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Scientists estimate that there are between 75 and 100 trillion cells in the body of an adult
There are 3 main factors that influence differentiation in animal cells:
The contents of the cell’s cytoplasm.
Mitosis ensures that daughter cells receive identical sets of chromosomes. However,
the contents of the cytoplasm may differ in each daughter cell. For example, one
daughter cell may have more storage vacuoles than the other – this will allow the cell to
use more energy as it grows.
Environmental conditions, such as temperature.
Such conditions, such as temperature and the presence or absence of certain nutrients.
For example, in Siamese cats, only cells that develop at cool temperatures produce dark
hair colours (the tips of the cat’s feet, tail, ears, and nose are usually cooler than the
rest of the body).
Chemical contaminants in the environment have also been shown to trigger abnormal
development. In humans, approximately 90% of problems in developing embryos can be
traced to environmental factors, including a mother’s exposure to harmful substances,
such as heavy metals.
As a cell matures, more of its genes get turned off or on by the effects of other cells
or environmental conditions. One combination of active and inactive genes will produce a
skeletal muscle cell. A different combination will produce a nerve cell.
The influence of neighbouring cells.
Nearby cells in the organism’s body have one of the biggest influences on what a cell will
When cells are close to one another, the substances produced by one cell can sometimes
diffuse through a neighbouring cell’s membrane. These substances can then change how
the information in the DNA of the second cell gets expressed.
Types of Tissues
A) Epithelial:
Line the surfaces of the body, both as a body covering and between internal organs.
It is made up of cells with strong connections between adjoining cell membranes, so they form
Some types:
Skin epithelia – made up of thin, flat cells that form sheets and act as a semi-permeable
barrier between the inside and the outside of a body.
Columnar epithelia – made up of columns of cells that line the small intestine, the stomach,
and glands. They may secrete mucus; and they have finger-like projections called cilia, which
help to move and absorb materials.
Skin epithelia
Columnar epithelia
B) Muscle:
Designed to change their shape; they act by shortening or lengthening.
Some types:
Skeletal muscle – made up of cells that line up in the same direction, making the tissue looked
striped, or striated. They attach to bone, making it possible for the body to move. It is
found in limbs (arms and legs) and where the body needs support (lower abdomen and back).
Smooth muscle – made of cells that are tapered at both ends and do not have a striated
appearance. They are found in blood vessels and the walls of internal organs (like the
esophagus and stomach). They contract more slowly than skeletal muscles, but its action can
be sustained for a long time.
Cardiac muscle – made of cells whose nuclei sometimes appear to be between cells. They are
branched and unevenly striated. They contract as a single unit. It is only found in the heart.
C) Nervous:
Made up of cells called neurons, which have finger-like projections to receive and transfer
They coordinate body actions.
They are varied in their actions – some relay signals form the brain or spinal cord to muscles and
glands; others detect information from their environment (like the heat from a hot stove) and
trigger the body’s responses.
D) Connective:
It strengthens supports, protects, binds, or connects cells and tissues.
It consists of cells in an extra-cellular matrix that can range from a liquid (in blood), to elastic
materials that can stretch (ligaments), to mineral deposits (in bone).
Some types:
Bone – made up of cells surrounded by calcium – hardened tissue through which blood vessels
run. They are needed for movement, support, and protection.
Fat (adipose tissue) – made up of large, tightly packed cells. They are found under the skin
and around organs. They are needed for energy storage, padding, and insulation.
Blood – includes red blood cells, white blood cells, and platelets within a straw-coloured liquid
matrix called plasma. It transports nutrients and oxygen; it clots when the skin is cut; and it
attacks invaders such as bacteria and viruses.
Fat (adipose tissue)
Stem Cells
are unspecialized cells that can produce various specialized cells.
Stem cells are found in animals and humans. They are similar to meristematic cells that
are found in plants – both produce more cells so that the organism can continue to grow
Human organs are formed in the embryo and the body cannot produce new ones. Mammals
can replace only small amounts of tissue (such as for bone or skin repair).
As the human embryo develops, it initially has totipotent stem cells. These stem cells can
become any kind of cell in the body.
As the embryo continues to develop, its stem cells become pluripotent. These stem cells
are less versatile; they are capable of producing many, but not all, types of cells.
Late in development and after birth, people have only adult stem cells, which can produce
only specific kinds of cells. For example, only skin stem cells can produce cells to repair
skins, and only bone cells can repair bone.
