Cardiac Muscle Fibers

Recall that cardiac muscle fibers function like those of skeletal muscles, but the fibers connect in branching

Heart Transplants

When Tina Orbacz was pregnant with her second child in 1990, she attributed her increasing fatigue to her pregnant state. But a month after her son's birth, she was even more exhausted. Plus, she had lost weight during her pregnancy, not gained it. Finally, cardiologists discovered that she was suffering from heart failure due to a birth defect, called an atrial septal defect, that weakened the tissue between the atria. Because her heart was failing, and a complication of this disorder is lung failure, Tina Orbacz, at the age of twenty-four years, found herself waiting for a heart-lung transplant.

In a heart transplant, the recipient's failing heart is removed, except for the posterior walls of the right and left atria and their connections to the venae cavae and pulmonary veins. The donor heart is similarly prepared and is attached to the atrial cuffs remaining in the recipient's thorax. Finally, the recipient's aorta and pulmonary arteries are connected to those of the donor heart.

Tina was lucky. Her condition stabilized, and she was able to survive the three-year wait until a sixteen-year-old who died in an accident donated a heart and lungs. Because she was young and her cell surface molecules closely matched those of the donated heart and lungs, Tina has done extraordinarily well. Today, she exercises regularly and leads a normal life.

Because of the shortage of donor hearts and the severity of the illnesses that lead to heart failure, many potential recipients die waiting for a transplant. A mechanical half-heart, called a left ventricular assist device (LVAD), can often maintain cardiac function long enough for a donor heart to become available.

This happened to Mike Dorsey, a forty-one-year-old father of six. Although he had to stay in the hospital to wear his LVAD, the device worked so well that he was able to exercise and help with office work while awaiting his transplant. The LVAD enabled him to increase his physical fitness, which contributed to the success of his eventual heart transplant six months later. A few patients in England too ill to receive transplants are surviving with permanently implanted LVADs. ■

Aortic area

Tricuspid area

Aortic area

Tricuspid area

Functional Syncytium Heart

Pulmonary area

Bicuspid area

Figure 15.17

Thoracic regions where the sounds of each heart valve are most easily heard.

Figure 15.17

Thoracic regions where the sounds of each heart valve are most easily heard.

Pulmonary area

Bicuspid area networks. Stimulation to any part of the network sends impulses throughout the heart, which contracts as a unit.

A mass of merging cells that act as a unit is called a functional syncytium. (funk'shun-al sin-sish'e-um). Two such structures are in the heart—in the atrial walls and in the ventricular walls. These masses of cardiac muscle fibers are separated from each other by portions of the heart's fibrous skeleton, except for a small area in the right atrial floor. In this region, the atrial syncytium and the ventricular syncytium are connected by fibers of the cardiac conduction system.

O Describe the pressure changes that occur in the atria and ventricles during a cardiac cycle.

^9 What causes heart sounds? ^9 What is a functional syncytium?

□ Where are the functional syncytia of the heart located?

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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