7Vulva is the collective term for the external female genitalia. A pair of slender skin folds, the labia minora, border the openings,A pair of thick, fatty ridges, the labia majora, protect the vaginal opening.A thin piece of tissue called the hymen partly covers the vaginal opening.Several female reproductive structures are important in sexual arousal, and stimulation of them can produce highly pleasurable sensations.The vagina, labia minora, and a small erectile organ called the clitoris all engorge with blood and enlarge during sexual activity.The clitoris consists of a short shaft supporting a rounded glans, or head, covered by a small hood of skin called the prepuce.The clitoris, especially the glans, has an enormous number of nerve endings and sensitive to touch.
9From each testis, sperm pass into a coiled tube called the epididymis, which stores the sperm while they continue to develop.2. Sperm leave the epididymis duringEjaculation (the expulsion of sperm-containing ﬂuid from the penis).3. At that time, muscular contractions propel the sperm from the epididymis through another duct called the vas deferens.4. The vas deferens joins a short duct from a gland,The seminal vesicle.The two ducts unite to form a shortEjaculatory duct, joinsits counterpart conveying sperm from the other testis.5. Each ejaculatory duct empties into the urethra, which conveys bothurine and sperm out through the penis,Student Misconceptions and Concerns1. Students might need to be reminded about the changes in surface-to-volume ratios as organisms increase in size. As any organism gets larger (maintaining the same proportions) the need for a circulatory system coupled with a respiratory system increases, since the increase in surface area does not keep up with the increase in volume.2. Students might not realize that closed circulatory systems are capable of greater pressures when fluids remain confined to limited spaces.Teaching Tips1. If you have not included Chapter 20 in your course, you may want to show your class Figure 20.13A. This figure provides a general demonstration of the types of systems required by organisms too large to exchange all materials at the surface of the body.2. A gastrovascular cavity, seen in cnidarians and flatworms, absorbs and distributes nutrients throughout the organism’s body. The word root vascula (meaning “little vessel”) represents the circulatory function of these systems. As noted in Module 23.1, gastrovascular cavities are not effective in larger animals.3. The following analogy to a house might help students distinguish between open and closed circulatory systems. The flow of air through a home with a blower furnace is an open system, in which the furnace propels air through ducts that open into rooms, and the air is later collected by vents that channel air back to the furnace. In this open system, air pressure and currents are generally low. In contrast, the plumbing systems of most homes are much more like a closed system in which water, under high pressure, is contained in pipes. The analogy is not perfect, because water pipes do eventually open up into sinks and bathrooms, before draining into the sewage system.4. Challenge students to explain why closed circulatory systems have evolved in squids and octopuses, but not in clams or snails. The greater amount of muscular activity in squids and octopuses may have favored these more efficient systems of delivery.5. To help students understand the need for a circulatory system, consider this analogy. Small islands are like small animals: No inner part is very far from the edges. However, large countries, like large animals, have considerable interior areas located far from their borders. Therefore, large countries such as the United States, Canada, and China require an internal system of roads and railways to transport many goods from ocean ports to cities located deep in these countries. These roads and railways move materials from ports in the same way that blood and blood vessels move them from respiratory surfaces.6. There are many simple demonstrations of diffusion that can be performed. If you use a video imager or overhead projector, add a single drop of food coloring into a beaker of water with bright illumination. The slow dissipation of the dye will serve as a colorful and dramatic example of materials moving from a higher to a lower level of concentration.6. The human penis consists mainly of erectile tissue essential for insertion of the penis into the vagina. Like the clitoris, the penis consists of a shaft that supports the glans, or head. The glans is richly supplied with nerve endings and is highly sensitive to stimulation. A fold of skin called the prepuce, or foreskin, covers the glans.9
11Process of Ejaculation occurs in two stages. 1. At the peak of sexual arousal, muscle contractions in multiple glands force secretions into the urethra and propel sperm from the epididymis. At the same time, a sphincter muscle at the base of the bladder contracts, preventing urine from leaking into the urethra from the bladder. Another sphincter also contracts, closing entrance of the urethra into penis. The section of the urethra between the two sphincters ﬁlls with semen and expands.2. The expulsion stage, the sphincter at base of penis relaxes, admitting semen into the penis. At the same time, a series of strong muscle contractions around the base of the penis and along the urethra expels the semen from the body.
18About the time the secondary oocyte forms, the pituitary hormone LH (luteinizing hormone) triggers ovulation, the rupture of the follicle and expulsion of the secondary oocyte.The ruptured follicle then develops into a corpus luteum (“yellow body”).Unless fertilization occurs, the corpus luteum degenerates before another follicle starts to develop.
21Hormonal Events Before Ovulation Releasing hormone from the hypothalamus stimulates the anterior pituitary to increase its output of FSH and LH.FSH stimulates the growth of an ovarian follicle, in effect starting the ovarian cycle. In turn, the follicle secretes estrogen.As the follicle grows, secretes more and more estrogen, and the rising but still relatively low estrogen exerts negative feedback on the pituitary keeps blood levels of FSH and LH low.As the time of ovulation approaches, hormone levels change drastically, with estrogen reaching a critical peak (Part D) just before ovulation. This high level of estrogen exerts positive feedback on the hypothalamus makes the pituitary—secrete surges of FSH and LH
22Hormonal Events After Ovulation LH stimulates the completion of meiosis I, transforming the primary oocyte in the follicle into a secondary oocyte.It also signals enzymes to rupture the follicle, allowing ovulation to occur, and triggers the development of the corpusluteum from the ruptured follicle (hence its name, luteinizing hormone).LH also promotes the secretion of progesterone and estrogen by the corpus luteum.High levels of estrogen and progesterone in the blood following ovulation have a strong inﬂuence on both the ovary and uterus. The combination of the two hormones exerts negative feedback on the hypothalamus and pituitary, producing falling FSH and LH levels.The drop in FSH and LH prevents follicles from developing and ovulation from occurring during the post-ovulatory phase. Also, the LH drop is followed by the gradual degeneration of the corpusluteum.Near the end of the post-ovulatory phase, the corpus luteum stops secreting estrogen and progesterone---the hypothalamus --stimulate the pituitary to secrete more FSH and LH, and a new cycle begins.
23What happens in the human Control of the Menstrual CycleHormonal regulation of the menstrual cycle is simpler than that of the ovarian cycle.The menstrual cycle (Part E) is directly controlled by estrogen and progesterone alone.Starting around day 5 of the cycle, the endometrium thickens in response to the rising estrogen and, later, progesterone. When the levels of these hormones drop, the endometrium begins to slough off.Menstrual bleeding begins soon after, on day 1 of a new cycle.What happens in the humanovary and uterus in the absence of fertilization and pregnancy?Early in pregnancy, the developing embryo, implanted in the endometrium, releases a hormone (human chorionic gonadotropin, or hCG). This hormone acts like LH in that it maintains the corpusuteum, which continues to secrete progesterone and estrogen, keeping the endometrium intact.