Lecture 7 - Biophysics of the Circulation Flashcards by Emma Djabali

What are the biophysics of circulation?

Study of the forces that result in blood flow = hemodynamics

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BV for 170 lb patient?

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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%

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In what veins are there the most blood?

Small veins and venules

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What are resistant forces?

Forces that oppose blood flow

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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

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What does blood flow to a particular region of the body depend on?

2. Regional resistance

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How does total cross sectional area change from arteries to veins?

Aorta < arteries «&laquo_space;capillaries > venules > veins > vena cavae

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How does velocity change from arteries to veins?

Aorta > arteries&raquo_space;»> capillaries < venules < veins < vena cavae

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Velocity of blood in aorta?

~30 cm/s

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Velocity of blood in capillaries? Purpose?

~0.1 cm/s

To ensure that the exchange can take place

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Equation to relate velocity, flow, and area of blood in vessels?

Flow (Q) = V x A

V = blood velocity
A = cross-sectional area

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Is blood flow constant throughout the circulatory system?

YUP

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How are blood velocity and cross-sectional area related?

Inversely

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Largest vessel in body? A?

Aorta

A = 4 cm^2

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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

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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

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Based on Poiseuille’s Law, what is the most important factor affecting flow rate?

Radius of the vessel

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Does normal blood viscosity vary throughout the CV system?

NOPE

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Rearranging Poiseuille’s Law, what is the equation for resistance?

R = 8.η.L/π.r^4

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How does doubling the resistance affect the flow rate?

Halves it

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How does doubling the radius of the vessel affect flow rate?

16x

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What is total peripheral resistance (TPR)? Other name?

Cumulative resistance in the blood vessels of the systemic circulation = systemic vascular resistance

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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

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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?

1. 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?

1. Coronary arteries 2. Carotid arteries 3. 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?

1. High velocity flow 2. Local obstruction 3. Abrupt increase in diameter 4. Local obstruction + abrupt increase in diameter 5. Branch points in vessels 6. 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?

1. Arterioles 2. Metarterioles 3. Capillaries 4. 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