CVS Session 5: flow Flashcards
Describe the CVS in terms of series and parallel organisation
Systemic and pulmonary vascular system
Describe the vessels through which blood flows
Heart Large (elastic/conducting) arteries Medium (muscular/distributing) arteries Arterioles Metarterioles Capillaries Post-capillary venules Venules Medium veins Large veins Heart
Define flow
The volume of fluid passing a given point per unit time. It is proportional to the pressure difference between the ends of a vessel: higher pressure difference=higher flow
Define velocity
The rate of movement of fluid particles along the tube
What is resistance to flow determined by?
Nature of the fluid and vessel
Relationship between flow and velocity
Flow is constant at all points along a vessel
Velocity can vary if the radius of the tube changes
At a given flow, velocity is inversely proportional to cross sectional area:
-vessels with a small cross sectional area have a high velocity (e.g. aorta)
-vessels with a large cross sectional area have a low velocity (e.g. capillaries)
Where does blood flow fastest and slowest
Fastest where cross sectional area is least, e.g. aorta
Slowest in capillaries-allows time for gas and nutrient exchange
Laminar flow
In most vessels
Gradient of velocity from middle to edge: fastest flow in the centre, fluid is stationary at the edge
So cells flow down the middle and plasma down the outside
Turbulent flow
As mean velocity increases, turbulence eventually results
Velocity gradient breaks down: fluid tumbles down, resistance greatly increased
This occurs when there is:
-high flow velocity
-low viscosity
-irregular lumen of a vessel e.g. atherosclerosis
What determines flow in a vessel with constant pressure?
Mean velocity
What determines mean velocity?
- VISCOSITY of fluid
- mean velocity inversely proportional to viscosity
- in laminar flow fluid moves in concentric layers that slide over one another, with middle moving fastest. Viscosity is the extent to which fluid layers resist sliding over
- high viscosity=slower flow of central layers=lower velocity - RADIUS of tube
- mean velocity directly proportional to radius
- viscosity determines the slope of the velocity gradient. At a constant gradient, wider tube=faster flow in middle
Describe Poiseuille’s Law
Describes how flow is the product of mean velocity and cross sectional area
When flow is laminar and steady in blood vessels larger than arterioles, flow is:
-proportional to the difference between inflow vs. outflow, and to the fourth power of the radius
-inversely proportional to the length of the vessel and to the viscosity of the blood
Cause of hyperviscosity syndrome
- High plasma protein levels (treated by plasmapheresis)
- High rbc/wbc count (treated by phlebotomy)
Underlying cause is often neoplasia
Why can severe anaemia lead to functional murmurs?
High blood flow velocity
Redcued viscosity of blood due to low rbc
Relationship between pressure, flow and resistance
PRESSURE=FLOW X RESISTANCE
Resistance increases as viscosity increases; resistance decreases to the fourth power of an increase in radius
therefore
it is much harder to push blood through small vessels and to push thicker blood
If FLOW is fixed: the higher resistance, the greater the pressure change from one end of the vessel to the other
If PRESSURE is fixed: the higher resistance, the lower the flow
Resistances in series vs parallel
Vessels in SERIES: resistances ADD (total resistance=sum of individual resistances)
Vessels in PARALLEL: effective resistance is LOWER (reciprocal of total resistance=sum of reciprocals of individual resistances)
Why do capillaries offer little resistance when they are very narrow?
Although narrow, offer little collective resistance due to their parallel arrangment
