Download THE EXCRETION INTRODUCTION Waste substances are produced

THE EXCRETION
INTRODUCTION
Waste substances are produced as a result of the nutrition of the cells in our body. The
removal of waste from the body is called excretion. It is really a cleaning process in which the
blood is filtered to get rid of all the dangerous substances it may contain. The excretory system
also eliminates toxic substances that come from the exterior of the body such as the chemicals in
food additives, medicines, air pollution, smoking, cosmetics…
EXCRETORY ORGANS
We eliminate waste through different organs of the body: kidneys (urine), sweat glands
(sweat), lungs (CO2) and liver (bile).
- Sweat glands: Our skin has millions of sweat glands which produce sweat, a liquid similar
to urine. It is produced by filtering the blood that reaches the skin. Another function of the sweat
glands is to cool the skin: when the temperature is high, sweat increases. As it evaporates from the
skin, the skin is cooled.
- Liver: Some waste products and other toxic substances are added to bile, which drains into
the intestine. From here, it is expelled from the body.
- Lungs: They remove carbon dioxide (CO2) from the blood and expel it from the body.
- Urinary system: This is the most important part of the excretory system. It eliminates
waste products (mainly urea, ammonia and uric acid) and other toxic substances in the form of
urine. In adittion to its excretion function, it regulates the amount of water and the concentration
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and type of minerals in the blood (it expels non waste products that are in excess such as water and
mineral salts). Curiosity: these 4 excretory organs are the most known but there are many other forms to eliminate
toxic substances: the cells that cover the skin and the walls of the intestine are renewed very fast, being able to
accumulate toxic substances and when the cells are shed, the toxic substances are eliminated outside the body (some
eczema in the skin and some inflammatory bowel diseases are caused by the accumulation and elimination of
significant amounts of toxic substances by these pathways). Tears can also eliminate toxic substances. Besides toxic
substances can be accumulated in nails and hair (nail testing and hair testing can be used to detect drug consumption in
the last few months or weeks).
The urinary system is made up of two kidneys, two ureters, a urinary bladder and a urethra.
Kidneys filter the blood to produce urine. The ureters are two narrow tubes (around 28 cm long)
connecting the kidneys to the bladder. The urinary bladder is an elastic and muscular sac where
urine collects before it is expelled from the body thanks to the contraction of the bladder that
pushes the urine out via the urethra, a tube that connects the urinary bladder to the outside of the
body. In men the urethra is connected to the reproductive system and in women the urethra is
separated from the reproductive system. This means that in men urine and semen pass through it
(at different times) and the men’s urethra is longer than women’s urethra because it goes through
the penis. Exercise: What are the differences between men’s and women’s urethra?
Exercises:
What
substances
are
concentrated in urine than in blood? Why?
more
Why do proteins and glucose not appear in
urine?
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THE CIRCULATORY SYSTEM
INTRODUCTION
The circulatory system is often seen to include two separate systems: the cardiovascular
system, which circulates blood, and the lymphatic system, which circulates lymph. The essential
components of the human cardiovascular system are the blood, blood vessels and the heart, and the
main components of the lymphatic system are lymph, lymphatic vessels and lymphatic nodes.
THE CARDIOVASCULAR SYSTEM
It is made up of the blood, blood vessels and the heart. The heart pumps blood through the
blood vessels mainly to carry to all the tissues the nutrients (included O2) our cells need and to take
away from the tissues the waste produced by our cells (included CO2).
BLOOD
The average human adult has more than 5 litres of blood in his or her body. Blood is the
transport medium of nearly everything within the body. It transports hormones, nutrients, oxygen,
antibodies, and other important things needed to keep the body healthy. Blood is a thick, red liquid
composed of plasma and blood cells.
Blood plasma is a yellowish liquid made up of water containing dissolved molecules such as
nutrients (mineral salts, glucose, amino acids, vitamins…), waste products (CO2, urea, uric
acid…), important proteins such as antibodies, hormones…
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There are three types of blood cells:
Red blood cells or erythrocytes: These are the most common type of blood cell (around 5 million
per mm3 of blood). They are disc-shaped and have no nucleus. They contain haemoglobin, a red
molecule that contains an atom of iron. This molecule is responsible for the blood´s red colour
and transports oxygen from the alveoli of the lungs to the body´s cells, and carries CO 2 from
these cells back to the alveoli.
