Lecture 7 - Biophysics of the Circulation Flashcards
What are the biophysics of circulation?
Study of the forces that result in blood flow = hemodynamics
BV for 170 lb patient?
5L
List the distribution of BV throughout blood vessels.
- Veins: 64%
- Lungs: 9%
- Small arteries and arterioles: 8%
- Large arteries: 7%
- Heart in diastole: 7%
- Capillaries: 5%
In what veins are there the most blood?
Small veins and venules
What are resistant forces?
Forces that oppose blood flow
Compare the resistance in small arteries/arterioles and that in the capillaries?
The resistance in the small arteries/arterioles is due to the contracting vs in the capillaries it is static
What does blood flow to a particular region of the body depend on?
- CO
2. Regional resistance
How does total cross sectional area change from arteries to veins?
Aorta < arteries ««_space;capillaries > venules > veins > vena cavae
How does velocity change from arteries to veins?
Aorta > arteries»_space;»> capillaries < venules < veins < vena cavae
Velocity of blood in aorta?
~30 cm/s
Velocity of blood in capillaries? Purpose?
~0.1 cm/s
To ensure that the exchange can take place
Equation to relate velocity, flow, and area of blood in vessels?
Flow (Q) = V x A
V = blood velocity A = cross-sectional area
Is blood flow constant throughout the circulatory system?
YUP
How are blood velocity and cross-sectional area related?
Inversely
Largest vessel in body? A?
Aorta
A = 4 cm^2
What is Poiseuille’s Law?
Flow equation, that expands on the original Q = ΔP/R:
Q = π.(P1 - P2).r^4/(8.η.L)
r = vessel radius
η = fluid viscosity
L = length of the vessel
List the factors that affect resistance to flow of fluid through any system of tubes.
- η = fluid viscosity
- L = length of the vessel
- r = radius of the vessel
Based on Poiseuille’s Law, what is the most important factor affecting flow rate?
Radius of the vessel
Does normal blood viscosity vary throughout the CV system?
NOPE
Rearranging Poiseuille’s Law, what is the equation for resistance?
R = 8.η.L/π.r^4
How does doubling the resistance affect the flow rate?
Halves it
How does doubling the radius of the vessel affect flow rate?
16x
What is total peripheral resistance (TPR)? Other name?
Cumulative resistance in the blood vessels of the systemic circulation = systemic vascular resistance
What is the peripheral resistance unit (PRU)?
Unit of vascular resistance equal to the resistance that produces a pressure difference of 1 mmHg at a blood flow of 1 mL/min
Equation to calculate total peripheral resistance?
TPR = (aortic P - CVP)/CO
CVP = central venous P
Equation to calculate total pulmonary resistance (PVR)?
PVR = Pulmonary artery P - LA P/CO
Normal PVR?
0.002 PRU
How can TPR be approximated in a clinical situation?
TPR = MAP/CO
How to measure total resistance in series circuit?
RT = R1 + R2 + R3 + … + Rn
What is the pressure gradient Pi-Po equal to in series and parallel circuits?
The sum of all the pressure gradients: (Pi-P1)+(P1-P2)+(P2-Po)
What is the equation of the total flow of a circuit in parallel?
Qtotal = Q1 + Q2 + Q3 + … + Qn
How to measure total resistance in parallel circuit? Why?
1/RT = 1/R1 + 1/R2 + 1/R3 + … + 1/Rn
Because as we add more ways for the flood the flow, the resistance is decreased
What are the 2 types of blood flow? What comprises the majority of flow in the CV system?
- Laminar***
2. Turbulent
Describe laminar flow.
Streamlined, organized, and unidirectional laminae which barely mix make up the flow
What happens if resistance is increased in a regional circulation?
Blood will be diverted to other parts of the body
What happens if resistance is increased in the whole systemic circulation?
BP elevation
Describe the velocity of laminae in laminar flow. What does this create?
