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BIOLOGY UNIT 5: 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…
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.
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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 CO2 from these cells back to the
alveoli.
-
White blood cells or leukocytes: They protect the body from infections. There are
around 8,000 per mm3 of blood.
-
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.
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
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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).
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.
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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
re-oxygenation 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 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?
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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.
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.
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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
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).
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Exercise: What could happen in the tissues of sedentary people in the long term?
SOME COMMON CIRCULATORY DISEASES
Primero recuerda lo que vimos en el tema de la salud:
Enfermedades Cardiovasculares
Enfermedad no infecciosa, es la 1ª causa de muerte en países desarrollados (1 de cada 3
personas en países desarrollados mueren de esta enfermedad).
Son las que afectan al corazón y a los vasos sanguíneos como aterosclerosis,
hipertensión, trombosis, ictus e infartos.
La mala alimentación, el tabaco, estrés y sustancias químicas de la contaminación del
aire y de otros orígenes dañan los vasos sanguíneos.
En los vasos sanguíneos dañados se van acumulando placas endurecidas llamadas
placas de ateroma que contienen colesterol, grasas y calcio entre otros componentes.
La formación de estas placas se llama aterosclerosis.
Estas placas dificultan el paso de sangre, por lo que el corazón debe bombear sangre con
más presión, produciendo hipertensión. La hipertensión daña más las arterias lo que
conlleva a más aterosclerosis y la arteriosclerosis aumenta más la hipertensión, entrando
en un círculo vicioso donde el enfermo cada año tiene más aterosclerosis y más
hipertensión.
La hipertensión puede provocar la rotura de una arteria (hemorragias internas)
impidiendo que llegue la sangre a una zona del cuerpo como el cerebro (ictus o
accidente cerebrovascular) y la aterosclerosis también puede provocar que no llegue
sangre a una zona del cuerpo como el cerebro (ictus o accidente cerebrovascular) o el
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corazón (infarto de miocardio), debido a que las placas de ateroma son inestables, y si
se rompen, provocan la coagulación de la sangre. El coágulo o trombo obstruye
completamente el paso de la sangre (trombosis o embolia) y mueren las células de la
zona del cuerpo que no llega la sangre. Ejemplos: en una trombosis cerebral o en una embolia
cerebral (ambas son por obstrucción de una de las arterias que llevan sangre al cerebro) o en un derrame
cerebral (se rompe una arteria que llevaba sangre a una parte del cerebro) se mueren neuronas del cerebro
por falta de sangre (todos son ictus).
Para prevenir o curar las enfermedades cardiovasculares se recomienda dieta sana, realizar
ejercicio físico regular, no fumar, reducir el estrés y evitar en la medida de lo posible los
químicos que dañan las arterias.
- Atherosclerosis: is the formation of fatty plaques (atherosclerotic plaque) lining blood
vessels. It causes the arteries to narrow and restricts the blood flow. If a plaque ruptures a clot
can be produced (thrombosis).
- Hypertension: is a chronic medical condition in which the blood pressure in the arteries is
persistently elevated. High blood pressure is a major cause of atherosclerosis because it injures
the arteries.
- Heart Attack: is when a clot (thrombosis) in a coronary artery stops the blood flow to
some parts of the heart and cardiac muscle cells die.
- Stroke: is when a clot (thrombosis caused by atherosclerosis) or a hemorrhage (caused by
hypertension) in a brain artery stops the blood flow to some parts of the brain and neurons die.
HEALTHY HABITS
We must always follow the 3 main healthy habits for all the systems in our body: Eat a
healthy diet, do physical exercises and not to smoke. Remember that healthy habits (eat a wellbalanced diet, exercise daily, getting sufficient sleep at night, avoid consuming tobacco...) can
keep the body and immune system strong enough to fight against microorganisms that can
damage any systems. In addition to these general rules, we can be more specific in the
circulatory system:
- Reduce processed food because it contains an excess of sugar and salt and it could contain
trans fats. Trans fats and an excess of sugar is related to atherosclerosis and salt is related to
hypertension.
- Avoid tobacco, it increases hypertension and atherosclerosis.
- Exercise regularly, the heart is a muscle and every muscle needs training to improve strength
and resistance.
- Reduce stress, it increases hypertension and atherosclerosis.
- Reduce substances that injure the arteries (increases atherosclerosis) such as air pollution,
some medicines and food additives, alcohol, coffee, toxic substances…
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