The reproductive system

All organisms reproduce, obviously 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.

The male reproductive organs

The male reproductive system has two main functions: producing sperm and releasing testosterone into

the body.

Sperm are male gametes, or reproductive cells. When a male gamete meets a female gamete, they can

form a new organism. Sperm form when certain cells in the male reproductive system divide resulting in

cells with half the amount of DNA as a regular "body" cell, having just one set of chromosomes. Regular

body cells have two set of chromosomes. When males grow older, they produce millions of sperm each

day. The male reproductive system also maintains and transports and delivers sperm in a protective

fluid, known as semen.

Testosterone is the main sex hormone in males. During the teen years, testosterone causes the

reproductive organs to mature and other male traits to develop. During adulthood, testosterone helps a

man to produce sperm.

The male reproductive organs include the penis, testes, and epididymis.

o The penis is a cylinder-shaped organ. It

contains the urethra. The urethra is a tube that

carries urine out of the body. The urethra also

carries sperm out of the body.

o The two testes (singular, testis) produce

sperm and secrete testosterone. The testes are

found inside of the scrotum. 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 six 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 grow larger and

mature. The epididymis also stores sperm until they leave the body. The testes, being in the

scrotum outside the body, allow the temperature of the sperm to be maintained at a few degrees

lower than body temperature. This is necessary for the stability of these reproductive cells.

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o Other parts of the male reproductive system include the vas deferens and the prostate gland.

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. The prostate gland is located

beneath the bladder. Semen is a "milky" liquid that carries sperm through the urethra and out of

the body. In addition to sperm cells, semen contains sugars (fructose) which provide energy to

the sperm cells, and enzymes and other substances which help the sperm survive.

Female Reproductive Organs

The female reproductive organs include the vagina, uterus, fallopian tubes, and ovaries. The breasts are

not considered reproductive organs, even though they are involved in reproduction. They contain

mammary glands that give milk to feed a baby. The milk leaves the breast through the nipple when the

baby sucks on it.

o The vagina is a cylinder-shaped organ

found inside of the female body. One end of

the vagina opens at the outside of the body.

The other end joins with the uterus. During

sexual intercourse, sperm may be released

into the vagina. If this occurs, the sperm will

move through the vagina and into the uterus.

During birth, a baby passes from the uterus

to the vagina to leave the body.

o The uterus is a hollow organ with

muscular walls. The part that connects the

vagina with the uterus is called the cervix.

The uterus is where a baby develops until

birth. The walls of the uterus grow bigger as

the baby grows. The muscular walls of the

uterus push the baby out during birth.

o The two ovaries are small, oval

organs on either side of the uterus. Each

ovary contains thousands of eggs, with about

1-2 million immature eggs present at birth

and 40,000 immature eggs present at

puberty, as most of the eggs die off. The

eggs do not fully develop until a female has gone through puberty. About once a month, on

average one egg completes development and is released by the ovary. A woman will release an

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egg once each month until she is in her 40s. The ovaries also secrete estrogen, the main female

sex hormone.

o The two fallopian tubes are narrow tubes that open off from the uterus. Each tube reaches for

one of the ovaries, but the tubes are not attached to the ovaries. 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. If an egg is to be fertilized, this will occur in the

fallopian tube. A fertilized egg then implants into the wall of the uterus, where it begins to

develop. An unfertilized egg will flow through the uterus and be excreted from the body.

The female monthly cycle

The menstrual cycle is a series of changes in the reproductive system of mature females that repeats

every month. While the egg and follicle are developing in the ovary, tissues are building up inside the

uterus, the reproductive organ where the baby would develop. The uterus develops a thick lining

covered in tiny blood vessels. This prepares the uterus to receive an egg that could develop into a child

(a fertilized egg). This occurs during the first part of the cycle.

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The egg rests in a nest of cells called a follicle that protects the egg as it matures in the ovary. After a

couple of weeks, the egg bursts out of the follicle and through the wall of the ovary. This is called

ovulation, which usually occurs at the midpoint of a monthly cycle. The egg is swept into the fallopian

tube. If sperm is present, fertilisation may occur. As sperm can only survive in the fallopian tube for up

to a few days, fertilisation can only occur within those few days post-ovulation.