White blood cells or leukocytes: They protect the body from infections. There are around 8,000
per microlitre (mm3) of blood.
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Platelets or thrombocytes: These are not true cells, but cell fragments. They allow the blood to
coagulate which prevents haemorrhages. There are around 300,000 per mm3 of blood.
Blood cells are produced in the bone marrow, which is found inside certain bones. Some
leukocytes (lymphocytes) mature in the lymph nodes.
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The functions of blood are:
Blood carries oxygen and nutrients to all cells and takes away their waste products.
It defends our body from infections.
It contains platelets that can form a plug in a damaged blood vessel to prevent blood loss.
It regulates the body temperature, moving body heat from the warmer areas to the cooler ones.
It transports hormones around the body.
BLOOD VESSELS
There are three main types of vessel: arteries, capillaries and veins. Arteries carry blood from
the heart to the body’s tissues, while veins carry blood back from the tissues to the heart. Small
arteries are called arterioles and small veins are referred to as venules. The third and smallest type
of vessel, capillaries, form a network connecting the smallest arterioles with the smallest venules.
So the route is: heart → arteries → arterioles → capillaries → venules → veins → back to the
heart and the route starts again.
Arteries have relatively thick elastic walls that enable them to withstand the high pressure of
blood pumped from the heart. By the time the blood has been forced through capillaries and
arrived in veins, its pressure has dropped, so veins have thinner walls. Veins have one-way valves
to stop blood from running back the wrong way. Capillaries are so small that blood cells can only
move through them one at a time and their walls are very narrow (formed by a single layer of flat
cells), allowing substances (nutrients and waste) to be exchanged quickly and easily between the
blood and the tissues. Oxygen and food nutrients pass from these capillaries to the interstitial fluid
(liquid between the cells) of the tissues and waste pass from the interstitial fluid to the capillaries.
Cells exchange these substances with the interstitial fluid (the interstitial fluid is the intermediary
between blood and cells).
Exercise: Fill in the gaps (using the words arteries, capillaries or veins):
……………. carry blood from arterioles to venules, ………….. carry blood from the heart to
the body’s tissues and …………….. carry blood from the body’s tissues to the heart. …………….
have thick elastic walls, ………….. have thinner walls and ………………walls are very narrow to
allow the exchange of substances between blood and tissues. The blood pressure is high in ……….
and low in ……….……. Valves in the …..……….. prevent backflow of blood.
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THE HEART
The heart is a hollow organ made up of muscle tissue (cardiac muscle). The cardiac muscle
contracts and then relaxes, on average, 75 times per minute. Each contraction is called a
heartbeat.
Parts of the heart
It is divided into two unconnected parts, the right and the left, by an internal wall. Each part is
divided into two chambers: the upper chamber is the atrium (auricle) and the lower chamber is the
ventricle. The heart therefore has 4 chambers: two atria and two ventricles. Each atrium and
ventricle are connected by a valve (atrioventricular valve) to stop blood from running back the
wrong way. The valve between the right atrium and the right ventricle is called the tricuspid
valve, and the one between the left atrium and the left ventricle is called the mitral valve.
Vessels of the heart
Several veins lead to the atria: a superior and a inferior vena cava on the right and four
pulmonary veins on the left. Two arteries lead away from the ventricles: the pulmonary artery
on the right and the aorta on the left. At the beginning of each of these arteries are the pulmonary
valve and the aortic valve, respectively; these are called the semilunar valves. They prevent
blood from flowing back to the heart after it has left.
Blood Circulation: A double circuit
The human blood circulation is double because the blood has to pass through the heart twice in
order to complete its circuit. The heart pumps oxygenated blood to the body (systemic circulation)
and deoxygenated blood to the lungs (pulmonary circulation). In the human heart there is one
atrium and one ventricle for each circulation:
• Pulmonary circulation: The pulmonary circuit carries de-oxygenated blood to the lungs and
returns oxygenated blood to the heart. This circulation stars in the right ventricle, where the
pulmonary artery leads away from the heart and carries oxygen-depleted blood to the lungs for reoxygenation and removal of carbon dioxide in the dense network of capillaries that surrounds the
alveoli. The capillaries turn into venules, which join together to form four pulmonary veins which
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lead to the left atrium. From here the blood flows through the mitral valve to the left ventricle,
where systemic circulation begins.
