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The Cardiovascular System
The Heart

Introduction

A force is needed to keep the blood moving along the blood vessels and through the body's organs. This force has to be great enough to enable the blood to defy gravity and find its way to the brain and to reach the other organs under all conditions, yet not too forceful so as to damage delicate blood vessel and organ tissue.
The heart performs this function; blood pressure readings are an indication of how efficently the heart is performing this task.

Composition

The heart is a large muscular organ located slightly left of the middle of the chest. In men it weighs about 340 gm (l2oz) and a little less in women.
Essentially the heart consists of two pairs of chambers with muscular walls, that make them capable of contracting and dilating forcefully. Each pair has an upper Atrium and a lower thicker walled ventricle chamber which does most of the work of squeezing blood into the main arteries.
The four chambers work in unison with the help of four one way valves which ensure blood flows in the correct direction at all times and at the right pressure.

The heart actually pumps blood around two separate circuits.
Because blood constantly needs to be nourished with oxygen, one circulation specifically pumps blood to the lungs where large volumes of oxygen is absorbed into the blood before it is returned to the heart. This is called the pulmonary circulation.
The other circulation adds other nutrients and uses organs other than the lungs to cleanse the blood. This is called the systemic circulation.

How It Works

Blood returns to the left atrium chamber from the lungs through pulmonary veins having being enriched with oxygen. By way of a one way valve called the Mitral Valve, blood enters the left ventricle pushed by a contraction of the left atrium.
The left ventricle then contracts and simultaneously shuts the Mitral Valve so the blood can only go out through the open aortic valve into the Aorta. It then continues in the systemic arteries to capillaries where it gives up its oxygen and nutrients to tissues.
The blood returns to the heart from the body and brain in a large vein called Vena Cava. It goes into the right atrium. This contracts and the blood passes through a third valve called the Tricuspid Valve into the right ventricle.
A right ventricular contraction sends it out into the pulmonary artery, through the pulmonary valve, and through the lungs, where it has its oxygen renewed. It then returns to the heart in the pulmonary veins ready to start all over again.
This process is normally repeated 50—80 times every minute.

The Willing Worker

Every hour the heart relentlessly shifts about 90 gallons of blood around 60,000 - 80,000 miles of the body's blood vessels.
In an average life time the heart will beat about 2.4 billion times without stopping for a rest.
The heart is truly a well worked organ even when it is working in optimal state; It really could do without the extra overhead that high blood pressure presents.

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