisions. By the time it reaches the uterus up to five days after fertilization, it consists of a ball of cells called a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary]. Within another day or two, the blastocyst implants itself in the endometrium lining the uterus, and gestation begins.\r\n

If Fertilization Does Not Occur<\/h2>\r\nWhat happens if the secondary oocyte is not fertilized by a sperm as it passes through the oviduct? It continues on its way to the uterus without ever completing meiosis II. It is likely to disintegrate within a few days while still in the oviduct. Any remaining material will be shed from the woman\u2019s body during the next menstrual period.\r\n
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Pregnancy\u00a0and Childbirth<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"5280\"]Pregnancy[\/pb_glossary]<\/strong> is the carrying of one or more offspring from [pb_glossary id=\"6009\"]fertilization[\/pb_glossary] until birth. This is one of the major functions of the female reproductive system. It involves virtually every other body system including the cardiovascular, urinary, and respiratory systems, to name just three. The maternal organism plays a critical role in the development of the offspring. She must provide all the nutrients and other substances needed for normal growth and development of the offspring, and she must also remove the wastes excreted by the offspring. Most nutrients are needed in greater amounts by a pregnant woman to meet fetal needs, but some are especially important, including folic acid, calcium, iron, and omega-3 fatty acids. A healthy diet (see photo in Figure 18.7.4)<\/em>, along with prenatal vitamin supplements, is recommended for the best pregnancy outcome. A pregnant woman should also avoid ingesting substances (such as alcohol) that can damage the developing offspring, especially early in the pregnancy when all of the major organs and organ systems are forming.\r\n\r\n[caption id=\"attachment_5281\" align=\"aligncenter\" width=\"400\"]\"18.7.4 Figure 18.7.4 Eating a range of colourful fruits and vegetables helps ensure adequate intake of nutrients to support a healthy pregnancy.<\/em>[\/caption]\r\n

Trimesters of Pregnancy<\/h2>\r\nWhen counted from the first day of the last menstrual period, the average duration of pregnancy is about 40 weeks (38 weeks when counted from the time of fertilization), but a pregnancy that lasts between 37 and 42 weeks is still considered within the normal range. From the point of view of the maternal organism, the total duration of pregnancy is typically divided into three periods, called\u00a0[pb_glossary id=\"5283\"]trimesters[\/pb_glossary],<\/strong>\u00a0each of which lasts about three months. This division of the total period of gestation is useful for summarizing the typical changes a woman can expect during pregnancy. From the point of view of the developing offspring, however, the major divisions are different. They are the embryonic and fetal stages. The offspring is called an [pb_glossary id=\"5262\"]embryo[\/pb_glossary] from the time it implants in the uterus through the first eight weeks of life. After that, it is called a [pb_glossary id=\"5086\"]fetus[\/pb_glossary] for the duration of the pregnancy.\r\n

First Trimester<\/h3>\r\nThe first trimester begins at the time of [pb_glossary id=\"6009\"]fertilization[\/pb_glossary] and lasts\u00a0for\u00a0the next 12 weeks. Even before she knows she is pregnant, a woman in the first trimester is likely to experience signs and symptoms of pregnancy. She may notice a missed menstrual period, and she may also experience tender breasts, increased appetite, and more frequent urination. Many women also experience nausea and vomiting in the first trimester. This is often called \u201cmorning sickness,\u201d because it commonly occurs in the morning, but it may occur at any time of day. Some women may lose\u00a0weight\u00a0during the first trimester because of morning sickness.\r\n

Second Trimester<\/h3>\r\nThe second trimester occurs during weeks 13 to 28 of pregnancy. A pregnant woman may feel more energized during this trimester. If she experienced nausea and vomiting during the first trimester, these symptoms often subside during the second trimester. Weight gain starts occurring during this trimester, as well. By about week 20, the fetus is getting large enough that the mother can feel its movements. The photo in Figure 18.7.5 shows a pregnant woman at week 26, toward the end of the second trimester. (For comparison, the same woman is shown on the right at the end of the third trimester.)\r\n\r\n[caption id=\"attachment_5284\" align=\"alignright\" width=\"283\"]\"18.7.5 Figure 18.7.5 The same woman is shown in both photos: on the left at week 26 of the pregnancy, and on the right at week 40 of the pregnancy.<\/em>[\/caption]\r\n

Third Trimester<\/h3>\r\nThe third trimester occurs during weeks 29 through birth (at about 40 weeks). During this trimester, the uterus expands rapidly, making up a larger and larger portion of the woman's abdomen.\u00a0Weight\u00a0gain is also more rapid. During the third trimester, the movements of the fetus become stronger and more frequent, and they may become disruptive to the mother. As the fetus grows larger, its weight and the space it takes up may lead to symptoms in the mother such as back pain, swelling of the lower extremities, more frequent urination, varicose veins, and heartburn. By the end of the third trimester, the woman's abdomen often will transform in shape as it drops, due to the fetus turning to a downward position before birth so its head rests on the [pb_glossary id=\"5235\"]cervix[\/pb_glossary]. This relieves pressure on the upper abdomen, but reduces bladder capacity and increases pressure on the pelvic floor and rectum.\r\n

