The Circulatory System
The human circulatory system consists of the heart, a series of blood
vessels, and the blood that flows through them.
The Heart-
1. The heart is composed almost entirely of muscle is a hollow organ.
2. It is enclosed in a protective sac of tissue called the pericardium.
3. In the walls of the heart, there are two thin layers of epithelial
and connective tissue which sandwiches a thick layer of muscles
called the myocardium. The powerful contractions of the
myocardium pump blood through the circulatory system.
4. The heart muscle contracts on average 72 times a minute, pumping
about 70 ml of blood with each contraction.
5. The right side is divided from the left side by a septum which
prevents the mixing of oxygenated blood from the deoxygenated
blood.
6. On each side of the septum are two chambers. The upper chamber,
which receives the blood, is the atrium.
7. The lower chamber, which pumps blood out of the heart, is the
ventricle.
8. The heart has 4 chambers in all – two atria and two ventricles.
Circulation through the Body
1. The heart functions as two separate pumps.
2. The right side of the heart pumps blood from the heart to the
lungs. This pathway is known as pulmonary circulation.
3. In the lungs CO2 leaves the blood and oxygen is absorbed. The O2
rich blood then flows into the left side of the heart and is pumped
to the rest of the body. This pathway is called systemic circulation.
4. Blood that returns to the right side of the heart is O2 poor
because cells have absorbed much of the oxygen and loaded the
blood with CO2.
5. After this the blood again goes to the lungs.
Circulation through the heart.
1. Blood enters the heart through the right and left atria. As the
heart contracts, blood flows into the ventricles and then out from
the ventricles to either the body or the lungs.
2. There are flaps of connective tissue called valves between the
atria and the ventricles.
3. Blood moving from the atria holds the valves open.
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4. When the ventricles contract, the valves close, which prevents
blood from flowing back into the atria.
5. At the exits from the right and left ventricles there are valves
that prevent blood that flows out of the heart from flowing back in.
This system of valves keeps blood moving through the heart in one
direction.
Heartbeat- There are two networks of muscle fibres in the heart, one in
the atria and one in the ventricles. When a single fibre in either network
is stimulated and the network contracts as a unit, it is known as
heartbeat.
Each contraction begins in a small group of cardiac muscle cells- the
sinoatrial node (SA node). They are also called the pacemaker as they set
the pace for the heart.
How the impulse does spreads?
The impulse spreads from the network of fibres in the atria. It is picked
up by a bundle of fibres called the atrioventicular node (or AV node) and
carried to the network of fibres in the ventricles. When the network in
the atria contracts, blood in the atria flows into the ventricles. When the
muscles in the ventricles contract, blood flows out of the heart. This two
step pattern of contraction makes the heart a more efficient pump.
The path of the impulse
SA node atrial fibres AV node ventricular fibres
The two step flow of blood:
1. Step : Atria contracts ----------- blood flows into ventricle
2. Step : Ventricle contracts-------- blood flows out of the heart
Blood vessels-
Blood flows through the circulatory system in the blood vessels.
There are three types of blood vessels- arteries, capillaries, and veins.
Arteries- are the large vessels that carry blood from the heart to the
tissues of the body.
Except for the pulmonary arteries, all arteries carry O2 –rich blood. The
walls of the arteries contain connective tissue, smooth muscle and
endothelium. The elastic connective tissue allows an artery to expand
under pressure. Contractions of the smooth muscle regulate the diameter
of an artery.
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Capillaries- Capillaries are the smallest of the blood vessels. The walls
of the capillaries are only one celled thick. The capillaries bring nutrients
and oxygen to the tissues and absorb carbon dioxide and other waste
products from them.
Veins- the veins return blood back to the heart. Their walls also contain
connective tissues and smooth muscles. Large veins contain valves to
prevent the back flow of the blood. These valves open in only one
direction. The flow of blood is normally against the gravity in these veins.
So exercising improves the circulation of blood.
Blood Pressure
When the heart contracts, it produces a wave of fluid pressure in the
arteries. The force of the blood on the arteries’ walls is known as blood
pressure.
