Cardiac output and venous return

1. Cardiac output and venous return

2. Cardiac output • The quantity of blood pumped into the aorta • Amount of blood that flows through the circulation • Most important factor in relation to the circulation

3. Normal cardiac output • 5.6 L/min in young men when resting • 10-20 % less in women • Factors • Age • Level of body activity

4. Cardiac index • Cardiac output per sq. m of body surface area • Body weight around 70 kg = 1.7 sq. m body surface • Cardiac index = approximately 3 L/min/sq.m • Effects of age • Rapid increase (4L/min/sq.m) at age 10 • Decline thereafter (2.4 L/min/sq.m at age 80)

6. Venous return • Amount of blood flowing from the vein into the right atrium • Must be equal to cardiac output • Exception • Few heartbeats at a time for storage/removal of blood from heart and lungs

7. Control of cardiac output • Venous return • Primary controller • Peripheral factors • Not heart • Heart • Built-in mechanism to accommodate amount of blood that flows into the right atrium • Frank-Starlings law of the heart (pumping of blood) • Bainbridge reflex (heart rate)

8. Venous return • sum of all blood flow from peripheral system • Cardiac output • sum of all local blood flow regulation • Controlled by factors that control local flow of blood

9. Cardiac output = sum of the various factors controlling local blood flow = sum of local blood flow = venous return

10. Effects of total peripheral resistance • Variation in cardiac output under normal arterial pressure • Reciprocal of peripheral resistance • Increased peripheral resistance, decreased cardiac output • Ohm’s law

11. Plateau level in cardiac output • Amount of blood that a heart can pump out • Limited • Plateau around 13L/min when normal • 2.5 X above normal (5L/min) • Heart has a capacity to pump 2.5 X more blood than normal venous return before becoming a limiting factor • Abnormal condition • Hypereffective • Greater output • Hypoeffective • Lesser output

13. Hypereffective heart • Nervous stimulation • Sympathetic stimulation and parasympathetic inhibition • Greatly increased heart rate (180-200 beats/min) • Increased contractility of heart muscle by 2 X • Result • Raise in plateau level to 25L/min after sympathetic stimulation

14. Hypereffective heart • Heart hypertrophy • Increased workload • Increase in mass • Increase in contractile strength • Net results • Increased plateau output to 30-40L/min in marathon runners

15. Pathologically high cardiac output • Cause • Chronically reduced total peripheral resistance • Not by excessive excitation of heart • Excessive excitation of heart • Sudden increase in cardiac output • Lasts only for a short time • Increased blood flow to tissue triggers vasoconstriction • Increased capillary filtration of fluid • Net result = decreased venous return

17. Hypoeffective heart • Factors • Nervous excitation inhibition • Abnormal rhythm/rate of heart beat • Valvular heart disease • Hypertension • Congenital heart disease • Myocarditis • Cardiac anoxia • Damage to myocardium

19. Low cardiac output • Abnormalities that decrease pumping effectiveness • Damage to cardiac muscles • Cause cardiac shock • Abnormalities that decrease venous return • Decreased blood volume • Acute venous dilation • Vessel obstruction • Cause circulatory shock • Reduced amount of nutrients being delivered

20. Role of nervous system • Maintenance of arterial blood pressure when cardiac output increases • Essential to achieve high cardiac output • Increased local blood flow via dilation of blood vessel • Increase arterial pressure during exercise • Increase cardiac output