Factors Affecting Blood Flow Flashcards
What are some factors affecting blood flow?
- Length of tube
- Viscosity of liquid flowing in tube
- Pressure gradient across length of tube
- Cross sectional area of tube
- Diameter of tube
What can the pouseille equation calculate and how does it change when values in the equation change?
Flow (G)
- If you increase a value in top, flow increases.
- If you increase a value on bottom, flow decreases.
Why is the equation flawed?
It works with a few assumptions:
- Flow is laminar
- Flow is non-pulsatile
- Flow through a uniform, straight pipe.
What are some determinants of flow?
- Pressure gradient; from high to low pressure
- Resistance- proportional to diameter
- As branching increases down the arterial tree, resistance increases and flow is reduced
What are examples of defects that may occur due to lack of blood flow?
- Critical limb ischaemia
- Reynauld’s
- Coronary artery disease
What are some defects highlighting importance of viscosity and how can risk of these be reduced?
- Deep vein thrombosis
- Dehydration; blood viscosity increases so flow decreases
- To reduce risk; -movement to encourage venous return
- hydration and reduce alcohol
- compression socks and affect pressure gradient
How does local control of blood pressure work?
- A well developed system which modulates local blood pressure/flow and can override central control systems.
- Independant of nerves/hormones
- Modified by local changes in TPR
- Metabolites, blood gases and endothelium derived factors are involved.
How do we control our blood pressure?
Both centrally and peripherally
What are baroreceptors and where are they found?
- Present in carotid sinus and aortic arch
- Respond to stretching (pressure changes)
- More sensitive around MAP
- Input to NTS in brain
What does a decrease in arterial pressure cause?
Decrease in arterial baroreceptors firing causing:
- Increase in sympathetic outflow to heart, arterioles and veins.
- Decrease in parasympathetic outflow to heart.
What are veins and how is venous flow important?
- Veins; a low pressure resevoir system which contains around 70% of total blood volume.
- Venous pressure/flow important in determining tissue fluid balance (in capillaries) and blood return to the heart.
What is the cross section and pressure gradient like?
Cross sectional area large but pressure gradient is small.
How is flow maintained in veins?
- Valves direct blood to heart (stop backflow)
- Skeletal muscle pump (have perastalic effect to move blood back to heart)
- Respiratory movements aid venous return
- Sympathetic nerves; noradrenaline constricts veins therefore increased venous return to the heart.
What is preload and what happens if it increases?
Venous return to right ventricle is termed preload
If preload increases, the heart has to work harder to pump the blood out. This can be a problem in: -heart failure, -coronary artery disease (angina)
How can preload be decreased and what are treatment aims?
Can be reduced by nitroglycerin.
-Objective of treatment in angina is venodilation, as this will reduce preload, cardiac output and therfore cardiac work.
