Further Reading / Supplemental Links
Ana Maria Rodriguez. Edward Jenner: Conqueror of Smallpox (Great Minds of Science). Enslow Publishers, 2006.
Lorrie Klosterman. Immune System (The Amazing Human Body). Benchmark Books, 2008.
Steve Parker. Defend Yourself: The Immune System. Raintree, 2006.
http://www.niaid.nih.gov/final/immun/immun.htm http://www.acm.uiuc.edu/sigbio/project/lymphatic/index.html; http://www.niaid.nih.gov/final/immun/immun.htm
http://www.niaid.nih.gov/publications/immune/the_immune_system.pdf
http://en.wikipedia.org/wiki
Vocabulary
antibody
Large, Y-shaped protein that binds with an antigen.
antigen
Any protein that triggers an immune response; unlike any protein that the body makes.
immune response
The specific third line of defense against pathogens; involves the immune system.
immune system
System that protects the body from pathogens and other causes of disease.
immunity
Ability to resist a pathogen because cells of the immune system "remember" the pathogen from a previous infection or vaccination.
lymph
Yellowish fluid that leaks out of tiny vessels into spaces between cells in tissues.
lymph nodes
Small, oval structures located along lymphatic vessels that filter pathogens from lymph.
lymphocytes
Type of white blood cells involved in an immune response.
vaccination
Deliberate exposure to a pathogen in order to bring about immunity without causing disease.
Points to Consider
What do you think is the role of the reproductive system?
Do you know what organs and other structures make up the reproductive system?
Do you know how they differ between males and females?
Chapter 22: Reproductive Systems and Life Stages
Lesson 22.1: Male Reproductive System
Lesson Objectives
State the functions of the male reproductive system.
Identify and describe the male reproductive organs.
Explain what sperm are and how they are produced.
Check Your Understanding
How does sexual reproduction occur?
What happens during meiosis?
What are gametes?
Dogs have puppies. Cats have kittens. All organisms reproduce, including humans. Like other mammals, humans have a body system that controls reproduction. It is called the reproductive system. It is the only human body system that is very different in males and females. The male and female reproductive systems have different organs and different functions.
Functions of the Male Reproductive System
The male reproductive system has two main functions: producing sperm and secreting testosterone. Sperm are male gametes. Gametes were introduced in the Cell Division, Reproduction, and DNA chapter. Gametes are sex cells that may unite to form a new organism. Sperm form when certain cells in the male reproductive system divide by meiosis. Mature males produce millions of sperm each day.
Testosterone is the main sex hormone in males. You read about hormones in the Controlling the Body chapter. Hormones are chemicals that control many body processes. Testosterone has two major roles.
During the teen years, testosterone causes the reproductive organs to mature. It also causes other male traits to develop. For example, it causes hair to grow on the face.
During adulthood, testosterone is needed for a man to produce sperm.
Testosterone is not the only sex hormone that males secrete. Males also secrete small amounts of estrogen, the main female sex hormone. Scientists think that estrogen is needed for normal sperm production in males.
Male Reproductive Organs
The male reproductive organs include the penis, testes, and epididymis. These organs are shown in Figure below. The figure also shows other parts of the male reproductive system.
Figure 22.1
This drawing shows the organs of the male reproductive system. It shows the organs from the side. Find each organ in the drawing as you read about it in the text.
The penis is a cylinder-shaped organ. It contains the urethra. The urethra is a tube that carries urine out of the body. The role of the urethra in reproduction is to carry sperm out of the body.
The two testes (singular, testis) are egg-shaped organs. They produce sperm and secrete testosterone. The testes are contained in the scrotum. As you can see from Figure above, the scrotum is a sac that hangs down outside the body. The scrotum also contains the epididymis.
The epididymis is a tube that is about 6 meters (20 feet) long in adults. It is tightly coiled, so it fits inside the scrotum. It rests on top of the testes. The epididymis is where sperm mature. The epididymis also stores sperm until they leave the body.
Other parts of the male reproductive system include the vas deferens and prostate gland. Both of these structures are shown in Figure above. The vas deferens is a tube that carries sperm from the epididymis to the urethra. The prostate gland secretes a fluid that mixes with sperm to help form semen. Semen is a milky liquid that passes through the urethra and out of the body.
Sperm and Sperm Production
Sperm are tiny cells. In fact, they are the smallest cells in the human body. A sperm cell is shown in Figure below. What do you think a sperm cell looks like? Some people think that it looks like a tadpole. Do you agree?
Figure 22.2
This drawing of a sperm shows its main parts. What is the role of each part? How do you think the shape of the sperm might help it to swim?
Sperm
A sperm has three main parts. They are the head, midpiece, and tail. Each part plays an important role in reproduction.
