Page 728 - Fundamentals of anatomy physiology

Figure 20–4 The Heart Wall. Pericardial cavity Chapter 20 The Heart 715
Heart wall
Myocardium Parietal pericardium
(cardiac muscle tissue) Dense fibrous layer
Areolar tissue
Cardiac muscle cells Mesothelium
Connective tissues Artery
Vein
Endocardium
Endothelium Epicardium
Areolar tissue (visceral pericardium)
Mesothelium
Areolar tissue

Atrial a A diagrammatic section through the heart wall, showing the
musculature relative positions of the epicardium, myocardium, and
endocardium. The proportions are not to scale; the thickness
Ventricular of the myocardial wall has been greatly reduced.
musculature

b Cardiac muscle tissue forms Internal Anatomy and Organization
concentric layers that wrap around the
atria or spiral within the walls of the ventricles. Next let’s examine the major landmarks and structures visible

Cardiac Muscle Tissue 20on the interior surface of the heart. In a sectional view, you can

As noted in Chapter 10, cardiac muscle cells are interconnected see that the right atrium communicates with the right ventricle,
by intercalated discs (Figure 20–5). At an intercalated disc, the and the left atrium with the left ventricle (Figure 20–6a,c). The
interlocking membranes of adjacent cells are held together by atria are separated by the interatrial septum (septum, wall),
desmosomes and linked by gap junctions (Figure 20–5b). Inter- and the ventricles are separated by the much thicker interven-
calated discs transfer the force of contraction from cell to cell tricular septum. Each septum is a muscular partition. Atrio-
and propagate action potentials. Table 20–1 provides a quick ventricular (AV) valves are folds of fibrous tissue that extend
review of the structural and functional differences between into the openings between the atria and ventricles. These valves
cardiac muscle cells and skeletal muscle fibers. Histological permit blood to flow only in one direction: from the atria to
characteristics that distinguish cardiac muscle cells from skel- the ventricles.
etal muscle fibers include (1) small size; (2) a single, centrally
located nucleus; (3) branching interconnections between cells; The Right Atrium
and (4) the presence of intercalated discs.
The right atrium receives blood from the systemic circuit
&T i p s T r i c k s through the two great veins: the superior vena cava (VE. -na
The term intercalated means “inserted between other ele- KA. -vuh; venae cavae, plural) and the inferior vena cava. The
ments.” So, intercalated discs appear to have been inserted superior vena cava opens into the posterior and superior por-
between cardiac muscle cells. tion of the right atrium. It delivers blood to the right atrium
from the head, neck, upper limbs, and chest. The inferior vena
cava opens into the posterior and inferior portion of the right
atrium. It carries blood to the right atrium from the rest of the
trunk, the viscera, and the lower limbs. The cardiac veins drain-
ing the myocardium return blood to the coronary sinus, a