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Biological Sciences
Peter A.Bowler

e109 10/29/12 When the muscle returns to its usual length, what is the mechanism for the action potentials to stay at the “normal” frequency? Answer: spindle response depends on speed of stretch as well as amount of stretch—both static and dynamic response Blood moves from areas of higher pressure to lower pressure Mean systemic blood pressure (mmHg) Aorta: highest blood pressure Pressure undergoes a gradual decline Venae cavae: lowest blood pressure Fluid flow through a tube depends on the pressure gradient -Fluid flows only if there is a positive pressure gradient -delta P = pressure difference (P1-P2) -delta P/ delta L = pressure gradient -analogy to height vs. slope in mountain +lower slope vs. steeper slope +nonzero pressure gradient means that the fluid is going to be pushed through the tube No pressure gradient, so no flow Flow depends on delta P(Pressure difference), not absolute P(and not the ratio of P!) -higher pressure difference, greater flow down tube -flow is equal if length of two tubes is equal and pressure difference is the same -Note that pressure is higher in top figure, so same flow rate but different stress on blood vessel walls Flow is proportional to (radius)^4, so a vessel with twice the radius 2x2x 2 x 2 = 16x the volume flow rate -delta P is exactly the same in the two cases as is length (delta P is a fixed parameter); only the diameter is different -“flow” in this context means volume flow rate: volume/time -Resistance is inversely proportional to (radius)^4: Resistance = 1/(radius)^4 -Flow is inversely proportional to resistance: Flow = 1/resistance -pressure differences are set and will not change or be different in this example; only the flow rate will be different
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