The head of the sperm contains the nucleus. Within the nucleus are the chromosomes. Remember, in humans, the nucleus of the sperm cell contains 23 chromosomes. The head also contains enzymes that help the sperm break through the cell membrane of an egg. You will read more about this process in Lesson 3.
The midpiece of the sperm is packed with mitochondria. Mitochondria are structures in cells that produce energy (discussed in the Cells and Their Structures chapter). Sperm use the energy produced in the midpiece to move.
The tail of the sperm rotates like a propeller. This pushes the sperm forward. A sperm can travel about 0.8 meters (30 inches) per hour. This may not sound very fast, but don’t forget how small a sperm is. For its size, a sperm moves about as fast as you do when you walk briskly. You can see how a sperm’s tail rotates to propel it forward by watching the animation at http://www.stanford.edu/group/Urchin/sperm-l.htm.
Sperm Production
The process of producing sperm starts in the testes and ends in the epididymis. The entire process takes up to two months. It begins when special cells in the testes undergo mitosis. The special cells make identical copies of themselves that continue to go through the process of sperm formation, while the original cells remain to produce more sperm in the future. The copies of the original cells divide by meiosis, producing cells called spermatids. The spermatids have half the number of chromosomes as the original cell. However, they are still immature and cannot move on their own.
The spermatids are transferred from the testes to the epididymis. In the epididymis, they gradually become mature. They grow a tail. They also lose some of the cytoplasm from the head. Once they mature, they are able to “swim.” The mature sperm are stored in the epididymis until it is time for them to leave the body. To watch an animation of all these steps of sperm production, visit http://www.pennhealth.com/health_info/animationplayer/sperm_production.html
Sperm leave the epididymis through the vas deferens (Figure above). As they travel through the vas deferens, they pass by the prostate and other glands. The sperm mix with fluids from these glands, forming semen. The
semen travels through the urethra and leaves the body through the penis. A teaspoon of semen may contain as many as 500 million sperm!
Lesson Summary
The main functions of the male reproductive system are to produce sperm and secrete testosterone.
Male reproductive organs include the penis, testes, and epididymis.
Sperm are male gametes that form in the testes and mature in the epididymis.
Review Questions
Knowledge and Comprehension
What are sperm?
What is the main sex hormone in males?
Which organs produce sperm and secrete testosterone?
What is the function of the tail of a sperm?
Arrange the following structures in the order that sperm pass through them: urethra, epididymis, vas deferens.
Explain what testosterone does in males.
Contrast the roles of the testes and penis in reproduction.
How do sperm differ from semen? How are the two related?
Explain why sperm production is not completed when spermatids have been produced.
Why is the epididymis needed for reproduction in males?
Further Reading / Supplemental Links
CK–12.org, High School Biology, Chapter 40, Lesson 1.
http://www.eurekalert.org/pub_releases/2007-05/wuso-eii051007.php
http://www3.interscience.wiley.com/journal/118662957/abstract?CRETRY=1&SRETRY=0
http://www.kidshealth.org/parent/general/body_basics/male_reproductive.html
http://www.livescience.com/health/060208_hyper_sperm.html
http://www.nytimes.com/2007/06/12/science/12angi.html
http://en.wikipedia.org/wiki/Vas_deferens
Alex Firth. What’s Happening to Me?: Boys Edition. Usborne Books, 2007.
Frank C. Hawkins and Greta Laube. The Boy’s Body Guide: A Health and Hygiene Book for Boys 8 and Older. Boys Guide Books, 2007.
Kate G. Pfeifer. American Medical Association’s Boy’s Guide to Becoming a Teen. Jossey-Bass, 2006.
Vocabulary
epididymis
Male reproductive organ where sperm mature and are stored until they leave the body.
penis
Male reproductive organ that carries urine and sperm out of the body.
semen
Milky liquid that contains sperm and secretions of glands; passes through the urethra and out of body.
sperm
Male gametes or sex cells.
testosterone
Main sex hormone in males.
testes
Male reproductive organs that produce sperm and secrete testosterone.
Points to Consider
The production of sperm by males is just one part of the process of human reproduction.
The production of eggs by females is another part of the process.
Do you know which organs in females produce eggs? Do you know how eggs are produced?
Besides producing eggs, what do you think might be other functions of the female reproductive system?
Lesson 22.2: Female Reproductive System
Lesson Objectives
State the functions of the female reproductive system.
Identify and describe the female reproductive organs.
Explain what eggs are and how they are produced.
Outline the monthly cycle of the female reproductive system.
Check Your Understanding
Where is the pituitary gland?
What is its role in the endocrine system?
What are FSH and LH?
