Lecture 17: Heamodynamics and vascular function Flashcards
Describe MABP = CO x TPR
MABP = Fairly constant CO = Total blood flow, set by the bodies needs, individual tissue is highly regulated by response to tissue needs. TPR = Increasing resistance increases upstream pressure to maintain flow.
Describe how resistance influences flow?
Resistance increases and reduces flow to the local vascular bed
BUT
Pressure increases in the vessels prior.
Parallel circuits see no change in total flow as blood is shunted elsewhere.
Also, Q=p/tpr…. and resistance in parallel doesn’t change much. So think about the consequences of this.
Where does the greatest pressure drop occur?
Small arteries/arterioles, highest resistance (variable) results in largest pressure drop.
How is the Change in pressure determined across a vascular system?
Also what is the general equation for pressure?
Change P = Pa-Pv (Pv usually close to 0 and thus ignored)
P = CO x R
What are 3 notes about pressures across the vascular system?
- Major pressure drop occurs in arteries and small arterioles
- The inverse relationship between blood flow velocity and cross sectional area {V < 1/A]
- The maximal cross-sectional area and minimal flow rate in the capillaries.
What is the ohms low of blood flow?
Blood flow (Q) = Change in pressure / resistance
What is blood flow (Q) equal to?
Cardiac output
What is another equation for cardiac output?
CO = (Pa-Pv)/TPR
What is bloodflow/CO proportional to?
- Directly proportional to the pressure gradient
2. Inversely proportional to TPR (most impacted by precapillary resistance vessels)
Describe resistance in a series;
R(total) = Summation of resistances
i.e aortic stenosis = change in resistance (as its in series)
Greatest contribution from arterial system thus changing its resistance has greatest effect on total
Describe resistance in parallel;
R(total) = 1/R + 1/R etc
i.e Restricted renal art. blood flows elsewhere but no change in resistance
What are two important principles regarding resistance in parallel?
- The total resistance of a network of parallel vessels is less than the resistance of the vessel having the lowest resistance. i.e an arrangement of parallel vessels reduces resistance to flow.
- When many are in parallel, changing the resistance of a small number of vessels will have little effect on the total resistance for the segment
How does parallel resistance influence local flow?
Little effect on total flow, but big effect on local flow. i.e can restrict blood to specific tissue and flow will move to the other parallel vessels
What is poiseuilles equation?
Q (flow) = P(change) * [ (3.14 x r^4/8nL]
n = blood viscosity L = Length of vessel
i.e flow is dependant on pressure, tube geometry, and viscosity of the fluid. [In a glass tube]
How can poiseuilles law be rearranged and whats the importance of this?
Q = P(Change) / R
Thus
R = 8nL / 3.14*r^4
i. e Resistance is proportional to
1. Tube length
2. Viscosity of fluid
and inversely proportional to
3. THe radius raised to the power of four
Take home: A small change in the radius of blood vessels has a large effect on resistance.
Whats the impact of resistance being inversely proportional to the radius raised to the power of four?
Drugs that dilate the vessels therefore have powerful effect on blood flow
What is viscosity?
A measure of the friction between adjacent layers of fluid as they slide over one another, increasing viscosity decreases flow
What are the determinants of blood viscosity?
- Temperature (colder, blood gets thicker)
- Heamatocrit (the ratio of red cell volume to total cell volume) (inc. this inc. viscosity) (blood loss replaced with saline, which decreases viscosity)
- Shear rate (velocity)
- Vessel diameter (very small vessels appear to decrease viscosity(Axial streaming))
Describe how shear rate influences blood viscosity;
At very low velocities blood becomes non-newtonian
- Viscosity is no longer independent of shear rate.
- Viscosity increases as cells aggregate (sludging of blood)
What 3 things do poiseuilles equation assume?
- Steady laminar flow
- Rigid straight tube
- Newtonian fluid
We do not have rigid straight tubes, we have distensible vessels, describe what this means? and describe two differing instances
Increasing pressure results in:
- Distension of blood vessel
- Thus reduced resistance
- Increased blood flow
Decreasing pressure results in:
- Reduced stretch of vessels
- Increased resistance
- Decreased blood flow
What does auto regulation mean for Q=P/TPR
Blood flow is not directly proportional to the pressure gradient.
In many vascular beds there are local mechanisms to ensure blood flow remains relatively constant following changes of pressure.
What are four things the contractile state of vascular smooth muscle controlled by?
- Myogenic mechanisms i.e stretch, pH
- Vascular endothelial cells (NO)
- Neural
- Circulating hormones i.e ANG2`
Describe why shear stress influence vessels;
Endothelium is very sensitive to shear stress;
Inc. flow = inc. shear stress
- NO release
- Vasodilation
Potential damage
Do we have laminar flow?
We can have turbulent flow from;
- High fluid densities
- large tube diam
- high flow velocities
- low fluid viscosities
- abrupt variations in tube dimensions
- Irregularities in the tube walls
What is reynolds number? and its equation
A predictor of turbulent flow, when Re is greater than 2000-3000
Re = pvD/n
p = density v = mean velocity of flow D = diam of tube
What increases Re?
An increase in diam, velocity or decrease in viscosity will lead to increase in Re
What is velocity of blood inversely proportional to?
For constant Q,
v = 1/A
Inversely proportional to cross sectional area
What is bernoullis principle?
Conservation of energy
States that: Total energy of laminar flow without resistance is constant and equals the sum of
- Pressure energy
- Potential energy
- Kinetic energy
(Some energy lost to resistance)
i.e velocity can affect pressure.
How does bournoullis principle come into effect when there is a narrowing of a vessel?
When there is a narrowing of a vessel:
- V increases
- Kinetic energy increases
- Total energy remains constant
BUT P must drop in this narrow part..
Describe how stenosis impacts vascular function?
Stenosis = narrowing
Bournoullis principle; Increased velocity and decreased pressure
Increased velocity then into post stenosis widening lumen will in turn create turbulent flow as Re has increased.
What is the ability of blood vessels to change tone also known as?
Compliance
