Resp 4 - Gas Exchange + Lung Mechanics 1

Question 1

Explain why intrapleural pressure is below atmospheric pressure:

Answer 1

The lungs and chest wall recoil in opposite directions

Question 2

What happens to the pleural pressure during inspiration?

Answer 2

Decreases as chest wall expands

Question 3

Define ‘Resting Expiratory Level’:

Answer 3

State of equilibrium between lung recoil and chest wall recoil at the end of quiet respiration

Question 4

Which muscles are involved in inspiration from the resting expiratory level?

Answer 4

Contraction of the diaphragm and the external intercostal muscles

Question 5

Which muscles are involved in expiration from the resting expiratory level?

• Hint: Passive vs Forced*

Answer 5

Passive = Passive recoil of lungs = no muscles involved

Forced = Contraction of the abdominal muscles and internal intercostal muscles

Question 6

Name the neve which controls inspiration:

Answer 6

Phrenic nerve

C3, C4, C5 keeps the diaphragm alive

Question 7

Name the accessory muscles used in forced inspiration:

Answer 7

• Sternocleidomastoid
• Scalene
• Serratus anterior
• Pectoralis major

Question 8

Name the accessory muscles used in forced expiration:

Answer 8

• Internal Intercostals
• External Obliques
• Internal Obliques
• Rectus abdominus

Question 9

Define compliance:

Answer 9

The stretchiness of the lungs: ^ stretchy = ^ compliant

Change in volume per unit pressure change

Question 10

How would abdominal laparoscopic surgery affect the compliance of the lungs?

Answer 10

Decrease compliance

- due to increased intra-abdominal pressure

Question 11

Define surface tension, and how it relates to compliance:

Answer 11

Attraction of water molecules to each other

Increased surface tension = decreased compliance

Question 12

How does surfactant affect surface tension and compliance?

Answer 12

Surfactant reduces surface tension, by breaking H-bonds (detergent), therefore increasing compliance

Question 13

Which cells secrete surfactant?

Answer 13

Type II alveolar pneumocytes

Question 14

What is surfactant made up of?

Answer 14

90% phospholipids

10% Surfactant protein A

Question 15

What is the Law of Laplace?

Answer 15

Pressure = (2 x Surface Tension) / Radius of bubble

Question 16

What prevents small alveoli collapsing into larger alveoli (via Law of Laplace)?

Answer 16

Surfactant

Question 17

What is Newborn Respiratory Distress Syndome?

Answer 17

• Cyanosis, grunting, intercostal and subcostal reccesion seen in premature babies < 30 weeks, due to lack of surfactant = high surface tension = tendency of alveoli to collapse

Question 18

Why is most of the resistance to breathing in the upper respiratory tract?

Answer 18

As each branching point increases the number of airways in parallel = Decreases resistance

Question 19

Why is the combined resistance of the airways low?

Answer 19

Airways are connected in parallel = decreases resistance

Question 20

List the 3 factors affecting the rate of diffusion across the air-blood interphase:

Answer 20

1) Area available for exchange (usually ~70m^2)
2) Resistance to diffusion
3) Gradient of partial pressure

Question 21

What is the normal diffusion rate across the air-blood interphase in the lungs?

Answer 21

~ 5 L/min

Question 22

What is the distance between the blood and the air in alveoli?

Answer 22

~ 0.6 um

Question 23

How does emphysema decrease diffusion between the air and blood interphase in the lungs?

Answer 23

Destruction of alveolar walls = large air spaces = decreased total surface area for gas exchange

Question 24

How does pulmonary oedema decrease diffusion between the air and blood interphase in the lungs?

Answer 24

Fluid in the interstitium and alveoli increase the length of the diffusion pathway

Question 25

How does interstitial lung disease decrease diffusion between the air and blood interphase in the lungs?

Answer 25

Excessive deposition of collagen in interstitial space + thickening of alveolar walls = lengthened diffusion pathway

Question 26

List the 5 layers making up the diffusion pathway at the air-blood interphase in the lungs:

Answer 26

1) Alveolar wall 2) Interstitial fluid 3) Capillary endothelium 4) Plasma 6) Red blood cell membrane

Question 27

Why does gas exchange depend on the partial pressure gradient across the diffusion barrier?

Answer 27

Gases move down partial pressure gradients

Question 28

Give the normal values of PO2 in: - Inspired air - Alveolar air - Arterial blood - Mixed venous blood

Answer 28

Inspired air = 20.8 kPa Alveolar air = 13.3 kPa Arterial blood = 13.3 kPa Mixed venous blood = 5.3 kPa

Question 29

Give the normal values of PCO2 in: - Inspired air - Alveolar air - Arterial blood - Mixed venous blood

Answer 29

Inspired air = 0.04 kPa Alveolar air = 5.3 kPa Arterial blood = 5.1 kPa Mixed venous blood = 6.1 kPa

Question 30

How do we measure diffusion resistance across the air-blood interphase in the lung?

Answer 30

TLCO = Transfer Factor for Carbon Monoxide - Patient takes 1 deep breath of air, 14% helium and 0.1%CO - Collect exhaled air, and calculate how much CO was transferred from alveoli to blood - CO used because of very high affinity for Haemoglobin

Question 31

Define anatomical dead space:

Answer 31

The volume of the conducting airways (up to and including terminal bronchioles) Normally ~ 150ml

Question 32

How does adrenaline affect the anatomical dead space in the airways?

Answer 32

Increases due to bronchodilation

Question 33

How do we measure the anatomical dead space?

Answer 33

Nitrogen washout test - Breathe in 100% O2 - Exhale for as long as possible - Should exhale all residual N2 in airways (normal air is 78% N2), indicating how much residual gas sits in airways

Question 34

Define alveolar dead space:

Answer 34

The volume of air in the alveoli not taking part in gas exchange

Question 35

Why may some people have more alveolar dead space than others?

Answer 35

Some may have dead/damaged/poorly perfused alveoli

Question 36

Define physiological dead space:

Answer 36

Anatomical + Alveolar dead space | = The volume of air in the conducting airways and alveoli not taking part in gas exchange

Question 37

Define pulmonary ventilation rate:

Answer 37

The total rate of air movement into and out of the lungs = Dead space ventilation + Alveolar ventilation = Tidal volume x respiratory rate ~ 750 ml/min

Question 38

How can you calculate dead space ventilation?

Answer 38

Dead space volume x respiratory rate