AQA AS Biology Unit 1: The Heart

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  • Created on: 22-08-13 16:20
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The Heart
Humans have a double circulatory system with a 4-chambered heart.
The heart is a muscular pump.
It has a left pump and a right pump:
Pump Function
Left Deals with oxygenated blood
Pumps blood to the rest of the body
Systematic circulation
Right Deals with deoxygenated blood
Pumps blood to the lungs
Pulmonary circulation
Each pump has two chambers:
Chamber Properties and Function
Atrium Thin muscular wall
Elastic and stretches as it collects blood
Pumps blood a short distance to the
ventricle
Ventricle Thick muscular wall
Pumps blood a large distance to either the
lungs or the rest of the body
Why have two separate pumps?
Blood has to pass through tiny capillaries in the lungs in order to
present a large surface area for the exchange of gases.
In doing so, there is a large drop in pressure and so the blood flow
to the rest of the body would be very slow.
Mammals therefore have a system in which the blood is returned to
the heart to increase its pressure before it is distributed to the rest of the body.
It is essential to keep the oxygenated blood in the pump on the left hand side separate from
the deoxygenated blood in the pump on the right.

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Between each atrium and ventricles are valves that prevent the backflow of blood into the
atria when the ventricles contract.…read more

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Each of the four chambers of the heart is served by large blood vessels that carry blood
towards or away from the heart.
The ventricles pump blood away from the heart and into the arteries.
The atria receive blood from the veins.…read more

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The Cardiac Cycle
The heart undergoes a sequence of events that is repeated about 70 times per minute.
This is known as the cardiac cycle.
There are two phases to the beating of the heart:
Contraction - systole
Relaxation ­ diastole
Cardiac muscle is myogenic which means that it can contract on its own.
Contractions are initiated within the heart by the sino-atrial node located in the right
atrium.…read more

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Contraction of the Atria ­ Atrial Systole
The muscle of the atrial wall contracts, forcing the remaining blood that they contain into
the ventricles.
The blood only has to be pushed a very short distance and
therefore the muscular walls of the atria are very thin.
During this stage, the muscle of the ventricle walls remain
relaxed.
Contraction of the Ventricles ­ Ventricular Systole
After a short delay to allow the ventricles to fill with blood, their walls contract
simultaneously.…read more

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Valves in the Control of Blood Flow
It is important to keep blood flowing in the right direction through the heart around the
body. This is achieved mainly by the pressure created by the heart muscle.
Blood, as with all liquids and gases, will always move from a region of higher pressure to one
of lower pressure.
There are, however, situations within the circulatory system when pressure differences
would result in blood flowing in the opposite direction from that which is desirable.…read more

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Cardiac Output
Cardiac output is the volume of blood by one ventricle of the heart per minute.
It is usually measured in dm³ min -1.…read more

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Controlling the Cardiac Cycle
Cardiac muscle is myogenic ­ its contraction is initiated from within the muscle itself, rather
than by the nervous impulses from outside.
Within the wall of the right atrium of the heart is a distinct group of cells known as the
sinoatrial node.
It is from here that the initial stimulus for contraction originates.
The sinoatrial node has a basic rhythm of stimulation that determines the beat of the heart.
For this reason it is often reffered to as the pacemaker.…read more

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Heart Disease
Coronary heart disease affects the pair of blood vessels, the coronary arteries which supply
the heart muscle with the glucose and oxygen that it requires for respiration.
Blood flow through these vessels may be impaired by the build-up of fatty deposits known
as atheroma.
If blood flow to the heart muscle is interrupted, it can lead to myocardial infarction.
Atheroma
Atheroma is a fatty deposit that forms within the wall of an artery.…read more

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Thrombosis
If an atheroma breaks through the lining of the blood vessel, it forms a rough surface that
interrupts the otherwise smooth flow of blood.
This may result in the formation of a blood clot, or thrombus, in a condition known as
thrombosis.
This thrombus may block the blood vessel, reducing or preventing the supply of blood to
tissues beyond it.…read more

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