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.