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OCR AS Biology
The heart
External…read more

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Internal…read more

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Arteries, Veins and Valves
The heart is a double pump. This means the blood is pumped around two circuits, the pulmonary
circuit, to the lungs and back, and the systemic circuit all the way around the body and back.
Coronary arteries lie over the surface of the heart to carry oxygenated blood to the heart muscle
There are four main arteries and veins you need to know about;
· The vena cava deliverers deoxygenated blood, from the body, to the right atria.
· The pulmonary artery delivers deoxygenated blood from the right ventricle to the lungs.
· The pulmonary vein delivers the oxygenated blood from the lungs to the left atria.
· The Aorta delivers the oxygenated blood from the left ventricle to the body.
There are four main valves you need to know about
· The Tricuspid valve stops deoxygenated blood in the right ventricle from going back to the right
· The pulmonary semi Luna valve prevents the blood in the pulmonary arteries from going back to
the heart as the ventricle relaxes
· The bicuspid valve stops oxygenated blood from the left ventricle from going back to left atrium
· The Aorta semi Luna valve prevents blood in the Aorta from going back to the heart as the ventricle
Inside the ventricles are string like tendinous cords, These attach the valves to the walls of the
ventricles and prevent the flimsy valves from turning inside out.…read more

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Four Chambers
Right Atria and Left Atria; These have thin muscle walls because they do not need
to create as much pressure. They only need to push blood into the ventricles.
Right Ventricle; The walls are thicker than the atria. This enables the ventricle to
pump blood out of the heart. However they are much thinner than left ventricle.
This is because the right ventricle pumps deoxygenated blood out of the heart to
the lungs. The lungs are close to the heart and also contain lots of very fine
capillaries and alveoli's with very thin walls. The pressure of the blood must be
kept down to prevent capillaries in the lungs from bursting.
Left Ventricle; The walls of the left ventricle can be two or three times thicker
than those of the right ventricle. The muscle creates sufficient pressure to push
the blood out of the aorta and around the systemic circulation…read more

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The cardiac Cycle
Filling phase; Diastole
While both the atria and the ventricles are relaxing blood flows into the atria
through the open atrioventricular valves and into the ventricles.
Atrial Contraction; Atrial Systole
The heart beats when the atria contract. Both the left and the right atria contract
together, the slight increase in pressure pushes blood from the atria in the
ventricles ensuring the ventricles are full. Blood fills the atrioventricular valve flaps
causing them to snap shut (lub sound)
Ventricular Contraction; Ventricular Systole
Now there is a short period when all four valves are closed. The walls of the
ventricles contract. The pressure in the ventricles raises very quickly. The
contraction starts at the apex of the heart and works its way up. The semi lunar
valves open and blood is pushed out of the heart. As the ventricles start to relax
the semi lunar valve close (dub sound)…read more

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