Researchers have found that marrow transplanted from a healthy animal could restore blood cells
in an animal that had undergone radiation therapy. Radiation, which can destroy rapidly dividing
cells, is commonly used as a cancer treatment. Sometimes, radiation also destroys the patient’s
normal bone marrow cells. Then, a bone marrow transplant is needed.
Are unspecialized cells that can become any one of an organism’s
body cells.
These embryonic stem cells are only found in embryos that are very young – less than 1
week old.
Under laboratory conditions, these totipotent stem cells are able to keep dividing for a
year, or longer, without ever differentiating.
Embryonic stem cells are sometimes called “source” or “starter” cells because the can
become any of the roughly 300 different types of human body tissue. This makes them
valuable for research and medical treatments.
Scientists obtain embryonic stem cells from eggs fertilized in vitro (outside the womb).
Sources are usually unused embryos from fertility clinics. However, obtaining these stem
cells destroys the embryo. Some people consider this use to be taking a human life.
Recent studies have shown that some adult stem cells (such as those from skin) can be
transformed (or induced) into becoming pluripotent stem cells. However, most induced
pluripotent stem cells are created using viruses – which could potentially damage the stem
cell’s DNA.
Organs and Systems
Common Medical Imaging Technologies Used in Diagnosis
Produced by transmitting a
wavelength of electromagnetic
radiation through the body to expose
photographic film on the other side.
CT or CAT scan
(computerized axial tomography)
Produced by taking X-rays of very
thin “slices” of a body part that can
be reconstructed by a computer into
a three-dimensional image.
(medical sonography)
MRI scan
(magnetic resonance imaging)
Produced by directing high frequency
sound waves at a part of the body.
Show real-time movement of body
parts (like the heart)
Produced using radio signals in a
magnetic field to create images of
body parts.
Human Organ Systems
The Digestive System:
Digestion begins in the mouth, where the teeth begin the mechanical breakdown of food into
smaller pieces. An enzyme in saliva, called amylase, starts the chemical breakdown.
After the food is swallowed, it passes through the pharynx into a muscular tube called the
esophagus. At this point, the food is in small chunks. The muscular walls of the esophagus
contract and relax, pushing each chunk of food along until it gets to the stomach.
In the stomach, the food chunks are surrounded by gastric juices that are secreted by the
epithelial tissue that lines the stomach. These juices include hydrochloric acid and the enzyme
pepsin. One of the reasons why gastric juices must be acidic is that pepsin (which breaks down
proteins) needs an acidic environment in which to function.
The stomach lining also secretes mucus, which protects the stomach wall from breaking down in
the presence of those protein-digesting juices.
Nerves in the stomach wall sense the presence of food and signal the stomach’s muscle tissue to
mix the contents, continuing the mechanical breakdown. Due to actions of the gastric juices and
the churning of the stomach muscles, the partially digested food breaks down into liquid.
Once the meal is fully mixed, a round muscle at the bottom of the stomach – called the sphincter
– relaxes and some of the contents of the stomach are released into the small intestine. The
first metre of the small intestine is called the duodenum (which is where most digestion takes
place). It has small tubes called ducts that connect to the pancreas, liver, and gall bladder.
These organs release more digestive enzymes into the duodenum, which completes the chemical
breakdown of the food.
When the digested food moves into the remaining length of the small intestine, it is ready to be
absorbed into the body. The small intestine is covered with millions of interior folds called villi
and microvilli – these act to maximize the surface area over which nutrients and water can be
absorbed into the bloodstream.
The final organ in the digestive system is the large intestine, which includes the colon, rectum,
and anus.
In humans, the large intestine has a larger diameter and a shorter length than the small intestine.
The large intestine’s main functions are to absorb water, vitamins, and various salts from the
digested food and to eliminate undigested food through the anus as feces.
The large intestine also contains bacteria that can help finish the process of breaking down food
and also produce essential nutrients, such as vitamin K.
The Excretory System:
Water and other materials absorbed through walls of the large intestine move into blood vessels.
As blood passes through the kidneys, it is filtered and wastes are removed. As the blood if
filtered, urine containing water and unnecessary salts is formed.
The urine is stored in the bladder. When the bladder is full, the urine is flushed from the body.
The Circulatory System:
The circulatory system picks up and transports nutrients and oxygen to cells, and carries wastes
to the organs responsible for eliminating them from the body.
When the heart contracts, it produces pressure on the blood in the circulatory system. This
pressure pushes blood through the body. Flexible flaps of tissue called valves are found
throughout the circulatory system, including the heart and veins. They open when blood is pushed
through them, and then close to prevent blood from flowing backward.
Blood that returns from the body is deoxygenated (has had its oxygen removed by body cells)
and carries the body’s carbon dioxide waste from cellular respiration.
