Physiology of Heart


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Cardiac Output and Venous Return

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Physiology of Heart

  1. 1. PHYSIOLOGY Cardiac Output, Venous Return and Their Regulation
  2. 2. Cardiac Output: It is the quantity of blood pumped into the aorta each minute by the heart or the quantity of blood that flows through the circulation each minute. Cardiac output = Heart Rate Stroke Volume Venous return: It is the quantity of blood flowing from the veins into the right atrium each minute. Venous Return = Cardiac Output
  3. 3. Increase in Cardiac Output The factors which increases the cardiac output are:  Heart  Venous Return  Arterio-Venous Fistula  Blood Volume Mean Systematic Filling Pressure
  4. 4. Metabolic Rate  Exercise Rate  Pathology  Size of the body (Cardiac Index) Person’s age
  5. 5. Role Of Frank-Starling Mechanism Frank-Starling Law: It states that “ Within physiologic limits, the heart pumps all the blood that returns to it by the way of veins.” Increased Venous Return Cardiac muscles stretches to greater length Ventricular muscle contracts with greater force Increased Cardiac Output
  6. 6. EFFECTS Stronger Contractions: Increased heart volume stretches muscles and causes stronger contraction. Heart Rate: Stretch increases heart rate as well. S-A Node: Direct effect on rhythmicity of the node to increase heart rate as much as 10-15%. Bainbridge Reflex: It gives reflexes to the vasomotor center and then back to the heart by the way of sympathetic nerves and vagi, increases the heart rate.
  7. 7. Cardiac Output Regulation The venous return to the heart is the sum of all the local blood flows through all the individual tissue segments of the peripheral circulation. At each increasing level of work output during exercise, the oxygen consumption and the cardiac output increase in parallel to each other. To summarize, cardiac output is determined by the sum of all the various factors throughout the body that control local blood flow. All the local blood flows summate to form the venous return, and the heart automatically pumps this returning blood back into the arteries to flow around the system again.
  8. 8. Relation of Cardiac Output and Stroke Volume Stroke Volume: Volume of blood that is ejected by each ventricles during systole is called Stroke Volume Output.  As the stroke volume increases the cardiac output also increases.  Stroke Volume depends upon 1. End diastolic Volume 2. Contractility
  9. 9. Limitations For The Cardiac Output There are definite limits to the amount of blood that the heart can pump, which is expressed quantitatively in the form of cardiac output curves. Normal: The plateau level of this normal cardiac output curve is about 13 L/min. Hypoeffective: The lowermost curves are for hypoeffective hearts that are pumping at levels below normal. Hypereffective heart plateau – 5 L/min Hypereffective: The uppermost curves are for Hypereffective hearts that are pumping better than normal. Hypoeffective heart plateau – 20 L/min
  10. 10. Hypoeffective Heart In this the heart is pumping at a very slow rate and the cardiac output decreases.  Factors that can cause hypoeffectivity are following: 1. Increased Arterial Pressure 2. Inhibition of Nervous Excitation 3. Abnormal Heart Rhythm 4. Coronary Artery Blockage 5. Valvular Heart Disease 6. Congenital Heart Disease 7. Myocarditis 8. Cardiac Hypoxia
  11. 11. Hypereffective Heart  In this the heart is pumping at a high rate and the cardiac output increases.  Two type of factors can make the heart a better pump than normal: 1. Nervous Stimulation: It involves Sympathetic Stimulation and Parasympathetic Inhibition. 2. Hypertrophy of The Heart Muscles: It involves the increased mass and contractile strength due to exercise and causing hypertrophy which allows increased
  12. 12. Pathologically High or Low Cardiac Output In healthy humans, the average Cardiac Outputs are surprisingly constant from one person to another. However. Multiple clinical abnormalities can cause either high or low cardiac outputs.
  13. 13. High Cardiac Output  High cardiac output is mostly caused by reduced total peripheral Resistance.  Following are some of the conditions that can decrease the Peripheral Resistance and at the same time increase the Cardiac Output to above the normal. 1. Beriberi 2.Arteriovenous Fistula 3.Hyperthyroidism 4. Anemia
  14. 14. Low Cardiac Output There are two factors that cause Low Cardiac Output: B. Abnormalities that cause the Venous Return to fall too low A. Abnormalities that cause pumping effectiveness of the Heart to fall too low
  15. 15. A. Cardiac Factors: Whenever the heart becomes severely damaged, regardless of the cause, its limited level of pumping may fall below that needed for adequate blood flow to the tissues. Some examples of this include: 1. Severe Coronary Blood 2. Myocardial Infarction 3. Severe Valvular Heart Disease 4. Myocarditis 5. Cardiac Temponade 6. Cardiac Metabolic Derangements
  16. 16. B. Decreased Venous Return: Anything that interferes with venous return also can lead to decreased cardiac output. Some of these factors are the following: 1.Decreased Blood Volume 2. Acute Venous Dilation 3. Obstruction of the Large Veins 4. Decreased Muscle Mass 5. Decreased Metabolic Rate of the Tissues
  17. 17. THANK YOU
  18. 18. Any Questions???