Chapter 12 – The Heart: Cardiac Structure & Specializations

Question 1

The human heart is a remarkably efficient, durable, and reliable pump that propels over 6000
liters of blood through the body daily and beats more than 40 million times a year, thereby
providing the tissues with a steady supply of vital nutrients and facilitating the excretion of waste
products.

The major categories of cardiac diseases considered in this chapter include :

• congenital heart abnormalities,
• ischemic heart disease,
• heart disease caused by systemic hypertension, heart disease caused by pulmonary diseases (cor pulmonale),
• diseases of the cardiac valves, and
• primary myocardial diseases.
• A few comments about pericardial diseases and cardiac neoplasms as well as cardiac transplantation are also offered.

Before considering details of
specific conditions, we will briefly review the anatomy of the normal heart, because many
diseases cause changes in the size and appearance of one or more of its components. We will
also discuss the principles of cardiac hypertrophy and failure, the common end points of many
different types of heart disease; these are essential for later discussion of disease processes.

Question 2

What is the weigt of the heart?

Answer 2

Heart weight varies with body height and weight; it normally averages approximately 250 to 300
gm in femalesand300 to 350 gm in males or roughly 0.4% to 0.5% of body weight.

Question 3

What is the usual thickness of the free wall of the right ventricle and left ventricle?

Answer 3

The usual
thickness of the free wall of the right ventricle is 0.3 to 0.5 cm, and that of the left ventricle 1.3
to 1.5 cm.

LV> RV

Increases in cardiac size and weight accompany many forms of heart disease

Question 4

Greater heart weight or ventricular thickness indicates ____________

Answer 4

hypertrophy

Question 5

What is dilation of the heart?

Answer 5

enlarged chamber size implies dilation.

Question 6

What is cardiomegaly?

Answer 6

An increase in cardiac weight or size or both (resulting from hypertrophy
and/or dilation) is termed cardiomegaly.

Question 7

The efficient pumping of blood by the heart to the entire body requires the normal function of
each of its key components:

Answer 7

• the myocardium,
• valves,
• conduction system,
• and coronary arterial circulation

Question 8

The pumping function of the heart is accomplished by the cardiac muscle, the myocardium,
composed primarily of a collection of specialized muscle cells called cardiac myocytes.

Question 9

Ventricular myocytes are arranged circumferentially in a spiral orientation and contract during
systole and relax during diastole.

The contractile unit is the sarcomere, an orderly arrangement of thick filaments composed principally of :

Answer 9

• myosin,
• thin filaments containing actin, and
• regulatory proteins such as troponin and tropomyosin.

Question 10

What is responsible for the striated appearance of the cardiac muscle?

Answer 10

Cardiac muscle cells contain strings of sarcomeres
in series, which are responsible for the striated appearance of these cells.

Contraction depends
on a coordinated ratcheting mechanism whereby each myosin filament pulls the neighboring
actin filaments toward the center of the sarcomere, leading to the shortening of the myocyte

Question 11

What determines the amount of force that is generated by the cardiac muscle?

Answer 11

The amount of force generated is determined by the distance each sarcomere shortens.

Moderate ventricular dilation during diastole increases the extent of sarcomere shortening and the force of contraction during systole.

With further dilation, however, there is a point at which
effective overlap of the actin and myosin filaments is reduced and the force of contraction
decreases sharply, as occurs in heart failure.

Question 12

Describe the difference between atrial myocyte.

Answer 12

Atrial myocytes are generally smaller and arranged more haphazardly than their ventricular
counterparts.

Some atrial cells have distinctive electron-dense granules in the cytoplasm called specific atrial granules; these are the storage sites of atrial natriuretic peptide.

Atrial natriuretic peptide can produce a variety of physiologic effects, including vasodilation, natriuresis, and diuresis, actions beneficial in pathologic states such as hypertension and congestive heart
failure

Question 13

What are the storage site of ANP?

Answer 13

Some atrial cells have distinctive electron-dense granules in the cytoplasm called specific atrial granules; these are the storage sites of atrial natriuretic peptide.

Atrial natriuretic
peptide can produce a variety of physiologic effects, including vasodilation, natriuresis, and
diuresis, actions beneficial in pathologic states such as hypertension and congestive heart
failure

Question 14

What are the atrial natriuretic peptide physiological effects?

Answer 14

Atrial natriuretic
peptide can produce a variety of physiologic effects, including :

• vasodilation,
• natriuresis, and
• diuresis,

actions beneficial in pathologic states such as hypertension and congestive heart failure

Question 15

What are intercalated disc?

Answer 15

• *Functional integration of cardiac myocytes** is mediated by structures called intercalated discs,
• *which link individual cells and contain specialized intercellular junctions that permit both**
• *mechanical and electrical (ionic) coupling.**

Question 16

What is found inside the intercalated disc?

Answer 16

Within the intercalated discs, gap junctions facilitate
synchronous myocyte contraction through electrical coupling by permitting relatively
unrestricted passage of ions across the membranes of adjoining cells.

Abnormalities in the
spatial distribution of gap junctions and their respective proteins in ischemic and myocardial
heart disease may contribute to electromechanical dysfunction (arrhythmia) and heart
failure.

Question 17

What are the four cardiac valves?

Answer 17

The four cardiac valves (tricuspid, pulmonary, mitral, and aortic) maintain unidirectional blood flow through the heart.

Question 18

The function of the valves depends on the integrity of what?

