Anatomy

The heart is a muscular organ enclosed in a fibrous sac, the pericardium, and located in the chest (thorax). The extremely narrow space between the pericardium and the heart is filled with a watery fluid that serves as a lubricant as the heart moves within the sac.

The walls of the heart are composed primarily of cardiac muscle cells and are termed the myocardium. The inner surface of the walls—that is, the surface in contact with the blood within the cardiac chambers— is lined by a thin layer of cells known as endothelial cells, or endothelium. (As we shall see, endothelial cells line not only the heart chambers, but the entire cardiovascular system.)

As noted earlier, the human heart is divided into right and left halves, each consisting of an atrium and a ventricle. Located between the atrium and ventricle in each half of the heart are the atrioventricular (AV) valves, which permit blood to flow from atrium to ventricle but not from ventricle to atrium (Figure 14-12). The right AV valve is called the tricuspid valve, and the left is called the mitral valve.

The opening and closing of the AV valves is a passive process resulting from pressure differences across the valves. When the blood pressure in an atrium is greater than that in the ventricle separated from it by a valve, the valve is pushed open and flow proceeds from atrium to ventricle. In contrast, when a contracting ventricle achieves an internal pressure greater than that in its connected atrium, the AV valve between them is forced closed. Therefore, blood does not normally move back into the atria but is forced into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle.

To prevent the AV valves from being pushed up into the atrium, the valves are fastened to muscular projections (papillary muscles) of the ventricular walls by fibrous strands (chordae tendinae). The papillary muscles do not open or close the valves. They act only to limit the valves' movements and prevent them from being everted.

The opening of the right ventricle into the pulmonary trunk and of the left ventricle into the aorta also contain valves, the pulmonary and aortic valves, respectively (Figure 14-12) (these valves are also col-

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

Pulmonary vein

Superior vena cava

Interatrial septum

Right atrium

Right AV valv

Inferior vena cava

Pulmonary vein

Superior vena cava

Interatrial septum

Right atrium

Right AV valv

Inferior vena cava

Pulmonary Trunk Function

Aorta

Pulmonary artery

Left atrium

Pulmonary vein

Pulmonary trunk

Left ventricle

Right ventricle

Pulmonary and aortic valves

Interventricular septum

FIGURE 14-12

Diagrammatic section of the heart. The arrows indicate the direction of blood flow. %

Right ventricle

Pulmonary and aortic valves

Aorta

Pulmonary artery

Left atrium

Pulmonary vein

Pulmonary trunk

Left ventricle

Left

AV valve

Interventricular septum

FIGURE 14-12

Diagrammatic section of the heart. The arrows indicate the direction of blood flow. %

lectively referred to as the semilunar valves). These valves permit blood to flow into the arteries during ventricular contraction but prevent blood from moving in the opposite direction during ventricular relaxation (Figure 14-13). Like the AV valves, they act in a purely passive manner. Their being open or closed depends upon the pressure differences across them.

Another important point concerning the heart valves is that, when open, they offer very little resistance to flow. Accordingly, very small pressure differences across them suffice to produce large flows. In disease states, however, a valve may become narrowed so that even when open it offers a high resistance to flow. In such a state, the contracting cardiac chamber must produce an unusually high pressure to cause flow across the valve.

There are no valves at the entrances of the superior and inferior venae cavae (plural of vena cava) into the right atrium, and of the pulmonary veins into the left atrium. However, atrial contraction pumps very little blood back into the veins because atrial contraction compresses the veins at their sites of entry into the atria, greatly increasing the resistance to backflow. (Actually, a little blood is ejected back into the veins, and this accounts for the venous pulse that can often be seen in the neck veins when the atria are contracting.)

Valve partly open Valve almost completely closed

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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  • hyiab
    Is the opening and closing of valves an active or passive process?
    8 years ago

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