• Systemic circulation: The systemic circuit carries oxygenated blood around the body to
deliver the oxygen and returns de-oxygenated (poor in oxygen) blood to the heart. This circulation
stars in the left ventricle and flows along the aorta, which splits into arteries, arterioles and then
into capillaries. These vessels lead to all body tissues and organs except the lungs. The blood in the
capillaries provides cells with the oxygen and nutrients they need, and takes away waste products
(mainly carbon dioxide). It then returns via the venules, veins and venae cavae, which lead to the
right atrium. From here the blood flows through the tricuspid valve to the right ventricle, where
pulmonary circulation begins.
Blood circulation in humans is double, closed and complete. Double because we have the
pulmonary and the systemic circulation, closed because the blood is always inside the vessels and
complete because oxygenated and deoxygenated blood are never mixed. The right and the left part
of the heart are separated by an internal wall, so the oxygenated blood in the left part does not mix
with the deoxygenated blood of the right part of the heart. The left ventricle is thicker (more
muscle) than the right ventricle because it has to pump the blood stronger (with more pressure) so
that the blood gets all the parts of the body except the lungs. Exercise: What are the main
differences between the pulmonary and the systemic circulation?
Exercise: Use arrows to indicate all the places that a red blood cell goes through since it is into
the right atrium until it gets there again. Besides, indicate the circuit that contains oxygenated
blood and the circuit that contains de-oxygenated blood and indicate the pulmonary and systemic
circuits.
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The Cardiac Cycle
The sequence of events in a single heartbeat is called the cardiac cycle. Since the average heart
beats about 75 beats per minute, the
length of each cardiac cycle is less than
one second. During the cycle the
chambers of the heart may be relaxed
(diastole) or contracted (systole) and
two sounds are produced: “lub” and
“dub”. Each cardiac cycle (heartbeat)
has 3 phases: atrial systole, ventricular
systole and diastole.
• Atrial Systole: The atria (left
atrium and right atrium at the same
time) contract, the atrioventricular
valves (tricuspid and mitral) open and
the blood moves into the ventricles.
• Ventricular Systole: The ventricles (left and
right at the same time) contract, the
atrioventricular valves (tricuspid and mitral) close
causing the first sound: “lub” and preventing the
blood from going back to the atria. The semilunar
valves (aortic valve and pulmonary valve) open,
letting the blood out through the aortic and
pulmonary arteries.
• Diastole: The ventricles and the atria are
relaxed. The semilunar valves (aortic valve and
pulmonary valve) close causing the second sound:
“dub” and preventing the blood from going back
to the ventricles. The blood goes into the atria
through the pulmonary veins and cavas.
THE LYMPHATIC SYSTEM
Water and substances from blood plasma filter out of the capillaries into the spaces between
the cells in the body tissues. These spaces are filled with a fluid known as the interstitial fluid.
Excess interstitial fluid is collected, and then returned to the bloodstream (veins), by the lymphatic
system. Lymph is a clear liquid transported by the lymphatic system and it is formed from the
interstitial fluid. Lymph also contains many lymphocytes, so the lymphatic system has a protective
function; in addition, it regulates the interstitial fluid levels in the tissues (the lymphatic system
drains off any extra fluid to stop the tissues from puffing up). Exercise: What are the two main
functions of the lymphatic system?
The lymphatic system is made up mainly of lymphatic vessels and lymphatic nodes. Lymph
travels along the lymphatic vessels until it is emptied in a vein close to the neck (subclavian vein).
Along this route there are lymph nodes, especially in the armpits, chest and abdomen. Lymph
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nodes contain many lymphocytes. Lymph is filtered through the lymph nodes before being
emptied into the blood. Bacteria, viruses and cancer cells picked up from the tissues by the lymph
are trapped in the lymph node. Lymphocytes can then attack and kill the bacteria. This is why your
lymph nodes tend to swell if you have an infection. Lymphatic vessels have valves (similar to the
valves in veins), which stop the lymph from running back the wrong way.
Lymphatic vessels have valves (similar to the
valves in veins), which stop the lymph from running
back the wrong way. The lymphatic system does not
have a pumping organ like the heart. So, the lymph
moves very slowly. It is propelled by the movement of
the body (the contraction of the muscles that surround
the lymph vessels pushes it, this is similar to what
happens in veins).
Exercise: What could happen in the tissues of sedentary people in the long term?
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