Download The Human Body - Background Notes 7-9

The Human Body Background notes 7
7. The digestive system: the ins and out
Our digestive system is a long tube that winds through the body from the mouth to anus. Food usually takes
about a day and a half to pass from one end to the other. If the digestive tract of an adult was stretched out,
it would measure almost nine metres in length. The digestive system breaks food down into particles that are
small enough to be absorbed into our blood. Each part of this tube and the organs that are attached to it, has
its own role in the digestion of food. The chemical energy that our bodies need comes from the food we eat.
The mouth and oesophagus
What might appear on our plate as a delicious meal is soon mashed beyond recognition by our teeth and
tongue. By the age of three we have 20 milk teeth. By the time we reach 20 years of age we may have a
total of 32 teeth. We have different shaped teeth to perform different tasks. We have incisors with sharp
edges for cutting; canines with pointed tips for tearing; and molars with flattened ridged surfaces for grinding.
Our teeth are covered by enamel, the hardest substance in the body. Proper oral hygiene is essential so that
the enamel doesn’t start to break down.
When we eat, our tongue positions food for chewing, mixes
in saliva, and shapes each mouthful into a bolus. The tongue
pushes this to the back of the mouth for swallowing. Saliva
moistens food to help us chew and swallow. The salivary
glands in our lower jaw release saliva when we see and
smell food. Saliva is mostly water however it contains
antibacterial substances, called lysozyme, that protect the
mouth against infection.
Saliva also contains the enzyme amylase that begins the
process of digestion of carbohydrate foods such as cereals,
breads, rice, fruits and vegetables. The chewed and
moistened bolus is pushed down the oesophagus to the
stomach by a wave of muscular contractions, called
peristalsis. When we vomit, peristalsis works in the opposite
direction to bring food from our stomach out of our mouth.
A bolus of food moves down the oesophagus by
peristalsis
The stomach
An empty stomach holds the contents of half a teacup but when full it can expand to hold half a bucketful.
The stomach’s lining has many folds that stretch out as the stomach fills and collapse as it empties. Millions
of tiny openings dot the lining of the stomach. These are the gastric pits, which are the source of gastric
digestive secretions. Glands at the bottom of each pit produce gastric juice. Gastric juice is made up of
watery mucous, hydrochloric acid and other digestive substances such as the enzyme called pepsinogen
that helps to breakdown the protein foods in our diet.
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The Human Body Background notes 7
Food may stay in the stomach for several hours soaking in
gastric juice and being broken down into smaller and smaller
pieces. The stomach’s muscular walls churn its contents
regularly to ensure thorough mixing. When the partly
digested food resembles thick soup, the stomach lets it out
through a little opening at one end. The partly digested
mixture is called chyme. The stomach is able to absorb
certain chemicals like aspirin and alcohol but it does not
absorb other digested food particles.
The small intestine
Partly digested food oozes into the small intestine where
digestive juices from our liver, gall bladder and pancreas are
squirted and mixed into the food. The small intestine is
where most digestion and absorption take place. Food
usually spends about six hours being digested in the small
intestine. The digested liquid slowly moves along by
muscular contractions
The lining of the small intestine has many folds. These folds
increase the surface area of the small intestine three fold.
Poking from these folds are millions of tiny finger-like
projections, called villi, which further increase the surface
area of the small intestine by another 10 fold. If the villi in a
small intestine were flattened out, they would form a surface
as large as a tennis court. Villi are full of tiny blood vessels.
As food is broken down into smaller and smaller pieces in
the small intestine, it moves across the walls of the villi into
the blood. Because the surface of the small intestine is so
large a lot of nutrients can be absorbed into the blood at one
time.
Goblet cells secrete mucous to protect the
stomach from acid attack. Source: Monash
University.
Finger-like projections called villi in the small
intestine. Source: University of Melbourne
Two important organs: Liver and pancreas
Blood carrying nutrients from our small intestine goes directly
to our liver. Our liver is a very important organ. It processes
and stores nutrients and sugar absorbed by our small
intestines. It destroys poisons and bacteria and breaks down
chemicals such as alcohol, so they are less harmful to the
body. The liver recycles dead red blood cells and processes
the red pigment into green bile. Soapy bile is stored in our
gall bladder. It is released from a tube into our small intestine
when we eat a fatty meal. Bile helps to break down fat into
small globules that can be absorbed across the villi, into the
bloodstream.
