# Lecture 7 - Biophysics of the Circulation Flashcards

1
Q

What are the biophysics of circulation?

A

Study of the forces that result in blood flow = hemodynamics

2
Q

BV for 170 lb patient?

A

5L

3
Q

List the distribution of BV throughout blood vessels.

A
1. Veins: 64%
2. Lungs: 9%
3. Small arteries and arterioles: 8%
4. Large arteries: 7%
5. Heart in diastole: 7%
6. Capillaries: 5%
4
Q

In what veins are there the most blood?

A

Small veins and venules

5
Q

What are resistant forces?

A

Forces that oppose blood flow

6
Q

Compare the resistance in small arteries/arterioles and that in the capillaries?

A

The resistance in the small arteries/arterioles is due to the contracting vs in the capillaries it is static

7
Q

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

A
1. CO

2. Regional resistance

8
Q

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

A

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

9
Q

How does velocity change from arteries to veins?

A

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

10
Q

Velocity of blood in aorta?

A

~30 cm/s

11
Q

Velocity of blood in capillaries? Purpose?

A

~0.1 cm/s

To ensure that the exchange can take place

12
Q

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

A

Flow (Q) = V x A

```V = blood velocity
A = cross-sectional area```
13
Q

Is blood flow constant throughout the circulatory system?

A

YUP

14
Q

How are blood velocity and cross-sectional area related?

A

Inversely

15
Q

Largest vessel in body? A?

A

Aorta

A = 4 cm^2

16
Q

What is Poiseuille’s Law?

A

Flow equation, that expands on the original Q = ΔP/R:

Q = π.(P1 - P2).r^4/(8.η.L)

‎η‎‎ = fluid viscosity
L = length of the vessel

17
Q

List the factors that affect resistance to flow of fluid through any system of tubes.

A
1. ‎η‎‎ = fluid viscosity
2. L = length of the vessel
3. r = radius of the vessel
18
Q

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

A

19
Q

Does normal blood viscosity vary throughout the CV system?

A

NOPE

20
Q

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

A

R = 8.η.L/π.r^4

21
Q

How does doubling the resistance affect the flow rate?

A

Halves it

22
Q

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

A

16x

23
Q

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

A

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

24
Q

What is the peripheral resistance unit (PRU)?

A

Unit of vascular resistance equal to the resistance that produces a pressure difference of 1 mmHg at a blood flow of 1 mL/min

25
Q

Equation to calculate total peripheral resistance?

A

TPR = (aortic P - CVP)/CO

CVP = central venous P

26
Q

Equation to calculate total pulmonary resistance (PVR)?

A

PVR = Pulmonary artery P - LA P/CO

27
Q

Normal PVR?

A

0.002 PRU

28
Q

How can TPR be approximated in a clinical situation?

A

TPR = MAP/CO

29
Q

How to measure total resistance in series circuit?

A

RT = R1 + R2 + R3 + … + Rn

30
Q

What is the pressure gradient Pi-Po equal to in series and parallel circuits?

A

The sum of all the pressure gradients: (Pi-P1)+(P1-P2)+(P2-Po)

31
Q

What is the equation of the total flow of a circuit in parallel?

A

Qtotal = Q1 + Q2 + Q3 + … + Qn

32
Q

How to measure total resistance in parallel circuit? Why?

A

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

33
Q

What are the 2 types of blood flow? What comprises the majority of flow in the CV system?

A
1. Laminar***

2. Turbulent

34
Q

Describe laminar flow.

A

Streamlined, organized, and unidirectional laminae which barely mix make up the flow

35
Q

What happens if resistance is increased in a regional circulation?

A

Blood will be diverted to other parts of the body

36
Q

What happens if resistance is increased in the whole systemic circulation?

A

BP elevation

37
Q

Describe the velocity of laminae in laminar flow. What does this create?

A

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

38
Q

Describe turbulent flow.

A

Disorganized flow in axial, radial, and circumferential directions characterized by eddies, whorls, stasis, and mixing of layers (but overall downstream)

39
Q

What is blood stasis?

A

Blood stagnation

40
Q

How does turbulent flow affect BP? Consequence?

A

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

41
Q

How does turbulent flow present on auscultation?

A

Can be heard as murmurs or vascular bruits

42
Q

What is Reynold’s number? Unit?

A

The point at which laminar flow transitions into turbulent flow

Dimensionless number

43
Q

Common vessels where turbulent flow and CVD occur?

A
1. Coronary arteries
2. Carotid arteries
3. Abdominal aorta
44
Q

Equation for Reynold’s number?

A

Re = ρ.D.v/η

```ρ = fluid density
D = vessel diameter
v = mean velocity
η = fluid viscosity```
45
Q

How can Reynold’s number predict flow? What to note?

A
• 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

46
Q

How does turbulent flow affect the flow associated with a particular pressure gradient?

A

Reduces it

47
Q

What is the density of blood? Does it vary?

A

1 g/mL

NOPE

48
Q

How does viscosity affect flow?

A

It organizes it

49
Q

What is viscosity?

A

Property of fluids that resists flow

50
Q

6 causes of turbulence?

A
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
51
Q

What does vessel caliber mean?

A

Vessel diameter

52
Q

Why does a local obstruction cause turbulence?

A

Because the vessel diameter increases beyond the area of occlusion (increase in Re)

53
Q

Why is turbulent so common in coronary arteries?

A

Large vessels with fast velocity blood flow and lots of branching PLUS the vessels are contorsioning as the heart beats

54
Q

Why is turbulent so common in carotid arteries?

A

Large vessels with lots of branching

55
Q

Why is turbulent so common in abdominal aorta?

A

Large vessel with lots of branching and high velocity

56
Q

What is wall tension? Unit?

A

Tendency for a longitudinal slit in the vessel wall to pull apart

Unit = force/unit length tangential to the vessel wall

57
Q

Equation for wall tension? What is this called?

A

La Place’s Law:

T = P.r

```P = transmural pressure
58
Q

Which vessels are least susceptible to bursting? Why?

A

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

59
Q

What is the transmural pressure of a vessel?

A

Pressure difference across a vessel wall (inside vs outside)

60
Q

How does wall thickness change with radius?

A

61
Q

BP in capillary?

A

35 mmHg

62
Q

4 vessels of microcirculation?

A
1. Arterioles
2. Metarterioles
3. Capillaries
4. Venules
63
Q

What is a metarteriole?

A

Short vessel that links arterioles and capillaries

64
Q

What allows large vessels to withstand their high wall tension?

A

Structural features: elastic fibers

65
Q

Explain how vessel size and wall tension applies to an aortic aneurysm.

A

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

66
Q

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?

A

In tube B, because larger diameter means less resistance and more flow (blood follows the pass of least resistance) => higher velocity

67
Q

When can you assume that velocity is faster in a smaller vessel?

A

When the vessels are in series and the flow rate is kept constant

68
Q

When can you assume that velocity is faster in a larger vessel?

A

When the flow rate is increased in one of the vessels (larger than the others)

69
Q

How does blood viscosity affect velocity?

A

Decreases it because of increased resistance

70
Q

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

A

81x

71
Q

What kind of exchange occurs at the level of the collecting venules?

A

Diapedesis