Respiratory Mechanics

Question 1

How is air flow generated?

Answer 1

by pressure gradients

Question 2

What is Poiseuille’s Law equation? (using flow rate)

Answer 2

Q = πPr^4 / 8ηl
Q = Flow rate
P = pressure
r = radius
η = fluid viscosity
l = length of tubing

Question 3

What is Poiseuille’s Law equation? (using resistance)

Answer 3

R = 8ηl / πPr^4
R = resistance
P = pressure
r = radius
η = fluid viscosity
l = length of tubing

Question 4

What is Poiseuille’s Law?

Answer 4

describes factors that affect resistance

Question 5

What does halving tube diameter do to resistance?

Answer 5

increases resistance by 16 times

Question 6

What components is blood pressure a product of?

Answer 6

Blood pressure = Cardiac Output (blood entering system) x Total Peripheral Resistance (size of tubes in system)

Question 7

What happens to the airways as lung volume increases?

Answer 7

conductivity + dilation of the airways increases

Question 8

What are the characteristics of small arteries + arterioles?

Answer 8

extensive smooth muscle - to regulate diameters + resistance to blood flow

Question 9

What are the characteristics of veins + venules?

Answer 9

highly compliant - act as a systemic blood reservoir

Question 10

Why does pressure in blood vessels fall across circulation?

Answer 10

viscous (frictional) pressure losses

Question 11

Which blood vessels present the most resistance to flow?

Answer 11

small arteries + arterioles

Question 12

Why is the relationship between blood pressure, cardiac output + resistance an approximation?

Answer 12

3 assumptions:
• steady flow (which does not occur due to the intermittent
pumping of the heart)
• rigid vessels
• right atrial pressure is negligible

Question 13

What is regulation of flow achieved by?

Answer 13

variation in resistance in the vessels while blood pressure remains constant

Question 14

What is MAP?

Answer 14

mean arterial pressure, also blood pressure

Question 15

How is MAP calculated?

Answer 15

• MAP = diastolic BP + (systolic - diastolic BP)/3

* MAP = diastolic BP + (pulse pressure/3)

Question 16

What 3 variables does resistance of a tube to flow depend on?

Answer 16

• fluid viscosity
• length of tube
• inner radius of tube

Question 17

What does Poiseuille’s equation emphasise?

Answer 17

importance of arterial diameter as a determinant of resistance

Question 18

How do small changes in vascular tone affect flow?

Answer 18

small changes in vascular tone = large changes in flow rate

Question 19

What are the 2 types of fluid flow?

Answer 19

• laminar

* turbulent

Question 20

What is laminar fluid flow?

Answer 20

• velocity is constant at any point
• flows in layers
• blood flows fastest closest to the centre of the lumen

Question 21

What is turbulent fluid flow?

Answer 21

• erratic blood flow
• forms eddys
• prone to pooling
• associated w pathophysiological changes to endothelial lining of blood vessels

Question 22

Where is blood pressure usually measured + why?

Answer 22

• measured on upper arm

* easily accessible + at heart-level

Question 23

How is blood pressure measured?

Answer 23

slow deflation of blood pressure cuff causes TURBULENT FLOW - heard with sthetoscope

Question 24

How can pulse pressure be calculated using BP measurements?

Answer 24

systolic BP - diastolic BP = pulse pressure (PP)

Question 25

What is the ratio of systole to diastole?

Answer 25

1 : 3

Question 26

What happens to transmural pressure + the airways during inspiration?

Answer 26

* transmural pressure is POSITIVE | * airways remain PATENT (open)

Question 27

What effect does severe transmural pressure have on different sized airways?

Answer 27

* collapse in small-to-medium airways | * no collapse in large airways due to cartilage rings

Question 28

What is compliance?

Answer 28

* tendency for tissue to distort under pressure | * change in volume / change in pressure

Question 29

What is elastance?

Answer 29

* tendency for tissue to recoil to its original volume | * change in pressure / change in volume

Question 30

Why do ventricular + aortic pressures differ?

Answer 30

* aortic valve closing * ventricular pressure falls rapidly * aortic pressure falls slowly

Question 31

What explains the ventricular + aortic pressure differences?

Answer 31

elasticity of the aorta and large arteries which act to buffer the change in pulse pressure

Question 32

What is arterial compliance?

Answer 32

ability of the arterial wall to distend + increase volume w/ increasing transmural pressure

Question 33

What happens to blood flow in the arteries during ejection?

