Control of cardiac output Flashcards
Equation for cardiac output
CO = HR x SV
CO = BP / TPR
What does stagnation of blood cause
Reduction of oxygen and CO2 exchange, doesn’t provide nutrients to tissues, leading to ischaemic damage and death.
What is preload?
During diastole, ventricles fill with blood. The more blood enters the heart, the more the walls of the heart get stretched. Affects force of contraction
The greater the preload, the greater the contraction, the higher the volume of blood ejected
What is end diastolic volume
Amount of blood in the heart at the end of relaxation, determines preload, determines force of contraction and ejection volume
What is Starling’s law of the heart
Energy of contraction of cardiac muscle is relative to the muscle fibre length at rest.
Why does stretching a muscle fibre at rest make it have a greater energy of contraction?
Myosin interacts with actin, brings Z bands close together. When stretched, more capacity for movement, less mechanical interference, and Z lines can move much more. More sites for calcium binding exposed in the stretched muscle.
Name the main factors that affect stroke volume
- Force with which the heart contracts
- Filling pressure (preload)
- Resistance to contraction (afterload)
How does Starling’s law show balance in the heart
As more blood returns to the right side of the heart, the right ventricle stretches. Greater energy of contraction, more blood ejected to the lungs, more blood from lungs to left side of heart to stretch left ventricle.
What is afterload?
(Laplace’s Law)
Opposing the contraction that ejects blood from the heart and is determined by wall stress directed through the heart wall.
Equation for wall stress (S)
S = T / W
Total tension in the wall is proportional to..
Pressure of blood in the chamber multiplied by the radius.
Why does radius affect wall stress/afterload
Thicker wall -> Larger ventricle radius -> More wall stress directed through heart wall -> More afterload
How does Starling’s law overcome Laplace’s law
Wall stress is overcome with an increased preload, so stretched heart although bigger will have a greater force of contraction.
What is baroreflex
Decreased sympathetic tone, decreases blood pressure
What is the energy of contraction
Amount of work required to generate stroke volume
