Page 97 - 86 human physiology part-2

Chapter 16

Corpus luteum forms.

Progesterone secretion is
prominent.
(*)Assuming a 28 day cycle.

There are two phases of the ovarian cycle the follicular phase and the luteal phase. In the follicular
phase about 10-25 follicles are taken from preantral or early antrial follicles to develop further. Seven
days later the dominant follicle is selected to develop to full maturity. This is the pre-cursor for
ovulation. Follicles themselves secrete FSH and estrogen, and these two hormones stimulate follicular
growth and development. Ovulation marks the beginning of the luteal phase. This is started by the wall
of the Graffian follicle to rupture and cause a flow of antral fluid that will carry the oocyte to the
ovary's surface. The ruptured follicle is then turned into a gland (corpus luteum). Which secretes
estrogens and progesterone. This is all triggered by and abrupt change in plasma LH levels. After
ovulation the released oocyte enters the uterine tube, where it will be either fertilized or discarded.

The uterine cycle operates in sync with the ovarian cycle and is divided into three phases. The first
phase in the menstrual phase. It is named the menstrual phase because in corresponds with the shedding
the the uterine lining or more commonly called menstruation. The corpus luteum degenerates causing
plasma estrogen and progesterone levels to decrease and in turn causes menstruation. Blood vessels in
the outer most layer of the endometrium constrict and decrease blood flow to the tissues killing these
tissues. After the tissues die they start to separate from the underlying endometrail tissues. Eventually
the dead tissue is shed. This shedding of the tissues ruptures blood vessels and causes bleeding. Now
we have the proliferative phase. During this phase the uterus renews itself and prepares for pregnancy.
The endomitrial tissue that is left after menstruation begins to grow. The endometrial glands grow and
enlarge causing more blood vessels. The cervical canal has glands that secrete a thin mucous that helps
deposited sperm. Estrogen promotes uterine changes in this phase. The last phase is the secretory
phase. This is where the endometrium is transformed to make it the best environment for implantation
and subsequent housing and nourishment of the developing embryo. By doing this the endometrium
will do things like have an enriched blood supply, begin to secrete fluids rich in glycogen, and even
form a plug at the end of the cervical canal so that microorganisms can not enter. These changes in the
uterus are caused by progesterone, due to the corpus luteum. At the end of the secretory phase the
corpus luteum degenerates, and progesterone levels fall. This will trigger menstruation.

Sexual Reproduction

Sexual reproduction is a type of reproduction that results in increasing genetic diversity of the
offspring. In sexual reproduction, genes from two individuals are combined in random ways with each
new generation. Sex hormones released into the body by the endocrine system signal the body when it
is time to start puberty. The female and male reproductive systems are the only systems so vastly
different that each sex has their own different organs. All other systems have "unisex" organs.

Reproduction is characterized by two processes. The first, meiosis, involves the halving of the 46
of chromosomes. The second process, fertilization, leads the fusion of two gametes and the restoration
of the original number of chromosomes: 23 chromosomes from the paternal side and 23 from the
maternal side. During meiosis, the chromosomes of each pair usually cross over to achieve genetic
recombination.

Sexual reproduction cannot happen without the sexual organs called gonads. Both sexes have

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