Cardiovascular system

•  Blood moves from area of higher pressure to an area of lower pressure. 
• The heart has four chambers.
• The two upper chambers are called receiving and together they are called the atrium. There is a right atrium and a left atrium.
• The right side of the heart receives blood from the systemic circulation specifically the vena cava.
• The left side received blood from pulmonary circulation specifically the aorta. 
• Vena Cava—> right atrium—-> tricuspid—> right ventricle Pulmonary valve (semilunar) —-> pulmonary trunk —> pulmonary artery —> lungs —> capillaries—> pulmonary veins — > left atrium—-> mitral valve —> left ventricle—> aortic valve —->aorta —> systemic circulation 
• The pericardium anchors the heart in place and keeps the heart from overfilling. Allows the heart to beat in a frictionless environment. 
• The AV valve prevents backflow from ventricles to atria during ventricular contraction.
• The semilunar valves prevents backflow from arteries to ventricles during ventricular relaxation. 
• Systole is the contraction of heart muscle. During Systole first the atria contract as one, then the ventricles.
• Diastole is the relaxation of heart muscle.
• Isovolumetric contraction is the period during systole when ventricles contract with all valves closed.
• Isovolumetric relaxation is period during diastole when the ventricles relax with all valves closed.
• Pacemaker potential is the gradual depolarization in pacemaker cells that leads to an action potential setting the heartbeat rhythm. 
• HCN channels in the heart generate and regulate pacemaker potentials in the SA node, setting and maintaining the heart rate by allowing ion flow during hyperpolarization.
• HCN channels in the nervous system regulate neuronal excitability, influence synaptic integrations, and contribute to synaptic plasticity. 
• Cardiac muscle contraction

  Cardiac muscle contraction is initiated by an action potential generated by the SA node, which goes through the heart's conduction system. Depolarization of cardiac muscle cells leads to Ca 2+ influx, which triggers further Ca2+ relapse from the SR. The calcium binds to troponin, causing cross-bridge formation between actin and myosin, resulting in muscle contraction. Th relaxation phase involves the removal of Ca 2+ from the cytoplasm, allowing muscles to return to its resting state. 
• Electrical conduction system 


  The sinus node generates an electrical stimulus. The atria are then activated. The electrical stimulus travels down through the conduction pathways and causes the heart's ventricles to contract and pump out blood. 
• The brief delay between generation of an action potential in the sinoatrial node and myocardial muscular contraction is due to the electrical impulse must travel first from the SA node to the AV node. 
• Cardiac potential of pacemaker cells:

  During phase 0 there is an inward Ca2+ current. During phase 3 there is a potassium outflow. During phase 4 sodium leaks inward via HCN and there is a slow rise in potential to threshold.  
• Cardiac action potential of non-pacemaker cells: 

  During phase 0 there is a fast sodium influx and rapid depolarization. During phase 1 sodium influx stops and potassium efflux starts, a brief repolarization. During phase 2 potassium efflux continues and calcium influx starts causing a plateau. During phase 3 (repolarization phase) calcium channels close and potassium channels are open causing a net potassium efflux. During phase 4 sodium gets pumped out and potassium gets pumped in by sodium potassium pump.  
• During atrial systole (contaction) the atria contact pushing blood into the ventricles. The AV valves (tricuspid and mitral) are open. The semilunar valves (pulmonary and aortic) are closed. 
• During Isovolumetric contraction the ventricles begin to contract but blood volume remains constant because all valves are closed. 
• During ventricular systole (Ejection) ventricles contract fully, ejecting blood into the pulmonary artery and aorta. The semilunar valves (pulmonary and aortic) are open. The AV valves (tricuspid and mitral) are closed. During this stage blood is pumped into the lungs. 
• During isovolumetric relaxation ventricles start to relax but blood volume remains constant because all valves are closed. 
• During ventricular diastole (passive filling) ventricles continue to relax and fill with blood passively from the atria. The AV valves (tricuspid and mitral) are open. The Semilunar valves (pulmonary and aortic) are closed. 
• During atrial systole there is an increase in atrial pressure which leads to a slight increase in ventricular pressure. 
• During Isovolumetric contraction there is a increase in ventricular pressure and atrial pressure decreases.
• During ventricular systole ventricular pressure peaks leading to ejection of blood, atrial pressure gradually increases as they fill. 
• During isovolumetric relaxation there is a drop in ventricular pressure with no volume change and atrial pressure remains high.
• During ventricular diastole there is low ventricular pressure and atrial pressure drops slightly as blood moves to the ventricles. 
• The heart sound is associated with the closing of heart valves. First sound occurs as AV valves close and signifies beginning of systole. The second sound occurs when the SL valve closes at the beginning of ventricular diastole. 
• An EKG records the electrical actiovoty of the heart over time. The p wave is atrial depolarization. The QRS complex represents ventricular depolarization. The T wave indicates ventricular depolarization. 
• The sympathetic system increases heart rate and force of contraction via the release of norepinephrine which activates beta 1 adrenergic receptors.
• The parasympathetic nervous system decreases heart rate and force of contraction via the release of acetylcholine which activates M2 receptors.
• The sympathetic system increases cAMP and PKA activity.
• The parasympathetic system decreases cAMP and PKA activity. 
• Arteries and veins are composed of 3 tunics the tunica intima, tunica media, and tunica externa.
• The tunica intima is made up of endothelial cells and functions as a selectively permeable barrier and regulation of blood vessel diameter.
• The tunica media is made up of smooth muscle cells and functions to strengthen the vessel and prevents blood pressure from rupturing them.