Week 06 Lect. 2 - Hemodynamics

Question 1

What is critical closing pressure?

What is its value?

Answer 1

internal pressure at which a blood vessel collapses and closes completely

~20 mmHG

(according to wikipedia, but I’ve seen different values)

Question 2

On a flow vs. pressure graph for human vessels, what are the two vessel types?

How do their graphs differ and why?

Answer 2

1. Lung-Type Vessels - constantly increasing slope as pressure increases beyond critical closing pressure
2. Kidney-Type Vessels - flow rises after CCP, but autoregulation via arterial smooth muscle causes a barely rising plateau from 50-150 mmHg, then flow increases more quickly after 150

Question 3

What are two reasons high pressures must be reach in blood vessels in order for the circulatory system to function?

Answer 3

1. RBCs are larger in diameter than capillaries - high pressures allow RBCs to deform and be pushed through capillary beds
2. Transmural Pressure - pressure on outside of vessels must be counteracted in order to open the vessels and alllow flow

Question 4

Why is there a constantly increasing slope to a flow vs. pressure graph of lung-type vessels?

Answer 4

• because these vessels are not rigid, they contain a high amount of elastic tissue
• as pressure increases, vessel radius also increases, which increases flow and decreases resistance

(to the fourth power, according to Hagen-Poiseuille equation)

Question 5

Explain the mechanism of arteriolar autoregulation of blood flow.

Answer 5

• Mechanosensitive Cation Channels - sense pressure increase + open
• Depolarization occurs
• VDCCs - open + allow calcium influx
• Ca²⁺ signal is induced
• Contraction occurs in smooth muscle
• Vessel Radius Decreases - resistance increases

Question 6

What conditions can cause increased hematocrit?

Answer 6

Polycythemia

Leukemia

Dehydration

Question 7

What is the viscosity of blood serum?

Answer 7

1.2 centipoise

Question 8

What is pulse pressure?

Answer 8

the difference betwen systolic and diastolic pressure

40 mmHg is normal pulse pressure

(considering 120/80 sys/dias)

Question 9

How is mean arterial pressure calculated?

Answer 9

(1xP_sys + 2xP_dias)/3 = MAP

• accounts for 1:2 ratio of time spent in sys:dias per cycle

Question 10

How should a pressure vs. time graph look for one cardiac cycle?

Where do systole/diastole begin + end?

And how long is each, considering a 75 bpm heart rate?

Answer 10

Systole - from just before the rise begins (aortic SL valve opens) until the incisura (indicating aortic SL valve closure), 0.27 seconds

Diastole - from the incisura until just before the next rise begins, 0.53 seconds

Total Cycle Time: 0.8 seconds

Question 11

What are the various mean arterial pressures throughout the systemic circulation (from aorta to veins)?

Answer 11

• Aorta: 100 mmHg
• Arteries: 85 mmHg
• Arterioles: 35 mmHg
• Capillaries: 15 mmHg
• Veins: 0 mmHg

Question 12

Considering the mean arterial pressures of various parts of the systemic circulation…

and the resistance equation of R = ΔP/Q

where is resistance highest?

Answer 12

Arterioles

because the mean pressure drop (ΔP) between arteries (~85 mmHg) and arterioles (~35 mmHg) is 50 mmHg, which is the largest ΔP between any two parts of the circulation

Question 13

What is the Hagen-Poiseuille equation?

What is its hemodynamic significance?

Answer 13

• flow is proportional to vessel radius to the fourth power

Question 14

What is Bernoulli’s law in relation to hemodynamics?

Answer 14

An increase in the speed of a fluid occurs simultaneously with a decrease in pressure.

A blockage in a main vessel (ex: abdominal aorta) near a branching point (ex: renal artery) will increase blood velocity, decrease the side pressure supplying the branch and thus decrease flow to organs supplied by the branch (ex: kidney).

Question 15

What is the resistance equation discussed in Varnai’s hemodynamics lecture?

And its main significance?

Answer 15

• Resistance is inversely proportional to vessel radius to the fourth power
• Thus vasodilation/constriction via arterioles is a very effective resistance/pressure regulating mechanism

Question 16

How can arterial pressure be calculated?

Answer 16

P_arterial = Cardiac Output x Total Peripheral Resistance

Question 17

What is the equation for Reynold’s number?

And its main significance?

Answer 17

high diameter/velocity, or low viscosity can cause turbulence

Question 18

What is LaPlace’s law?

And its physiological significance?

Answer 18

Since wall tension is related to vessel radius and wall thickness…

…in aneurysm, bulging of blood vessel wall causes an increased vessel radius and decreased wall thickness, leading to greatly increased wall tension and eventual rupture.

Question 19

What is Distensibility?

Its equation?

Answer 19

• the tendency of a vessel to increase in volume under pressure
• takes into account vessel’s original volume

Question 20

What is Compliance?

Its equation?

Answer 20

• also the tendence of a vessel to increase in volume under pressure
• ignores original vessel volume

Question 21

Approximately how much blood is stored via the elasticity of large arteries in each cardiac cycle?

Answer 21

40 ml

Question 22

What are the two components of resting tone in arteriolar smooth muscle?

Answer 22

1. Myogenic Tone - inherent to the smooth muscle
2. Sympathetic Tone - stimulated via the sympathetic ANS

Question 23

What are the two physical parameters affecting blood pressure?

Answer 23

1. Blood Volume
2. Compliance

Question 24

How does an increase in cardiac output affect blood pressure?

Answer 24

• with an increase in stroke volume at constant heart rate…
• P_sys increases greatly
• P_dias increases
• P_pulse increases
• P_mean increases

Question 25

How does an increase in total peripheral resistance affect blood pressure?

Answer 25

• P_sys and P_dias increase equally, so…
• P_pulse does not change
• P_mean increases

Question 26

How does a decrease in compliance (as in old age) affect blood pressure?

Answer 26

• P_sys increases
• P_dias decreases
• P_pulse increases greatly
• P_mean does not change

Question 27

How does a change in blood volume effect blood pressure?

Answer 27

increases cardiac output, so has same effect as that

Question 28

How does an increase in viscosity of blood affect blood pressure?

Answer 28

Increases total peripheral resistance, so has the same effect as that.