Define afterload and describe the physiological factors that may affect afterload

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College Answer

Definitions for afterload vary slightly amongst common physiology textbooks, and 
candidates were expected to mention any one commonly accepted definition. Essentially 
afterload is the resistance to ventricular ejection - the "load" that the heart must eject blood 
against and is related to ventricular wall stress (Law of Laplace, T=Pt.r/u). Candidates were 
expected to mention aortic valve and systemic vascular resistance, aortic impedance, blood 
viscosity, intathoracic pressure and relationship of ventricular radius and volume. 
Candidates generally did well, but few substantially good answers, with a lack of detail being 
the biggest limiting factor.