The heart is the size of a clenched fist and lies in the thorax between the
two lungs with its apex tilted slightly to the left. It is conical in shape and
is surrounded by the pericardium, a two-layered bag of pericardial fluid which
reduces friction when the heart is beating.
The human heart is muscular pump that is divided into 4
distinct chambers. The upper chambers are known as the atria while the lower
chambers are called ventricles. The atria collect blood that enters the heart
and pushes to the ventricles. On collection, the ventricles will push the blood
out of the heart and into the arteries onto other parts of the body.
The atria have comparatively thin walls as they are only
forcing blood into the ventricles which does not require much force. Ventricles,
however, are required to push blood out to the rest of the body at a higher
pressure hence explaining their relatively thicker walls.
How does the pumping action of the heart result?
Ever wonder how your heart goes “dub, lub, dub, lub”?
Look no further! In order to move blood around the heart, the heart chambers
undergo alternating period of relaxation (diastole) and contraction (systole),
hence allowing the chambers to fill up and pump blood respectively.
The two atria of the heart have to work simultaneously,
relaxing at the same time to receive blood from the veins. The right atrium
receives deoxygenated blood from the vena cava (superior vena cava and inferior
vena cava) while the left atrium receives blood from the pulmonary veins.
Contracting at the same time, the atria push blood into the ventricles. The
ventricles then contract simultaneously to force the blood from the left and
right ventricles into the aortic arch and pulmonary arch respectively.
A ventricular relaxation (diastole) and contraction
(systole) make up one heartbeat, with a short pause between 2 heartbeats. The
rate of beat is dependent upon each individual with relation to his weight and
size. The average adult heartbeat is around 72 times a minute.
What about the backflow of blood?
Blood is prevented from flowing back into the atria by
one-way valves. These are the tricuspid valves and the bicuspid valves that lie
between right atrium and right ventricle and left atrium and left ventricle
respectively. The closing of the valves produce the “lub” sound we hear of
the heartbeat. The backflow of blood from the arteries into the ventricles are
also protected by semi-lunar valves which closes when the ventricle relaxes, creating the “dub” sound.
Heart Valves
Valves act as one-ways sentry points that allow blood to
pass through between chambers and into the various blood vessels. There are 4
main heart valves to note of:
Tricuspid Valve The tricuspid valve is located between the right atrium and right ventricle.
When the valve opens, blood is pumped from the right atrium into the right
ventricle. The tricuspid valve prevents the reverse of blood flow back into the
atrium by closing during ventricular contraction (systole). The valve has three
flaps that are attached to the walls by chordae tendineae, permitting a one-way
route from the atrium to the ventricle.
Pulmonary Valve
The pulmonary valve prevents the backflow of deoxygenated blood from the
pulmonary arteries back into the right ventricle. The pulmonary trunk will leave
the heart and divide into two pulmonary arteries, one to each lung. This valve
is a semi-lunar valve that is found in the pulmonary arch.
Bicuspid Valve (Mitral Valve)
The bicuspid valve lies between the left atrium and left ventricle. It is
similar in function and structure to the tricuspid valve though it only has two
flaps. It prevents the backflow of oxygenated blood from the left ventricle into
the left atrium after the atrium has relaxed.
Aortic Valve
The aortic valve is a semi-lunar valve that can be found in the aortic arch.
Its main purpose is to act as a one-way gateway from the left ventricle into the
arteries. Blood entering the aortic arch is of very high pressure.
The heart is the size of a clenched fist and lies in the thorax between the
two lungs with its apex tilted slightly to the left. It is conical in shape and
is surrounded by the pericardium, a two-layered bag of pericardial fluid which
reduces friction when the heart is beating.
