Physiology of Pregnancy PDF

Pregnancy ,physical changes, placenta Laaraib Nawaz Fertilization Within an hour after sexual intercourse sperm would have travelled from the vagina through the cervix into the uterus and uterine tube. the acrosome on the head of the spermatozoa would be worn off, releasing...

Pregnancy ,physical changes, placenta Laaraib Nawaz Fertilization Within an hour after sexual intercourse sperm would have travelled from the vagina through the cervix into the uterus and uterine tube. the acrosome on the head of the spermatozoa would be worn off, releasing acrosin enzyme by the time sperm are attached to the outer coatings of the ovum. Cont..  About 50 spermatozoa are attached to the outermost coating called corona radiata using hydrolysis reaction aided by acrosin ,some of the sperm reach the inner coating called zona pellucida.  One of the sperm will eventually penetrate through zona pellucida and allow its cell membrane to fuse with cell membrane of ovum.  this causes a rapid electrical depolarization at the cell membarne of ovum, preventing other sperm entering the ovum. Cont..  Now meiosis II is reactivated in the cytoplasm of the ovum ,dividing the 46 chromosomes in the nucleus into 23 chromosomes for fertilization(uniting with another 23 chromosomes from the sperm) and 23 chromosomes to be eliminated along with the second polar body.  The head of the penetrated sperm is now detached from its mid piece and tail. It will then rupture releasing 23 chromosomes in the form of long strands of DNA molecule. Cont.. The chromosomes from the sperm and ovum now unite to form a complete set of genetic makeup for the offspring – 2 haploid cells are now joined to become a diploid cell called zygote. Fertilization is now complete. Pregnancy A zygote is formed about 12-24 hours after ovulation. This single cell still the same size as the original ovum continues to travel through the uterine tube toward the uterus by the action of cilia along the inner lining of uterine tube. About an hour after fertilization is complete mitotic cell division called the cleavage occurs dividing the zygote into a cluster of smaller cells. Cont.. By the time cleavage has produced 16 identical cells , it is called a morula which occurs about 2-3 days after fertilization. Cleavage continues along the journey through the uterine tube,by the time this cluster of cells has arrived at the uterus about 5-6 days after fertilization it is called a blastocyst which contains hundreds of small cells called blsatocyst which contains hunderds of small cells called blastomeres surrounding a hollow cavity called blastocoel. Cont.. The blastocyst releases digestive enzymes and embedds itself onto the thickened and vascularized endometrium layer –a process called implantation which occurs about 7 days after fertilization. The blastocyst is now called an embryo which continues to develop for the next 2 months until a fetus is formed. Soon after implantation layers of membrane begin to form outside embryo. Cont..  Chorion the innermost membrane which secretes a hormone called the Human Chorionic Gonadotropin (HCG) stimulates the corpus luteum in the ovary for the secretion of estrogens and progestrone until the placenta is fully developed and can secrete estrogens and progestrone.  Amnion the middle layer that secretes amniotic fluid for nourishing of the embryo.  Placenta the outermost membrane that protects the embryo and fetus allows exchange of nutrients and waste between fetal and maternal blood and secretes estrogens and progestrone to maintain pregnancy. HCG  In the first 3 months of pregnancy or first trimester HCG level is the highest and it declines in the last two trimesters. This is to ensure that corpus luteum is sustained and not degenrated into corpus albicans.  HCG is secreted by renal tubules into urine allowing pregnancy to be tested positive in a typical pregnancy test.  The high HCG level may be responsible for ‘morning sickness’ and other discomfort felt by pregnant female. Cont.. In the last two trimesters placental estrogens and progestrone cause the uterus and breasts to enlarge and during labor cause the vagina to stretch. The sharp decline of estrogens after birth will signal new ovarian and menustral cycles to begin. The sudden reduction of progestrone before birth removes the suppression of oxytocin from posterior pitutary gland resulting in the uterine contractions during the birth process. Total duration Pregnancy lasts for about 40 weeks (280 days after the last menustration or 266 days after fertilization) and ends with parturition. During the last 6 weeks of fetal development the fetus assumes the Vertex position where the head faces the cervix. Cont.. At the ned of pregnancy the fetus moves downward and its head causes pressure onto the dialating cervix ( the hormone RELAXIN from the ovaries stimulate the dilation of cervix and pubic symphysis. The pressure onto the cervix signals the hypothalamus which in turn stimulates the posterior pitutary gland for the release of oxytocin. Cont.. Oxytocin causes the myometrium layer made of smooth muscle to contract involuntarily,pushing the fetus downward. The downward movement of fetus exerts more pressure onto the cervix , a phenomenon called positive feedback untill the fetus is expelled from the uterus through the cervix and vagina to the outside. Hormonal changes during pregnancy  Following implantation cells of trophoblast embryonic cells that helps from placents begins to secrete HCG.  HCG maintains the corpus luteum which continues secreting estrogens and progestrone.  As the placenta develops it secretes large quantities of estrogen and progestrone: o Stimulate the uterine lining to cnotinue development. o Maintain the uterine lining o Inhibit secretion of FSH & LH from anterior pitutary gland o Stimulate development of memroy gland o Inhibit uterine contractions (progestrone) o Enlarge the reproductive organs (estrogen) Cont.. Relaxin from the corpus luteum also inhibits uterine contractions and relaxes the pelvic ligaments. The placenta secretes placental lactogen that stimulates breast development. Aldosterone from adrenal cortex promotes recabsorbption of sodium leading to fluid retention. Parathyroid hormone from the parathyroid gland hepls maintain a high concentartion of calcium due to high fetal demand for calcium.