The Heart as Two Pumps • The heart is really two pumps in tandem • The right heart sends blood to the lungs • The left heart gets blood back from the lungs and sends the blood to the systemic circulation • This is a bigger job because the systemic circulation is larger and has more gravity
Global Tissue OxygenationMade Ridiculously Simple 100% Venous Oxygen Delivery SvO2 = 75% 25% Arterial Oxygen Delivery Oxygen Consumption
Global Tissue OxygenationSimple Description • Each Hb molecule can carry four oxygen molecules • The hemoglobin in the blood picks up oxygen in the lungs • The hemoglobin sends the oxygen in the blood through the arteries to the tissues • The tissues do not extract 100% of the oxygen from the hemoglobin • 25% of oxygen is in the tissues, 75% in the veins • The Hb then goes back to the loading station
Global Tissue OxygenationDetailed Description • The lungs load each hemoglobin with 4 oxygen molecules. • Oxygen content is 20% of total volume. • At the tissue level, Oxygen extraction is a ratio of oxygen consumed (VO2 = 250 mL/min) to the amount delivered (DO2) = 25% • Thus 75% of oxygen delivered is returned to the venous side, i.e. normal SvO2 = 75%. • Oxygen consumption (VO2) is a function of cardiac output and the difference between arterial (Hb x SaO2 x 13.4) and venous oxygen content (Hb x SvO2 x 13.4). • Given the same CO and Hb, VO2 is analogous to the difference between arterial and venous oxygenation. • For example, 1 Hb will deliver 4 oxygen molecules to the tissue -> 1 oxygen molecule is consumed (VO2) by the tissue + 3 oxygen molecules are returned to the venous outflow.
Coronary CirculationDescription • The arteries and veins in the heart perfuse the heart with oxygen • The coronary arteries come off of the aorta at the place of the aortic valve • Left and right coronary arteries • Left almost immediately branches into the circumflex and the left anterior descending artery • Nurses the left side of the heart • Right • Both nourish the septum • Blood then goes into the capillaries and then the veins of the heart • Large vein that delivers the blood back to the heart is the coronary sinus
Cardiac Conduction System • Conduction system stimulates the myocardium to contract and pump blood • Conduction system usually controls the rhythm of the heart (unless the person has a pacemaker) • Heart has two conduction systems • One controls atrial activity • One that controls ventricular activity
Anatomy of the Conduction System SA Node AV Node Bundle of His Bundle branches Purkinje fibers Porth, 2007, Essentials of Pathophysiology, 2nd ed., Lippincott, p. 331.
SA Node • Pacemaker of the heart • Impulses originate here • Located in posterior wall RA • Fires at 60 -100 bpm • Responsible for the heart rate in the normal person • Impulse causes atrial contraction
AV Node • Connects the atria and ventricles, provides one way conduction • Would beat independently • Fires at 40 -60 bpm • Can assume pacemaker function if SA fails to discharge • There is a pause here • The speed of conduction in the AV node is influenced by the SNS (beta-1)
Purkinjie Fibers • Supplies the ventricles • Supplies the impulse to the cardiac muscle • Large fibers, rapid conduction for swift and efficient ejection of blood from heart • Large fibers – fast conduction • Small fibers – slow conduction • Fire 15-40 bpm • Only occurs if there is no input from the other areas • Assume pacemaker of ventricles if AV fails • HR reflects intrinsic firing of these structures
Action Potentials (AP) • Stimulus • The only intrinsic conduction in the heart is in the SA node • Any other conduction comes from depolarization of the muscle excitable tissues (muscle and conduction system) evokes an AP characterized by a sudden change in voltage resulting from transient depolarization and then repolarization. • AP’s are electrical currents involving the movement/flow of electrically charged ions at level of cell membrane. • AP’s are conducted throughout the heart, responsible for initiating each cardiac contraction.
SLOW SA & AV Nodes FAST Purkinje Fiber & Muscle
Types of Membrane Ion Channels that Contribute to Voltage Changes during the AP
Types of Membrane Ion Channels that Contribute to Voltage Changes during the AP • Fast Na+ channels • Rapid depolarization of muscles • Important in cardiac APs and Purkinje fibers • Slow Na+ channels • Pacemaker activity (SA, AV) • Potassium channels • Speedy repolarization
Three Phases of Action Potentials • Resting • Depolarization • Repolarization
Resting Phase • Membrane is relatively permeable to K+, but much less so to Na+ • Inside is negative, outside is positive
Cardiac Muscle Cell Firing • Cells begin with a negative charge: resting membrane potential • Calcium leaklets Ca2+ diffuse in, making the cell more positive Threshold potential Resting membrane potential Calcium leak
Depolarization Phase • Cell membrane becomes permeable to Na+ • Na+ enters cell, inside the cell is more +
Cardiac Muscle Cell Firing (cont.) • At threshold potential, more Na+ channels open • Na+ rushes in, making the cell very positive: depolarization • Action potential: the cell responds (e.g. by contracting) Action potential Threshold potential Resting membrane potential Calcium leak
Cardiac Muscle Cell Firing (cont.) • K+ channels open • K+ diffuses out, making the cell negative again (starting to repolarize), but Ca2+ channels are still allowing Ca2+ to enter • The cell remains positive: plateau Action potential PLATEAU Threshold potential Calcium leak
Repolarization Phase • Outward flow of positive charges, mainly K+ • Inside the cell is more negative • Assisted by Na+-K+ pump • Relatively slow method of repolarization • Potassium ions made a bigger, faster difference
Cardiac Muscle Cell Firing (cont.) • During plateau, the muscle contracts strongly • Then the Ca2+ channels shut and it repolarizes • The potassium channels opened a while ago so the potassium comes out, leading to repolarization Action potential PLATEAU Threshold potential Calcium leak
Cardiac Action Potentials • Unlike nerve cells, cardiac cells have five phases in their action potential • Phase 4 – the resting membrane potential. • Phase 0 – there is rapid depolarization • The QRS complex corresponds to this section • Phase 1 – there is a short repolarization (only observed in ventricular muscle) • Occurs right in the end of depolarization • Only observed in ventricular muscle • Phase 2 – the membrane potential remains depolarized in a plateau • When calcium is entering the cell, so further repolarization is prevented (because cell is more positive) • Phase 3 – the membrane potential becomes repolarized. • The T wave corresponds to the repolarization
Cardiac Muscle Action Potential 5 Phases Unlike nerve cells, cardiac cells have 5 phases in their action potential. Phase 0: Upstroke, rapid depolarization Phase 1: Early, short repolarization Seen only in ventricular muscle Phase 2: Plateau phase; membrane potential remains depolarized Phase 3: Final rapid repolarization Phase 4: Resting, diastolic repolarization
Cardiac Muscle Cell Contraction • During Phase 2, the plateau, calcium ion enters the muscle cell, causing it to contract strongly. • The strength of contraction is directly proportional to the number of calcium ions that enter the cell. • Calcium channel opening is controlled by voltage (the calcium channels only open when the membrane is at a certain voltage) and by beta1 receptors in the ventricular myocardium.