Book : Human Biology
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Posted by: CHELSEA
posted by
Writer CHELSEA

Female Reproductive Cycle

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Overview

Towards the end of puberty, girls begin to release eggs as part of a monthly period called the female reproductive cycle, or menstrual cycle (menstrual referring to "monthly"). Approximately every 28 days, during ovulation, an ovary sends a tiny egg into one of the fallopian tubes. Unless the egg is fertilized by a sperm while in the fallopian in the two to three days following ovulation, the egg dries up and leaves the body about two weeks later through the vagina. This process is called menstruation. Blood and tissues from the inner lining of the uterus (the endometrium) combine to form the menstrual flow, which generally lasts from four to seven days. The first period is called menarche. During menstruation arteries that supply the lining of the uterus constrict and capillaries weaken. Blood spilling from the damaged vessels detaches layers of the lining, not all at once but in random patches. Endometrium mucus and blood descending from the uterus, through the liquid creates the menstruation flow.

Description

The reproductive cycle can be divided into an ovarian cycle and a uterine cycle (compare ovarian histology and uterine histology in the diagram on the right). During the uterine cycle, the endometrial lining of the uterus builds up under the influence of increasing levels of estrogen (labeled as estradiol in the image). Follicles develop, and within a few days one matures into an ovum, or egg. The ovary then releases this egg, at the time of ovulation. After ovulation the uterine lining enters a secretory phase, or the ovarian cycle, in preparation for implantation, under the influence of progesterone. Progesterone is produced by the corpus luteum (the follicle after ovulation) and enriches the uterus with a thick lining of blood vessels and capillaries so that it can sustain the growing fetus. If fertilization and implantation occur, the embryo produces Human Chorionic Gonadotropin (HCG), which maintains the corpus luteum and causes it to continue producing progesterone until the placenta can take over production of progesterone. Hence, progesterone is "pro gestational" and maintains the uterine lining during all of pregnancy. If fertilization and implantation do not occur the corpus luteum degenerates into a corpus albicans, and progesterone levels fall. This fall in progesterone levels cause the endometrium lining to break down and sluff off through the vagina. This is called menstruation, which marks the low point for estrogen activity and is the starting point of a new cycle.

Fertilization Overview


Fertilization is the joining of a sperm and an egg. A sperm is a male gamete that is released into the vagina of a female during intercourse. In order for fertilization to occur there must be a mature ovum present. Every month one of the ovaries releases an egg which will meet one of the A 4 million sperm the male ejaculates into the vagina. The sperm swim through the cervix and into the uterus which lead to the fallopian tubes. This is where fertilization is most likely to take place. The high amount of sperm in the ejaculate is needed because only around 100 survive to enter reach the fertilization site. In order to penetrate the egg the sperm must first break through 2 barriers surrounding the ovum. The acromsome of sperm comes in contact with the corona radiata and releases digestive enzymes that break down a gelatinous layer around the egg called, the zona pellucida. Once a sperm reaches the plasma membrane of the egg it sets of a reaction that spreads across the membrane of the egg preventing other sperm from breaking through the egg membrane. Once the sperm reaches the inside of the egg it sheds its tail and the two nuclei fuse and now the 23 chromosomes from the egg and the 23 chromosomes of the sperm join and they become a zygote. Chromosomes contain all the information needed to determine the genetic structure of the new baby. Normally all human beings have two chromosomes that determine sex: A combination of X and Y makes a male or a combination of X and X makes a female. All ovum have X sex chromosomes where as sperm have both X or Y sex chromosomes. Therefore, the male gametes determine the sex of the baby.

Pregnancy and Lactation Overview


Mothers milk is ideal because it meets specific needs. Lactation is a neuroendocrine response in milk production sucking stimulates the sensory nerve endings in the nipples it sends stimulus tot he hypothalamus the hypothalamus stimulates anterior pituitary and prolactin is released. In milk let-down the sucking stimulates sensory nerves in the nipples this stimulates the hypothalamus in the hypothalamus this stimulates the posterior pituitary. This goes on to the release of oxytocin, because, when sucking occurs this stimulates contraction of the cells around the alveoli in the mammary cells milk then flows into the milk ducts causing milk let-down. Breast milk provides almost all the nutrients required for the first 4-6 months. It contains macronutrients like carbohydrates like lactose, fat such as high linoleic acid and protein like readily digest and absorbed alpha- lactalbumin. Breast milk also contains an adequate supply of vitamins and minerals, digestive enzymes, hormones and immunological factors. The first milk produced after birth is called colostrum this is synthesized during the end of pregnancy and 3-5 days of postpartum. This is very high in protein and low in fat and carbohydrate, and it contains immunoglobulins. This help the baby have a first bowel movement and prevent jaundice, and is different in color and is a different consistency. In some cultures they discard the colostrum because of the difference, but what they do not know is that it is the best thing for the baby. In breast milk the composition varies during feeding, over time and with development of the baby. When breast feeding there is three names for the composition of the milk. There is the fore milk, it is during the beginning of breast feeding, mid is the middled of feeding and hind which is the end of the feeding of he baby, it is high in composition of fat. When Breast feeding the female should consider the types of food that will be ate by her and the kind of diet she is consuming. If a female is on a low fat diet or if the foods like garlic broccoli and onions are eaten may affect the baby's preference for breast feeding. Also in the consumption of alcohol, caffeine, smoking, and medications a breast feeding mom should be discouraged of breast feeding.



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