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Sabtu, 17 Oktober 2009

Blood Supply of the Heart

Heart Structure and Blood Supply

It seems odd that the tissues making up the heart must have their own separate blood supply. You might think that the torrent of blood rushing through the heart every minute would more than adequately meet the needs of the organ. The walls of the heart, however, consist of layers of specialized muscle. These walls are quite thick—the wall of the left ventricle is often over 1 inch thick. Since the lining of the heart is watertight, the blood cannot seep through the layers of muscle to provide the nourishment essential to these constantly working masses. Blood is carried through the muscle layers that form the heart wall by means of the two coronary arteries. These two small vessels branch off the aorta just after it leaves the heart and curl back across the surface of the chambers, sending twigs through the walls (Fig. 4-1).
The coronary arteries are so named because of the supposed resemblance to a crown or “corona” of the little arteries as they encircle the heart. These arteries divide into smaller and smaller branches, like all blood vessels in the body, until they become so small that only one blood cell at a time can move through them. At this point the vessels are called capillaries. After the blood has passed through the capillaries, and the tissues have extracted the needed oxygen, it returns by way of veins, which become larger and larger until they, like all other veins in the body, empty into the right atrium. The veins from the wall of the heart, or coronary veins, empty into the right atrium through a structure called the coronary sinus.
The blood supply of the tissues in the wall of the heart is not very good; thousands of people die every year because of this curious fact. Most organs and tissues of the body have a “reserve” or collateral blood supply. Each finger, for instance, has two arteries, one on each side. These arteries are connected by many cross-channels, or collateral vessels. If the artery is cut on one side, the collateral or cross-connections from the artery on the other side would probably provide sufficient blood to maintain life in the tissues of the finger. The same “safety” feature is true in most of the major areas of the body. It is not true in the wall of the heart.
The coronary arteries tend to be end arteries, meaning that each branch follows its own course to some area of the heart muscle with relatively few connections to other branches nearby. If one of these coronary branches is plugged by hardening or by a blood clot, the muscle that depends on it for blood will die. A form of gangrene actually sets in. (Some people's coronary arteries have many more cross-connections than others. The more of these cross-connections an individual has, the less likely he or she is to die of coronary artery disease. In 10,000 or 20,000 years the process of evolution may result in a race with a good coronary blood supply by virtue of the early death of those without it.)
The names of the chief branches of the coronary arteries are important because they will be used repeatedly in this book. Learn them now; they're very simple.
There are two main coronary arteries leading out of the aorta—the right and left coronary arteries. After about an inch, the left coronary artery divides into two principal branches. The left anterior descending branch comes down the front of the heart, roughly along the septum between the two ventricles. The circumflex branch of the left coronary artery coils around the left side and back of the heart. The right coronary artery divides into a number of branches that course through the right chambers of the heart as well as through a large part of the left ventricle.

Note: There are four coronary arteries to remember:
The left main coronary artery (before it divides): LMCA.
The right coronary artery: RCA.
The left anterior descending branch of the left main coronary artery: LAD.
The circumflex branch of the left main coronary artery: LCA or LCirc.

Pumping Action of the Heart

Blood Flow Through the Heart
Blood is pumped through the chambers of the heart and out through the great vessels by a simple squeezing action of the heart chambers. You have probably seen a bulb syringe with a glass nozzle like the one pictured in Figure 3-1. Suppose it is full of water. If you squeeze forcefully, expelling the water, you would be imitating the contraction of a heart chamber. This is called systole (sis-toe-lee). After the syringe had been emptied, imagine that you placed the nozzle in a container of water and let the bulb expand so that it filled. This is what a heart chamber does when it relaxes and fills with blood. The movement is called diastole (die-as-toe-lee). You can picture the process by holding your left hand over your right, fists clenched. If your left hand represents the atria, your right hand will represent the ventricles. Now clench your left fist (the atria) while opening your right fist (the ventricles). This is what happens during atrial systole when the atria are pumping blood down into the ventricles. Next, open your left fist and clench your right. This is what happens during ventricular systole when the ventricles are pumping blood out into the two great arteries and the atria are refilling. By alternately opening and clenching your two fists you can similate the coordinated beat of the heart.
Note: The cycle of a heartbeat, in other words, goes through these stages:
Atrial systole: The atria contract, forcing the blood down into the ventricles.
Ventricular systole: The ventricles contract, forcing the blood out the pulmonary artery and aorta.
Atrial diastole: This starts during ventricular systole as the atria begin refilling with blood from the great veins.
Ventricular diastole: This takes place during atrial systole as blood from the atria fills the ventricles.

