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Ireland 3e Chapter Podcast, Chapter 18
The Reproductive System
The function of this system is species perpetuation. The reproductive
system produces gametes, provides a suitable place for the union of
the egg and sperm, nourishes the developing fetus, and produces the
sexual characteristics associated with being male or female.
Sexual reproduction requires two sexes, and results in the mixing of
genes. This mixing provides new genetic combinations that strengthen
the population. Natural selection is a selective force that operates on
these genes, so that one phenotype has a selective advantage over
others.
Gametes are the egg and sperm. They carry half the genetic material
of the species. Formation of these gametes occurs in the ovaries and
testicles, through the process of meiosis. Meiosis is very similar to
mitosis, normal somatic cell division, except that in prophase I tetrads
of homologous chromosomes form. These tetrads allow for crossing
over of genetic material, further enhancing the mixing of genes. In
anaphase I the tetrads are split and sent to two different cells. Meiosis
proceeds through telophase I, prophase II, metaphase II, anaphase II
and telophase II. The second round of division mirrors mitosis, but
because it began with 2 cells each carrying half the DNA, it ends with 4
haploid cells.
The male reproductive system includes a series of tubular organs that
produce, mature and deliver sperm. The testis are held in the scrotal
sac, ensuring that the temperature remains cool enough to produce
viable sperm. Sperm is produced within the testes. Once spermatids
are formed, they are released into the lumen of the seminiferous
tubules and travel to the epididymus where they mature. The
anatomy of a mature sperm, includes its acrosome and nucleus in the
head, the mitochoindria of the midpiece, and the flagellar tail. During
an ejaculatory event, sperm travel through the ductus deferens, on to
the ejaculatory duct and out the urethra of the penis. There are three
erectile tissues of the penis that become engorged with blood during
an erection.
The hormonal control of the male reproductive system begins in the
hypothalamus and pituitary gland, where FSH and LH are released.
FSH stimulates sperm production, and LH initiates testoterone
production.
The ovaries produce eggs. The uterine tubes capture the ovulated egg
and pass it along to the uterus. The vagina serves as the birth canal.
Within the ovaries, eggs are formed via oogenesis. Unlike sperm
production, only one egg is formed during meiosis. As eggs mature,
they pass through identifiable stages in the ovary. These stages,
beginning with a primordial follicle and continue past a secondary
follicle and a graffian follicle to ovulation. After ovulation, the follicular
cells remain on the ovary, producing hormones for 14 days. The
uterus responds to these hormones, thickening and secreting the
endometrial lining. The external anatomy of the female, including the
clitoris which is analogous to the penis. The mammary glands are part
of the female reproductive system, producing milk after the birth of
the infant.
The female reproductive system is governed by two separate hormone
cycles. The pituitary gland secretes follicle stimulating hormone and
leutenizing hormone, and in response, the ovaries secrete estrogen
and progesterone. Follicle stimulating hormone stimulates growth of
ovarian follicles. Growing follicles secrete estrogen that causes the
endometrial lining to thicken. The pituitary gland then secretes
leutenizing hormone, causing the most mature follicle to ovulate. The
ovary now produces progesterone, and the endometrial lining responds
by secreting nutritive fluids for a possible pregnancy. If no pregnancy
occurs, at the end of 14 days the ovary stops producing hormones,
and the endometrial lining sheds.
In both genders, orgasm is the culmination of sexual activity. The
human sexual response is composed of four phases: excitement,
plateau, orgasm and resolution.
Sexually transmitted diseases can be transmitted during sexual
contact. They include bacterial, viral, and protozoan diseases.
There are many options for couples who wish to avoid pregnancy. The
most reliable birth control method is abstinence. Surgical methods
include tubal ligations in females, or vasectomies in males. Hormonal
control such as birth control pills, depoprovera shots, norplant
hormonal implants, and birth control patches all control the female
reproductive cycle. Physical barriers to sperm reaching the egg
include both male and female condoms, as well as diaphragms.
Chemical methods of birth control prevent sperm from reaching the
egg by interfering with the motility of the sperm. Intrauterine devises
prevent implantation of a fertilized egg, and the morning after pill or
abortion removes implanted embryos. Natural family planning
includes predicting ovulation and avoiding introduction of sperm into
the female system when an egg is present.