The reproductive cycles of sexually-reproducing animals are influenced by a number of hormones, including follicle-stimulating hormone, leutinizing hormone, estrogen, testosterone, and progesterone, produced by the animals bodies. Additionally, reproductive cycles are influenced by environmental factors such as day length. Since the hormone melatonin is only secreted in the dark, animals bodies produce more melatonin in winter months when there is less daylight. Melatonin (or lack thereof) produces a number of physiological changes, including regulation of fertility in many species. In humans, melatonin levels drop 75% just before puberty, and it has been noted that blind girls whose brains/pineal glands do not receive light from their eyes have delayed onset of puberty. In many other species, melatonin controls sexual receptivity. For example, ewes only are receptive in fall/winter so the lambs will be born in spring and have all summer to grow. Typically, special sex pheromones are emitted to convey information about receptivity to members of the opposite sex or to regulate sexual cycles of other members of the same species.
Sperm production in males is under the control of follicle-stimulating hormone (FSH) and leutinizing hormone (LH). LH stimulates production of androgens (including testosterone) by the testes, while FSH stimulates the testes to produce sperm. As in females, production of FSH and LH are under the control of gonadotropin releasing hormone (GnRH) produced by the hypothalamus. Via a negative feedback loop, androgens control levels of GnRH, FSH, and LH. Additionally, androgens are responsible for development of the male reproductive organs during embryonic development, development of secondary sexual characteristics (beard, deeper voice) and behaviors (singing in birds), and production of sperm. Also via negative feedback loops, FSH and LH control production of GnRH. In human males, it appears that blood levels of all these hormones remain fairly constant, but in males of many other mammal species, blood levels of these hormones vary seasonally (hinting at involvement of melatonin in some way).
Thus, in a number of species of mammals, such as goats, deer, camels, and sheep, due to environmental and hormonal cues, the male periodically goes into rut, a periodic sexual excitement in the male that is analogous to estrous in the female. Billy goats (bucks) in rut stink worse that they do at other times. Buck deer are more likely to use their antlers to engage in sexual competition with other males. People who work with camels know that male camels in rut become more unpredictable, meaner than usual, and harder to work with and control.
In most species of mammals, the female has either an estrous cycle or a menstrual cycle. An estrous cycle is also called being in heat because the females body temperature rises a little. However, more pronounced than changes in temperature are noticable behavioral changes. If the female does not become pregnant, the endometrium is reabsorbed and (usually) there is no bleeding. In species in which the female has a menstrual cycle, the unused endometrium is shed and voided from the body as the menses or period or menstrual flow. Note that while this term was derived in the context of human females who happen to cycle in about the same length of time as a lunar month, it is also applied to other species whose cycles are not one month long. Most other species have cycles that are shorter or longer than a month, and in some species, the timing of ovulation and female receptivity is so tied to environmental cues (light, temperature, etc.) that they only cycle and mate at one or two specific times of the year. Human females are somewhat unusual because the females of most species are only receptive around the time of ovulation (release of a fertile egg). A human female is more receptive around the time of ovulation, but that is not the only time she is receptive. Human females generally are receptive to sexual activity throughout their cycles.
The average menstrual cycle in humans ranges about 20 to 40 days in length, with a statistical average of about 28 to 29 days. Some women experience regular cycles, while others are quite irregular. Much is understood about the hormones involved in the female cycle, but we are only beginning to understand the role of pheromones. It is well-known that the cycles of college women living in a dorm situation frequently adjust such that many of the women have their periods at the same time as each other. It has been shown experimentally that when perspiration from one woman is dissolved in alcohol and applied above the upper lip of other females (several times a week), their cycles all adjust to coincide with hers. There is also experimental evidence that frequent exposure (several times a week) to male perspiration (therefore, pheromones) can regulate a females cycle, causing irregular, short, or long cycles to become more regular and closer to the 28 to 29 day average. This would make sense evolutionarily because it would increase a womans chances of conceiving if she was in a situation where she had frequent contact with a stable, supportive mate who could provide her and her child with enough food to survive (at a time when she would be more vulnerable and less able to gather food herself, yet need more than usual).