The rhythmic contraction and relaxation of the ventricles does the work of pumping the blood: atrial contraction is much less important and, in fact, many patients live for years without any pumping action from the atria. If the ventricles stop beating, death follows within minutes.

Valves of the Heart


Valve Structure and Function

Like any pump, the heart has valves to keep the blood flowing in the right direction. Proper function of these small flaps of tissue spells the difference between good health and sickness, and often between life and death.
Almost everyone is familiar with the word valve. Very few people, however, really know what a valve is or what it does. Imagine pumping water through a pipe with a farm pump. To keep the water from flowing back toward the pump between strokes, you could place a valve in the pipe leading out of the pump. The simplest kind of valve would consist of two semicircular flaps hinged to open only one way—forward with the flow of water. These flaps would close the pipe completely when they swung shut. When the water flowed forward from the pump, the flaps of the valve would swing open allowing the water to pass. Between strokes the valves would snap shut if any water attempted to flow back toward the pump (Fig. 2-1).

Note: The heart is equipped with four sets of valves that function on this simple principle:
tricuspid valve
mitral valve
pulmonic valve
aortic valve

The valves between the atria and ventricles are called the atrioventricular (AV) valves. The AV valve leading into the right ventricle has three flaps and is called the tricuspid valve (a cusp is a valve flap or leaflet).
The AV valve that swings into the left ventricle is called the mitral valve. (It has two cusps and therefore looks something like a bishop's miter.)
Each of the outlet valves from the ventricles has three cusps. The valve at the entry to the pulmonary artery is called the pulmonic valve. The valve at the entry to the aorta is called the aortic valve.
As stated, an AV valve is located between each atrium and ventricle (Fig. 2-2). This valve opens downward into the ventricle. During diastole, or relaxation, the valves swing open, allowing the blood to flow down into the ventricles. When the ventricles contract, these valves snap shut, preventing any blood from flowing back up into the atria.



A valve is also located at the outlet from each ventricle into the great vessel leaving the chamber. When the ventricles contract, these valves are forced open; the blood rushes into the pulmonary artery and the aorta. When the ventricles relax, the valves close, shutting off any backward flow into the ventricles.
If the heart is to function efficiently, these valves must be absolutely watertight, or more properly, bloodtight. Further, they must open freely and widely to let the blood flow forward with the pumping action of the heart. If the valves leak or if they are partly closed by adhesions or hardening, the heart works against a mechanical load, often an impossible and fatal load, as will be discussed in later chapters.
Layers of the Heart
The heart does not simply hang freely in the chest cavity; around it is a loose protective sack of tissue called the pericardium. The heart lies inside this sack, which is loose enough to permit the heart to beat easily. Picture a turnip held in a heavy, double thickness plastic bag. This is about the way the heart looks inside the pericardium (Fig. 2-3).
If the pericardium is cut open, the surface of the heart itself appears shiny and reddish in color. You can actually peel away a thin, shiny membrane from the outer surface of the heart. This membrane is called the epicardium. The mass of the heart is muscle; under the epicardium is a thick layer of muscle called the myocardium, which forms the actual working part of the heart. The myocardium is thickest in the left ventricle; it is thinnest in the atria. The cells in the myocardium are a specialized type of muscle, different from anything else in the body.
The inside of the heart, or cavity, is lined with another smooth, shiny membrane much like the inside surface of the cheek. This thin membrane, called the endocardium, covers the inside of the chambers of the heart. It also covers the heart valves and the small muscles associated with the opening and closing of these valves.

