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