By convention, the first day of a womans period is considered to be day 1 of her cycle. The first 3 to 7 days are generally the menstrual flow phase, and during this time, all hormones involved are at low levels. There are five hormones involved in controlling the female cycle. These are gonadotropin releasing hormone (GnRH) secreted by the hypothalamus, follicle-stimulating hormone (FSH) and lutenizing hormone (LH) secreted by the pituitary gland, and estrogen and progesterone secreted by the ovaries. The following diagrams and explanation show how levels of these hormones and the changes they induce vary throughout a womans cycle.
The first half of a womans cycle is the proliferative phase (follicular phase), during which the endometrium starts to thicken. The pituitary secretes FSH which causes (usually one) follicle to mature and the ovaries to secrete estrogen. The ovarian estrogen secretion gradually increases until just prior to ovulation. This gradually supresses secretion of FSH and stimulates the hypothalamus to secrete a larger amount of GnRH which, in turn, triggers the pituitary to secrete a burst of LH, causing ovulation. During the proliferative phase, a womans body temperature is low, perhaps as low as 98.0° F (36.7° C) or less. Sometimes there is a slight rise near the end of the phase during the pre-ovulatory burst of LH before it dips again at ovulation. Throughout this phase, the cervical mucus becomes progressively clearer and thinner.
On about day 14 of an average 28-day cycle, ovulation, the rupture of the follicle and release of the egg, occurs in response to the surge of LH. LH stimulates the remaining follicle cells to form a corpus luteum after ovulation. Often at ovulation there is a sharp drop in the womans body temperature and her cervical mucus becomes very thin and clear and forms threads. Interestingly, both the low body temperature and the condition of the mucus are designed to prolong the life of and aid in the mobility of sperm to increase the chances that the egg being released will be fertilized.
The second half of the cycle is called the secretory phase in the uterus and the luteal phase in the ovaries. The endometrium continues to build up due to the secretion of progesterone (and estrogen) by the corpus luteum, and this prepares the uterus for a possible pregnancy. In case an egg has been fertilized, the growing embryo needs a warmer environment to do well, so the body temperature rises to 98.6° F (37.0° C) or higher. The cervical mucus becomes thick and paste-like, blocking any more sperm from entering the uterus.
If an egg is not fertilized, as the corpus luteum deteriorates at the end of a cycle, both estrogen and progesterone levels go very low. In a pregnancy, implantation triggers hormone production that keeps the corpus luteum secreting estrogen and progesterone for a while, but without that, the corpus luteum disintegrates and stops secreting these hormones. At this time, all of the hormone levels are low and decreasing. This frequently causes the mood swings and other symptoms of premenstrual syndrome (PMS), but PMS is also affected by diet, and large amounts of salt, sugar, yeast, and/or caffeine can make things worse.
Throughout this discussion, the roles of the five hormones that influence a womans menstrual cycle have been mentioned, but their interactions are quite complex and involve both positive and negative feedback loops. Reiterating the previous discussion specifically from the point of view of the female hormone cycle will hopefully aid in better understanding the roles played by these hormones.
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Copulation in most species of animals is preceded by various courtship rituals which help insure that the most genetically fit individuals pass on their genes. In humans, various activities including flirting and foreplay lead up to and physiologically stimulate and prepare the couples bodies for coitus, and in general, most women need considerably more stimulation over a longer period of time than many men, to physiologically prepare their bodies for sexual intercourse. Human mating physiology includes four phases in the sexual response cycle.
If the woman is just pre-ovulation or at ovulation, conception, the start of a new person, of pregnancy, may occur. Interestingly, there are some things a couple can do to increase their chances of having a baby of a given sex. It has been found that sperm which contain an X chromosome tend to live longer than Y sperm, so having intercourse a couple days prior to ovulation increases the chances of a girl baby. On the other hand, Y sperm swim faster than X sperm, so having intercourse right at ovulation can increase the chances of a boy baby. As previously mentioned, at the time of ovulation, a womans body temperature and cervical mucus are right to help sperm survive. The contractions caused by orgasm and the prostaglandins in semen help propel sperm up into the uterus, and from there, up the Fallopian tubes. An egg is fertilized near the end of the Fallopian tube, then finishes meiosis, the nuclei unite, and the embryo starts dividing as it begins to travel to the uterus.
