Page 36 - Cardiac Nursing

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12 PA R T I / Anatomy and Physiology

myocardium and much of the interventricular septum. The left The atrial circumflex branch is usually small in caliber but
anterior descending artery appears to be a continuation of the left sometimes is as wide as the remaining portion of the circumflex.
main coronary artery. It passes to the left of the pulmonic valve re- It runs along the left AV groove, perfusing the left atrial wall.
gion, courses in the anterior interventricular sulcus to the apex, In 45% of cases, the sinus node artery originates from the ini-
and then courses around the apex to terminate in the inferior por- tial portion of the circumflex; it runs cranially and dorsally, to the
tion of the posterior interventricular sulcus. Occasionally, the pos- base of the superior vena cava in the region of the sinus node. 31
terior descending branch of the right coronary artery extends This artery perfuses portions of the left and right atria as well as
around the apex from the posterior surface and the left anterior the sinus node.
descending artery ends short of the apex. The major branches of There are between one and four obtuse marginal branches.
the left anterior descending artery, in the order in which they These branches vary greatly in size. They run along the ventricu-
branch, are the following: lar wall laterally and posteriorly, toward the apex, along the obtuse
margin of the heart. The marginal branches supply the obtuse
1. First diagonal branch margin of the heart and the adjacent posterior wall of the left ven-
2. First septal branch tricle above the diaphragmatic surface.
3. Right ventricular branch The posterolateral branches arise from the circumflex artery in
4. Minor septal branches 80% of cases. These branches originate in the terminal portion
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5. Second diagonal branch of the circumflex artery and course caudally and to the left on the
6. Apical branches posterior left ventricular wall, supplying the posterior and di-
aphragmatic wall of the left ventricle.
The first diagonal branch is usually a large artery. It originates
close to the bifurcation of the left main coronary artery and passes The posterior descending and AV nodal arteries occasionally
diagonally over the free wall of the left ventricle. It perfuses the arise from the circumflex. When they do, the entire septum is sup-
high lateral portion of the left ventricular free wall. Several smaller plied by branches of the left coronary artery.
diagonal branches may exit from the left side of the left anterior
descending artery and run parallel to the first diagonal branch. Coronary Capillaries
The one referred to as the second diagonal branch takes its origin
approximately two thirds of the way from the origin to the termi- Blood passes from arteries into arterioles, then into capillaries,
nation of the left anterior descending artery. This second diagonal where exchange of oxygen, carbon dioxide, metabolic compounds,
branch perfuses the lower lateral portion of the free wall to the and waste materials takes place. The heart has a dense capillary net-
2
apex. work with approximately 3,300 capillaries per mm or approxi-
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The number of septal branches varies. The first septal branch is mately 1 capillary per muscle fiber. Blood flow through coronary
the first to exit the left anterior descending artery. The others are capillaries is regulated according to myocardial metabolic needs.
referred to as minor septal branches. The septal branches exit at a When myocardial cells hypertrophy, the cell radius increases.
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90-degree angle. They then course into the septum from front to The capillary network; however, does not appear to proliferate.
the back and caudally. Together, the septal branches perfuse two The same capillaries must perfuse a larger tissue mass. The diffu-
thirds of the upper portion of the septum and most of the inferior sion distance is increased. Thus, with hypertrophy, the mass of tis-
portion of the septum. The remaining superoposterior section of sue to be perfused is increased but the efficiency of exchange is di-
the septum is supplied by branches from the posterior descending minished.
artery, which usually derives from the right coronary artery.
There can be one or more right ventricular branches. One Coronary Veins
branch runs toward the conus branch of the right coronary artery;
it can anastomose into the circle of Vieussens. Most of the venous drainage of the heart is through epicardial
The final branches are the apical branches. These branches per- veins. The large veins course close to the coronary arteries. Two
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fuse the anterior and diaphragmatic aspects of the left ventricular veins sometimes accompany an artery. The major veins feed into
free wall and apex. the great cardiac vein, which runs alongside the circumflex artery,
becomes the coronary sinus, and then empties into the right
Circumflex Artery. The circumflex artery supplies blood to atrium (Fig. 1-13). An incompetent (incompletely shut) semilu-
parts of the left atrium and left ventricle. In 45% of cases, the cir- nar valve, called the valve of Vieussens, marks the junction be-
cumflex artery supplies the major perfusion of the sinus node; in tween the great cardiac vein and the coronary sinus. A similar
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10% of cases, it supplies the AV node. The circumflex artery ex- structure, the Thebesian valve, is also incompetent and is found at
its from the left main coronary artery at a near-right angle and the entry of the coronary sinus into the right atrium. Venous
courses posteriorly in the AV groove toward, but usually not blood from the right ventricular muscle is drained primarily by
reaching, the crux. If the circumflex reaches the crux, it gives rise two to four anterior cardiac veins that empty directly into the
to the posterior descending artery. In the 15% of cases in which right atrium, bypassing the coronary sinus (Fig. 1-12).
this occurs, the left coronary artery supplies the entire septum and The Thebesian veins empty directly into the ventricles
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possibly the AV node. The branches of the circumflex artery, in (Fig. 1-14). These are more common on the right side of the
order of origin, are as follows: heart, where the pressure gradient is favorable for such flow. Only
a small amount of venous blood is returned directly to the left
1. Atrial circumflex branch ventricle. When blood is returned to the left ventricle, this flow is
2. Sinus node artery a component of physiologic shunt, or unoxygenated blood enter-
3. Obtuse marginal branches ing the systemic circulation. Many collateral channels are found in
4. Posterolateral branches the venous drainage system.