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Biology UNIT 5
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UNIT 5B I O
ORGANS AND SYSTEMS
L O G Y
THE CARDIOVASCULAR SYSTEM
In this section you will learn about the structure and function of the heart, the blood vessels
and the events of the cardiac cycle. You will also investigate the effect of caffeine on the
heart rate in Daphnia, more commonly known as water fleas.
Many cells in a multicellular organism are not in direct contact with their surroundings. This
means that the organism cannot rely on diffusion alone to supply the cells of all its organs
with the nutrients and oxygen it needs to survive. The cardiovascular system, also known as
the circulatory system, allows more blood to circulate around the body to transport and
supply the essential nutrients that multicellular organisms need in order to maintain
homeostasis and to survive.
The cardiovascular system consists of the heart, blood vessels and blood (see Figure 5.11). It
is responsible for not only transporting nutrients and oxygen but also hormones and cellular
waste throughout the body.
Blood is a tissue and it always flows in blood vessels. The components that make up human
blood are erythrocytes, leukocytes, thrombocytes and plasma.
Blood is responsible for transporting:
o Oxygen from the lungs to body cells for aerobic respiration;
o Carbon dioxide from respiring cells back to the lungs to be exhaled;
o Nutrients from the intestines to body cells;
o Urea (waste product) from the liver to the kidney to be removed;
o Hormones from the endocrine glands to their target cells;
o Heat from the respiring tissue to organs to maintain body temperature or to the skin to
be lost.
Blood is also responsible for regulating:
o Body temperature;
o pH of body tissues;
o Volume of fluid in circulation.
Blood also protects us because it contains:
o Platelets that cause plotting, to prevent bleeding and entry of pathogens;
o Leukocytes that defend us against infection.
Characteristic Features of Blood Vessels
Blood is always in blood vessels, either arteries, arterioles, capillaries, venules or veins. The
vessels form a closed transport system which starts and finishes at the heart. We refer to the
mammalian circulation system because we have two separate circulations. The blood flows
from the heart to body tissues and back to the heart. This is known as systemic circulation. In
a separate circulation known as pulmonary circulation, blood flows from the heart to the
lungs to expel carbon dioxide and take in oxygen and then it returns back to the heart.
,Arteries and arterioles
Arteries carry blood away from the heart. The blood leaves the ventricles of the heart and
enters thick elastic arteries under high pressure. The artery lumen is small, and arteries have
thick walls that contain collagen, a fibrous protein, to help maintain the shape and volume of
the arteries under pressure. Artery walls also contain elastic tissue to enable them to
continuously expand and recoil to keep the blood pressure. This expansion and recoiling are
what you will feel as your pulse where an artery passes near the surface of the skin. Arteries
also contain smooth muscle that contracts, enabling the artery lumen to narrow if needed.
Arteries are lined with smooth endothelium tissue, reducing the friction as the blood flows
through the lumen. As blood flows further from the heart, these large elastic arteries become
smaller muscular arteries which carry the blood to the organs in the body. They contain less
elastic tissue and more smooth muscle than larger elastic arteries. Arteries then divide into
smaller arterioles around the endothelium. These eventually become capillaries.
Capillaries
Capillaries allow the exchange of materials between blood and the body’s cells via tissue
fluid. These are tiny vessels with very thin walls consisting of only one layer of endothelium
cells. This thin wall reduces the diffusion distance for the materials being exchanged. The
lumen of a capillary is very narrow, with a diameter that is the same size as a red blood cell.
The small diameter allows only one erythrocyte through at one time. This ensures that the red
blood cell has to squeeze through the capillaries which help it release the oxygen. Capillaries
spread throughout tissues, forming capillary networks, and it is here where the plasma of
blood, rich with nutrients and oxygen, is forced out through small gaps in the capillary walls.
This fluid that is forced out is known as tissue fluid and it also carries away the waste from
cellular activity. Capillaries link arterioles to venules.
Venules and veins
Capillaries become slightly large and form venules, which also have small diameters.
Venules join to make veins. Veins have a large lumen and their walls are thinner than
arteries. Veins have thinner layers of collagen, smooth muscle and elastic tissue as they do
not need to constrict and recoil. Veins have valves to help prevent the back flow of blood as it
makes its way back to the heart. The action of the surrounding skeletal muscle can flatten
veins. This also helps force the blood back to the heart.
Table 5.8 states the role of the major blood vessels found in the body.
, Blood Vessel Artery or Vein Role
To deliver deoxygenated
Vena Cava Vein blood from the body into the
right atria of the heart.
To deliver oxygenated blood
Pulmonary Vein Vein the lungs into the left atria of
the heart.
To transport deoxygenated
Pulmonary Artery Artery blood away from right
ventricle in the heart to the
lungs to collect oxygen.
To transport oxygenated
Aorta Artery blood away from left
ventricle in the heart to the
rest of the body.
To supply the cardiac
Coronary Artery Artery muscle with its own supply
of oxygen.
Table 5.8 – Role of the major blood vessels.
Structure and Function of the Heart
Your heart is about the size of your fist when it is clenched, and it has a mass if about 300g. It
is in thoracic cavity between the lungs and behind the sternum, enclosed in a fibrous bad
made from inelastic connective tissue called the pericardium. The heart is a muscular double
pump divide into two halves, each of which contain two chambers.
The wall of the heart is made from mainly myocardium which consists of cardiac muscle.
Cardiac muscle contracts to make your heartbeat. The coronary arteries seen in Figure 5.12
lie on the surface of the heart. They carry oxygenated blood to the heart muscle itself.
The heart is divided into four chambers.
The two upper chambers are called the
atria. They have thin walls and they sit
above the two lower chambers called
ventricles. Ventricles are thicker walled
chambers. Deoxygenated blood flows
into the right atrium from the vena cava
(a main vein); at the same time the left
atrium received oxygenated blood from
the lungs via the pulmonary vein. From
here, the blood blows into the lower
ventricles through atrioventricular
valves. The atrioventricular valve
between the left atria and left ventricle
is known as the bicuspid valve. The tricuspid is the valve between the right atrium and right
ventricle. These valves are thin flaps of tissue attached to the ventricles via tendinous chords
to stop the valves from turning inside out. When the ventricles are full of blood and ready to
contract, the valves close to stop the back flow of blood into the atrium.
The heart is divided into the right and left side and the ventricles are separated by a wall of
muscle called the septum. This stops the oxygenated and deoxygenated blood coming into
contract with each other.
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