Chapter 21 - Cardiovascular System: Heart
 
 Overview
     The heart is the pump that maintains the circulation of blood.
     This circulation of blood can be divided into two circuits:
   1. Pulmonary circuit carries deoxygenated blood from the heart to the lungs and back to the heart.
   2. Systemic circuit carries oxygenated blood from the heart to all the other peripheral tissues and back to the heart.
     Blood flows through each of these circuits in sequence.
 
Pericardium  
     The heart is located in the mediastinum in a cavity called the pericardial cavity.
     The percardium is the serous membrane lining the cavity. The pericardium is a continuous membrane but the part that attaches to the surface of the heart is the  visceral pericardium (a.k.a. epicardium) and the part that lines the outer wall of the cavity is the parietal pericardium.
     The outer surface of the parietal pericardium is reinforced by a layer of dense, irregular connective tissue called the fibrous pericardium.
     A small amount of pericardial fluid (10 to 20 ml) within the pericardial cavity reduces friction as the heart moves within the cavity.
 
Structure of Heart Wall 
  The heart wall can be divided into three layers:
  1. Epicardium (visceral pericardium) is described above
  2. Myocardium consists of multiple interlocking layers of cardiac muscle tissue with associated connective tissue, blood vessels and nerves.
  3. Endocardium is the inner lining of the heart. It is continuous with the endothelium that lines the blood vessels.
 
Cardiac Muscle Tissue 
     Cardiac muscle cells form a branching, network of cells bound strongly to one another by intercalated discs.
     Intercalated discs are regions of close attachment between cardiac cells that enable the cardiac muscles to work together during a contraction. This is because desmosomes at the discs physically bind the cell membranes; contractile fibers within the cells in turn attach to the desmosomes; and gap junctions at the discs enables rapid communication between cells.
 
Fibrous Skeleton 
     The fibrous skeleton of the heart consists of four fibrous rings that surround the valve orifices and two triangular fibrous connections between these rings, the right and left fibrous trigones. The valves include the aortic and pulmonary semilunar valves and the right and left atrioventricular valves.
     The fibrous skeleton physically supports the valves, provides independent attachment for the atrial and ventricular myocardium, and acts an electrical insulator between the atria and ventricles.
 
Orientation and Superficial Anatomy of Heart 
  1. The heart lies slightly to left of midline.
   The base of the heart is formed mainly by the left atrium, and, to a small extent, by the back part of the right atrium. The apex is the inferior rounded tip.
  2. The heart at oblique angle to longitudinal axis.
   Because of this tilt, the apex points obliquely toward the left.
   This tilt in the orientation of the heart also causes the horizontal and vertical borders of the heart to be as follows:
  1. The superior border is formed by the base.
  2. The right border is formed by the right atrium.
  3. The inferior border is formed by the inferior wall of the right ventricle.
  4. The left border is formed by the left ventricle and a small portion of the left atrium.
  3. The heart rotated slightly toward left.
   Because of this rotation:
     The surface of the heart underneath the sternum and the ribs on the left side, the sternocostal surface, is that of the right atrium and ventricle.
     The surface of the heart that rests on the diaphragm, which curves directly behind the heart, is called the diaphragmatic surface, and is formed by the posterior walls of the right and left ventricles
     The atria have thin muscular walls that are distended as they receive blood. When contracted, the anterior walls form flaps that are called auricles. A deep groove between the atria and ventricles is called the coronary sulcus. The boundary between the right and left ventricles is indicated externally by a shallow groove on the anterior surface, the anterior interventricular sulcus, and the posterior surface, the posterior interventricular sulcus.
 
Internal Anatomy and Organization of the Heart   
     The atria and ventricles are side by side and share a common wall (like adjoining apartments). The common wall between the atria is the interatrial septum and the common wall between the ventricles is the interventricular septum.
     The internal anatomy of the four chambers reflects the functional demands placed on each chamber as blood flows through it.
 