Recent work on human pheromones has turned up some interesting effects they may have on pregnancy. It is not unusual in our busy society, for a couple who desire a baby to keep track of where the woman is in her cycle, and plan to have intercourse at the right time, yet, despite no physical problems, many of these couples still fail to conceive. Recent work on human pheromones suggests that this is not enough. Apparently the womans body may need continued exposure to the mans pheromones, such as would be acheived by the close contact during coitus, several times a week during the last two weeks of her cycle to make implantation more likely. Again, evolutionarily, this would make sense in terms of signaling the presence of a pair-bonded male who could provide her and her developing child with enough food to survive the pregnancy. It would, then, make sense that lack of continued pheromonal stimulation (as in when the couple go back to their busy schedule until next month at the appointed time or in a one night stand) would signal the absence of a male that could mean increased hardship and lower chances of survival if pregnancy were to result. However, in the interest of passing on ones genes, evolutionarily, the possibility of pregnancy is not totally ruled out, just diminished.
If an egg is fertilized, fertilization occurs in the far end of the Fallopian tube near the ovary, and as the embryo travels down the tube, cell division starts. By the time it reaches the uterus (about a week later), it is a hollow ball or blastocyst, which has about 100 cells. The embryo, specifically the placenta, starts producing hormones which prevent the corpus luteum from disintegrating, and hormones from the corpus luteum maintain the endometrium until the placenta is large enough to secrete more hormones on its own. Note that whereas contraceptives, as their name suggests, prevent conception (union of egg and sperm to form a new person), intrauterine devices (IUDs) as well as morning after pills and some other drugs, including Depo-Provera, work to prevent implantation of the week-old, genetically-unique, growing embryo thereby causing it to die, and thus are abortifacients.
When the embryo implants in the endometrium, it starts to form the placenta. A human placenta attains the size of a dinner plate. The placenta serves as the site of transfer of nutrients and wastes between the maternal and fetal blood, and when large enough, it secretes hormones to maintain the endometrium, thus the pregnancy. Keep in mind that the two blood supplies do not come into contact with each other, but chemicals can be passed back and forth.
In the first trimester (the first three months) of pregnancy, most organogenesis, development of body organs occurs. The babys heart starts beating at about four weeks (when the mothers first period is about two weeks late and shes beginning to suspect she might be pregnant). Because so much critical growth and development takes place in the first few weeks before she even knows shes pregnant, it is very important that she be well nourished (including vitamin supplements if needed, especially folic acid to prevent neural tube defects) before she even tries to get pregnant. By the end of the eighth week (when her second period is two weeks overdue) all of the major body structures and organs of an adult are present in at least rudimentary form, so the embryo is now called a fetus. The rest of the time during pregnancy is mostly just growth. Note that because of organogenesis, the first trimester is the time when the growing baby is the most sensitive to adverse effects of drugs like alcohol, tobacco, and caffeine, viruses like German measles, and lack of necessary nutrients such as folacin. During the first trimester, the placenta secretes HCG to maintain the corpus luteum which continues to secrete the estrogen and especially progesterone needed to maintain the endometrium. The cervical plug forms to protect the growing baby from foreign invaders, and the womans breasts begin to enlarge in preparation for lactation, or nursing. The level of HCG is so high in the blood that some is excreted in the urine. Most pregnancy tests look for the presence of HCG in urine. As the sex organs begin to develop, the embryo starts with rudimentary forms of both male and female systems, and only one set continues to develop while the other atrophies. Also the initial development of external genitalia is similar and it can be difficult to tell the sex of a baby early on.
In the second trimester growth occurs. The baby is very active, and eventually these movements can be felt by the mother. The baby can hear by the fourth month, and when born recognizes not only moms voice, but also dads voice (if hes around) and/or any other frequently-heard, familiar sounds, perhaps including things like any musical instruments the mother plays, etc. During this trimester, HCG production declines and the corpus luteum stops producing progesterone as it deteriorates. As this is happening, the placenta takes over and begins to secretes progesterone itself to maintain the endometrium.
