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
• Normal cardiac output– 5.6 L/min in young men when resting
• 10-20 % less in women
– Factors• Age• Level of body activity
• 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)
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
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)
• 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
Cardiac output = sum of the various factors controlling local blood flow = sum of local blood flow = venous return
• 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
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
• 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
• 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
• 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
• 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
• 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