The reproductive system is the only system that is not essential to a person's survival. It is, however, essential to the survival of the species. Because cells replicate continually, they are subject to errors of replication, either due to the inborn rate of error or because of environmental agents such as chemical pollutants or radiation. This makes the reproductive organs susceptible to diseases associated with genetic damage. These range from cancer, which affects the individual, to birth defects and mutations, which affect a person's offspring. Some pollutants are thought to mimic the sex hormones, affecting reproduction and development.
The main reproductive organs are the gonads: the testes in the male and the ovaries in the female. The gonads have two functions: production of haploid gametes by meiosis and the production of the steroidal sex hormones. The male gametes are sperm and the female gametes are ova (singular, ovum). In addition, there are a number of accessory reproductive organs. These include the ducts that transport sperm and ova, and associated glands.
Each ejaculation releases 2 to 5 mL of semen, the fluid containing the sperm. Semen normally contains between 20 and 100 million sperm per milliliter. The sperm consists of a 5-p.m head containing the nucleus, a midpiece containing mitochondria, and a 55-p.m-long flagellum, or tail, which can propel the sperm at a speed of 1 to 4 mm/min. The seminal vesicles and the prostate gland produce much of the liquid that makes up semen, which includes fructose to provide energy for the sperm, and alkalinity to counter the acidity of the vagina. When the male becomes sexually excited, nerves of the autonomic nervous system produce dilation of arterioles in the penis, causing blood to enter that organ faster than it can leave. This causes the erectile tissue to become engorged, causing an erection. The male then inserts the penis into the female's vagina in the act of copulation, injecting the semen by rhythmic contractions called ejaculations.
The ovaries contain about 400,000 ova arrested in prophase of meiosis I since birth. Some 400 of these will eventually develop completely, one per menstrual period. Cilia along the fallopian tubes transport the ova to the uterus. Fertilization may occur in either of these organs, and development takes place mostly in the latter.
The main sex hormone in the male is testosterone. It is produced mainly by the testes. It is part of a group of hormones produced by both males and females in the adrenal cortex that have a masculinizing effect. Thus, they are collectively called androgens ("man-maker"). The androgens other than testosterone are relatively low in potency. The main sex hormones in the female are progesterone and a group of hormones called the estrogens. The most important estrogen is called estradiol.
Production of sex hormones is controlled by a series of hormones originating with the brain stimulating the hypothalamus to release gonadotropin-releasing hormone (GnRH). GnRH is released in pulses and stimulates the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Although these names are based on their functions in females, they are produced in both sexes.
In males, FSH stimulates the testes to produce sperm and another hormone called inhibin. The inhibin inhibits FSH production, thus forming a negative feedback loop to control sperm production. LH stimulates the testes to produce testosterone. The testosterone inhibits GnRH and LH secretion, forming a negative feedback control loop. Testosterone has many functions, including (1) combining with FSH to stimulate sperm production, (2) affecting aggressive behavior and sexual desire through the CNS function, (3) stimulation of protein synthesis and muscle growth, (4) stimulation of secondary sex characteristics such as facial hair, (5) maintaining accessory organs, and (6) in fetuses it stimulates the formation of male reproductive system. In contrast to females, GnRH production in males is relatively constant from hour to hour or day to day, thus keeping its effects also at a constant level.
The hormone levels in females, on the other hand, varies in a cycle with a period of 22 to 35 (average 28) days. This cycle, together with associated physiological changes, is called the menstrual cycle. The menstrual cycle is unique to humans, monkeys, and apes. Females who menstruate may be receptive to males at any part of the cycle, although they can only conceive at certain points. Other mammals follow a simpler estrous cycle, in which the female will only copulate at certain times, called estrus or heat. Some animals, such as dogs and foxes, enter estrus only once per breeding season. Others, especially the mammals in tropical regions, have repeated estrus during the breeding season. The estrous cycle provides control over the season in which young are born.
The menstrual cycle consists of two ovarian phases or three uterine phases. In the fol-licular phase the structure containing the ovum develops into a follicle, and the ovum continues its meiosis to produce a mature ovum, or egg cell. The follicular phase ends with ovulation, when the ovum erupts from the surface of the ovary and enters the fallopian tube. This event marks the beginning of the second, luteal phase of menstruation, in which the remains of the follicle, now called the corpus luteum (yellow body), produces hormones that prepare the uterus for pregnancy. If pregnancy does not occur, the corpus luteum degenerates after about 12 days and the luteal phase ends.
Simultaneous with the ovarian phases, the uterus goes through its own cycle (Figure 9.15). The uterus is a muscular organ that supports and nourishes the fetus. It is lined with a glandular mucosa called the endometrium, which performs the nourishing function. The first phase of the uterine cycle is menses, in which the thickened endome-trium from the previous cycle deteriorates. Over a period of 3 to 5 days it is sloughed off and discharged along with 35 to 50 mL of blood. This endometrial sloughing is called menstruation. Menses is followed by the proliferative phase, in which the endometrium regenerates inside the uterus and produces a glycogen-rich mucus. Menses and the pro-liferative phase coincide with the follicular phase of the ovarian cycle. After ovulation, the uterus enters the secretory phase (simultaneous with the ovarian luteal phase), in which
Levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in blood plasma.
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