Childbirth<\/h2>\r\nNear the time of birth, the [pb_glossary id=\"5285\"]amniotic sac[\/pb_glossary] \u2014 a fluid-filled membrane that encloses the [pb_glossary id=\"5086\"]fetus[\/pb_glossary] within the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] \u2014 breaks in a gush of fluid. This is commonly called \u201cbreaking water.\u201d Labour usually begins within a day of this event, although it may begin prior to it.\u00a0[pb_glossary id=\"5286\"]Labour[\/pb_glossary]<\/strong> is the general term for the process of childbirth in which regular uterine contractions push the fetus and placenta out of the body. Labour can be divided into three stages, which are illustrated in Figure 18.7.6: dilation, birth, and afterbirth.\r\n
    \r\n \t
  1. During the dilation stage of labour, uterine contractions begin and become increasingly frequent and intense. The contractions push the baby\u2019s head (most often) against the cervix, causing the cervical canal to dilate, or become wider. This lasts until the cervical canal has dilated to about 10 cm (almost 4 in.) in width, which may take 12 to 20 hours \u2014 or even longer. The cervical canal must be dilated to this extent in order for the baby\u2019s head to fit through it.<\/li>\r\n \t
  2. During birth, the baby descends (usually headfirst) through the cervical canal and vagina, and into the world outside. This is the stage when the mother generally starts bearing down during the contractions to help push out the fetus. This stage may last from about 20 minutes to two hours or more. Usually, within a minute or less of birth, the umbilical cord is cut, so the baby is no longer connected to\u00a0the placenta.<\/li>\r\n \t
  3. During the afterbirth stage,\u00a0the placenta\u00a0is delivered. This stage may last from a few minutes to a half hour.<\/li>\r\n<\/ol>\r\n[caption id=\"attachment_5287\" align=\"aligncenter\" width=\"700\"]\"18.7.6 Figure 18.7.6 The three stages of labour are dilation of the cervix, birth of the baby, and delivery of the afterbirth (placenta).<\/em>[\/caption]\r\n\r\n
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    Breastfeeding<\/h1>\r\n<\/div>\r\nAlthough the [pb_glossary id=\"5288\"]breast<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0are not classified as organs of the reproductive system, they nonetheless may play an important role in\u00a0reproduction. The physiological function of the female breast is\u00a0<\/span>[pb_glossary id=\"5289\"]lactation[\/pb_glossary],<\/strong> or the production of breast milk to feed an infant. This function is illustrated in Figure 18.7.7. Besides nutrients, breast milk provides hormones, antibodies, and other substances that help ensure a healthy start after birth.<\/span>\r\n\r\n[caption id=\"attachment_5290\" align=\"aligncenter\" width=\"400\"]\"18.7.7 Figure 18.7.7 The physiological function of the human breast is to provide nourishment and other substances to an infant.<\/em>[\/caption]\r\n\r\nThe Figure 18.7.7 (above) shows the correct way for an infant to suck the breast to stimulate the letdown of milk from the mammary glands (lips flanged, baby's mouth on the nipple symmetrically). The letdown of milk when an infant sucks at the breast is one of the few examples of a [pb_glossary id=\"2962\"]positive feedback[\/pb_glossary] loop in the human organism. Sucking causes a release from the posterior [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary] of the hypothalamic hormone [pb_glossary id=\"3093\"]oxytocin[\/pb_glossary]. Oxytocin, in turn, causes milk to flow from the alveoli in the breasts where milk is produced, through the milk ducts, and into the milk sacs behind the areola. You can trace this route of milk through the breast in Figure 18.7.8. The baby can suck the milk out of the sacs through the nipple, where they converge. The release of milk stimulates the baby to continue sucking, which in turn keeps the milk flowing.\u00a0 Oxytocin is also an important hormone for maternal-child attachment.\r\n\r\n[caption id=\"attachment_5291\" align=\"aligncenter\" width=\"350\"]\"18.7.8 Figure 18.7.8 Sucking the nipple allows the baby to drain the milk ducts, and continued sucking results in the release of more milk into the ducts.<\/em>[\/caption]\r\n\r\n
    \r\n\r\nFemale Sex Hormones<\/span>\r\n\r\n<\/div>\r\nFemale\u00a0reproduction\u00a0could not occur without sex hormones released by the ovaries. These hormones include estrogen and progesterone.\r\n