The pancreas is another important organ that aids in the
digestion of food. It produces the enzyme amylase to digests
carbohydrates, proteases to digest protein and lipases to
digest fats. It produces the hormones, insulin and glucagon, The liver processes and stores nutrients; destroys
poisons and bacteria; breaks down chemicals;
that help regulate the level of sugar in our blood and it also
recycles dead red blood cells and makes bile.
secretes an alkaline substance, sodium bicarbonate
(Na2CO3) which neutralises the acid secretions from the
stomach.
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The Human Body Background notes 7
The large intestine
The large intestine is primarily a drying and storage organ. The
wall of the large intestine also has many folds in it to increase
its surface area. The large intestine absorbs water from the
sloppy mixture of waste that comes in from the small intestine.
To keep the waste contents moving along, the large intestine
has many mucus-producing cells in its lining. Mucus lubricates
the passage of wastes and protects the lining of the large
intestine from acids and gases.
The final segment of the large intestine is the anal canal, which
finishes at the anus. A round muscle, called a sphincter, allows
us to voluntarily open and close our anus when we need to. The
left over solids that pass out of the digestive tract are called
faeces and these contain undigested cellulose, bilirubin,
bacteria, salt and water.
Millions of bacteria live in our large intestine. Many of these
bacteria keep our gut healthy and help process waste material.
Unfortunately these bacteria also produce about half a litre of
foul-smelling gas a day. Bacteria give faeces its distinctive
brown colour. The green bile, which is secreted into the
intestines from the liver, turns brown after being digested by
bacteria in the bowel. Sometimes harmful bacteria infect our
digestive tract. The worm-like appendix is where immune cell
are produced to fight off the invaders.
E.coli bacteria (above) & lactobacillus
(below) are common bacteria found in our
large intestines. Source: AMGEN Australia
and Yakult Australia Pty Lty
When things go wrong
Gallstones
Crystals can sometimes form in bile if cholesterol levels are
high. These crystals are called gallstones. Gallstones can
cause extreme pain and sometimes even damage to the gall
ducts if they begin to move through it. Sometimes they have to
be removed.
What is heartburn?
When food travels to the end of the oesophagus a valve, called
a sphincter, opens to let it enter the stomach. Heartburn occurs
when some of the acidic contents of the stomach refluxes back
up through a weak valve and burns the oesophagus.
Gall stones. Source: Royal Melbourne
Hospital
Gastric ulcers
Bacterial infection is the cause of some stomach ulcers. Bacteria live in the stomach lining, destroying its
protective layer. This infection may develop into a stomach ulcer.
Diarrhoea
We get diarrhoea when food waste rushes through the large intestine, leaving little time for water to be
reabsorbed. This water is passed through the anus in very liquid faeces.
Some bacteria can irritate the large intestine and affect its normal function, producing diarrhoea. Prolonged
diarrhoea can cause dehydration.
Constipation
We get constipated when food waste travels too slowly through our large intestine. In this case, too much
water is absorbed and the faeces become hard and dry. Constipation can be caused by not drinking enough
water and by not eating enough high-fibre foods.
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The Human Body Background notes 7
In order to survive, many things must go into our bodies and
many things must also come out. Our heart pumps blood to
every part of our body, delivering a constant supply of
nutrients, oxygen and hormones (chemical messengers that
tell the body what to do). Our blood also picks up our bodies
waste so that it is disposed of by our liver, kidneys, skin and
lungs. An endless flow of blood carries all of these
substances through our circulation.
Our respiratory system: we need oxygen
Our cells need oxygen, all the time. They produce carbon
dioxide, a waste that must be removed. We obtain oxygen
and expel carbon dioxide by breathing. When we breathe in,
our diaphragm contracts (pulling our chest cavity downward)
and air is drawn into our lungs. Our lungs contain millions of
tiny air spaces, called alveoli. Alveoli walls are a single-cell
thick and are covered in tiny blood vessels. This enables the
gases to move across concentration gradients, between the
alveoli and blood.
Oxygen moves out of the lungs and attaches to red blood
cells in the surrounding blood vessels. At the same time,
carbon dioxide gas leaves these vessels and enters the
alveoli spaces. When we breathe out, our diaphragm relaxes
and this air is forced out of our lungs.
