Functional Anatomy
The heart has an exacting task because 60%, 80%, and 100% of the total volume of blood in humans, dogs, and horses, respectively, passes through the heart each minute. This efficient pumping requires coordinated contraction because asynchronous contraction (fibrillation) of heart muscle, especially of the ventricles, is ineffectual and is rapidly fatal.
The sinoatrial node, the pacemaker, generates a wave of excitation that normally spreads to the atrial muscle to reach the atrioventricular node (Figs. 7.15/2 and 7.17A and B). It has the highest rate of spontaneous activity when relieved of external stimuli but is regulated by the fine balance of accelerating sympathetic and retarding vagal inputs. Following a short delay at the atrioventricular node to permit completion of atrial contraction, the impulse then spreads to the ventricular muscle through the atrioventricular conducting tissue. Although ventricular contraction is almost synchronous, the subendocardial layer, which includes the papillary muscles, gains a slight lead.
FIG. 7.19 Branching of the left coronary artery of the heart, viewed from the left. The left auricle has been shortened. 1, Pulmonary trunk; 2, left coronary artery; 2', paraconal interventricular branch; 2", circumflex branch; 3, great cardiac vein (continued by the coronary sinus on the right side of the heart); 3', paraconal interventricular tributary of 3.
Blood enters the atria for as long as the pressure within the veins exceeds that within the heart. Several factors of uncertain and varying magnitude contribute to the venous pressure. The force exerted upstream (vis a tergo) is the summation of the following: the residual pressure imparted to the blood by ventricular contraction; the forces exerted by muscles, visceral activity, and arterial pulsation; and the contraction of the diaphragm (the so-called abdominal pump) expelling blood from the caudal vena cava and its large tributaries within the abdomen.
The downstream force (vis a fronte) oscillates between a negative aspirating effect (provided by thoracic expansion and atrial relaxation) and a positive pressure developed on atrial systole. A lateral pressure may be exerted by contraction of the muscular coat of the great veins. Gravity also plays a part, sometimes assisting and sometimes impeding flow according to posture. Much blood flows directly into the ventricles through open atrioventricular ostia, and only a "topping-up" effect is exerted by the atrial contraction, which coincides with the last stage of ventricular relaxation. When the atria do contract, some blood may reflux into veins (despite the conjectured throttle mechanism already mentioned) as evidenced by a jugular pulse mostly seen in cattle.The pulmonary and aortic (arterial) valves are closed during ventricular relaxation. Ventricular contraction closes the atrioventricular valves while papillary muscles prevent eversion of the cusps into the atria. As the contraction develops, blood forces the arterial valves open, and the conducting arteries are expanded by this sudden input. The right ventricular lumen is squeezed in a bellows action in which the outer wall is drawn toward the septum (Fig. 7.24), and the more cylindrical left ventricle contracts radially and in length, generating a more powerful effect.
Auscultation Closure of the heart valves produces distinctive sounds that are audible on auscultation, and these sounds provide valuable information about the condition of the valves. Because of intervening tissues of varying densities, the projections of the heart valves on the chest wall are not necessarily the spots (puncta maxima) where the sounds are most clearly heard. As a rough guide, the pulmonary, aortic, and left atrioventricular valves are best auscultated over the third, fourth, and fifth ribs of the left side, and the right atrioventricular valve is best auscultated over the fourth rib on the right. The arterial valves are somewhat dorsal to the atrioventricular valves. Percussion is also used as a means of evaluating the size of the heart. The quality of cardiac dullness contrasts with the high pitch obtained when percussion is performed over the lungs. The boundary of the cardiac area is not sharply defined because the lung tissue covering the heart varies in thickness about the cardiac notch.