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Physical
Perhaps one of the greatest miracles of nature, is the creation of the human heart - a sophisticated pump designed
to work without fault for approximately 90 odd years. In one lifetime, it will beat over 3.1 billion times and
shift over 50 million gallons of blood in 80 years. If it misses just one beat, you will feel it as a momentary
light palpation in the chest. It weighs approximately 325 grams (11.46 ounces - 0.72 lbs) in males and 275 grams
in females.The heart pumps around 70 ml of blood per stroke (about 2.4 onces), and beats approximately 72 times
a minute.
Location and Activity
The heart is located in the middle of the breast just beneath the breastbone which protects it. Slightly more of
the heart projects out to the left hand side (as perceived by the heart owner). In many ways, it should be considered
part of the lung/heart couplet as its location right next to the lungs is vital to its working.
The heart has two pumping circuits: one is pumping the blood out through the Aorta artery and into the main arteries
and around the body; the other is pumping blood in a circuit through the lungs where blood is resupplied with Oxygen
and returned to the heart to be transferred to the primary circuit. The heart provides the motion to force oxygen
enriched blood through a massive network or arteries, blood vessels, capillaries and veins - thus supplying all
organs and elements of our body with food and energy.
Construction
The heart has 4 main chambers called the right atrium, left atrium, right ventricle, and left ventricle. These
are muscular bags that contract to
squeeze/push blood onwards. The left ventricle (chamber) has the thickest walls as it does more of the pumping.
The chambers are arranged in pairs with the 2 atrium consisting of thin walls receiving blood from the veins. Each
atrium pumps blood through a valve into its associated thicker-walled ventricle, which in turn - pumps the blood
into a main artery. The sequence of operation is as follows:-
- 1) oxygenated blood from the lungs arrive to the
heart in the pulmonary veins
- 2) It enters the Left Atrium
- 3) The Atrium contracts and pushes the blood through
the mitral valve into the left ventricle.
- 4) The mitral valve closes and the left ventricle
contracts pushing the blood out through the aortic valve into the aorta artery.
- 5) Blood returns depleted of Oxygen from the body
through a large vein into the right atrium.
- 6) The right Atrium contracts, pushing the blood
through tricuspid valve into the right ventricle.
- 7) The right ventricle contracts - sending the
blood out through the pulmonary valve into the pulmonary artery through the lungs where it is re-oxygenated.
- 8) It returns to the heart (1) above and the cycle
is repeated.
The 4 valves operate automatically opening and closing the valves to ensure pumping continues in one direction.
The Valves
The Pulmonary and Aortic valves are very similar with each having three leaf-like cusps made of tough, yet thin,
fibrous tissue. The mitral and tricuspid valves are more complicated but similar in structure, with the mitral
valve possessing two leaf-like cusps, and the tricuspid valve having three. These valves sit in in a ring with
the free edges touching each other and able to close the passage between the ventricle and atrium. These edges
are also attached to a set of fine strings (chordae tendinea) passing down into the ventricle. These prevent the
valve from springing backwards when it is under pressure (like stays).
Timing
The timing of the valves opening and closing synched to the 4 chambers contracting and pumping is a critical process.
With each heartbeat, both the atria contract together charging the ventricles with blood prior to the ventricles
contracting together. Initial control is affected by the sinuatrial node in the right atrium. impulses from it
pass through the muscles of both atria causing them to contract. At the junction of the atria and the ventricles,
is another secondary node (the atrioventricular node) which delays the impulses before passing them on and down
through a bundle of conducting fibres (called the Bundle of His) into the ventricles, causing them to contract
after the two atria.
Oddities
After heart transplants, some receipients have reported changes in their behaviour such that they begin to favour
activities or foods which the donor had favoured while alive. This has led to speculation that the heart may contain
a degree of memory cells retaining traits from the donor. No scientific evidence has yet been found to support
this theory. All cells do in fact possess some kind of latent memory but this is at a low level of functionality
more to do with cell reproduction and immunity.
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