The lungs, heart and excretory organs
Sinuses: filtering air
Our nose and sinuses moisten, warm and filter air as we
breathe it in. Dust, bacteria and other particles are trapped
by nose hairs and by mucus secreted by glands in our
sinuses. Mucus is either blown out of the nose or is
swallowed.
Windpipe: channeling air
Air passes down the trachea (windpipe) towards our lungs.
The trachea branches to enter each lung. Tiny hairs called
cilia sweep foreign particles trapped in mucus towards the
throat, for swallowing. Smoking destroys cilia so coughing is
the only way to remove mucus from the lungs.
Alveoli in lung tissue
Larynx: directing air
Our larynx acts as a switch, directing food and air down the
correct passageways. A spoon-shaped cartilage, the
epiglottis, seals off the air passage when we swallow. If food
enters the trachea, the epiglottis initiates a cough. Coughing
pushes the food in a rush of air that can travel at 70 kph.
Vocal cords: vibrating air
Inside the larynx, vocal cords vibrate as air passes up
between them. Like guitar strings, varying the length and
tension of the vocal cords changes the pitch of the sound
produced. The loudness of a voice depends on the force of
the air-stream rushing across the vocal cords.
Our nose and mouth connect to the back of the
throat.
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The Human Body Background notes 8
8. The circulatory system: the round trip
Circulation of blood
Our blood supply never stops. Our cells demand nutrients and oxygen constantly.
Our heart pumps blood around our body at 2.5 litres a minute. Blood surges away
from the heart to the tissues of our bodies in arteries and returns in veins. A round
trip from the heart to the toes and back takes a minute. The watery part of blood
(plasma) carries dissolved nutrients and wastes. Floating in the plasma are billions
of cells. Most of the cells are red blood cells, which carry oxygen. Some cells are
white blood cells, which attack intruders like bacteria.
The heart is two pumps joined together. The right side of the heart pumps blood,
low in oxygen, to the lungs. The left side of the heart pumps blood, filled with
oxygen from the lungs, out of the heart to the rest of the body. Blood flows into the
heart through the vena cava and the pulmonary artery, via the left and right atria
respectively. It then flows down into the left and right ventricles. As the ventricles
contract they force blood out of the heart through the pulmonary vein and the aorta,
to the lungs and the body, respectively.
Blood flow through arterial
circulation and the heart.
Circulation through the heart, blood vessels and chambers.
Blood vessels form a circuit
Our bodies contain over 100,000 km of blood vessels. Arteries carry blood from the heart around the body.
They get smaller and smaller as they branch out. The smallest arteries become tiny capillaries, only one cell
wide. In the capillaries, nutrients and oxygen are exchanged for wastes. Capillaries eventually become veins,
which carry blood back to the heart.
Arteries: from the heart
Under high pressure, blood travels away
from the heart through muscular arteries that
have thick elastic walls. This movement of
the artery walls can be felt as a pulse. Most
arteries, except those that take blood to the
lungs, carry red, oxygenated blood.
Cross section of connective tissue. Source:
University of Melbourne
.
Veins: back to the heart
Veins have thin walls. They carry blood back
to the heart under relatively low pressure.
Most veins, except those that take blood
from the lungs, carry deoxygenated blood,
which appears blue. Veins have valves to
stop blood flowing backwards.
Capillaries: so thin
Capillaries are so narrow that red blood cells
travel through them in single file. Capillary
walls are only one cell thick. The walls of
capillaries are so thin that nutrients, wastes
and hormones can easily pass between the
cells of the body and the blood.
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The Human Body Background notes 8
Our excretory system
Cells dump their wastes into our bloodstream. Most of
these wastes are removed from our blood by our kidneys.
Kidneys: the filtering unit
Each kidney has over a million tiny blood-filtering units
called nephrons. Blood rushes into the nephrons at high
pressure and is filtered through a cluster of tiny capillary
membranes called a glomerulus.
A glomerular capillary membrane is more than one
hundred times more permeable than other capillaries in
the body. Blood cells and plasma proteins are too large
to fit through these tiny filtering units, however, plasma
containing liquid wastes is filtered through into a
collecting chamber called Bowman’s capsule.
