Cardiac Cycle

Lecture 6 ? The Cardiac Cycle Cardiac Cycle And Cardiac Output (Slide 1) 1. Cardiac cycle defined (components) a. A cardiac cycle is defined as a single period of atrial and ventricular contraction and relaxation. 2. Cardiac output defined (components) 3. Why is cardiac output important? How is it measured? 4. How does cardiac output relate to circulation? Cardiac Cycle And Cardiac Output (Slide 2, Fig 14.17) 1. The cardiac cycle consists of two phases. a. Diastole is the ventricular filling phase. b. Systole is the ventricular emptying phase. 2. Isovolumetric refers to ?same volume.? There are two phases that are isovolumetric. a. The first phase is isovolumetric contraction, which occurs when the ventricles contract. b. The second phase is isovolumentric relaxation, which occurs when the ventricles relax. 3. Rapid filling and reduced filling occur during diastole when the ventricles are filling. 4. Rapid ejection and reduced ejection occur during systole when the ventricles are emptying. 5. The electrocardiogram at the bottom of the figure shows how an ECG would picture various parts of the cardiac cycle. 6. There are four valves in the heart, two atrioventricular valves (AV valves), one aortic valve and one pulmonary valve. These open and close to regulate the volume and pressure in the ventricles. They open or close due to the pressure gradient across them. For example, if the pressure in the left ventricle is lower than the pressure in the aorta, the aortic valve remains closed. 7. You can compare the graphs of ventricular volume, ventricular pressure and valves opening/closing. 8. There are four phases in the cardiac cycle: a. Phase 1 is ventricular filling. The blood returning to the heart flows through the atria, through the AV valves and into the ventricles using its own pressure. It does not require any pumping. This phase is mid-to-late diastole. b. Phase 2 is isovolumentric contraction. The ventricles contract, which raises the internal pressure. When the pressure inside the ventricles exceeds the pressure of the atria, the AV valves close. The pulmonary and aortic valves remain closed because the pressure is not high enough yet. This is the beginning of systole. c. Phase 3 is ventricular ejection. The pressure inside the ventricles has risen high enough so that the pulmonary and aortic valves open. Blood is forced through the valves and into the aorta and pulmonary artery. This is the end of systole, and the beginning of diastole. d. Phase 4 is isovolumetric relaxation. Ventricular myocardium is relaxed. Ventricular pressure is too low to keep the aortic and pulmonary valves open, but it is too high to keep the AV valves open. So all valves are closed, and the heart is relaxed. This is the onset of diastole. Left Ventricular Pressure-Volume Relations During A Single Cardiac Cycle (Slide 3) 1. The yellow boxes on this slide show the opening and closing of valves. 2. Point F is the closing of the aortic valve. From point F through A, B and C, that segment is diastole. 3. Points C, D, E and F represent systole. 4. Segments AF and CD are the isovolumetric contraction and relaxation of the ventricles. The valves are closed, so no blood is flowing, and no volume is changing. 5. Each individual segment means something: a. Segment AB is the relaxation of the ventricles. Even though blood is filling them, the pressure falls. This is the end of diastole. b. Segment BC is the beginning of systole. The pressure rises again, and ventricular pressure exceeds atrial pressure, causing the mitral valve to close. c. Segment CD is the isovolumetric contraction of the ventricle. d. Segment DE is the rapid ejection phase of diastole. Ventricular pressure rises because it is contracting. e. Segment EF is the reduced ejection phase of systole. Ventricular pressure falls because there is little blood left in the ventricles. f. Segment FA is isovolumetric relaxation of the ventricles. It is the beginning of diastole. Contractile Work Done By The Left Ventricle Involves Pressure And Volume, Kinetic Energy, And The Product Of Tension And Time (Slide 4) 1. The heart has to do work, which in terms of physics, is force times distance. 2. The heart has to give blood kinetic energy in order to move it from tiny arterioles into the heart. This accounts for 5-6% of the work the heart does. 3. There is a small amount of work that is done on the heart by the blood. As it fills the blood during diastole, it pushes against the walls of the myocardium, stretching myofibrils and causing them to contract. This accounts for a portion of the work. 4. Blood loses energy as it travels through the body in the form of heat. Even though this is wasted energy, the blood must be reheated before leaving the heart. This accounts for another portion of the work the heart does. 5. The largest portion of the work the heart does, about 80-90%, is because of tension. a. Tension times time index (T x ?t) b. This is the amount of time the heart spends generating tension. c. This time is mainly during reduced and rapid ejection as well as isovolumetric contraction and relaxation. Summary And Analysis (Slide 5) 1. A cardiac cycle consists of systole and diastole phases. 