Haemodynamics Flashcards
Define cardiac output
Sum of all local tissue blood flows
What is velocity proportional to
Inversley proportional to area. As the area increases the velocity stays the same. Flow is consistent throughout
Mean arterial pressure equals
TPR * CO
Ohms law
Flow equals the change in pressure/resistance
What is the average mean arterial blood pressure
100mmHg
Cardiac output formula
CO= (PA-PV)/TPR
But typically venous pressure is ) so:
CO= MAP/TPR
What is a typical pathology leading to increased venous pressure (and thus the need to include venous pressure when calculating CO
Heart failure
Parallel resistance consequences
Little effect on total flow, big effect on local flow
Poiseuille law
Flow depends the driving pressure, tube geometry and the viscosity of the fluid
What is the factor that has the biggest influence on flow
The radius of the vessel
What are the determinants of blood viscosity
Temperature and haematocrit
Also velocity. Increased velocity results in an apparent decrease in velocity due to RBC accumulating centrally in the vessel.
Velocity also can decrease in very small vessels as less RBC fit in
Define heamatocrit
The ratio of red cell volume in a sample of peripheral blood
Why is blood flow not directly proportional to pressure
Because resistance is variable
The contractile state of vascular smooth muscle is controlled by…
Vascular endothelial cells (release NO) Sympathetic control (NA) Circulating hormones (vasopressin, angiotensin II)
What induces release of NO
Increased flow increases heamodynamic stress, results in endothelial cells releasing NO
Factors that may result in turbulent flow
High fluid densities Large tube diameters High flow velocities Low fluid viscosities Abrupt variation in tube dimensions Irregularities in the tube walls
What is the use of reynolds number and what variables is it based on
A number we can use to predict wether we will get turbulent flow or not. Based on density of fluid, mean velocity of flow, diameter of the tube, viscosity
Describe Bernoulli’s principle
That energy remains constant. Energy in blood flow is derived from pressure energy (heart pumping), gravitational energy, and kinetic energy. If one of these goes up the other/s go down. E.g. in stenosis velocity increases
What is transmural pressure
The pressure inside the vessel minus the pressure outside the vessel
