cardiac output and venous return
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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 - PowerPoint PPT PresentationTRANSCRIPT
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
• 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