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Source 1: Circulatory
System (High School Biology)

Although the
heart plays one of the biggest roles in the circulatory system, the rest of the
system is also very important as it is what transports blood to and from the
heart to keep the body alive.

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Blood is
transported throughout the body by blood vessels. Blood vessels are complex
structures but can be simplified quite significantly. The two main components
of blood vessels are the arteries and the veins. Arteries carry oxygenated
blood away from the heart, while veins carry deoxygenated blood to the heart.
Other blood vessels found in the circulatory system are arterioles and venules.
Arterioles are blood vessel that branch off arteries and lead to capillaries.
Venules are similar to arterioles but branch off veins and also lead to
capillaries. Finally, capillaries are the smallest of the blood vessels that
connect arterioles and venules and play an important role in the exchange of
gases such as oxygen. The average size of each type of blood vessel can be found
below in Table 1.  1.


Table 1: All blood vessels and their approximate diameter.

Blood Vessel

Size (Diameter)








Source 2: Early Bird
Body System

endothelium can be found throughout the entire circulatory system. It is the
thin permeable layer of cells found on the inner walls of all blood vessels.
Permeability is the ability of a membrane to allow certain molecules to be
transported through it. Permeability of the endothelium is important because
oxygen, nutrients and white blood cells need to be transported through the
blood vessels to reach other organs.


The three
main functions of blood include transport, protection and regulation. Blood
transports many different nutrients around the body, blood plays a key role in protection
as it decreases inflammation, carries antibodies that destroy bacteria and
transports platelets that initiate clotting and finally, blood is useful in regulating
the body. For example, the blood along with the nervous system is responsible for
regulating pH and water balances.  2.


Source 3: Circulatory
System Dynamics

blood vessels are responsible for many different tasks in the human body, they
are not involved in regulating the transport of blood. The transportation of
blood is controlled by the nervous system. The nervous system is in charge of
telling the blood vessels when to constrict and when to dilate. When the body
is hot, vasodilation occurs, which is when the diameter of the blood vessels
increases allowing more blood to reach the surface of the skin to cool the body
down. When the body is cold, vasoconstriction occurs, which is when the
diameter of the blood vessels decreases, keeping heat within the body.


Pressure is the amount of force against the walls of the blood vessels by the
circulating blood.

pressure decreases as blood vessels go from arteries to arterioles to
capillaries, and then veins.

pressure generally however, refers to the pressure in the arteries since they
are under the most strain. Blood pressure can be measured in two phases;
systolic and diastolic pressure. Systolic pressure is the pressure measured at
the highest point, beginning of the cardiac cycle while diastolic pressure is
the pressure measured at the lowest point, the resting phase of the heart.


Although the
circulatory is considered one large system, there are many subsystems within.
The pulmonary circuit is the part of the circulatory system that transports
blood to and from the heart. The pulmonary circuit consists of the pulmonary vein
that carries oxygenated blood from the lungs to the heart, while the pulmonary
artery that carries deoxygenated blood from the heart to the lungs. The second
circuit is the coronary circulation, which are the vessels that deliver oxygen
to the heart itself. Finally, there is the systemic circulation, the rest of
the circulatory system that carries oxygen throughout the body 3.


Source 4: Blood Components

important component of the circulatory system are the valves found in veins. Since
valves can also be found in the heart, the term venous valve is used to
describe the elastic flap like structures in the rest of body. The circulatory
system is designed in away that allows for blood to only travel through the
heart and body in one direction. To prevent the back flow of blood, valves can
be used.


Like stated
above, blood is the liquid medium responsible for carrying many different
dissolved substances throughout the body. For instance, hormones, nutrients
such as water, glucose, amino acids and minerals and finally waste substances
such as carbon dioxide and urea. Blood is composed of three main elements;
plasma, formed elements and platelets. Plasma is similar to seawater in which
it is water based but contains many salts such as sodium chloride. The formed
elements consist of red and white blood cells. Red blood cells are responsible
for the transport of oxygen and removal of carbon dioxide. Hemoglobin can be
found within red blood cells and bonds to oxygen as a means of transportation.
White blood cells are the cells that protect the body against disease and
infection. Finally, platelets are very small cells that respond to injury and
help blood clot 4.

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