Download Cardiac output and venous return

Cardiac output and venous
return
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
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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
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