Cardiovascular System

• The heart wall consists of three layers: endocardium, myocardium, and epicardium
• the muscle cells of the heart are interconnected by intercalated discs
• intercalated discs contain gap junctions that allow electrical impulses to pass from one cell to another
• The heart is located in the thoracic cavity in the mediastinum; within its own pericardial cavity
• Trace blood flow from the body in the heart

  Vena cava - right atrium - tricuspid valve - right ventricle - pulmonary semilunar valve - pulmonary arteries - lungs - pulmonary veins - left atrium - Mitral valve - left ventricle - aortic semilunar valve - aorta
• With the contraction of papillary muscles the chordae tendineae tighten preventing the valves cusps from everting and prevents backflow
• The left ventricle walls are thicker because they pump blood throughout the body to locations where the resistance to blood flow is greater
• Semilunar valves open when pressure in the ventricles exceeds the pressure in the arteries
• Coronary circulation is the flow of blood through the many vessels that pierce the myocardium of the heart. This circulation delivers oxygenated blood and nutrients to, and removes carbon dioxide and wastes from, the myocardium.
• Deoxygenated blood returns to the right atrium primarily through the principal vein called the coronary sinus.
• The many anastomoses of the coronary circulation provide detours for blood flow if an area becomes blocked.
• The heart contains autorhythmic cells that repeatedly generate spontaneous action potentials that the trigger heart contractions.
• The components of this system are the sinoatrial (SA) node (pacemaker), atrioventricular (AV) node, atrioventricular bundle (bundle of His), right and left bundle branches, and the conduction myofibres (Purkinje fibres).
• Signals from the autonomic nervous system and hormones, such as epinephrine, modify the heartbeat, but do not establish rhythm
• A normal ECG consists of a P wave, a QRS complex, and a T wave
• A recording of the electrical changes that accompany each cardiac cycle (heartbeat) is called an electrocardiogram 
• The P-Q (PR) interval represents the conduction time from the beginning of atrial excitation to the beginning of ventricular excitation.
• The S-T segment represents the time when the ventricular contractile fibres are fully depolarized, which is during the plateau phase of the impulse.
• The T wave represents repolarization of the ventricles.
• The QRS complex represents the rapid spread of the action potential through the ventricles.
• The P wave represents arterial depolarization - the spread of the impulse from the SA node over the atria
• The sound of a heartbeat is due primarily to the turbulence in blood flow caused by the closure of the valves, rather than by the contraction of the heart muscle.
• The first sound is created by the blood turbulence associated with the closing of the atrioventricular valves, which occurs soon after the beginning of ventricular systole.
• The second sound represents the closing of the semilunar valves near the end of the ventricular systole.
• End diastolic volume is the volume of blood in the ventricles at the end of diastole
• End systolic volume is the volume of blood in the ventricles at the end of systole or contraction
• the ventricular ejection phase
• Hypertension is persistent high blood pressure
• Hypotension is low blood pressure