How does the heart work step by step?

The heart as a pump

The heart is a pump that rhythmically contracts and relaxes. Think of the principles at work in a hand operated water pump. When the handle is depressed, pressure is put on the water and it exits through a spigot. When the handle is raised, a vacuum is created and water is drawn in from the well. The flow of the water forces open a little “trap door,” a valve, which opens in one direction only. Once the water is drawn up into the pumping apparatus, it cannot flow backward into the well, only outward to the spigot.

This is the way the heart works. It is the pump in charge of circulating oxygen, water, and nutrients through the bloodstream to all of the body’s tissues, including its own. The heart wall is a muscle, and just like any other muscle, it needs oxygen, water, and nutrients to function. But unlike other muscles for example, the ones in the arms or legs, which, when resting, need only a little oxygen the heart muscle needs a good supply of oxygen every second of your life, in order to continue beating. The heart beats about 70 times each minute, sometimes faster, if you are physically active. (This means that the heart of an eighty five year old person has beaten over 3,127,320,000 times!) The more active you are, the greater your need for oxygen.

Two pumps in one 

The heart is actually a double pump, each side quite distinct from the other. In this cross section of the heart (Figure 1-1) we see that there are four chambers. The two chambers on the right side receive blood returning from all parts of the body through a very large vein, the vena cava. This blood enters the top chamber, called the right atrium (atrium is a Latin word meaning the first room or chamber entered in a house); from here it goes to the lower chamber, the right ventricle, from which it is pumped through the pulmonary artery to the lungs to exchange its carbon dioxide for fresh oxygen.
This fresh blood, ready to feed oxygen to all the tissues, comes back to the left side of the heart, goes into the upper chamber, the left atrium, then down through a valve to the left ventricle, which pumps blood into the aorta, a very large artery that then subdivides into many smaller and smaller arteries throughout the body The left side of the heart is completely separate from the right side. The contraction phase of the heart is called systole, and the relaxation phase, when the chambers expand, drawing blood into the atria and ventricles, is called diastole.

The heart as part of the cardiovascular system 

The heart is linked to the lungs by major blood vessels to allow for a continuous exchange of oxygen and for the elimination of carbon dioxide, the by products of body energy production. Blood vessels, from large to very small, reach each tissue of the body. The vessels you could call them pipes that bring oxygen rich blood from the lungs to the various parts of the body are called arteries. When the arteries become very small, they are called capillaries, a name derived from the Latin capillus, meaning hair, because the capillaries are as fine as human hair. The blood vessels that return blood from the tissues to the lungs are called veins.

The coronary arteries 

Arteries forming a crown around the heart muscle bring oxygen and other nutrients to the heart. Because of their crownlike shape, they have been called coronary arteries (corona in Latin means crown) . The coronary arteries are the lifeline that supplies what the heart needs to stay alive. Just as if you stop breathing for longer than a few moments, you suffocate and die, so the heart muscle cannot be without an open lifeline for more than a few minutes without suffering damage.

The heart valves 

There are four valves in the heart, which prevent backward blood flow after the chambers contract and send blood forward to the ventricles, the pulmonary arteries, and the aorta. These valves are the tricuspid valve, which controls blood flow from the right atrium to the right ventricle; the pulmonary valve , which is the gate between the right ventricle and the pulmonary artery; the mitral valve , which allows blood to pass from the left atrium to the left ventricle; and the aortic valve , which allows blood to pass out from the left ventricle to the aorta.

The electrical system 

All the pumping of the heart is controlled by a finely tuned electrical system. Let’s return to the point where oxygen depleted blood returns to the right side of the heart, into the right atrium. The right atrium also contains an electrical signal generator called the sinoatrial node (SA node). This node acts as a kind of natural pacemaker: it releases regular electrical impulses, which cause the right atrium to contract. (Remember, the heart is primarily a muscle, and its response to an electrical impulse is to contract.) Impulses from the brain, traveling through the nervous system, also affect the timing of the SA node’s signal, but it has been found that the node can operate on its own if necessary.

When the electrical signal reaches the muscle of the atrium, the whole chamber contracts, forcing the blood inside the chamber past the tricuspid valve and into the right ventricle. The electrical impulse contacts another electrical node, the atrioventricular node (AV node), and stimulates the ventricle to contract, now forcing blood into the pulmonary arteries and toward the lungs. Almost simultaneously the left atrium and then the left ventricle contract, forcing oxygenated blood into the arteries to circulate through the body. This entire cycle of signal transmission and contraction of the heart’s chambers (with ensuing relaxation as well) takes only about one second! You can see that if the electrical signal malfunctions, it can affect the entire blood and oxygen pumping system.

Blood pressure 

Blood pressure is controlled mainly by the contraction and relaxation of the heart muscle, the shape and size of arteries, and proper kidney function. The most important cause of high blood pressure is constriction of the many miles of small arteries, called arterioles. The kidneys also affect blood pressure since they control the amount of fluids in the body by retaining or excreting water and certain minerals in solution in the blood, such as potassium and sodium; when water is retained, the volume of blood increases and this causes blood pressure to rise. Increased fluid volume and arteriole constriction are additive and raise blood pressure even more.

How the heart can break down

Damage to any one of the components of the heart can cause it to lose pumping efficiency, resulting in insufficient delivery of oxygen to various organs and especially to the heart itself. Breakdowns can occur in the delivery of electrical signals to the heart muscle; in the mechanical pumping action of the heart, including malfunctioning of the heart valves; and in the coronary arteries that make up the circulatory network of blood vessels that feed the heart muscle.

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