2. The ventricles fill during diastole and empty during systole. 3. Both systole and diastole can be further divide into rapid or reduced filling, rapid or reduced emptying and isovolumetric contraction and relaxation. 4. At the resting heart rate in adults, about 72 contractions per minute, diastole lasts about twice as long as systole. The Right And Left Ventricles have Different Pumping Mechanisms For Ejecting Blood (Slide 6, Fig 14.5) 1. The right ventricle is like an appendage to the left ventricle. The wall between the right and left ventricles is called the interventricular septum. 2. The right ventricle performs similarly to the left ventricle. It has a thin freewall. 3. The left ventricle has a freewall 4-5 times thicker than that of the right ventricle. The left ventricle also has two layers of striated muscle. The inner layer spirals from the base to the apex and becomes papillary muscle. The outer layer is circular muscle. 4. When the ventricles contract, they contract slightly differently: a. The right ventricle contracts from apex to base as well as from the freewall to interventricular septum. b. The left ventricle contracts from apex to base, but its circumference contracts toward the center. Cardiac Output (Fig 14.1, 14.20) 1. Cardiac output refers to the volume of blood that leaves the ventricles every minute. The volume that leaves the two ventricles has to be the same because if one ventricle has more blood that the other, then the other ventricle will eventually run out of blood to pump. 2. End diastolic volume (EDV) refers to the volume of blood in the ventricles at the end of diastole. 3. Cardiac output = heart rate x stroke volume Q = HR x VS Some Of The Direct And Indirect Determinants Of Stroke Volume, Therefore Of Cardiac Output (Slide 7, Fig 14.29) 1. Stroke volume is directly determined by afterload, end-diastolic volume and ventricular contractility. a. Afterload refers to the pressure against which ventricles must contract to release the blood. A larger afterload means lower stroke volume. b. End-diastolic volume (EDV) refers to the maximum ventricular volume attained during the cardiac cycle. It is attained just before ejection. A larger EDV means a higher stroke volume. c. Ventricular contractility refers to a change in the force of ventricular contractions. A larger contractility means a higher stroke volume. 2. Stroke volume is also indirectly affected by many factors. a. Ventricular end-diastolic pressure refers to the pressure in the ventricles at the end of diastole. The myocardium is compliant tissue, meaning it can expand and contract like a rubber band. The less compliant a tissue is, the lower the stroke volume b. Norepinephrine is the major neurotransmitter than innervates the heart. It is released by sympathetic nerves. It has three effects: positive ionotropic (increases contractility), positive chronotropic (increases heart rate) and positive dromotropic (increases conduction velocity at the AV node). c. Filling time refers to the time it takes to fill the ventricles. The longer it takes means a lower stroke volume. d. Mytropolaps refers to the resistance against valves. The opening and closing of valves influences blood flow. Disease slows blood movement through valves and decreases stroke volume. Why And How Do We Measure Cardiac Output? How Is It Changed In Different Physiological States? (Slide 8) 1. Cardiac output is an important determinant of the health of the cardiovascular system. 2. It is measured variously in clinics and experimental laboratories. 3. Blood flow can be redistributed to meet the metabolic demands of different organs. Fick?s Principle For Estimating Cardiac Output, Or Blood Flow, In Any Organ (Slide 9) 1. Fick?s Principle is based on the law of conservation of mass. 2. When measuring the flow of a fluid across a segment, if the final concentration of solute is lower than the initial concentration, something in between those two points took that solute away. If the final concentration is higher, something was added. If it is the same, nothing happened. 3. Here is a typical math problem using Fick?s Principle. a. Assume the O2 content in the pulmonary artery is 15ml O2 per 100ml blood while the content in the pulmonary vein is 20ml O2 per 100ml blood. b. The difference between the two is 5ml per 100ml blood. c. On average, there is 250ml O2 per 100ml blood coming into the lungs. d. So 250/5 = 50 units. So you need 50-100ml units of blood passing into the lungs each minute. e. 50 x 100ml = 5000ml. This is the cardiac output. Distribution Of Blood Flow, i.e. Cardiac Output, During Resting Conditions And During Conditions Of Increased Physical Activity (Slide 10, Fig 15.15) 1. Blood flow can be redistributed throughout the body wherever it is needed the most.