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Transcript
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