Cardiac Output Graphics are used with permission of: adam.com (http://www.adam.com/) Benjamin Cummings Publishing Co (http://www.awl.com/bc)
Page 1.
Introduction
• Cardiac output is the amount of blood pumped out by each ventricle in one minute. • Cardiac output can increase markedly to meet the demands placed on our body, whether dashing to catch a bus or riding a mountain bike,
Page 2. Goals • To recognize that cardiac output varies directly with heart rate and stroke volume. • To identify factors that modify heart rate and stroke volume, and to indicate how they change cardiac output.
Page 3.
Cardiac Output Definition
• Cardiac Output (CO) = The volume of blood ejected from the left or right ventricle into the aorta or pulmonary trunk per minute. • Cardiac output depends on: 1. Heart rate 2. Stroke volume Cardiac Output CO =
Page 4.
= HR
Heart Rate X
X
Stroke Volume SV
Heart Rate Definition
• Heart rate (HR) is the number of times the heart beats in one minute, averaging 75 beats per minute (bpm) in the adult at rest.
Page 5.
Stroke Volume Definition
• Stroke volume (SV) is the amount of blood pumped by each ventricle with each heartbeat, averaging 70 ml per beat in the adult at rest.
Page 6.
SV = EDV - ESV
• Stroke volume represents the difference between end diastolic volume (EDV) and end systolic volume (ESV). • By the time diastole ends, each ventricle has filled up with blood. This amount of blood is the end diastolic volume or EDV. • The amount of blood ejected during the systole is the stroke volume. At the end of systole the volume of blood remaining in each ventricle is the end systolic volume or ESV. • Each ventricle normally contains about 120 ml of blood by the end of diastole. At the end of systole about 50 ml of blood are left in each ventricle. This means that 70 ml of blood were pumped out o f each ventricle during systole. Stroke Volume SV = ~70 ml/beat
Page 7.
= =
End-Diastolic Volume EDV ~ 120 ml/beat
End-Systolic Volume ESV -
~50 ml/beat
Cardiac Output Demonstration
** Let this animation run for a full minute until it is done.
This will give you some appreciation for the amount o f blood that is pumped each minute by the heart at rest - about 5.25 liters!
** Now is a good time to go to quiz question 1: • Click the Quiz button on the left side of the screen. • After answering question 1a and 1b, click the Back to Topic button on th e left side of the screen. • To get back to where you left off, click on the scrolling page list at the top of the screen and choose "8. Regulation of Cardiac Output".
Page 8.
Regulation of Cardiac Output
• The key factor regulating stroke volume is the amount of stretching that occurs to ventricular cardiac muscle prior to ventricular contraction. The more cardiac muscle stretches, the more forcefully it contracts. These stronger contractions increase stroke volume.
**
Please note, there is a lot of important information on this page. Go through this page slowly and try to make sense of it.
Interactive Physiology
• Fill out this chart, making note of the reasons for the increase or decrease: Affect on Heart Rate Affect on Stroke Volume Affect on Cardiac Output Increased Sympathetic Stimulation Increased Parasympathetic Stimulation Increased Venous Return Slow Heart Rate Extremely Fast Heart Rate Exercise Sudden Drop in Blood Pressure Rising Blood Pressure Sudden Drop in Blood Volume Excess Calcium • Increased Sympathetic Stimulation - Increased sympathetic stimulation (due to fright, anger, etc.) increases the heart rate. It also increases stroke volume by increasing contractility, which results i n more complete ejection of blood from the heart (lower ESV). • Increased Parasympathetic Stimulation - Parasympathetic activity increases after a crisis has passed. This reduces heart rate and stroke volume from their high levels, bringing cardiac output back to normal. • Increased Venous Return - Cardiac muscle fibers are stretched by increased blood volume returning to the heart (increased venous return and EDV). Increased stretch results in greater force of contraction, which increases stroke volume. • Slow Heart Rate - Slow heart rate allows for more time for ventricular filling, increasing EDV and therefore stroke volume. • Extremely Fast Heart Rate - Extremely rapid heart rate results in low venous return and therefore decreased stroke volume. • Exercise - Exercise activates the sympathetic nervous system, increasing heart rate, contractility, and stroke volume. Both the higher heart rate and squeezing action of skeletal muscles on veins increase venous return, contributing to increased stroke volume. • Sudden Drop in Blood Pressure - A sudden drop in blood pressure results in low venous return and therefore decreased stroke volume. However heart rate increased due to sympathetic activity, and normal cardiac output is maintained. • Rising Blood Pressure - Rising blood pressure reduces sympathetic activity, decreasing heart rate. High blood pressure also increases arterial pressure which ventricles must overcome before semilunar valves open, increasing ESV and decreasing stroke volume. Reduced cardiac output helps bring blood pressure down to normal levels. • Sudden Drop in Blood Volume - A sudden drop in blood volume (eg. due to severe blood loss) results in low venous return and therefore decreased stroke volume. Sympathetic activity increases heart rate, maintaining cardiac output. • Excess Calcium - Excess calcium can lead to spastic heart contractions, an undesirable condition. Calcium also increases stroke volume by enhancing contractility.
Interactive Physiology
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Page 9.
Heart Videos
• Effect of Epinephrine on a Frog's Heart: Epinephrine is normally released when there is increased sympathetic activity. • Effect of Acetyl Choline on a Frog's Heart: Acetyl choline is a parasympathetic neurotransmitter.
Page 10.
Summary
• Cardiac Output = Heart Rate X Stroke Volume • Heart rate is increased by sympathetic nerve activity and epinephrine. Heart rate is decreased by parasympathetic nerve activity.
** Now is a good time to go to quiz questions 2 and 3: • Click the Quiz button on the left side of the screen. • Click on the scrolling page list at the top of the screen and choose "2. Heart Rate Regulation". • Work through quiz questions 2-3.