The third trimester is a time of rapid growth. The baby tends to move less just because the uterus has become so crowded. Estrogen levels in the mothers blood reach their highest in the last weeks of pregnancy, and as this increases, it triggers the formation of oxytocin receptors in the uterus. Late in pregnancy, fetal cells produce increasing amounts of oxytocin, another hormone, as does the pituitary. Oxytocin is a powerful smooth muscle stimulant, and due to the receptors which have formed in the uterus, causes uterine contractions. The high estrogen levels also stimulate the placenta to make prostaglandins (similar to those that cause cramps) that also stimulate contractions. The hormonal induction of labor is a positive feedback loop. Oxytocin and the prostaglandins cause uterine contractions that, in turn, stimulate the release of more oxytocin and prostaglandins. This is partially due to pressure of the babys head against the cervix that both dilates the cervix and acts on cervical nerve endings to stimulate the production of oxytocin, which causes stronger contractions that cause the babys head to be pressed more strongly against the cervix, etc.
There are three stages in the birth process, including:
After the baby is born, the umbilical cord is clamped and cut, often before the placenta is delivered. When the baby is born, much of his/her blood is still circulating in the placenta, and as (s)he begins to breath, that blood is naturally brought back into the body, aided by the pulsating of the umbilical cord. For optimum health of the baby, it is important that as much of the babys blood as possible be out of the placenta and into the babys body before the cord is cut, thus the person doing this should wait until the cord stops pulsating before clamping and cutting it. Many medical staff people try to rush through this procedure and cut the cord too soon.
After birth, decreasing progesterone and estrogen and increasing oxytocin help reduce the uterus to its normal size. It has been found that laying the baby on the mothers abdomen immediately after birth will stimulate the production of oxytocin, helping shrink the uterus faster. The babys sucking on the mothers nipples stimulates nerve endings that also stimulate the production of oxytocin, so immediate postpartum nursing can also help return uterus to proper size.
The hormones oxytocin and prolactin trigger the production of milk. While these are present initially in quantities sufficient to begin lactation, the babys sucking triggers further release of these hormones, in yet another positive feedback loop. At first, colostrum is produced. This nutrient-rich substance contains lots of antibodies, etc. important to the babys health for the first few days and its very important that the baby have the benefit of this nutrition. Also, nursing is important in the bonding process, and for that reason, too, its very important to let a baby nurse immediately after birth. There is evidence that human babies imprint on the smell of their mothers nipples. Instinctively, when a newborn is first trying to nurse, (s)he will turn his/her mouth toward anything that brushes his/her cheek. Frequently, in hospital setting, well-meaning nurses trying to help can actually distract and confuse the baby by touching the babys face, thus making it harder for the baby to find the nipple. In contrast, brushing the babys cheek with the nipple will cause the baby to turn toward it. About 2 to 3 days after the birth, the mothers real milk will begin to be produced. This is a specially-designed food source that exactly matches the nutritional needs of a baby, thus every effort should be made to make use of mothers milk rather than artificial substitutes which often are nutritionally lacking.
For couples who are trying to avoid creating a baby, a number of birth control methods that prevent fertilization (contraceptives) are available, but not all methods of birth control fit into that category. In one method of natural birth control, the woman must have very regular, predictable cycles and must keep a very accurate, daily record of her body temperature (first thing in the morning before getting out of bed) and condition of her cervical mucus in order to determine when ovulation is occurring. The couple must, then, cooperate in being willing to forego a few minutes of pleasure within several days of ovulation, in the interest of building a strong, long-term, loving relationship. Since many womens cycles are not that predictable and/or many women do not wish to keep such detailed records and/or many couples would rather not have to consult a calendar, a variety of other methods of preventing ovulation or preventing sperm from reaching the egg are available. Some of the hormone pills prevent ovulation, and many of the barrier methods attempt to prevent sperm from getting up into the uterus, either by introducing a physical barrier between the penis and the uterus (cervical cap, diaphragm, condom) or by killing the sperm (spermicidal chemicals inserted into the vagina). However, none of these methods is a substitute for willpower and personal responsibility, and will not prevent conception if the people involved do not choose to discipline themselves to make it work. If a barrier or foam is neglected or used improperly or if a woman doesn't take the pills on schedule, a baby is a likely outcome. Birth control methods that permanently make reproduction impossible are called sterilization, and include things like vasectomy, tubal ligation, and hysterectomy.