Nephron showing the Bowmans capsule the convoluted
loop of Henle and the collecting ducts which drain into a
ureters.
This filtrate travels into a long tube called the loop of
Henle, where most of the water and salt in the filtered
plasma is absorbed back into our blood. The filtrate is
then drained into a collecting duct which leads into a
ureter and into the bladder. The liquid wastes and excess
water becomes urine. Every day, our kidneys filter 1500
litres of blood to produce 1.5 litres of urine.
Ureters: tubes for moving
Urine flows from the collecting tube in the kidneys to the
bladder through two tubes called ureters.
Sometimes salts in urine can crystallise, forming a kidney
stone. A large stone may block a ureter and prevent urine
drainage. Pressure builds up in the kidney, causing
severe pain. Kidney stones can be shattered by
ultrasound.
A magnified view of Bowman’s capsule, with glomerulus
– showing magnified glomerular filtering membrane.
Bladder: storage unit
Our bladder is a collapsible muscular sac that stores urine until it is convenient to urinate. The urge to urinate
is felt when the bladder contains about 200mL of urine. However, a bladder can hold up to a litre if
necessary.
Urethra: tubes for letting it out
Urine leaves our bodies through the urethra. A man’s urethra is about 20 cm long and also carries semen.
A woman's urethra is about 4 cm long. Bacterial infection can quickly spread from a woman’s urethra to the
bladder and kidneys. Prompt medical attention is needed to prevent kidney damage.
Urine: waste
Urine consists of 96% water. Urea and creatine are cellular waste products produced from the breakdown of
proteins. Their concentration in the urine varies with the amount of protein eaten in the diet and cellular
activity. Urine also contains uric acid, which is produced from the breakdown of cellular nucleotides from old
cells. The amount of ammonia in the urine depends on the amount of acid substances that are being
metabolised by the kidneys. Salts such as sodium, chlorine, calcium, potassium, phosphorus and sulphates
are also excreted by the kidneys and the urinary system
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The Human Body Background notes 8
When things go wrong: blocked passageways
Asthma
Asthma is often triggered by allergens such as dust mites or cat fur. Airways become inflamed and swollen,
reducing airflow. The symptoms of this disease include shortness of breath and wheezing.
Atherosclerosis
High cholesterol levels in the blood may cause arteries to become blocked, a condition known as
atherosclerosis.
If the blockage occurs in the brain it can cause a stroke. If the blood supply to the heart is blocked it can
cause a heart attack. People who smoke and eat fatty food are much more likely to suffer from
atherosclerosis.
Polycystic kidney disease
This inherited disease affects 1 in 500 people. Cysts develop in the kidneys and can eventually cause them
to fail. The symptoms are difficulty in breathing, swelling in the abdomen and diarrhoea. The cysts are not felt
until a person's 40s and may not be a problem until his or her 60s.
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The Human Body Background notes 9
9. The muscular and skeletal systems: the power within
We walk, talk, chew, smile, breathe and blink. We also lift,
carry, stretch and strain. All of our movements depend on
our musculoskeletal system, a bony skeleton with muscles
attached. Our musculoskeletal system enables us to
perform an enormous range of tasks. From threading a
needle to chopping wood, our bones, joints and muscles
work together for precision and strength.
The skeletal system
Our skeleton is the body's supporting framework. When
muscles pull on the skeleton, movements are made. A
skeleton has bones, joints and cartilage and makes up
20% of our body weight. Joints are the meeting point of
adjoining bones that can move. Smooth cartilage covers
the ends of bones at joints and enables them to slide over
each other. Cartilage is also found in parts of the skeleton
that need to bend and be flexible.
Bones provide structure and strength; by weight, they are
stronger than steel. To stay strong, bone tissue is
constantly renewed. When bones break, they repair
themselves. Soft tissues, like the brain and heart, are
protected by bone. Bones store minerals for use by the
body. Bone marrow produces new red and white blood
cells.
The Human Skeleton
Bones
The human skeleton can be divided into two main groups of bones. The first group includes the bones that
protect and support our vital organs and tissues in the head and chest regions of the body, including the
skull, spine and ribs.
The second group supports our limbs and enable us to move. They include the bones of the shoulders,
arms, pelvis and legs.
