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# Hemodynamics

### Cardiology Hemodynamics

Question | Answer |
---|---|

What are the components of blood? | Plasma (liquid component); cellular components (RBCs, WBCs, and platelets) |

Hematocrit | volume of RBCs in blood (bottom 45% usually in centrifuged blood) |

Plasma | Liquid component; contains variety of proteins (albumin, globulin), electrolytes, hormones, enzymes, blood gases |

What is the mathematical formulation for Ohm's law (or an extension of) in fluid mechanical terms? | Q = (P2-P1)/R , where Q is the volumetric flow rate, P2-P1 is the pressure drop, and R is the resistance provided by the vessels. IMPORTANT: flow is a function of pressure CHANGE, not absolute pressure! |

What assumptions does Ohm's law for fluid flow assume about the cardiovascular system (that is, why does not approximate it perfectly)? | Assumes blood vessels rigid tubs and blood is a perfect Newtonian fluid |

What is the mathematical formulation of Poiseuille's law? | Q = (pi * (P2-P1) * r^4) / (8 * n * l) where P2-P1 is the pressure drop across the two ends of the pipe (blood vessel), r is the radius of the tube, n is the viscosity of the fluid, and l is the length of the vessel |

What assumptions does Poiseullie's law assume? | Assumes that there is laminar flow: the maximum velocity is in the middle, and the velocity towards the edges of the tube is near 0 (due to drag), creating semi-parabolic flow. |

What are murmurs? | Result of localized turbulence, where some energy is lost due to noise (usually due to structural defect); normal blood flow is laminar and NOT TURBULENT, thus, NORMAL BLOOD FLOW IS SILENT |

What is the mathematical formulation for the Reynold's number? | Re = (d*v*D)/n , where d is the density of the fluid, v is the velocity of the fluid, D is the diameter of the tube, and n is the viscosity of the fluid. |

What is the critical value for the Re in blood (that is, at what Reynold's number will you get turbulent flow?) | ~1000; at values >1000, you will have turbulent flow, while at values <<1000, you will have laminar flow |

By what factor can arterioles change their radius? Thus, what is the change in the possible flow that passes through? | By a factor of 4 (constrict up to 1/4 of their diameter); Can decrease flow by a factor of 4^4 = 256 times |

If hematocrit increases, what does that do to viscosity? To flow? | Viscosity increases --> flow decreases |

Compare the blood flow rate of patients suffering from anemia to those suffering from polycythemia | Anemic patients have LESS hematocrit --> faster blood flow, while patients with polycythemia have MORE hematocrit --> slower blood flow |

Distole | time during which cardiac muscles relax |

Systole | time during which cardiac muscles contract |

How long does a distole last relative to a systole? | Distole lasts ~2x long as systol (e.g. if heart rate is 67 bpms, cardiac cycle is 900 msec --> distole = 600 msecs, systole = 300 msecs) |

How is energy transferred from the heart to the arteries (i.e. where does the energy of the pressure wave go?) | High pressure blood from the contraction of the ventricles transfers 95% of energy to artery walls (fibrous and elastic); that energy gets turned into kinetic energy via elastic recoil |

What is the advantage of storing energy from ventricles into the elastic (potential) energy of the arteries? | Limits the drop in arterial blood pressure during diastole; further down, arterial tree can function as elastic reservoir for blood flow from arteries to capillaries (allowing for blood flow to be continuous) even if flow is pulsatile |

Compliance | Tendency of a hollow organ to resist recoil toward its original dimensions. C = (V2-V1)/(P2-P1), where V2-V1 is the stroke volume and P2-P1 is the pulse pressure |

Mean arterial pressure (MAP) | Driving force for fluid entering arterial circulation |

How would you estimate mean arterial blood pressure (MAP) based on diastolic and systolic pressure? | MAP = 2/3 (diastolic P) + 1/3 (systolic P); based on the fact that diastole lasts twice as long as systole, though this is inaccurate for high heart rates for this reason |

What is the formulation of Ohm's law for flow in terms of the Mean arterial pressure and the total peripheral resistance? | MAP = CO * TPR , where MAP is the mean arterial pressure, CO is the cardiac output, and TPR is the total peripheral resistance |