Kaplan's 2008 Kaplan USMLE Step 1 Home Study Program-Brand New Volume PDF

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EDV). Similarly, on the cardiac ouput curve, an index of performance, the cardiac output, is plotted as a function of an index of filling, the right atrial pressure(Figure I-4-l7A). b. Like the peak systolic pressure-volume curve, the cardiac output curve illustrates Starling'slaw of the heart. q3 p (! ,. Figure l-4-17. The cardiac output-venous return graph. (A, cardiac output curve; B, venous return curve; C, interactionof cardiac output and venous return curves) c. Conversely,it is shifted downward and rightward by negative inotropic agents,increasesin afterload, or decreasesin heart rate, all of which decreasethe cardiac output at a given right atrial pressure.

However,this phase2 Caz*influx causesthe releaseof a much larger amount of Ca2*from the sarcoplasmicreticulum (SR). The amount of Ca2*releasedfrom the SR, and hencethe force generatedby the contraction, is determinedby the amount of extracellular Ca2*entering the cell during phase2, the time that haselapsedsincethe previous action potential (which, in turn, determineswhetherthe Ca2*releasechannelshaverecovered their ability to respondto Ca2*),and the sizeof the SR Ca2+stores. 52 Physiology c. Regulation of the sarcoplasmic reticulum Ca2*AIPase.

Phase0 depolarization is accomplishedby the slow inward Ca2*current (rather than the rapid inward Na* current) in slow fibers. In fact, slow response APs can be observedin fast fibers if the rapid inward Na* current is blocked by inactivating the Na* channels. Such inactivation can be achievedpharmacologically(procainamide, quinidine,lidocaine) or by making the RMP lessnegative(hyperkalemia,myocardial injury or ischemia). The conduction velocity refers to the speed of impulse propagation and is determined by various characteristics of the cell and also by the phase 0 slope and amplitude.

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2008 Kaplan USMLE Step 1 Home Study Program-Brand New Volume III: Organ Systems Book 1 by Kaplan


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