Physio_20.txt

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c_sopkovich
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59988
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Physio_20.txt
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2011-01-15 18:43:38
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Physiology
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Chapter 20 Cardiac output and venous return
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  1. venous return
    • The quantity of blood that flows from the veins into the right atrium each minute
    • The venous return and the cardiac output must be equal
  2. Four factors that directly affect cardiac output
    • 1. level of body metabolism
    • 2. exercising
    • 3. age, with increasing age body activity diminishes
    • 4. size
  3. Average CO
    • men 5.6
    • women 4.9
    • For adults the average is stated to be 5
  4. cardiac index
    • the cardiac output in proportion to body surface area
    • CO/BSA
    • normal is around 3 L/min/M^2
    • Peaks at around 10 years of age, 4 L/min/M^2
  5. Stretch of the right atrium affect?
    • positive inotropic affect
    • positive chronotropic effect, bainbridge sympathetic reflex
  6. When does the heart become the limiting factor in cardiac output
    when venous return becomes more then the heart can handle
  7. Venous return in the sum of
    all local blood flow through all the individual segments of tissue
  8. Blood flow in tissue increases in proportion to
    tissue metabolism
  9. How PVR affects CO
    reciprocally, when PVR increases CO decreases
  10. Ohm's law
    • Q = arterial pressure / Total peripheral resistance
    • flow = pressure / resistance
  11. Conductance and resistance are reciprocal
  12. What amount of venous return can the heart pump without special stimulation
    • 13 L/min
    • can max out higher or lower if it is hypereffective or hypoeffective respectfully
  13. Two factors that can cause hypereffectivness
    • nervous stimulation
    • hypertrophy of the heart muscle
  14. Two ways nervous stimulation increases effectiveness
    • increases heart rate, pos chronotrop, up to 180 - 200 beats/min
    • increases the strength of contraction, pos inotrop
    • can raise output to 25 L/min
  15. Factors that cause hypoeffectivness
    • coronary blockage
    • sympathetic inhibition
    • abnormal rate and rhythm
    • Valvular disease
    • increased arterial pressure
    • congenital heart disease
    • cardiac hypoxia
  16. Without nervous control
    • you heart cannot respond to changes in arterial pressure
    • no compensation
  17. Tissue metabolites act?
    on arterioles
  18. Distinguishing feature of diseases that increase co
    • all result from reduced total peripheral resistance
    • non result from increased excitement of the heart
  19. diseases that decrease peripheral resistance and increase CO
    • beriberi - caused by insufficient thiamine vitamin, B1. This lack cause the cell to lose the ability of oxidative phosphorilation and compensate by increasing blood supply
    • AV fistula - decreases total peripheral resistance
    • hyperthyroidism - increased metabolism of tissues. metabolic byproducts are released that causes dilation
    • anemia - reduced viscosity of the blood, decreased delivery of O2 to the tissues
  20. Two categories of conditions that decrease CO
    • abnormalities that cause pumping effectiveness to fall too low
    • those that decrease venous return
  21. Five things that damage the heart and lower CO
    • CAD
    • valve disease
    • myocarditis
    • cardiac tamponade
    • metabolic derangments
  22. Cardiogenic shock
    when the CO falls so low that the tissues begin to suffer nutritional deficiency
  23. Four factors that decrease venous return
    • decreased blood volume, peripheral vascular pressure falls to a level too low to force blood back to the heart
    • acute venous dilation - when the sympathetic nervous system suddenly becomes inactive
    • obstruction of large veins -
    • decreased tissue mass of skeletal muscle - aging and long period of inactivity. decreases oxygen consumption and blood flow needs resulting in decreased output.
  24. An increase in intrapleural pressure
    • shifts the cardiac output curve to the right
    • Right atrial pressure must increase in proportion to the increase in intraplerual pressure
  25. 5 factors that can alter the intrapleural pressure
    • cyclic changes during respiration
    • breathing against negative pressure shifts the curve to the left
    • positive pressure breathing, shifts to the right
    • opening of the thoracic cage increases pressure from -4 to 0
    • cardiac tamponade, as the chambers of the heart fill the external pressure rises
  26. Three principle factors that affect venous return from the systemic circulation
    • right atrial pressure; exerts a backward flow to impede return
    • mean systemic filling pressure;
    • resistance to blood flow
  27. mean systemic filling pressure
    • pressure of rt atrium when blood flow stops
    • usually around 7
    • when arterial and venous pressure come to equilibrium
  28. rt atrial pressure bill plateau at negative pressure
    • this is caused by the collapse of the veins entering the chest
    • sucks the walls of the veins together
  29. Mean circulatory filling pressure
    • when blood flow is stops this is the pressure in the circulation
    • greater the blood volume the greeted the circulatory filling pressure
    • at zero when volume is unstressed
    • increased by sympathetic stimulation, decreased compliance
    • decreased by parasympathetic
  30. systemic and circulatory filling pressures are almost always equal
    The greater the system is filled the easier it is for blood to return to the heart
  31. The pressure gradient of venous return
    the greater the difference between systemic filling pressure and right atrial pressure the greater the venous return
  32. Where does most resistance to venous return occur
    • in the veins
    • some in arterial and small arteries
    • In the veins when there is resistance blood begins to dam up, there is little increase in pressure because of the veins are highly distensible
  33. A decrease in venous resistance =
    a increase in venous return
  34. What is the maximum pressure the rt atrial pressure can rise to
    the mean systemic filling pressure
  35. Affect of an increase in blood
    • increase in the mean systemic filing pressure, shifts venous return curve to the right
    • distends blood vessels and decreases resistance to venous return , increases slope of venous return curve
  36. Three compensatory effects in response to increased blood volume
    • increased cardiac output increases capillary pressure so fluid begin to transude out. decreases volume
    • stress-relaxation of the veins; increased pressure causes veins to distend thus reducing systemic filling pressure
    • increased resistance through tissue causing return to decrease
    • over a period of 10-40 minutes these effects will return cardiac output to normal
  37. Effects of sympathetic stimulation on cardiac output
    • increased strength of contraction
    • increases systemic filling pressure, and increases resistance to venous return
    • venous return curve is shifted to the right and slope is decreased
  38. What can shift the cardiac output curve to the right
    increase in intraplerual pressure
  39. Hexamethonium
    blocks sympathetic nervous system
  40. effects of opening an AV fistula
    • decrease resistance to venous return thus increasing the slope of venous return
    • slight increase in cardiac output due to the decrease in peripheral resistance
    • after sympathetics kick in the CO output curve is increased and the venous return curve is shifted right from the constriction of the veins and increase in systemic filling pressure
    • After a couple weeks volume has increased shifting the venous return curve farther right, increase in systemic filling pressure, and an increase in CO
  41. Fick principle
    CO = O2 absorbed by the lungs / AV O2 difference

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