Answer 33

blood enters arteries FASTER than it leaves them

Question 34

How does pressure change when the aortic valve closes?

Answer 34

ejection ceases but due to elastic recoil of arteries, pressure falls slowly + there is diastolic flow in the downstream circulation

Question 35

What is the Windkessel effect?

Answer 35

large central arteries (e.g. aorta) are elastic = acts as a reservoir during systole + stores some ejected blood forced out during diastole

Question 36

What effect does reducing arterial compliance have on the Windkessel effect?

Answer 36

damping effect of Windkessel effect is reduced + pulse pressure increases

Question 37

What is venous return facilitated by?

Answer 37

* skeletal muscle pumps in lower limbs | * respiratory muscle pumps in thorax

Question 38

What is varscosity?

Answer 38

incompetent valves cause dilated superficial veins in the leg

Question 39

What is oedema?

Answer 39

prolonged elevation of venous pressure (even with intact compensatory mechanisms) causes oedema in feet

Question 40

What is an aneurysmal disease?

Answer 40

vessel walls weakening causing balloon-like distention = aneurysm

Question 41

What effect do vascular aneurysms have on the radius of a blood vessel?

Answer 41

increases radius

Question 42

What effect do vascular aneurysms have on the forces acting on a blood vessel?

Answer 42

for the same internal pressure, the inward force exerted by muscular wall must INCREASE

Question 43

Why do aneurysms rupture?

Answer 43

force exerted by muscle wall must increase but muscle fibres have weakened - force needed can't be produced so aneurysm will expand + rupture

Question 44

What is the difference between venous + arterial compliance?

Answer 44

venous compliance = arterial compliance x 10-20 times

Question 45

If arterial compliance decreases, what effect does this have on blood pressure?

Answer 45

* systolic increases | * diastolic decreases

Question 46

What does increasing smooth muscle contraction do to venous volume + pressure?

Answer 46

* decreases venous volume | * increases venous pressure

Question 47

How does ventilation differ across the lungs?

Answer 47

* less ventilation at the top | * more ventilation at bottom

Question 48

Why is there less ventilation towards the top of the lungs?

Answer 48

* intrapleural pressure is MORE negative * GREATER transmural pressure gradient * alveoli larger + less compliant

Question 49

Why is there more ventilation towards the bottom of the lungs?

Answer 49

* intrapleural pressure is LESS negative * SMALLER transmural pressure gradient * alveoli smaller + more compliant

Question 50

How does perfusion differ across the lungs?

Answer 50

* lower flow rate towards top | * higher flow rate towards bottom

Question 51

Why is there lower flow rate towards the top of the lungs?

Answer 51

* Lower intravascular pressure (gravity effect) * Less recruitment * Greater resistance

Question 52

Why is there higher flow rate towards the bottom of the lungs?

Answer 52

* Higher intravascular pressure (gravity effect) * More recruitment * Less resistance

Question 53

What varies more between base and apex; ventilation or perfusion?

Answer 53

perfusion

Question 54

What does V/Q ratio stand for?

Answer 54

ventilation/perfusion ratio

Question 55

What is V/Q ratio used for?

Answer 55

to assess to efficiency + adequacy of the matching of ventilation + perfusion

Question 56

What is a V/Q ratio?

Answer 56

ratio of: • amount of air reaching the alveoli per minute : • amount of blood reaching the alveoli per minute

Question 57

What are ventilation + perfusion the main determinants of?

Answer 57

blood O2 + CO2 concentration

Question 58

How is V/Q ratio measured?

Answer 58

ventilation/perfusion scan

Question 59

What can a V/Q mismatch cause?

Answer 59

type 1 respiratory failure

Question 60

What is a typical V/Q ratio value of the lung as a whole?

Answer 60

0.8

Question 61

How does V/Q ratio vary in the lung?

Answer 61

ratio value varies depending on position within lung

Question 62

How would COPD affect compliance?

Answer 62

emphysemic breakdown of structural lung tissue INCREASES compliance

Question 63

How does COPD affect resistance?

Answer 63

bronchitic swelling of the airways + mucus hyper-secretion INCREASES resistance to airflow

Question 64

Why doesn't resistance continue to increase as airways get smaller?

Answer 64

resistance to airflow depends on no. of parallel pathways present - many many alveoli vs. two bronchi...

Question 65

What happens to airway resistance as lung volume increases?

Answer 65

airway resistance decreases

Question 66

Why does airways resistance decrease as lung volume increases?

Answer 66

airways distend as lungs inflate + wider airways have lower resistance