Laminae nearest the wall is stationary because its contact with the wall will cause the most resistance
Velocity gets higher and higher toward the center => this creates a parabolic velocity front of the flow
Describe turbulent flow.
Disorganized flow in axial, radial, and circumferential directions characterized by eddies, whorls, stasis, and mixing of layers (but overall downstream)
What is blood stasis?
Blood stagnation
How does turbulent flow affect BP? Consequence?
Reduces the flow associated with a pressure gradient and increases pressure in the vessel (corrected for by myogenic regulation)
Regions with turbulence are more prone to vascular disease
How does turbulent flow present on auscultation?
Can be heard as murmurs or vascular bruits
What is Reynold’s number? Unit?
The point at which laminar flow transitions into turbulent flow
Dimensionless number
Common vessels where turbulent flow and CVD occur?
- Coronary arteries
- Carotid arteries
- Abdominal aorta
Equation for Reynold’s number?
Re = ρ.D.v/η
ρ = fluid density D = vessel diameter v = mean velocity η = fluid viscosity
How can Reynold’s number predict flow? What to note?
- Re<2000 => laminar flow
- Re>2000 => turbulent flow
Note: the number 2000 varies, but this is a pretty good approximation of when the shift happens
How does turbulent flow affect the flow associated with a particular pressure gradient?
Reduces it
What is the density of blood? Does it vary?
1 g/mL
NOPE
How does viscosity affect flow?
It organizes it
What is viscosity?
Property of fluids that resists flow
6 causes of turbulence?
- High velocity flow
- Local obstruction
- Abrupt increase in diameter
- Local obstruction + abrupt increase in diameter
- Branch points in vessels
- Vessel contorsion
What does vessel caliber mean?
Vessel diameter
Why does a local obstruction cause turbulence?
Because the vessel diameter increases beyond the area of occlusion (increase in Re)
Why is turbulent so common in coronary arteries?
Large vessels with fast velocity blood flow and lots of branching PLUS the vessels are contorsioning as the heart beats
Why is turbulent so common in carotid arteries?
Large vessels with lots of branching
Why is turbulent so common in abdominal aorta?
Large vessel with lots of branching and high velocity
What is wall tension? Unit?
Tendency for a longitudinal slit in the vessel wall to pull apart
Unit = force/unit length tangential to the vessel wall
Equation for wall tension? What is this called?
La Place’s Law:
T = P.r
P = transmural pressure r = vessel radius
Which vessels are least susceptible to bursting? Why?
Small vessels in the microcirculation (arterioles, capillaries, venules) because their small radii protect them from a large wall tension even though transmural pressure in these can be large
What is the transmural pressure of a vessel?
Pressure difference across a vessel wall (inside vs outside)
How does wall thickness change with radius?
Large radius => thinner wall
BP in capillary?
35 mmHg
4 vessels of microcirculation?
- Arterioles
- Metarterioles
- Capillaries
- Venules
What is a metarteriole?
Short vessel that links arterioles and capillaries
What allows large vessels to withstand their high wall tension?
Structural features: elastic fibers
Explain how vessel size and wall tension applies to an aortic aneurysm.
Larger vessels, like arteries are more susceptible to bursting because they have a large radius, so a large wall tension, and if the wall is weakened, then the radius will get even larger and eventually the artery will burst
If a tube X splits into 2 parallel tubes, A and B, with A having a smaller radius than B, in which tube will velocity be higher? Why?
In tube B, because larger diameter means less resistance and more flow (blood follows the pass of least resistance) => higher velocity
When can you assume that velocity is faster in a smaller vessel?
When the vessels are in series and the flow rate is kept constant
When can you assume that velocity is faster in a larger vessel?
When the flow rate is increased in one of the vessels (larger than the others)
How does blood viscosity affect velocity?
Decreases it because of increased resistance
How does tripling the radius of the vessel affect flow rate?
81x
What kind of exchange occurs at the level of the collecting venules?
Diapedesis