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