The total cross-sectional area between the aorta and the capillaries increases about 500-fold according to Table 15.1. The veins and venules contain the greatest volume of blood in the systemic vascular system (64%), while only 6% of total blood volume exists in the capillaries and 14% in the aorta, arteries, and arterioles. On the other hand, the blood volume in the pulmonary vascular bed is equally distributed among the arterial, capillary, and venous vessels. The venae cavae have a larger cross-sectional area compared to the aorta, resulting in slower flow velocity in the venae cavae. This information is supported by Figure 15.3, which provides data on blood volume distribution. Patients with hyperthyroidism (Graves’ disease) often experience elevated basal metabolism and arteriolar vasodilation. This reduction in arteriolar resistance leads to pulsatile flow in the capillaries, as observed in the fingernail beds of affected individuals.

After studying this chapter, students should be able to answer the following questions:
1. How does the action potential contribute to excitability and contraction in heart muscle?
2. What is automaticity, and how does it differ from excitability? How do abnormalities in these properties contribute to arrhythmias?
3. What is the structural basis of the electrocardiogram?
4. How are the concepts of preload and afterload, initially developed for skeletal muscle, applied to the contraction of the heart?
5. What role does Ca++ play in excitation-contraction coupling?
6. What changes occur in atrial and ventricular pressures and volumes during a cardiac cycle, and how do they relate to the electrocardiogram?
7. How does the relationship between end-diastolic volume and left ventricular developed pressure define the Frank-Starling law of the heart and regulate the force of cardiac contraction?
8. What is the pressure-volume loop of the left ventricle, and how does it define changes in left ventricular function?
9. How is cardiac metabolism linked to O2 consumption, and how are these processes affected by changes in cardiac work?

The heart cells, similar to neurons, are excitable and generate action potentials, which initiate contraction and determine heart rate. Disorders in electrical activity can lead to serious and sometimes life-threatening disturbances in cardiac rhythm. This section discusses the electrical properties of cardiac cells and how they are responsible for the electrocardiogram (ECG). The initiation of contraction based on the electrical properties of cardiac cells is explored in a later section.

The Cardiac Action Potential.
Table 15.1 ) indicate that between the aorta and the capillaries, the total cross-sectional area increases about 500-fold. The volume of blood in the systemic vascular system is greatest in the veins and venules (64%). Only 6% of total blood volume exists in the capillaries, and 14% of total blood volume is found in the aorta, arteries, and arterioles. In contrast, blood volume in the pulmonary vascular bed is about equally divided among the arterial, capillary, and venous vessels. The cross-sectional area of the venae cavae is larger than that of the aorta. Therefore, the velocity of flow is slower in the venae cavae than in the aorta (see
Fig. 15.3
Aortaandlargearteries3006.0Smallarteries4008.0Capillaries3006.0Smallveins230046.0Largeveins90018.0Total420084.0
Arteries1302.6Capillaries1102.2Veins2004.0Total4408.8Heart (End-Diastole) 3607.2
DatafromBoronWF,BoulpaepEL.Medical Physiology. 2nded.Philadelphia:ElsevierSaunders;2009.*Valuesrefertoa70-kgwoman.
Inapatientwithhyperthyroidism(Graves’ disease), basalmetabolismiselevatedandoftenassociatedwitharteriolarvasodilation.Thisreductioninarteriolarresistancediminishesthedampingeffectonpulsatilearterialpressureandismanifestedaspulsatileflowinthecapillaries,asobservedinthefingernailbedsofpatientswiththisailment.
Upon completion of this chapter, the student should be able to answer the following questions: 1.
How does the action potential contribute to excitability and contraction in heart muscle?
2.
What is automaticity, and how does it differ from excitability? How do derangements of these properties contribute to arrhythmias?
3.
What is the structural basis of the electrocardiogram?
4.
How are the concepts of preload and afterload, developed for skeletal muscle, applied to the contraction of the heart?
5.
What is the role of Ca++ in excitation-contraction coupling?
6.
What changes occur in atrial and ventricular pressures and volumes during a cardiac cycle, and what is their temporal relation to the electrocardiogram?
7.
How does the relation between end-diastolic volume and left ventricular developed pressure define the Frank-Starling law of the heart and regulate the force of cardiac contraction?
8.
What is the pressure-volume loop of the left ventricle, and how does it define changes in left ventricular function?
9.
How is cardiac metabolism linked to O2 consumption, and how are these processes affected by changes in cardiac work?
Electrical Properties of the Heart
The cells of the heart, like neurons, are excitable and generate action potentials. These action potentials initiate contraction and thus determine the heart rate. Disorders in electrical activity can induce serious and sometimes lethal disturbances in cardiac rhythm.
In this section, the electrical properties of cardiac cells are described. In addition, how these electrical properties account for the electrocardiogram (ECG) is considered. The initiation of contraction as a result of the electrical properties of cardiac cells is considered in a later section.
The Cardiac Action Potential