What is haemodynamics
The physical factors that govern blood flow
What is Darcy’s Law
Flow in the steady state is linearly proportional to the pressure difference between two points
What are the 3 patterns of flow in circulation
- Laminar flow - normal arteries and veins
- Turbulent flow – ventricles, ascending aorta
- Single-file flow – capillaries
What is resistance
Resistance to steady flow along a vessel is:
- Proportional to tube length and fluid viscosity
- Inversely proportional to tube radius raised to the 4th power
What is compliance
The change in volume per unit
change in distending pressure
What has a higher compliance, veins or arteries?
Whats the advantage of this?
- Veins
- Veins can accommodate a large increase in blood volume after a small BP increase
What does distending pressure do to vessel walls
Stretches them
What happens if the forces are unbalanced
The wall ruptures
Whats the law of LaPlace
Radius and wall thickness influence tension to withstand transmural pressure
What is microcirculation
The circulation of blood through the smallest blood vessels: the arterioles, capillaries and venules
What is autoregulation
The intrinsic adjustment of blood flow to a tissue of specific vascular bed such that the flow meets the local requirements at any given point in time
What are the 2 mechanisms that can change blood flow
- Change in diameter of the arterioles
- Altering the contraction of precapillary sphincters
What is metabolic control
- Relationship between rate of metabolism and rate of blood flow
- Intrinsic property of the microcirculation
What are the factors affecting metabolic control
- Oxygen
- Metabolic products
- Endothelium-derived relaxing factors (EDRFs)
What is myogenic control
Isolated, perfused organs can maintain a constant rate of blood flow through its vascular bed over a wide range of perfusion pressures
Describe long-term autoregulation
- May develop over a period of weeks to months
- Provoked in the heart by gradual, partial occlusion of a coronary vessel
- Chronic exposure to low pO2 elicits a similar response throughout the body
What is transcapillary solute exchange
- Lipophilic solutes (e.g. O2, CO2) enter/leave capillaries via the transcellular route
- Hydrophilic solutes cross through the intercellular clefts
What are starling forces
Govern the passive exchange of water between microcirculation and interstitial fluidW
What’s the lymphatic system
- Drainage system
- Helps maintain fluid balance in the body
- Defends the body from infections
