Human Reproduction
In the man’s body, the pituitary gland in the brain sends a hormone,
called follicle-stimulating hormone, (FSH) to the testicles, which are the
male sex glands. FSH signals the testicles to produce sperm, which are
the male reproductive cells. Sperm are so small they can only be seen through a microscope. Each
tiny sperm has a tale that allows it to move. After sperm are produced in
the testicles, they travel through a tube called the vas deferens to the
seminal vesicle, which holds the mature sperm.
A gland called the prostate makes seminal fluid, which is the fluid that
will transport the sperm in the urethra. The urethra is a tube that extends
from the bladder, through the prostate, then through the penis to the
outside of the man’s body. During sexual activity, the prostate gland becomes as full of fluid as it
can comfortably hold, and it contracts (squeezes). As the prostate
contracts, it draws sperm from the seminal vesicle. Seminal fluid mixed
with sperm is called semen. The contractions of the prostate gland force
the semen through the urethra and out the tip of the penis. This is called
ejaculation. The ejaculate (semen) is a teaspoon to a tablespoon of
thick milky liquid and contains millions of microscopic sperm.
During sexual activity, but before ejaculation, the penis becomes
engorged with blood. This causes the penis to become firm and erect,
and is called the male erection. The male erection allows the penis to
pass into the vagina of the woman. When a man has an erection a valve closes between the prostate gland
and the bladder to prevent urine from being released when semen is
ejaculated. Practically speaking, this means that a man cannot urinate
and ejaculate at the same time. To begin the reproductive cycle in a woman’s body, a hormone, called
follicle-stimulating hormone, (FSH) is produced by the pituitary gland in
the brain. FSH travels through the bloodstream to the ovaries, causing a
few of the many tiny follicles there to begin ripening, or maturing. Each
follicle is a tiny ring of cells with a tiny ovum, or egg, inside. As the
follicles ripen, they produce another hormone, called estrogen. Estrogen
causes the cervix, the opening of the uterus, to begin making moisture
and mucus. This is when a woman notices wetness or mucus outside
the vagina. This mucus is not a sign of disease. It is a natural part of the
cycle and indicates that the ova are maturing in the ovaries. If
intercourse occurs at this time, the mucus will protect the sperm from the
acidity of the vagina, assuring that the sperm will still be alive at
ovulation. Estrogen also causes the endometrium, the lining of the uterus, to grow
soft and spongy, forming a kind of bed that can, in the event of
pregnancy, hold and nourish a baby. Finally, estrogen signals the
pituitary gland in the brain that some follicles have matured in the
ovaries. When the pituitary gland receives the message that some follicles have
matured, it produces another hormone, leutenizing hormone, or LH. LH
causes one of the ripening follicles to release its ovum into a fallopian
tube. This is called ovulation.
Once ejaculation has deposited the sperm in the women’s vagina, they
swim up the vagina to the uterus. They do not know which fallopian tube
may contain the ovum, and so they swim both ways. When a sperm
penetrates the ovum, it is called fertilization.
Dividing and maturing as it goes, the fertilized ovum is moved slowly
along the fallopian tube toward the uterus by the cilia, tiny hairs inside
the fallopian tubes. In six or seven days, the fertilized ovum implants in
the endometrium (the lining of the uterus), which is the bed that has
been created to receive it. Menstruation does not occur, because the
fertilized ovum, growing and developing in the uterus, creates hormones
that maintain the uterine lining. The woman realizes she has conceived
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