Kamis, 15 Oktober 2009

Structure and Function of the Normal Heart

Before you begin to learn about heart disease, you must learn how the normal heart is constructed and how it functions. This is easier than you might think, because the heart is a surprisingly simple organ. An hour's easy reading will give you all the information you need to begin.
The Chambers of the Heart and their Connections
The heart is a hollow organ divided into four chambers, two on the top and two on the bottom (Fig. 1-1). Study this simple diagram until you know it as well as your own name: it's basic to everything else in the book.
The top two chambers are thin-walled structures that act primarily as holding chambers for the blood. They are called atria. This is the plural of the Latin word atrium, meaning “anteroom” or “porch,” and, in fact, these chambers do act as entryways to the great chambers below. The ventricles are large, thick-walled chambers that do the real work of pumping the blood. (This name comes from the Latin ventriculum, meaning a “cavity” or “pouch.”)
Look again at Figure 1-1 and note the wall, or septum, that divides the left atrium from the right atrium and the left ventricle from the right ventricle. This wall of tissue is much like the septum in your nose that separates the two nostrils. The important thing to remember about the heart's septum is that it is absolutely watertight, or, more properly, “bloodtight.” Normally, no blood can pass through this septum from one side to the other. (It took the human race about 4,000 years to discover this simple fact. The ancient Greeks and Romans were convinced that blood somehow oozed through the septum from one side to the other. It doesn't.)
Physicians commonly refer to the right atrium and right ventricle together as the right heart and to the left atrium and left ventricle as the left heart.


The Motion of the Blood Through the Heart
The function of the heart can be described as a simple pump that forces blood forward by squeezing, in exactly the way that a bulb syringe forces out fluid when it's compressed.
The alert reader will at once ask, “If the blood doesn't flow from one side of the heart to the other through the septum, how does it ever move forward?” The answer to that question eluded philosophers and scientists until the English medical doctor William Harvey, in the early seventeenth century, discovered the simple circuit that is the basis of all modern cardiology.
The blood moves from the right heart to the left heart by way of the lungs. In other words, the right heart pulls the blood out of the veins and pumps it into the lungs. The left heart pulls the blood out of the lungs and pumps it on to the body.
(The outraged squalling of Harvey's contemporaries and the hoots of disbelief that greeted this profound truth are amusing to contemplate; they are also a little frightening.) Thus the heart and lungs together form a machine that takes oxygen out of the air, dissolves it in the blood, and pumps it to the tissues of the body.

Back to Structure: How Are the Heart and Lungs Connected?
The blood that has completed its course through the tissues of the body flows back to the heart through the veins. The veins come together, growing larger, like streams combining into a river, until they end in two great veins that empty into the top and bottom of the right atrium. The word cava in Latin refers to something large or cavelike; hence the vein that empties into the top of the right atrium is called the superior vena cava, or, literally, “large top vein.” The great vein that empties into the bottom of the right atrium is logically called the inferior vena cava (Fig. 1-2). Blood flows from the right atrium down into the right ventricle and out to the lungs through the pulmonary artery (Fig. 1-3).




Within the lungs, the pulmonary artery branches into ever smaller arteries until it ends in a mass of capillaries—tiny vessels just wide enough to let one blood cell through at a time (Fig. 1-4). After the blood has been oxygenated it flows back to the heart through the four veins that empty into the left atrium (Fig. 1-5). Since these veins flow from the lungs to the heart they are called the pulmonary veins.




From the left atrium, blood flows down into the left ventricle and then out the aorta to the body (Fig. 1-6).
You must be thoroughly familiar with this circuit and with the names and function of the great vessels of the heart and lungs. The great vessels of the heart is the term used to include both arteries and veins.
Note: The great vessels of the heart are as follows:
The superior and inferior vena cavae, that empty all the blood from the body into the right atrium.
The pulmonary artery, which carries blood from the right ventricle to the lungs.
The pulmonary veins, which carry oxygenated blood from the lungs to the left atrium.
The aorta, or great artery, which carries the oxygenated blood out of the left ventricle to the body.