|4538 - How Sex Works||05/06/2005 - 09:16:47|
From a biological standpoint, the goal of sex is to merge two sets of genetic information, one from the father and one from the mother, to make a baby that is genetically different from either parent.
In this article, we will explore the biology of sex -- otherwise known as human sexual reproduction. We will examine the body's sexual organs, the biological cycles of sex, and the process of fertilization.
In reptiles and mammals (including humans), fertilization takes place inside the body of the female (internal fertilization). This technique increases the chances of successful sexual reproduction. Because we use internal fertilization, our sexual organs are specialized for this purpose. Let's take a closer look at the sexual organs in males and females.
Male Sex Organs
The testes are housed outside of the main part of the male's body, in a sac called the scrotum. This location is important because in order for the sperm to develop properly, they must be kept at a slightly lower temperature (95 to 97 degrees Fahrenheit, 35 to 36 degrees Celsius) than normal body temperature (98.6 F, 37 C).
The immature sperm travel from each testis to a coiled tube on the outer surface of each testis called the epididymis, where they mature in about 20 days. The sperm exit the body through the penis.
The penis is made of soft, spongy tissue. When engorged with blood during sexual excitation and intercourse, the spongy tissue stiffens and causes the penis to become erect, which is important for the penis's main function -- to place the sperm inside the female.
Male sex organs
Delivering Sperm Cells
One last male organ is a tiny, pea-sized set of glands inside the body at the base of the penis, called the bulbourethral or Cowper's glands. During sexual excitation, and just prior to the ejection of sperm (ejaculation), the Cowper's glands secrete a tiny amount of fluid that neutralizes any traces of acidic urine that may be leftover in the urethra. It is also believed that these secretions may lubricate the penis and female tract during sexual intercourse.
Female Sex Organs
The two ovaries are the major female sex organs, the counterpart of the male testes. The ovaries make the eggs, or oocytes, which are the female gametes, and produce estrogen, the female sex hormone. Estrogen causes female secondary sexual characteristics such as pubic hair, breast development, widening of the pelvis and deposition of body fat in hips and thighs. The ovaries are located in the abdomen.
Female sex organs
Eggs develop inside the ovary and are released upon ovulation into a tube (the oviduct or Fallopian tube) lined with fingerlike projections. The egg travels through the Fallopian tube, where fertilization can take place, to a muscular chamber called the uterus.
The uterus is where the baby develops. It is made of smooth muscle and is normally about the size and shape of a small pear turned upside down. During pregnancy, it can stretch to about the size of a basketball to hold the developing baby. The base of the uterus (neck of the pear) is a muscular wall called the cervix. In the cervix is a tiny opening, about the size of a pinhead, called the external os. The external os is filled with a thick plug of protein (mucus) that serves as a barrier to the entrance of the uterus. The cervix leads into a smooth-muscle-walled tube called the vagina, or birth canal.
The vagina connects the uterus to the outside of the body, and its opening is covered by the labia. The vagina receives the male's penis during sexual intercourse and delivers the baby during childbirth. The vagina is normally narrow (except around the cervix), but can stretch during intercourse and childbirth.
Finally, two sets of glands, the greater vestibular gland (Bartholin's gland) and the lesser vestibular gland, are located on either side of the vagina and empty into the labial folds of skin. The secretions from these glands lubricate the labial folds during sexual excitation and intercourse.
Development of Sex Organs
Now, let's take a look at some other organs that are important to human sexual function.
The nerve cells time-release small, low-level spurts of GnRH every 90 minutes, which causes the anterior pituitary to secrete small pulses of LH and FSH. The sex hormones from the testes/ovaries give feedback to the hypothalamus and anterior pituitary to regulate the secretion of GnRH, LH and FSH -- this interplay is called the negative feedback control system. The chemical interplay between the hypothalamus, anterior pituitary gland and the testes/ovaries is important for sexual development, maintaining sexual function and sexual reproduction. An error in this chemical interplay can be a cause of infertility.