Bones are shaped according to the role that they carry out in the body. Different shaped bones do different
things. There are long bones like the femur (thigh), round bones like the patella (kneecap) and flat bones like
those found in the skull. There are also irregular shaped bones such as vertebrae. Some bones are locked
together and do not move, while others move at a joint.
(Left to right) : Cross section of the shaft of long bone. Source: Monash University; Micrograph of compact bone that shows the dark
osteons that house the bone cells (osteocytes).Source: University of Melbourne.
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What’s inside bone?
A long bone is made up of an outer layer of compact
bone with spongy bone at the ends. A cavity in the centre
is filled with bone marrow.
Hard compact bone is made up of tightly packed rods of
bone. It is riddled with canals and passageways used by
nerves, blood vessels and lymphatic vessels.
Soft bone marrow is a jelly-like substance often found
within spongy bone at the ends of long bones. Blood cells A magnified image of a single bone cell called an
are replenished from stem cells that exist in the marrow. osteocyte. Source: National University Hospital of
Singapore
Osteocytes are cells that secrete the mineral rich bone.
They are tiny spider-shaped cells found between the
canals of compact bone. If they die, bone begins to break
down.
Hyaline cartilage cells produce a more flexible
substance than bone cells do. Cartilage provides support
as well as flexibility for the body. It covers the ends of
long bones, supports the tip of the nose and ears and
also provides the flexible support of the trachea.
A magnified image of hyaline cartilage. Source:
Monash University
Our Joints – Fixed and Mobile
Movement of the skeleton occurs at joints, places where bones meet.
The structure of our joints enables us to move the way we do. Some joints allow bending movements; some
allow twisting. In these mobile joints, the bones are protected by cartilage. The smooth cartilage and a
slippery fluid lubricate a joint's movement. Gristly bands, called ligaments, are attached to the bones around
a joint, holding them in position. Not all joints move. Some bones, like the skull plates, are locked rigidly in
position.
What do the joints do ?
The hinge joint allows for bending of the elbow and knee.
The ball-and-socket joint gives the shoulder and hips the greatest range of movement.
The gliding joints in the hands and feet are restricted by ligaments and make very small movements.
The saddle joint allows the thumb to rock back and forth and side to side. It allows pinching and gripping.
The rotating joint is responsible for turning the head from side to side.
hinge joint
ball and socket
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saddle joint
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The Human Body Background notes 9
Muscles mean movement
No part of our body moves without muscles.
Muscles move the food in our intestines.
They even make the irises of our eyes open
or close to adjust to the light. Movements,
which are beyond our conscious control, are
made by smooth muscles. Cardiac muscle,
found only in the heart, causes it to contract.
Skeletal muscles, the muscles attached to
bones, cause all voluntary movements to the
body. They tend to work in pairs, relaxing
and contracting in turn to move a joint.
How do muscles contract and relax?
When a muscle cell receives a signal, the
filaments within it slide over each other and
cause the muscle to contract. As the
filaments release and slide apart again, the
muscle relaxes.
Skeletal muscle and smooth muscle cells
are supplied with nerve endings that control
their activity. However, most cardiac muscle
cells contract automatically. A section of the
heart, called the pacemaker, sends out
electrical signals that pass from one muscle
cell to another and cause the cells to
contract at the same time.
Bundles of muscles
Muscles are made up of hundreds or thousands of very long,
cylindrical cells joined together. Some skeletal muscle cells can be
up to 30 cm long, but they are thinner than hair.
Skeletal muscle: appear to have stripes under the microscope
Smooth muscle does not have stripes. The cells are tapered and
elongated.
Skeletal muscle.
Cardiac muscle is made up of branched cells that help electrical
signals pass through quickly, causing the heart to contract
rhythmically.
When Things Go Wrong:
Osteoporosis
Osteoporosis is a disease that causes bones to become brittle.
Women over 40 and men over 60 are at the highest risk of
developing osteoporosis. Adequate calcium in the diet and plenty of
exercise can reduce the chances of developing osteoporosis.
Smooth muscle.
Arthritis
Osteoarthritis, the most common type of arthritis, is caused by
overuse of a joint. It affects such weight-bearing joints as knees and
hips. Cracks in the cartilage lining may cause it to become swollen
Cardiac muscle
and painful. Rheumatoid arthritis is a disease that leads to
inflammation in the joints. They become very swollen and movement
is severely hampered.
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