Control of Cardiac Output


How filling pressure and inotropic state govern cardiac output
Hannah Tribe
Flashcards by Hannah Tribe, updated more than 1 year ago
Hannah Tribe
Created by Hannah Tribe over 8 years ago

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Question Answer
What 2 factors determine the energy of contraction? Starling's Law and contractility of the myocardium
What does Starling's Law state? "The more the cardiac myocytes and stretched at rest, the more they will contract."
What does this mean? If more blood enters the heart, the ventricular myocytes will stretch more so when they contract, they will do so with more force so the volume of blood ejected also increases.
What is the mechanism behind Starling's Law? It is a property of the cardiac muscle itself. When more blood fills the ventricles, the actin and myosin filaments are stretched further apart. This allows more Ca++ ions to bind to actin as there is fewer myosin fibres in the way. More Ca++ bound means more cross-bridges can form. (SV increases)
What does Starling's Law do? (4) 1. Makes sure L and R outputs balance 2. Causes drop in CO during haemorrhage (helps preserve blood volume), and restores it when IV blood given 3. Causes CO drop when standing, as blood pools in legs = less venous return 4. Increases SV during exercise, as increased HR causes increased venous return
What is LaPlace's Law? "As the chamber gets smaller, the pressure exerted by the walls inwards increases" P = (2 S w)/r
What do P, S, w and r refer to in LaPlace's Law? P = pressure inside the chamber S = wall tension w = wall thickness r = radius of chamber
What does LaPlace's Law do? (3) 1. Helps ejection, as when ventricles contract, r is decreasing so pressure increases. 2. Opposes Starling's Law, because and increased pre-load would stretch the ventricles, which decreases r and thus decreases pressure. 3. Contributes to heart failure. When there is pathology that prevents efficient myocyte contraction, blood remains in the ventricles which increases the radius, which means not enough pressure is generated to maintain a normal SV.
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