Answer 18

leaflets or cups

Their function depends on the mobility, pliability, and structural integrity of their delicate flaps, called leaflets (in the tricuspid and mitral valves) or cusps (in the aortic and pulmonary valves, also known as the semilunar valves).

Question 19

All four valves have a similar, layered architecture:

Answer 19

All four valves have a similar, layered architecture:

• a dense collagenous core (fibrosa) close to the outflow surface and continuous with valvular supporting structures,
• a central core of loose connective tissue (spongiosa),
• a layer rich in elastin (ventricularis or atrialis depending on which chamber it faces) below the inflow surface, and an endothelial covering.

The collagen is responsible for the mechanical integrity of a valve.

The valve is populated throughout by interstitial cells, which produce and continuously repair the extracellular matrix (especially collagen), allowing the valve to respond and adapt to changing mechanical conditions.

Question 20

The valve is populated by what cells and what is the function?

Answer 20

The valve is populated throughout by interstitial cells, which produce and continuously repair the extracellular matrix (especially collagen), allowing the valve to respond and adapt to changing mechanical conditions.

Question 21

The function of the semilunar valves depends on the integrity and coordinated movements of
what?

Answer 21

The function of the semilunar valves depends on the integrity and coordinated movements of
the cuspal attachments.

Thus, dilation of the aortic root can hinder coaptation of the aortic valve cusps during closure, yielding regurgitation.

In contrast, the competence of the
atrioventricular valves depends on not only the leaflets and their attachments, but also on tendinous connections to the papillary muscles of the ventricular wall.

Left ventricular dilation, a
ruptured tendon, or papillary muscle dysfunction can all interfere with mitral valve closure,
leading to regurgitation.

Question 22

Because the cardiac valves are thin enough to be nourished by diffusion from the heart’s blood, normal
leaflets and cusps have only scant blood vessels limited to the proximal portion.

T or F

Answer 22

True

Question 23

Pathologic
changes of valves are largely of three types:

Answer 23

• damage to collagen that weakens the leaflets, exemplified by mitral valve prolapse;
• nodular calcification beginning in interstitial cells, as in calcific aortic stenosis;
• and fibrotic thickening, the key feature in rheumatic heart disease (see later).

Question 24

Coordinated contraction of the cardiac muscle depends on propagation of electrical impulses,
which is accomplished by specialized excitatory and conducting myocytes within the cardiac
conduction system that regulate heart rate and rhythm.

Key components of the conduction
system include

Answer 24

• (1) the sinoatrial (SA) pacemaker of the heart, the SA node, located near the junction of the right atrial appendage and the superior vena cava;
• (2) the AV node, located in the right atrium along the atrial septum
• ; (3) the bundle of His, which courses from the right atrium to the summit of the ventricular septum; and its major divisions
• (4) the right and left bundle branches, which further arborize in the respective ventricles through the anteriorsuperior and posterior-inferior divisions of the left bundle and the Purkinje network

Question 25

Describe the cells of the conducting system of the heart.

Answer 25

The cells of
the specialized cardiac conduction system depolarize spontaneously, enabling them to function
as cardiac pacemakers.

Question 26

What sets the pace in the conducting system of the heart and why?

Answer 26

Because the normal rate of spontaneous depolarization in the SA node
(60 to 100 beats/minute) is faster than the other components, it normally sets the pace.

Question 27

What serves as the gatekeeper in the conducting system of the heart and why?

Answer 27

The AV
node serves as a kind of “gatekeeper”; by delaying the transmission of signals from the atria to
the ventricles,itensures that atria contraction precedes ventricular contraction.

Question 28

What triggers the start of cardiac cycle and controls the rate of firing of the SA node?

Answer 28

The autonomic nervous system (the same part of the nervous system involved in blood
pressure control) controls the rate of firing of the SA node to trigger the start of the cardiac
cycle.

Autonomic inputs can increase the heart rate to twice normal within only 3 to 5 seconds,
and are important in cardiac responses to exercise or other states associated with increased
oxygen demand.

Question 29

To meet their energy needs, cardiac myocytes rely almost exclusively on what?

Answer 29

To meet their energy needs, cardiac myocytes rely almost exclusively on oxidative phosphorylation, which is manifest by the abundant mitochondria that are found in these cells. [5]

Oxydative phosphorylation requires oxygen, making cardiac myocytes extremely vulnerable to ischemia .

A constant supply of oxygenated blood is thus essential for cardiac function.

Question 30

Most of the myocardium depends on nutrients and oxygen delivered via the the_________ ?

Answer 30

Most of the myocardium depends on nutrients and oxygen delivered via the the coronary arteries, which arise immediately distal to the aortic valve, initially running along the external surface of the heart (epicardial coronary arteries) and then penetrating the
myocardium (intramural arteries) .

These small penetrating arteries yield arterioles and,
ultimately, provide a rich network of capillaries enveloping individual cardiac muscle cells.

Question 31

The three major epicardial coronary arteries are

Answer 31

• (1) the left anterior descending (LAD) and
• (2) the left circumflex (LCX) arteries, both arising from branches of the left (main) coronary artery, and
• (3) the right coronary artery.

Question 32

Branches of the LAD are called _______

Answer 32

“diagonal” and “septal perforators,”

Question 33

The branches of the LCX are:

Answer 33

and those of the LCX are “obtuse marginals.”

Question 34

Most coronary arterial blood flow to the myocardium occurs during when?

Answer 34

ventricular diastole, when the microcirculation is not compressed by cardiac contraction