The first path that the blood takes is from the heart’s right atrium to its right ventricle.
From the right ventricle, blood is pumped through the pulmonary artery to the lungs. There,
the blood eliminates carbon dioxide and picks up oxygen. This oxygenated blood then goes back
to the heart – to the left atrium, and then to the left ventricle. From the left ventricle, it is
pumped out through the aorta (a huge artery) to the rest of the body.
Veins carry blood back toward the heart, and the blood is deoxygenated.
Arteries carry blood from the heart throughout the body, and the blood is oxygenated.
Mammals have what is known as a closed circulatory system – meaning that the blood stays in the
arteries and veins.
Blood Flow in the Heart
Heart Structure
Arteries and veins are connected by capillaries. Capillaries are extremely small, thin-walled
blood vessels. They are only one epithelial cell thick! The areas where the branches of capillaries
are close to one another are called capillary beds.
The capillaries bring blood into close contact with the tissues in organs throughout the body.
They are able to diffuse oxygen and nutrients throughout the body, as well as pick up carbon
dioxide and waste from the body’s cells.
Heart Disease and Stroke:
Every year, there are approximately 70 000 heart attacks in Canada – and about 25% will result
in death.
The two most common causes of heart disease are hypertension (high blood pressure) and
arteriosclerosis (thickening of the walls of the arteries with plaque – this narrows the
passageway for blood).
Hypertension and arteriosclerosis can cause the formation of blood clots. If a blood clot breaks
free, it may flow to a coronary artery and block a vessel, causing a heart attack.
If the blood clot reaches the brain, it may block a vessel there, causing a stroke.
When a stroke occurs, parts of the brain are damaged due to oxygen deprivation.
A type of surgery, called angioplasty, uses a balloon or laser to try to open clogged arteries.
The Respiratory System:
The organs of the respiratory system are responsible for the body’s gas exchange – bringing
oxgyen into the body and getting rid of carbon dioxide.
The respiratory system is connected to the circulatory system – one system could not do its work
without the proper functioning of the other.
When you breathe, muscle contractions cause your rib cage to move up and out and your
diaphragm to move down. This causes air to be pulled into your body through your nose or
This air passes by epithelial cells that have microscopic, hair-like projections called cilia. These
cells also secrete mucus. The mucus and cilia help keep foreign particles, such as dust and
bacteria, out of your body.
Once air passes through your nose and pharnyx (passageway from your nose to your throat), the
air moves into the trachea (aka: your windpipe). A muscular flap, the epiglottis, opens so air can
pass. When food is in the passage, the epiglottis closes so that no food gets into the trachea.
The air passes into branching tubes called the bronchi (singular: bronchus). Each bronchus then
continues to branch into smaller tubes called bronchioles. Muscle tissue in the walls of the
bronchioles allows the diameters of the tubes to contract and relax in order to control the
amount of air entering the lung.
After entering the bronchioles, the air passes into increasingly smaller tubes until it ends up in
tiny sacs called alveoli. In the alveoli, the air is only 1/1000th of a millimetre away from the
bloodstream. This thin layer of epithelial tissue keeps most inhaled bacteria and viruses out of
the bloodstream, but still allows gases to cross (oxygen out of the lungs and carbon dioxide back in).
Your red blood cells contain a protein called hemoglobin, which attaches to oxygen molecules.
When deoxygenated red blood cells pass by the alveoli, the hemoglobin they contain causes them
to pick up oxygen. The oxygenated blood then travels throughout the body, delivering oxygen to
cells that need it for cellular respiration.
When your chest muscles and diaphragm relax, you exhale and the carbon dioxide in the alveoli is
carried out of your body.
Your body constantly monitors the amount of carbon dioxide being carried in the blood. When
the level of carbon dioxide is too high, your breathing rate increases so this waste product can be
eliminated more quickly.
Diseases Related to Smoking:
One problem is that smoking damages the cilia, preventing them from sweeping foreign particles
out on cells of the respiratory system.
Cigarette smoke is filled with more than 4000 chemicals, including over 40 that are known
carcinogens (cancer-causing substance).
Cigarette smoke has over 1000x the level of carbon monoxide that is known to be harmful to
human health.
Cigarette smoke also contains tar, which accumulates in the lungs. Tar is responsible for many
respiratory problems.
People who smoke a pack of cigarettes a day absorb about 250 mL of tar into their lungs each
Maintaining Healthy Systems
Prenatal Care and Ultrasound:
Since sound waves from an ultrasound do not penetrate deeply into tissues, they are generally
considered safe, even for a developing fetus.