    Estrogen<\/h2>\r\nBefore birth, [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] is released by the gonads in female [pb_glossary id=\"5086\"]fetuses<\/span>[\/pb_glossary]<\/span>\u00a0and leads to the\u00a0development\u00a0of\u00a0female reproductive organs. At [pb_glossary id=\"5155\"]puberty[\/pb_glossary], estrogen levels rise and are responsible for sexual maturation, and for the development of female secondary sex characteristics (such as breasts). Estrogen is also needed to help regulate the\u00a0[pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary]\u00a0and [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] throughout a woman\u2019s reproductive years. Estrogen is produced primarily by follicular cells in the ovaries. During pregnancy, estrogen is also produced by\u00a0the [pb_glossary id=\"5242\"]placenta[\/pb_glossary]. There are actually three forms of estrogen in the human female: estradiol, estriol, and estrone.<\/span>\r\n
      \r\n \t
    1. Estradiol<\/strong>\u00a0is the predominant form of estrogen during the reproductive years. It is also the most potent form of estrogen.<\/li>\r\n \t
    2. Estriol<\/strong>\u00a0is the predominant form of estrogen during pregnancy. It is also the weakest form of estrogen.<\/li>\r\n \t
    3. Estrone<\/strong>\u00a0is the predominant form of estrogen in post-menopausal women. It is intermediate in strength between the other two forms of estrogen.<\/li>\r\n<\/ol>\r\n

      Progesterone<\/h2>\r\n[pb_glossary id=\"3411\"]Progesterone[\/pb_glossary]<\/strong>\u00a0stands for \u201cpro-gestational\u00a0hormone.\u201d It is synthesized and secreted primarily by the corpus luteum in the ovary. Progesterone plays many physiological roles, but is best known for its role during pregnancy. In fact, it is sometimes called the \u201chormone of pregnancy.\u201d Among other functions, progesterone prepares the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] for pregnancy each month by building up the uterine lining. If a pregnancy occurs, progesterone helps maintain the pregnancy in a number of ways, such as decreasing the maternal immune response to the genetically different embryo, and decreasing the ability of uterine muscle tissue to contract. Progesterone also prepares the [pb_glossary id=\"3432\"]mammary glands<\/span>[\/pb_glossary]<\/span>\u00a0for [pb_glossary id=\"5289\"]lactation[\/pb_glossary] during pregnancy, and withdrawal of progesterone after birth is one of the triggers of milk production.<\/span>\r\n
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      Feature: Myth vs. Reality<\/h1>\r\n<\/div>\r\nThere are many myths associated with pregnancy. Most are harmless, but some may put the pregnant woman or fetus at risk. As always, knowledge is power.\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
      Myth<\/span><\/th>\r\nReality<\/span><\/th>\r\n<\/tr>\r\n
      \"You should avoid petting your cat during pregnancy.\"<\/em><\/td>\r\nCat feces may be contaminated with microscopic parasites that can cause a disease called toxoplasmosis. Pregnant women who contract this disease are at risk of stillbirth, miscarriage, or giving birth to an infant with serious health problems. Pregnant women should not have contact with a cat\u2019s litter box or feces, but petting a cat poses no real risk of infection.<\/td>\r\n<\/tr>\r\n
      \"You should not dye your hair during pregnancy, because the chemicals can harm the fetus.\"<\/em><\/td>\r\nWhereas some chemicals (such as certain pesticides) have been shown to be associated with birth defects, there is no evidence that using\u00a0hair\u00a0dye during pregnancy increases this risk.<\/td>\r\n<\/tr>\r\n
      \"A pregnant woman needs to eat for two, so she should double her pre-pregnancy caloric intake.\"<\/em><\/td>\r\nThroughout a typical pregnancy, a woman needs only about 300 extra calories per day, on average, to support her growing fetus. Most of the extra calories are needed during the last trimester, when the fetus is growing most rapidly. Doubling her caloric intake during pregnancy is likely to cause too much weight gain, which can be detrimental to her baby. Babies that weigh much more than the average 7.5 pounds (3.4 kg) at birth are more likely to develop diabetes and obesity in later life.<\/td>\r\n<\/tr>\r\n
      \"Women who are pregnant have strange food cravings, such as ice cream with pickles.\"<\/em><\/td>\r\nSome women do have food cravings during pregnancy, but they are not necessarily cravings for strange foods or unusual food combinations. For example, a pregnant woman might crave starchy foods for a few weeks, or she may be put off by certain foods that she loved before pregnancy.<\/td>\r\n<\/tr>\r\n
      \"A pregnant woman has skin that glows.\"<\/em><\/td>\r\nPregnancy can actually be hard on the skin and its appearance. Besides stretch marks on the abdomen and breasts, pregnancy may lead to spider veins, varicose veins, new freckles, darkening of moles, and acne flare-ups. In addition, as many as 75 per cent of pregnant women experience chloasma, which is the emergence of blotchy brown patches of skin on the face due to high estrogen levels. Chloasma is often referred to as the \u201cmask of pregnancy.\u201d<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n
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      18.7 Summary<\/span><\/h1>\r\n<\/header>\r\n
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