  Right Atrium 
   The right atrium receives deoxygenated blood from superior vena cava, inferior vena cava and coronary sinus.
   Parallel ridges of muscle called pectinate muscles are found in the interior of the right auricle.
   In the fetus, there is an oval opening called the foramen ovale in the interatrial septum. Forty-eight hours after birth, the foramen ovale closes and becomes a oval depression called the fossa ovalis.
  Right Ventricle  
   The deoxygenated blood from right atrium enters the right ventricle through an opening bounded by three flaps, or cusps, of the right atrioventricular (tricuspid) valve. The free edges of the cusps are attached to strings of connective tissue called the chordae tendineae. The chordae tendineae are anchored to the ventricular wall by papillary muscles that are conical projections from the wall.
   The inner surface of the right ventricles has irregular, muscular folds called trabeculae carneae.
   The moderator band is a band of muscle that extends from the interventricular septum to the anterior wall of the right ventricle.
   The superiorly right ventricle tapers into a funnel-shaped passage, the conus arteriosus, where blood is ejected into the pulmonary trunk.
   Backflow of blood is prevented by three half-moon flaps attached to the base of the pulmonary trunk that forms the pulmonary semilunar valve.
   The pulmonary trunk branches into the right and left pulmonary arteries.
  Left Atrium 
   The left (2) and right (2) pulmonary veins drain oxygenated blood from the lungs into the left atrium.
   The auricle of the left atrium lacks pectinate muscles.
   The oxygenated blood enters the left ventricle through a valve with two flaps, the left atrioventricular (a.k.a. bicuspid, mitral) valve.
  Left Ventricle 
    The left ventricle, as explained below, has a thicker wall than the right. It also differs from the right in lacking a moderator band and having more prominent trabeculae carneae.
   As oxygenated blood is ejected from the left ventricle is passes through the aortic semilunar valve and enters the ascending aorta.
   Sac-like expansions at the base of the ascending aorta are called aortic sinuses. 
   The aortic sinuses prevent the flaps of the semilunar valves from adhering to the walls of the aorta and permits blood to flow unimpeded into the right and left coronary arteries that originate from the aortic sinuses.
 
Structural Differences Between Right and Left Ventricles
     The right ventricle pushes blood through pulmonary circuit at lower pressure and its wall is thinner than left ventricle. The left ventricle needs to generate 6 to 7 times as much force and as a consequence has a much thicker wall.
      In cross section, the wall of the left ventricle is thick and circular while thinner wall of the right ventricle is like a pouch attached to the wall of the left ventricle.
 
Coronary Blood Vessels
 
   The coronary circulation supplies blood to heart tissue.
   The right and left coronary arteries, as mentioned above, come off ascending aorta at the aortic sinuses.
  Right Coronary Artery 
   The right coronary artery curves across anterior surface of heart following coronary sulcus. It branches include:
  1. Atrial branches that supply the myocardium of the right atrium.
  2. Ventricular branches include the right marginal branch that extends toward the apex along the anterior surface of the right ventricle, and the posterior interventricular branch that descends toward the apex along the posterior interventricular sulcus.
  3. Branches to the conducting system include branches to the sinoatrial and atrioventricular nodes.
   Left Coronary Artery  
   The left coronary artery supplies the left atrium and ventricle and contributes to the supply to the interventricular septum. As it reaches the anterior surface it divides into two branches:
  1. The circumflex branch curves to left in coronary sulcus.
  2. The anterior interventricular branch descends along anterior interventricular sulcus.  
   Cardiac Veins
   The great cardiac vein and middle cardiac vein are found in the anterior interventricular sulcus and posterior interventricular sulcus, respectively. Both  veins drain blood into the coronary sinus that is a thin-walled vein that lies in the posterior coronary sulcus.
 
Cardiac Cycle
   The cardiac cycle represents the endless repetition of heart contraction followed by heart relaxation. The phase of heart contraction is systole; while the phase of heart relaxation is diastole.
  Coordination of Heart Contractions
     The cardiac cycle is generated automatically because cardiac muscles possess the property of automaticity or autorhythmicity by which the stimulus for contraction originates within the cells themselves.
      In order for the four chambers of the heart to work efficiently as pumps their cycles need to be precisely coordinated. The atria need to contract prior to the ventricles and the atria and ventricles need to contract simultaneously.
     Contractions are coordinated by conducting cells that are part of the conducting system of the heart. There are two types of conducting cells:
  Nodal cells establish the rate of cardiac contraction.
  Conducting cells distribute the contractile stimulus throughout the myocardium.
  Conducting System of Heart 
     The sinoatrial node is the cardiac pacemaker of the cardiac cycle. This node is located near the entrance of the superior vena cava in the wall of the right atrium.
     This node has the most rapid autorhythmicity, as isolated cells generate 80 -100 impulses per minute. Because of the dominance of parasympathetic innervation, the actual heart rate is slower at 70 – 80 beats per minute.
  An abnormally low heart rate is called bradycardia.
  An abnormally high heart rate is called tachycardia.
     The impulse that originates from the SA node travels to the atrioventricular (AV) node by way of internodal fibers that stimulate the myocardial cells of the atria to contract. The impulse passes more slowly through the AV node which is located in the interatrial septum near the opening of the coronary sinus.
     The impulse slows as it goes through the AV node and travels through the interventricular septum by way of the AV bundle (bundle of His). The AV bundle divides into right and left bundles that go to the apex. Then branches at the apex radiate on inner surfaces of ventricles and Purkinje cells finally distribute the impulse to the myocardial cells of the ventricles.
  Electrocardiogram  ECG (EKG)
     The electrical impulses generated and distributed by the conducting system create electrical impulses that spread through the body. These impulses can be measured by electrodes placed on the surface of the body. Depending on the placement of the leads, various traces are produced that reflect the electrical activity of the heart.
     A typical trace produces:
   P waves – accompany depolarizations of atria.
  QRS complex - associated with ventricular depolarizations.
   T waves - associated with ventricular repolarizations.