Ultrasound is commonly used in the first few months of pregnancy to make sure a fetus’s heart is
beating normally, and that it has its major organs.
Preventative Health Care:
Preventative health care includes steps individuals can take to guard their own health, such as
eating well and exercising regularly.
It also includes Public
Health Strategies
which are a co-ordinated effort to track,
research, and reduce the incidence of specific health problems in a population.
A public health strategy that has greatly improved human health is the use of
vaccinations to control the spread of deadly diseases.
is the process of giving a vaccine by mouth or injection to provide active
immunity against a disease.
For example, a worldwide vaccination program for smallpox has virtually eliminated the
disease. The last known case of smallpox was recorded in Somalia in 1977.
Infectious diseases are caused by pathogens (a disease-causing agent such as a virus, and some
kinds of bacteria and fungi). Organs, such as your skin, prevent most of these invaders from
getting into your body. However, if a pathogen does enter your body, your immune system then
tries to attack and destroy the invader, usually with success.
The body’s first response to an invader is to send a flow of fluid containing white blood
cells called phagocytes, and dissolved substances from the blood to the site of the
infection. This causes inflammation, a swelling and redness in the area. The job of the
phagocytes is to fight infection. Any material that the body considers foreign and that
stimulates this response is called an antigen. In particular, proteins on the surfaces of
pathogens are antigens that trigger this response.
Bone marrow is another important part of the immune system. White blood cells and many other
disease-fighting molecules are manufactured in the bone marrow.
Among these molecules are specialized proteins called antibodies. Each antibody
identifies and attaches to a specific antigen. This attachment prevents the invader
from infecting the rest of the body or signals to other parts of the immune system that
this intruder needs to be destroyed.
Stopping the Spread of Disease
Many other infectious diseases do not have vaccinations available to control them. These
diseases need to be controlled with different kinds of public health strategies, which are usually
focussed on containing the spread of the disease.
In the spring of 2003, severe acute respiratory syndrome (SARS) entered Canada when
a person who was infected in Hong Kong brought the virus back home to Toronto.
SARS causes fluid to fill a patient’s lungs, making it nearly impossible for the person to
get enough oxygen.
Because the SARS virus is readily transferred to others through the air, the disease
quickly spread and caused a public health crisis in Ontario.
By the end of the epidemic that lasted 4 months, the SARS virus had spread to 432
people, and 44 of whom died.
Acquired immunodeficiency syndrome (AIDS) is caused by the human immunodeficiency
virus (HIV), which attacks the immune system itself.
This means that when other pathogens enter the body, the immune system is unable to
mount an attack.
HIV/AIDS has been directly responsible for the deaths of more than 25 million people
The federal government’s strategy on HIV/AIDS includes keeping track of how many
people have the disease. It also includes research, educational programs to prevent
more people from acquiring HIV/AIDS, and help for people who have the disease.
Canada had its first case of West Nile virus in 2002.
Although most people who get infected have no symptoms, a few become seriously ill or
The Public Health Agency of Canada now co-ordinates a strategy to try to reduce
people’s exposure to the virus. The strategy includes identifying the presence of the
disease in various animals and, where it is present, informing local authorities. Local
authorities may then use pesticides to reduce mosquito populations and provide
educational programs to the public.
Not all health strategies are directed at infectious diseases. For example, many public health
strategies are directed at a non-infectious disease that 2 out of every 5 Canadians will get at
some point in their lives: cancer.
One of the best ways to prevent cancer is to avoid the substances (carcinogens) and
situations that cause cells to mutate in this way. Many public health strategies
encourage these preventative measures, such as not smoking cigarettes, in order to
avoid lung cancer. Another preventative measure would be to avoid excessive exposure
to the sun, or tanning beds, in order to avoid skin cancer.
Some situations were beyond people’s control. For example, for years, people didn’t
know that their houses or workplaces contained asbestos insulation (a carcinogen).
Programs are now in place to remove this insulation wherever it is discovered.
are tests used to detect cancer cells at an early stage of
the disease so that it can be treated more effectively.
Depending on the type of cancer, early detection and treatment allows 65-95%
of the individuals to survive.
Some cancer screening programs are done routinely as part of a person’s annual
physical checkup. For example, doctors remove a small sample of cells from a
woman’s cervix using a technique called a PAP smear. These cells are then
examine for abnormalities that might indicated the presence of cancer. PAP
smears are now part of a regular health checkup for adult women. After PAP
tests were introduced in Canada nearly 50 years ago, the incidence of death due
to cervical cancer declined dramatically.