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Figure

(a) A normal ECG. In this set of drawings (b-h), the yellow areas of the hearts indicate where depolarization is occurring, and the green areas indicate where tissues are repolarizing; the portion of the ECG pattern produced at each step is shown by the continuation of the line on the graph paper.

Figure

A prolonged QRS complex may result from damage to the A-V bundle fibers.

Figure

A prolonged QRS complex may result from damage to the A-V bundle fibers.

Two other factors that influence heart rate are temperature change and certain ions. Rising body temperature increases heart action, which is why heart rate usually increases during fever. On the other hand, abnormally low body temperature decreases heart action.

Of the ions that influence heart action, the most important are potassium (K+) and calcium (Ca+2). Potassium affects the electrical potential of the cell membrane, altering its ability to reach the threshold for conducting an impulse (see chapter 10, p. 376). The sar-coplasmic reticula of cardiac muscle fibers have less calcium than do the sarcoplasmic reticula of skeletal muscle fibers. Therefore, cardiac muscle depends more on extracellular (blood-borne) calcium than does skeletal muscle (see chapter 9, p. 303). Although homeostatic mechanisms normally maintain the concentrations of these ions within narrow ranges, these mechanisms sometimes fail, and the consequences can be serious or even fatal. Clinical Application 15.2 examines abnormal heart rhythms.

Homeostatic Mechanisms Skeletal System
— Sympathetic trunk

Figure

Autonomic nerve impulses alter the activities of the S-A and A-V nodes.

Shier-Butler-Lewis: I IV. Transport I 15. Cardiovascular System I I © The McGraw-Hill

Human Anatomy and Companies, 2001

Physiology, Ninth Edition

Arrhythmias

Each year, thousands of people die from a fast or irregular heartbeat. These are types of altered heart rhythm called arrhythmia. John Thomas was almost one of them — several times.

In a crowded Boston train in December 1990, thirty-six-year-old Thomas lost consciousness for about eight seconds, as his heart fibrillated, small areas of the myocardium contracting in an uncoordinated fashion (fig. 15A). As a result, the myocardium failed to contract as a whole, and blood was no longer pumped.

Fibrillation is cardiac chaos. Atrial fibrillation is not life threatening because the ventricles still pump blood, but ventricular fibrillation, which struck Thomas, is often deadly. Ventricular fibrillation can be caused by an obstructed coronary artery, toxic drug exposure, electric shock, or traumatic injury to the heart or chest wall.

Fortunately, Thomas had a device called a defibrillator surgically implanted a year earlier, after he suffered a cardiac arrest. The device sends out an electrical jolt during fibrillation, which interrupts the abnormal heart rhythm, allowing the heart to resume a normal beat. Thomas's defibrillator saves his life several times a year. Today, many public places, such as airplanes, have defibrillators.

An abnormally fast heartbeat, usually more than one hundred beats per minute, is called tachycardia. Increase in body temperature, nodal certain drugs or hormones, heart disease, excitement, exercise, anemia, or shock can all cause tachycardia. Figure 15B shows the ECG of a tachy-cardic heart.

Bradycardia means a slow heart rate, usually fewer than sixty beats per minute. Decreased body temperature, nodal stimulation by parasympathetic impulses, or certain drugs may cause bradycardia. It also may occur during sleep. Figure 15C shows the ECG of a bradycardic heart. Athletes sometimes have unusually slow heartbeats because their hearts have developed the ability to pump a greater-than-normal volume of blood with each beat. The slowest heartbeat recorded in a healthy athlete was 25 beats per minute!

A premature beat occurs before it is expected in a normal series of cardiac cycles. Cardiac impulses origi nating from unusual (ectopic) regions of the heart probably cause a premature beat. That is, the impulse originates from a site other than the S-A node. Cardiac impulses may arise from ischemic tissues or from muscle fibers irritated by disease or drugs.

A heart chamber flutters when it contracts regularly, but very rapidly, such as 250-350 times per minute. Although normal hearts may flutter occasionally, this condition is more likely to be due to damage to the myocardium (fig. 15D).

Any interference or block in cardiac impulse conduction may cause arrhythmia, the type varying with the location and extent of the block. Such arrhythmias arise because certain cardiac tissues other than the S-A node can function as pacemakers.

The S-A node usually initiates seventy to eighty heartbeats per minute, called a sinus rhythm. If the S-A node is damaged, impulses

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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