Blood pressure decreases when the heart relaxes, but the system still
remains under pressure.
Blood pressure is measured with a device called Sphygmomanometer. The
normal blood pressure of a healthy person is 120/80. The first number is
the systolic pressure – the force felt in the arteries when ventricles
contract. The second number is the diastolic pressure- the force of the
blood felt in the arteries when the ventricles relax.
Regulation of the blood pressure-
The body normally regulates blood pressure in two ways:
1. Sensory receptors at several places in the body detect the level of
blood pressure, sending impulses to the medulla oblongata region of
the brain stem. When the pressure is too high, the autonomic
nervous system releases neurotransmitters that cause the smooth
muscles in blood vessel walls to relax, lowering the blood pressure.
2. When the blood pressure is too low, neurotransmitters are
released that elevate blood pressure by causing these smooth
muscles to contract.
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Diseases of the Circulatory System
High Blood pressure
Atherosclerosis
Blood and the Lymphatic System
Blood is a type of connective tissue containing both dissolved substances
and specialized cells. Blood is composed of blood cells suspended in a
liquid called blood plasma. Plasma, which comprises 55% of blood fluid, is
mostly water (90% by volume), and contains dissolved proteins, glucose,
mineral ions, hormones, carbon dioxide (plasma being the main medium for
excretory product transportation), platelets and blood cells themselves.
to carry oxygen, instead of hemoglobin. Blood is circulated around the
body through blood vessels by the pumping action of the heart. Arterial
blood carries oxygen from inhaled air to the tissues of the body, and
venous blood carries carbon dioxide, a waste product of metabolism
produced by cells, from the tissues to the lungs to be exhaled.
Blood Cells
The blood cells present in blood are mainly red blood cells (also called
RBCs or erythrocytes) and white blood cells, including leukocytes and
platelets (also called thrombocytes).
Red Blood Cells
The most abundant cells in blood are red blood cells. These contain
hemoglobin, an iron-containing protein, which facilitates transportation of
oxygen by binding to this oxygen gas and greatly increasing its solubility
in blood. Mammals have red blood, which is bright red when oxygenated,
due to hemoglobin.
White Blood Cells
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White blood cells help to resist infections and parasites, and platelets
are important in the clotting of blood.
Platelets and Blood Clotting
Platelets, or thrombocytes, are the cells circulating in the blood that are
involved in the formation of blood clots. Dysfunction or low levels of
platelets predisposes to bleeding, while high levels, although usually
asymptomatic, may increase the risk of thrombosis. An abnormality or
disease of the platelets is called a thrombocytopathy
Blood Clotting Problems
T\he lymphatic System-
A network of vessels, nodes, and organs called the lymphatic system
collects the fluid that is lost by the blood and returns it back to the
circulatory system. This fluid is known as lymph.
Lymph collects in lymphatic capillaries and slowly flows into larger and
larger lymph vessels. Like large veins, lymph vessels contain valves that
prevent lymph from flowing backward. Ducts collect the lymph and return
it to the circulatory system through two openings in the superior vena
cava.
Along with the lymph vessels are lymph nodes which act as filters,
trapping bacteria and other micro organisms that cause disease. When
large number of micro organisms are trapped then the node becomes
swollen.
Lymph vessels are also responsible for absorbing nutrients like fats and
fat soluble vitamins from the digestive tract and carry them to the blood.
Thymus and Spleen also have important roles in the lymphatic system.
The lymphocytes called T-cells mature in the thymus before they can
function in the immune system. The spleen helps to cleanse the blood and
removes damaged blood cells from the circulatory system. It also
harbours phagocytes that engulf and destroy bacteria and other micro
organisms.
Assignment:
Due : 25th of April
1. You are a drop of blood in a human body. Explain your journey
through the body in detail. Also explain your role in the survival of
the body.
You will be judged equally on
Your accuracy of the information provided
The details provided
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The language used
Creativity in explaining the working of the system
2. There will be a quiz due next week on the circulatory system.
3. Answer the following questions:
What is blood clotting and how is it done?
What is the difference between artery and veins?
What are the various blood types?
How is blood pressure regulated?
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