The ruptured follicle, now called the corpus luteum, will secrete the hormone progesterone. This

hormone keeps the endometrium from breaking down. If the egg is fertilised, the egg makes its way

through the fallopian tube into the uterus, where it imbeds into the thick lining called endometrium,

which will be maintained to help nourish the egg. When this occurs, the monthly cycle stops and it does

not resume until the pregnancy is over.

If a sperm does not enter an egg, the corpus luteum will break down and disappear. Without

progesterone, the endometrium will also break down and be shed. Blood and other tissues from the

lining break off from the uterus. They pass through the vagina and out of the body. This is called

menstruation or menstrual period and is the process in which the endometrium of the uterus is shed

from the body. It lasts about 4 days, on average. When the menstrual period ends, the cycle repeats.

The average length of the cycle (time between menstrual periods) is about 28 days, but there is no

“normal” cycle length 1 .

Menstruation starts during puberty and continues until their mid- or late- forties. Then women go

through menopause, a period during which their menstrual cycles slow down and eventually stop,

generally by their early fifties. After menopause, women can no longer reproduce naturally because their

ovaries no longer produce eggs.

Taming the cycle: how does the pill work?

In 1960 the combined oral contraceptive pill was approved in the US for preventing pregnancy, ushering

in a new era of hormonally-based birth control. Now, in addition to the original “pill” women can choose

from an overwhelming variety of oral contraceptives, from different hormone combinations and doses to

different pill schedules that result in skipping or stopping menstruation entirely.

The shedding of uterine lining leading to the observed bleeding is the menstrual phase. As bleeding

ends, Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) show small peaks above their

normal low levels, leading to thickening of the uterine lining and maturing of oocyte(egg)-containing

follicles in the ovary. Meanwhile, the estrogen, Estradiol (E), is steadily rising to a peak a few days

before ovulation, after which point FSH and LH spike once more, triggering the release of a mature

oocyte. Levels of Progesterone (P) rise for the next week, preparing the uterine lining for implantation of

an embryo. If no implantation occurs, both hormone levels drop sharply, triggering menstruation. If

1 Some people think that the average length of a menstrual period is the same as the “normal” length. They assume that shorter or longer menstrual periods are not normal. In fact, menstrual periods can vary from 1 to 8 days in length. This is usually normal.

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fertilization and implantation do occur, levels of P and E remain high throughout pregnancy, suppressing

the spikes of FSH and LH that drive ovulation.

Because of the intricate feedback loops controlling the menstrual cycle, artificially altering one or two

hormones can affect the entire cycle. This fundamental knowledge has been used both to suppress the

cycle, with the goal of contraception, as well as to stimulate ovulation with the hopes of promoting

fertility. Two ideal targets for hormonal contraception would be the hormones FSH and LH, which play

dual roles in the normal menstrual cycle, first re-starting the cycle after menstrual bleeding and then

triggering ovulation at the mid-point of the cycle. Because FSH and LH are required to trigger ovulation,

artificially blocking these two hormones would therefore suppress ovulation. Why then are the hormones

P and E commonly used in the Pill instead? It is generally easier to add something to a biological system

than to remove something, and since P and E suppress the release of FSH and LH these are logical

choices for halting the sequence of events that lead to ovulation. Because both P and E are at high

levels throughout pregnancy, some people describe taking the pill as “tricking the body into thinking it’s

pregnant”. While not entirely accurate, this statement does have some truth to it.

The standard combined oral contraceptive consists of both hormones P and E, taken daily for three

weeks, followed by a week of no pills (or placebo pills) that trigger what’s known as “breakthrough

bleeding”, induced by the drop in hormones. Although the outward appearance is the same (menstrual

bleeding in week 1 of a 28 day cycle) the constant high levels of P and E for a woman taking the Pill

actually abolish the normal hormonal cycling that underlies ovulation. The Pill therefore replaces the

normal menstrual cycle with an artificial cycle (3 weeks of “mimicking pregnancy”, followed by 1 week

with breakthrough bleeding).

There are two kinds of hormonal birth control pills: (1) the combination pill which contains estrogen and

progestin and (2) the progestin-only pill (known as the minipill). Combo pills are significantly more

effective than progestin-only pills and have the added benefit of less breakthrough bleeding. However,

some women cannot tolerate estrogen and prefer the progestin-only pill. Both types of pills are available

in several different brands, each of which have slightly different blends of hormones.

