Factors Affecting Blood Flow: Pouseille's Law Flashcards
What is Reynaud’s?
A disorder in which the fingers or toes suddenly experience decreased blood circulation
What are the different factors controlling the flow of blood through blood vessels?
- Length of the ‘tube’
- Viscosity of the liquid flowing in the ‘tube’
- Pressure gradient across the length of the ‘tube’
- Cross sectional area of the ‘tube’
What is Poiseuille’s equation?
Q= (deltaP)(pi)(r^4)/8nl
Q= blood flow Delta P = Change in pressure Pi = constant r^4 = Radius raised to the power of 4 Eta (n) = Viscosity L = length of tube
What are the assumptions made about flow through blood vessels in the Poiseuille’s equation to allow it to work?
- Flow is laminar
- Flow is non-pulsatile
- Flow through a uniform, straight pipe (much more complex than this)
What conditions can be caused by increased viscosity of the blood?
- Deep vein thrombosis
- Venus pooling/stasis due to immobility
- Increased blood viscosity so decreased blood flow due to dehydration
- A thrombus developing in a vein = the thrombus can break off and lodge somewhere else like the lungs which can be very serious
How can you reduce the risk of increasing the viscosity of your blood?
- Movement to encourage venous return
- Hydration and reduce alcohol
- Compression socks
In which blood vessels is flow velocity the lowest?
Capillaries
Flow in blood vessels is determined by the pressure gradient. What does the blood flow from?
From high to low pressure
Flow in blood vessels determined by resistance. What is the resistance proportional to?
- Proportional to diameter of blood vessel
- As branching increases down the arterial tree, resistance increases and flow is reduced
From which blood vessel is there the biggest drop in blood pressure?
- As blood passes from large arteries to arterioles
In which blood vessels is velocity highest?
- Large arteries then large veins
In which blood vessels is cross sectional area largest?
Capillaries
What system are veins, and how much of the total blood volume is within the veins at any one time?
- A low pressure reservoir system which contains around 70% of the total blood volume
What is venous pressure/flow important in determining?
- Tissue fluid balance in capillaries and blood return to the heart
What are present in the veins to prevent the backflow of the blood to endure the flow is kept in one direction towards the heart?
Valves
In veins the cross sectional area is large, but what is small?
The pressure gradient
What kind of pump is present in veins?
Skeletal muscle pump
What effect does noradrenaline have on the veins?
- Constricts veins therefore increasing the venous return to the heart
What is preload?
- The venous return to the right ventricle - how much blood you fill up the heart with
If preload increases the heart has to work harder to pump the blood out. What can this be a problem in?
- Heart failure
- Coronary artery disease - angina
Effects of nitrates is primarily venodilation. How does this reduce cardiac work?
- Reduces preload (filling) on the heart so reducing cardiac work
What is active hyperaemia?
Changes in O2/CO2/cellular metabolites can dilate the arterioles
- Relaxation of blood vessels
What is reactive hyperaemia?
- Increase in blood flow after it is temporarily interrupted/blocked
Flow can be maintained by flow autoregulation. What does this mean?
The intrinsic capacity of arterioles to respond to changes in arterial pressure by a change in the diameter of the blood vessels so as to maintain constancy of the blood flow
