Resp 4 - Gas Exchange + Lung Mechanics 1 Flashcards
Explain why intrapleural pressure is below atmospheric pressure:
The lungs and chest wall recoil in opposite directions
What happens to the pleural pressure during inspiration?
Decreases as chest wall expands
Define ‘Resting Expiratory Level’:
State of equilibrium between lung recoil and chest wall recoil at the end of quiet respiration
Which muscles are involved in inspiration from the resting expiratory level?
Contraction of the diaphragm and the external intercostal muscles
Which muscles are involved in expiration from the resting expiratory level?
- Hint: Passive vs Forced*
Passive = Passive recoil of lungs = no muscles involved
Forced = Contraction of the abdominal muscles and internal intercostal muscles
Name the neve which controls inspiration:
Phrenic nerve
C3, C4, C5 keeps the diaphragm alive
Name the accessory muscles used in forced inspiration:
- Sternocleidomastoid
- Scalene
- Serratus anterior
- Pectoralis major
Name the accessory muscles used in forced expiration:
- Internal Intercostals
- External Obliques
- Internal Obliques
- Rectus abdominus
Define compliance:
The stretchiness of the lungs: ^ stretchy = ^ compliant
Change in volume per unit pressure change
How would abdominal laparoscopic surgery affect the compliance of the lungs?
Decrease compliance
- due to increased intra-abdominal pressure
Define surface tension, and how it relates to compliance:
Attraction of water molecules to each other
Increased surface tension = decreased compliance
How does surfactant affect surface tension and compliance?
Surfactant reduces surface tension, by breaking H-bonds (detergent), therefore increasing compliance
Which cells secrete surfactant?
Type II alveolar pneumocytes
What is surfactant made up of?
90% phospholipids
10% Surfactant protein A
What is the Law of Laplace?
Pressure = (2 x Surface Tension) / Radius of bubble
What prevents small alveoli collapsing into larger alveoli (via Law of Laplace)?
Surfactant
What is Newborn Respiratory Distress Syndome?
- 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
Why is most of the resistance to breathing in the upper respiratory tract?
As each branching point increases the number of airways in parallel = Decreases resistance
Why is the combined resistance of the airways low?
Airways are connected in parallel = decreases resistance
List the 3 factors affecting the rate of diffusion across the air-blood interphase:
1) Area available for exchange (usually ~70m^2)
2) Resistance to diffusion
3) Gradient of partial pressure
What is the normal diffusion rate across the air-blood interphase in the lungs?
~ 5 L/min
What is the distance between the blood and the air in alveoli?
~ 0.6 um
How does emphysema decrease diffusion between the air and blood interphase in the lungs?
Destruction of alveolar walls = large air spaces = decreased total surface area for gas exchange
How does pulmonary oedema decrease diffusion between the air and blood interphase in the lungs?
Fluid in the interstitium and alveoli increase the length of the diffusion pathway
How does interstitial lung disease decrease diffusion between the air and blood interphase in the lungs?
Excessive deposition of collagen in interstitial space + thickening of alveolar walls = lengthened diffusion pathway
List the 5 layers making up the diffusion pathway at the air-blood interphase in the lungs:
1) Alveolar wall
2) Interstitial fluid
3) Capillary endothelium
4) Plasma
6) Red blood cell membrane
Why does gas exchange depend on the partial pressure gradient across the diffusion barrier?
Gases move down partial pressure gradients
Give the normal values of PO2 in:
- Inspired air
- Alveolar air
- Arterial blood
- Mixed venous blood
Inspired air = 20.8 kPa
Alveolar air = 13.3 kPa
Arterial blood = 13.3 kPa
Mixed venous blood = 5.3 kPa
Give the normal values of PCO2 in:
- Inspired air
- Alveolar air
- Arterial blood
- Mixed venous blood
Inspired air = 0.04 kPa
Alveolar air = 5.3 kPa
Arterial blood = 5.1 kPa
Mixed venous blood = 6.1 kPa
How do we measure diffusion resistance across the air-blood interphase in the lung?
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
Define anatomical dead space:
The volume of the conducting airways (up to and including terminal bronchioles)
Normally ~ 150ml
How does adrenaline affect the anatomical dead space in the airways?
Increases due to bronchodilation
How do we measure the anatomical dead space?
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
Define alveolar dead space:
The volume of air in the alveoli not taking part in gas exchange
Why may some people have more alveolar dead space than others?
Some may have dead/damaged/poorly perfused alveoli
Define physiological dead space:
Anatomical + Alveolar dead space
= The volume of air in the conducting airways and alveoli not taking part in gas exchange
Define pulmonary ventilation rate:
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
How can you calculate dead space ventilation?
Dead space volume x respiratory rate