These two kinds of hormonal birth control are available in other forms besides pills. The combination

formula is also available as a patch and a vaginal ring. The progestin-only formula is also available in

intramuscular shots, an implant, and in intrauterine devices. Some women may prefer these other forms

of hormonal birth control because they can be taken less often (and consequently are easier to

remember). While birth control pills must be taken everyday, the patch is only applied once per week,

the vaginal ring only once per month, and the intramuscular shot only once every three months. An IUD

is inserted into the uterus and can prevent pregnancy for five years or more.

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Hormonal birth control pills and devices are only available by prescription. Women may want to ask a

gynecologist or women's health care provider for information about different kinds of birth control,

including which methods would be best for them personally.

Fertilization and implantation

Sexual reproduction happens when a sperm and an egg cell combine together. This is called fertilisation.

Sperm are released into the vagina during sexual intercourse. They “swim” through the uterus and enter

a fallopian tube. This is where fertilisation normally takes place.

If the sperm breaks through the egg’s membrane, it will immediately cause changes in the egg that

keep other sperm out. This ensures that only a single sperm can penetrate an egg. The sperm and egg

each have only half the number of chromosomes as other cells in the body. This is because when they

combine together, they form a cell with the full number of chromosomes. The cell they form is called a

zygote. The zygote is the first cell with two sets of chromosomes, one from each parent. A human

zygote has two sets of 23 chromosomes, for a total of 46 chromosomes (23 pairs). The zygote slowly

travels down the fallopian tube to the uterus. As it travels, it divides many times. It forms a hollow ball

of cells.

After the ball of cells reaches the uterus, it fixes itself to the side of the uterus. This is called

implantation. It usually happens about a week after fertilisation. Now the implanted ball of cells is ready

to continue its development into a baby boy or girl.

Pregnancy and childbirth

While a woman is pregnant, the developing baby may be called an embryo or a fetus. Do these mean

the same thing? No, in the very early stages the developing baby is called an embryo, while in the later

stages it is called a fetus. The most obvious change, however, is an increase in body size.

By the 28 th week, the fetus is starting to look much more like a baby. Babies born after the 28 th week are

usually able to survive 2 . The fetus rapidly puts on body fat and gains weight during the last couple of

months. By the end of the 38 th week, all of the organs are working, and the fetus is ready to be born.

This is when birth normally occurs. A baby born before this time is considered premature.

During pregnancy, other structures also develop inside the mother’s uterus. They are the amniotic sac,

placenta, and umbilical cord

The amniotic sac is a membrane that surrounds the fetus. It is filled with water and dissolved

substances, known as amniotic fluid. Imagine placing a small plastic toy inside a balloon and then filling

the balloon with water. The toy would be cushioned and protected by the water. It would also be able to

2 However, they need help breathing because their lungs are not yet fully mature. A baby should not be delivered prior to this

time, unless absolutely necessary. A baby born prior to week 28 will need considerable medical intervention to survive.

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move freely inside the balloon. The amniotic sac and its fluid are like a water-filled balloon. They cushion

and protect the fetus. They also let the fetus move freely inside the uterus.

The placenta is a spongy mass of blood vessels. Some of the vessels come from the mother. Some come

from the fetus. The placenta is attached to the inside of the mother’s uterus. The fetus is connected to

the placenta by a tube called the umbilical cord. The cord contains two arteries and a vein. Substances

pass back and forth between the mother’s and fetus’s blood through the placenta and cord. Oxygen and

nutrients pass from the mother to the fetus 3 . Carbon dioxide passes from the fetus to the mother.

During childbirth, a baby passes from the uterus, through the vagina, and out of the mother’s body.

Childbirth usually starts when the amniotic sac breaks. Then, the muscles of the uterus start contracting.

The contractions get stronger and closer together. They may go on for hours. Eventually, the

contractions squeeze the baby out of the uterus. Once the baby enters the vagina, the mother starts

pushing. She soon pushes the baby through the vagina and out of her body.

As soon as the baby is born, the umbilical cord is cut. After the cord is cut, the baby can no longer get

rid of carbon dioxide through the cord and placenta. As a result, carbon dioxide builds up in the baby’s

blood. This triggers the baby to start breathing. The amniotic sac and placenta pass through the vagina

and out of the body shortly after the birth of the baby.