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WEEK 21:
Haemodynamics:
| Question | Answer |
|---|---|
| haemodynamics | physical laws governing pressure/ flow relationships in blood vessels |
| how do RBCs increase flow resistance | by rubbing up against vessel wall |
| why do RBC travel in the middle of the vessel | minimise interactions with endothelium to minimise resistance |
| flow equation | PΔ/R |
| P | pressure gradient between arteries and veins (created by pumping action of heart) |
| flow is directly related to | pressure difference (Pi-Po) |
| the greater the difference in pressure the | greater the flow (Darcy's law of flow) |
| R | resistance (measure of the degree to which the blood vessels resist flow of blood through it) |
| flow is inversely related to | resistance (R) AND/OR vessel length |
| Q | flow (ml/min) |
| ΔP | pressure difference across a vessels |
| r | internal radius of measured vessel |
| L | length of vessel |
| η | viscosity of blood |
| what do radius (r), length (L) and viscosity (η) all contribute to | flow resistance |
| how does the vascular smooth muscle cells of arteries and arterioles control blood flow | contract and relax |
| blood flow is proportional to what (r) | r^4 so a 2 fold change in radius causes a 16 fold change in flow |
| a fluids viscosity is measured relative to | water |
| haematocrit | measures percentage of whole blood volume that is occupied by RBCs |
| normal values for Hct | 41%-53% for males and 36%-46% for females |
| vascular resistance represents | afterload to the LV and determines how hard the heart must work to generate output |
| systemic vascular resistance (SVR) equation | SVR = (MAP- CVP) / CO |
| MAP | (mean arterial pressure) time averaged value that recognises that arterial pressure rises and falls in stepw ith cardiac cycle |
| CVP | central venous pressure |
| elastic arteries have (2) | smooth muscle layers and are rich in elastin fibres |
| resistance vessels are (2) | arteries and arterioles - where largest drop is pressure occurs |
| capillaries permit communication between blood and cells via | junctional clefts between adjacent cells and fenestrations |
| low pressure conduits | venules and veins |
| as venules progress towards the heart what happens | they fuse with each other |
| vein walls being thin means | they are highly distensible and able to accommodate large volumes of blood |
| under resting conditions, most of the total blood volume resides where | venous compartment - creating a reservoir that is used to boost ventricular preload and CO |
| preload | load that is applied to a myocyte and establishes muscle length before contraction begins (how much the heart muscle is stretched before it contracts) |
| in LV the preload equates with | volume of blood entering the chamber during diastole (EDV- end diastolic volume) |
| why does blood reside in venous compartment at rest | since walls are thin and readily distend to accommodate volume |
| what happens when VR (vascular resistance) is reduced | CO reduces too |
| disadvantage of veins thin walls | allow them to collapse easily when intraluminal pressure falls |
| vascular smooth muscles cells in vein walls are innervated by and contract during | sympathetic activation |
| venoconstriction effects (3) | mobilises blood reservoir, reduces overall capacity, and decreases transit time |
| describe mobilisation as a result of venoconstriction | venoconstriction raises venous pressure and drives blood out of reservoir. Valves ensure blood is forced towards the heart where it preloads the LV and increases CO through the Frank-Starling mechanism |
| describe capacity as a result of venoconstriction | venoconstriction decreases internal diameter of veins which decreases capacity |
| describe transit time as a result of venoconstriction | reducing system capacity means blood is transferred meaning less time (transit time) to get reoxygenated |
Created by:
kablooey
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