As mentioned above, some birth control methods like IUDs and some drugs prevent an already-fertilized and growing embryo from implanting in the uterus, thus are abortifacients, agents that cause abortions. RU-486 is specifically meant to kill an unborn baby, and must be taken in conjunction with other drugs that cause uterine contractions to expel the dead baby. According to patient literature which some of my female students received along with Depo-Provera injections, this drug works by thinning the endometrium of the woman’s uterus so that when/if a week-old blastocyst-stage embryo reaches her uterus, it will have no place to implant, and will, thus, die. While some people feel that abortion should legally be available as a means of correcting mistakes, it is not a contraceptive and should not be routinely used as such. Legal or not, abortion is not a substitute for personal willpower and responsibility for ones own actions. In early abortions, a suction device is used to dismember and remove the baby. In mid-term abortions, special tongs are used to dismember the baby and remove the pieces. In late-term abortions, some of the amniotic fluid is replaced with a strong salt solution to try to burn/poison/kill the baby, and the woman is then given drugs to induce labor. In late-term, partial birth abortions, the baby is manipulated into the breech birth position, then labor is induced. At the end of the delivery, the baby’s head is held inside the mothers vagina (so the birth isnt official) while a suction device is inserted at the top of the babys spine to remove his/her brain. When the baby is dead, the head is allowed to be born. Numerous studies have shown that women who go through these procedures often suffer from depression, sometimes severe, as a result. Studies have even found that years later, often without consciously realizing the cause, many of these women suffer depression especially on the anniversary of the date they had the abortion and/or on what would have been their child’s birthday. If you would like to see what actually goes on in an abortion, get hold of a copy of the movie Eclipse of Reason.
The only totally effective method of preventing making a baby (short of sterilization) is abstinence, yet in our culture, were bringing up a whole generation of young people who are being taught that they couldnt possibly be expected to have the personal willpower and responsibility for their actions to make that work. In previous generations when young people were taught that they were capable of abstinance and were expected to abstain from sexual intercourse until marriage, the rates of teen-age pregnancies among unwed mothers were much lower.
A number of sexually transmitted diseases (STDs) are caught/spread by having sexual intercourse with multiple partners, thus are easily avoided if a person so chooses. Some of these diseases are caused by viruses, while others are bacterial in origin. For example, “genital warts,” or human papiloma virus (HPV) is spread by sexual contact or by lying, naked, on an unsterilized tanning bed. HPV has been associated with cervical cancer in women, often necessitating a hysterectomy, perhaps at a young-enough age to preclude any opportunity of having children.
One interesting aspect of this is that while our government has been telling people that use of condoms will prevent STDs, current research indicates thats not entirely true. The latex of which condoms are made, just like any chemical, has pores or holes in between the molecules of which it is made. These holes are small enough that relatively large sperm cells are larger than the holes, so usually dont get through. However, viruses like the AIDS virus are several powers of 10 smaller in size than a large sperm cell, and are smaller than the size of the holes in the latex, thus can go through them and infect someone! Since the holes in the latex are not exactly lined up and its not a straight shot going through them, that does somewhat lessen the chance that an AIDS (or other) virus can find its way through. Thus, while use of condoms may reduce the chances of getting AIDS or some other virus, they do not prevent these diseases.
Other than unusual cases like hemophiliacs who need blood transfusions, the vast majority of people can control their chances of contracting AIDS if they choose to do so. Your chances of being exposed to AIDS are directly dependent on your lifestyle. If you choose to have sexual intercourse with many partners and have no idea with whom else theyve had intercourse, it is very likely that you will get AIDS. If you choose to save sexual intercourse for a special, lifelong commitment to one partner who feels the same way, and youre both the only ones youve ever had sex with (and neither of you does IV drugs, etc.), neither of you will be exposed to AIDS.
Also, note that the relatively-thicker walls of a woman’s vagina were designed to take the physical stress of sexual intercourse, while the relatively-thinner walls of a person’s rectum were not. Thus, “anal sex” frequently causes tears in the walls of the rectum which, if one’s partner is infected with an STD, can serve as entry sites for that infection. The tears from anal sex can significantly damage the rectum, which may necessitate painful, corrective surgery to “fix” accumulated rectal damage. If the person’s immune system has been “destroyed” by an HIV infection, the healing process following such surgery can be complicated by increased chances of infection.
Copyright © 1996 by J. Stein Carter. All rights reserved.
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