Production of Sex Cells
Each cell in your body contains a set of chromosomes from your mother (her egg) and your father (his sperm). When you look at human chromosomes, they are X-shaped, except for the male Y chromosome. When your body produces sex cells (sperm or egg, depending on whether you are a male or female), your body must reduce the number of chromosomes by half to go into the sex cells. To do this, it randomly sorts chromosomes from both sets in one cell division and then reduces them by half in another. Therefore, each sperm or egg that your body produces is unique and different -- it contains a different mix of your mother's and father's genes. This is why two brothers in the same family can look and act totally different from one another even though they come from the same parents -- it all depends on which genes (chromosomes) were randomly chosen when producing the sex cells of the mother and father.
Although women are capable of conceiving a child from puberty to menopause, the timing of sexual intercourse is critical to the success of sexual reproduction. Women have an ovulatory cycle, or menstrual cycle, made up of complex physiological and biochemical changes that are linked to the timing of reproduction.
In the first phase of the menstrual cycle, a follicle (the cell complex that surrounds and nurtures the egg) grows in the ovary, while the lining of the uterus builds up to receive a fertilized egg. At mid-cycle, when the egg is ready, the ovary releases the egg. This stage is called ovulation, and is the optimum time for fertilization to take place. The egg enters the Fallopian tube on its way to the uterus. In the last phase of the menstrual cycle, one of two things happens: If the egg is fertilized, it continues on to the uterus, attaches to the lining and pregnancy begins; if the egg is not fertilized, the uterus sheds its lining (and the now-dead egg), menses begins and another menstrual cycle ensues.
The man's brain sends nerve impulses to the blood vessels in his penis and tells the arterioles to dilate and the venules to constrict. The blood flow engorges the spongy tissue of his penis, causing it to become erect. As the couple engages in intercourse, the man inserts his erect penis into the woman's vagina. As intercourse continues, the man reaches a point at which muscle contractions in the epididymis, prostate and seminal vesicles propel semen from the penis into the woman's vagina (ejaculation) at the base of the uterine cervix. Muscle contractions in the woman's body periodically dip her cervix into the semen.
Once the semen is deposited at the base of the uterus, the sperm begin a long journey to fertilization.
Once the sperm have traversed the cervical mucus, they travel up the moist lining of the uterus into the Fallopian tubes (only one of the Fallopian tubes contains an egg, so many sperm travel in the wrong direction). Fewer than 1,000 sperm out of the millions in the semen actually reach the Fallopian tubes.
Many sperm surround the egg in the Fallopian tube. The head of each sperm (acrosome) releases enzymes that begin to break down the outer, jelly-like layer of the egg's membrane, trying to penetrate the egg. Once a single sperm has penetrated, the cell membrane of the egg changes its electrical characteristics (depolarizes). This electrical signal causes small sacs just beneath the membrane (cortical granules) to dump their contents into the space surrounding the egg. The contents swell, pushing the other sperm far away from the egg (cortical reaction). The other sperm die within 48 hours. The cortical reaction ensures that only one sperm fertilizes the egg.
The fertilized egg is now called a zygote. The depolarization caused by sperm penetration results in one last round of division in the egg's nucleus, forming a pronucleus containing only one set of genetic information. The pronucleus from the egg merges with the nucleus from the sperm. Once the two pronuclei merge, cell division begins immediately.
The dividing zygote gets pushed along the Fallopian tube. By approximately four days after fertilization, the zygote has about 100 cells and is called a blastocyst. When the blastocyst reaches the uterine lining, it floats for about two days, finally implanting in the uterine wall by the sixth day after fertilization. Once implanted, the blastocyst secretes hCG, which rescues the corpus luteum and signals that a successful pregnancy has begun.
The implanted blastocyst continues developing in the uterus for nine months. As the baby grows, the uterus stretches until it is about the size of a basketball.
In contrast, the two daughter cells that remain after a fertilized egg undergoes its first division may separate and divide independently of each other. When this happens, they remain loosely connected while in the Fallopian tube, and the two blastocysts implant together in the uterine wall. They develop into two separate embryos. Because these embryos came from the same fertilized egg, they share identical genetic material and are called identical twins.
Contraception and STDs
See FDA: Birth Control Guide for details, including information on the effectiveness of various contraception methods.
Sexual activity carries certain risks of diseases caused by:
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