Circulatory system
     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. 
     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.
Orientation and Superficial Anatomy of the Heart 
  1. The heart lies slightly to left of the midline.
   The base of the heart is the broad superior portion of the heart that includes the origin of the major blood vessels by which blood enters and leaves the heart. The apex is the inferior rounded tip.
  2. The heart is at an oblique angle to the longitudinal axis.
   Because of this tilt, the apex points obliquely toward the left.
  3. The heart is rotated slightly toward the left.
Internal Anatomy and Organization of the Heart ( Heart Anatomy and Animations)
     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.
  Right Ventricle  
   The deoxygenated blood from the 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 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 oxygenated blood enters the left ventricle through a valve with two flaps, the left atrioventricular valve (mitral valve).
  Left Ventricle 
   As oxygenated blood is ejected from the left ventricle is passes through the aortic semilunar valve and enters the ascending aorta.
Structural Differences Between Right and Left Ventricles
     The right ventricle pushes blood through the pulmonary circuit at lower pressure and its wall is thinner than that of the 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 the thinner wall of the right ventricle is like a pouch attached to the wall of the left ventricle.
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 the 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.