Flow and Pressure Flashcards
Why do we need a cardiovascular system
Diffusion is the only other means of transporting and it has its limitations ie. if organism is small enough, SA: V is high which means that the diffusion of O2 in and CO2 out is possible versus multi-organ organisms- NOT POSSIBLE as diffusion would be too slow
Describe the CV system in humans
Closed system
2 pumps in a series
- Pulmonary - low resistance, low pressure ( in series with cystemic)
- Systemic - High resistance, high pressure ( in parallel with each other)
Terminology
Filling of the heart—> determined by the central venous pressure
The filling pressure = PRELOAD
Resistance to flow—> Vascular bed of the systemic circulation
total peripheral resistance- determines the pressure load on the left heart
Flow out of the heart—>
SV- volume per beat
CO- volume per minute
CO= SV x Heart Rate
Flow into the heart—-> Venous return - volume per minute which means it equals the cardiac output
Movement of substances in the blood and to the cellS
Bulk flow- transport within the Blood/Air due to the pressure difference
Passive Diffusion- movement of the specified substance down a concentration gradient
Ficks Law: Rate of diffusion
Area over which diffusion occurs
Concentration difference
Distance over which it has to travel
Ficks Law: How easily a substance diffuses
Temperature
Solubility of a substance
square root of the molecular weight
Rate of diffusion =
( Concentration diff x Area over which diffusion occurs xs solubility) / (distance x square root of the molecular weight )
Flow in tubes- Darcy’s Law
Flow is proportional to the pressure difference ( P1-P2)
Flow is inversely proportional to the resistance to flow
When P1 is greater than P2 the flow will be down the pressure gradient
Flow = (P1 -P2) / R
What can affect the resistance ? Hence suggest a better formula
A small change in diameter –> large change in resistance therefore flow
R = 8VL / pi x r^4
Flow = (P1 -P2) x pi x r^4 /( 8VL)
Comment on viscosity
The thicker the fluid is the higher the viscosity
Red Cell Mass and plasma proteins —> Blood = viscous
Laminar Flow
Viscous drag at the sides of the tube slows down the fluid
Fastest movement is in the centre
More apparent as the radius becomes smaller
Axial Streaming
Cells tend to become alligned in the fastest moving part of the liquid
Reduces the viscosity of the fluid
Comment on the flow of RBCs
Red blood cells= 7 micrometers
Capillaries = 6 micrometers in diameter
Cells fit the capillary like a plug- slips through easily- viscosity similar to plasma
Comment on the flow of tubes- turbulence
High velocity, sharp edges, branch points in large tubes disrupts the laminar flow—> leads to turbulence—> increase in resistance due to increase in vibrations
High velocity is due to the narrow heart valves
High velocity can also be due to narrow airways
which causes murmurs and wheezes
Flow in flexible tubes
Distensible vessel- increasing pressure, means than tubes passively dilate, diameter increases, decreasing resistance, increasing flow
Distensible vessel and myogenic tone means that they contract passively in response to pressure to maintain the flow
Resistance in series
R total = R1 + R2
Adding pressure will increase RT
Resistance in parallel
1/RT = 1/R1 + 1/R2
Adding pressure will decrease RT
How is blood flow regulated?
Blood flow through each organ is regulated independently of others and even indapendantly of mean arterial blood and Independent control of resistance
Scenario A - P1—>P2 is constant
Flow will be P1—> P2
If Ra ( a resistance artery) increases, decrease flow to that area of capillaries
Total pressure is maintained- flow to other capillaries beds stays the same
Means that its flow can be controlled independently of other tissues by constriction and dilation of the BV
Scenario B - P1 —> P2 is variable
If Ra ( a resistance artery) increases, decrease flow to that area of capillaries
Total pressure is not the same so flow increases to the other places - means that the total flow does not decrease
MABP =
CO X TPR
Effect of branching on pressure
Increases in resistance in parallel therefore total resistance drops but enough to maintain flow
Explain why resistance decreases from venues to veins
Although there are fewer in parallel resitstnance should increase,
As diameter increases, the resistance decreases again