Notes on Quiz Questions: Quiz Question #1a: Cardiac Output • This question asks you to calculate cardiac output based on stroke volume and heart rate. If you get stuck, remember the equation: Cardiac Output = Heart Rate X Stroke Volume Also remember that there are 1000 milliliters in 1 liter. Quiz Question #1b: Soda Bottles • This question asks you to illustrate the cardiac output. Quiz Question #2: Heart Rate Regulation • This question asks you to predict what will happen to heart rate in various situations. Quiz Question #3: Stroke Volume Regulation • This question asks you to predict what will happen to stroke volume in various situations.
Study Questions on Cardiac Output: 1. (Page 3.) Define cardiac output. 2. (Page 3.) What two factors does cardiac output depend on? 3. (Page 3.) What is the mathematical relationship between cardiac output, heart rate, and stroke volume. 4. (Page 4.) Define heart rate. 5. (Page 4.) What is the average heart rate in an adult at rest? 6. (Page 5.) Define stroke volume. 7. (Page 5.) What is the average stroke volume in an adult at rest? 8. (Page 6.) Define end diastolic volume. 9. (Page 6.) Define end systolic volume. 10. (Page 10.) What is the mathematical relationship between end diastolic volume, end systolic volume, and stroke volume? 11. (Page 6.) If the ESV is 50 ml and the EDV is 120 ml, what is the stroke volume? 12. Page 7.) If the heart rate is 75 beats per minute and the stroke volume is 70 ml per beat, then what is the cardiac output? 13. (Page 8.) What's the relationship between cardiac muscle stretch and force of contraction? What effect does this have on stroke volume? 14. (Page 8.) What's the relationship between venous return and stroke volume? 15. (Page 8.) What is the effect of increased sympathetic activity on heart rate and stroke volume? How does this effect cardiac output? 16. (Page 8.) What is the effect of increased parasympathetic activity on heart rate and stroke volume? 17. (Page 8.) What is the effect of increased venous return on heart rate and stroke volume? effect cardiac output? Interactive Physiology 3
How does this
18. (Page 8.) What effect does a slow heart rate have on stroke volume? 19. (Page 8.) What effect does a fast heart rate have on stroke volume? 20. (Page 8.) What is the effect of exercise on heart rate and stroke volume? How does this effect cardiac output? 21. (Page 8.) What is the relationship between blood pressure and sympathetic activity? What effect does this have on heart rate? 22. (Page 8.) What is the effect of a sudden decrease in blood pressure on heart rate and stroke volume? 23. (Page 8.) What is the effect of a sudden increase in blood pressure on heart rate? 24. (Page 8.) What is the effect of a sudden increase in blood pressure on stroke volume? 25. (Page 8.) What is the effect of a sudden drop in blood volume on heart rate and stroke volume? 26. (Page 8.) What is the effect of an increase in calcium on heart rate and stroke volume?
Answers to Questions on Cardiac Output: 1. The volume of blood ejected from the left or right ventricle into the aorta or pulmonary trunk per minute. 2. Heart rate and stroke volume. 3. Cardiac Output (CO) = Heart Rate (HR) X Stroke Volume (SV) 4. The number of times the heart beats in one minute. 5. 75 beats per minute. 6. The amount of blood pumped by each ventricle with each heartbeat. 7. 70 ml 8. The amount of blood in each ventricle at the end of diastole. 9. The amount of blood in each ventricle at the end of systole. 10. Stroke Volume (SV) = End-Diastolic Volume (EDV) End-Systolic Volume (ESV) 11. Stroke Volume = End-Diastolic Volume End-Systolic Volume ~70 ml/beat = ~ 120 ml/beat ~50 ml/beat 12. Cardiac Output = Heart Rate X Stroke Volume 5250 ml = 75 beats/min X 70 ml/min 13. The more cardiac muscle stretches, the more forcefully it contracts. These stronger contractions increase stroke volume. 14. The greater the venous return, the more blood arrives at the heart and the more the heart muscle is stretched, resulting in a greater stroke volume. The lower the venous return, the less blood arrives at the heart and the heart muscle is stretched less, resulting in a lower stroke volume. 15. Sympathetic stimulation increases the heart rate and makes the heart contract with a greater force, which increases stroke volume. Cardiac output increases. 16. Normally the parasympathetic nervous system is active, keeping heart rate and stroke volume at normal levels. 17. If more blood is coming into the heart, the heart muscle will stretch, so it will contract with a greater force increasing the stroke volume but not the heart rate. Because stroke volume increases, cardiac output also increases. 18. If heart rate decreases, then stroke volume will have to increase to maintain a normal cardiac output. 19. If heart rate increases, then stroke volume will have to decrease to maintain a normal cardiac output. 20. Exercise stimulates the sympathetic nervous system, which increases the heart rate and makes the heart contract with a greater force, which increases stroke volume. Cardiac output increases. 21. A decreased blood pressure increases sympathetic activity, increasing the heart rate. An increased blood pressure decreases sympathetic activity and decreases the heart rate. 22. If there is a sudden drop in blood pressure, there is less venous return and stroke volume decreases. Heart rate increases due to increased sympathetic activity and cardiac output is maintained as a result. 23. Get less sympathetic activity so heart rate decreases. 24. High pressure in the arteries leaving the heart causes a decreased stroke volume since the semilunar valve opens only when pressure in the ventricle exceeds pressure in the arteries leaving the heart. 25. A drop in blood volume decreases blood pressure which increases sympathetic activity, causing an increase in heart rate. Since there is less blood, stroke volume decreases. 26. Increased calcium increases the number and force of contractions, increasing both heart rate and stroke volume.
Interactive Physiology
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