Introduction
Most of the male reproductive organs are outside the body. In contrast, most of the female reproductive organs are inside the body. The male and female organs also look very different. They have different functions, as well. However, two of the functions of the female reproductive system parallel the functions of the male reproductive system. Like the male system, the female system produces gametes and a major sex hormone.
Functions of the Female Reproductive System
One function of the female reproductive system is to produce eggs. Eggs are female gametes, and they are produced in the ovaries. Mature females release only one egg at a time. Eggs actually form before birth. However, they do not fully develop until later in life. This will be discussed later in this lesson.
Another function of the female system is to secrete estrogen. Estrogen is the main sex hormone in females. Estrogen has two major roles.
During the teen years, estrogen causes the reproductive organs to mature. It also causes other female traits to develop. For example, it causes the breasts to grow.
During adulthood, estrogen is needed for a woman to release eggs.
The female reproductive system has another important function. It supports a baby as it develops before birth. It also gives birth to the baby at the end of pregnancy.
Female Reproductive Organs
The female reproductive organs include the vagina, uterus, Fallopian tubes, and ovaries. These organs are shown in Figure below. The breasts are not shown in this figure. They are not considered reproductive organs. However, they are involved in reproduction. They contain mammary glands that secrete milk to feed a baby. The milk leaves the breast through the nipple when the baby sucks on it.
Figure 22.3
This drawing shows the organs of the female reproductive system. It shows the organs from the front. Find each organ in the drawing as you read about it in the text.
The vagina is a cylinder-shaped organ. One end of the vagina opens at the surface of the body. The other end joins with the uterus. During sexual intercourse, sperm may be deposited in the vagina. The sperm move through the vagina and into the uterus. During birth, a baby passes from the uterus through the vagina to leave the body.
The uterus is a hollow organ with muscular walls. The narrow part of the uterus where it connects with the vagina is called the cervix. The uterus is where a baby develops until birth. The walls of the uterus expand as the baby grows. The muscular walls of the uterus push the baby out during birth.
The two ovaries are small, oval organs on opposite sides of the uterus. Each ovary contains thousands of eggs. The eggs do not fully develop until a female has gone through puberty. About once a month, an egg completes its development and is released by the ovary. The ovaries also secrete estrogen.
The two Fallopian tubes are narrow passages that open off the uterus. Each tube reaches one of the ovaries. However, the tubes are not attached to the ovaries. Notice in Figure above that the end of each Fallopian tube by the ovary has “fingers.” They sweep an egg into the Fallopian tube. Then the egg passes through the Fallopian tube to the uterus.
Eggs and Egg Production
When a baby girl is born, her ovaries contain all the eggs they will ever produce. However, the eggs are not fully developed. They develop only after she starts having menstrual periods at about age 12 or 13. Just one egg develops each month. This usually continues until a woman is in her 40s.
Eggs
Eggs are very big cells. In fact, they are the biggest cells in the human body. An egg is about 30 times as wide as a sperm cell. It is large enough to see without a microscope. Like a sperm cell, the egg contains a nucleus with half the number of chromosomes as other body cells. Unlike a sperm cell, the egg contains a lot of cytoplasm, which is why it is so big. The egg also does not have a tail.
Egg Production
Egg production takes place in the ovaries. The process occurs in several steps. It begins before a girl is born. Before birth, special cells in the ovaries undergo mitosis. The daughter cells then start to divide by meiosis. However, they only go through the first of the two cell divisions of meiosis at that time. They remain in this state until the female has gone through puberty.
In a mature female, an egg develops in an ovary about once a month. The drawing in Figure below shows how this happens. As you can see from the figure, the egg is enclosed in a nest of cells c
alled a follicle. The follicle and egg grow larger and go through other changes. After a couple of weeks, the egg bursts out of the follicle and through the wall of the ovary. This is called ovulation. The moving fingers of the nearby Fallopian tube sweep the egg into the tube. The empty follicle then changes into a structure called a corpus luteum.
Figure 22.4
This diagram shows how an egg and its follicle develop in an ovary. After it develops, the egg leaves the ovary and enters the Fallopian tube. The empty follicle becomes a structure called a corpus luteum.
KEY:
Undeveloped eggs
Egg and follicle developing
Egg and follicle developing
Ovulation
Empty follicle changing into corpus luteum
Corpus luteum breaking down
If a sperm unites with the egg while it is passing through the Fallopian tube, the egg finally completes meiosis. This results in two daughter cells that are different in size. The smaller cell is called a polar body. It contains very little cytoplasm. It soon breaks down and disappears. The larger cell is the egg. It contains most of the cytoplasm. You can watch an animation of all the steps of egg production at http://www.pennhealth.com/health_info/animationplayer/egg_production.html