Physiology of Heart

Cardiac Output and Venous Return

1. Cardiac Output, Venous Return and Their Regulation

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. 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. Metabolic Rate Exercise Rate Pathology Size of the body (Cardiac Index) Person’s age

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. 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. Cardiac Output Regulation

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. A s the stroke volume increases the cardiac output also increases. Stroke Volume depends upon End diastolic Volume Contractility

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. 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: Increased Arterial Pressure Inhibition of Nervous Excitation Abnormal Heart Rhythm Coronary Artery Blockage Valvular Heart Disease Congenital Heart Disease Myocarditis Cardiac Hypoxia

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 Cardiac Output .

12. Pathologically High or Low Cardiac Output

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. Beriberi Arteriovenous Fistula Hyperthyroidism Anemia

14. Low Cardiac Output

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: Severe Coronary Blood Myocardial Infarction Severe Valvular Heart Disease Myocarditis Cardiac Temponade Cardiac Metabolic Derangements

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 : Decreased Blood Volume Acute Venous Dilation Obstruction of the Large Veins Decreased Muscle Mass Decreased Metabolic Rate of the Tissues

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