Gas Exchange Flashcards
Describe how the nose, nasal passages and upper airways are adapted
- Moisten, warm and filter incoming air
- Hairs are the first line of defence
Describe the trachea
- Large diameter
- Thin wall
- Reinforced with c-shaped rings of cartilage- this gives flexibility without wall collapse
- Negative pressure inside, so hyaline cartilage keeps it open
What is the second line of defence in the airway?
- Muco-ciliary escalator
What is the third line of defence in the airway?
- White blood cell moving through tissue and digesting proteins of connective tissue as they go
- Proteins include collagen and elastin
What enzymes digest proteins of connective tissue in airways?
- Collagenase and elastase
- Found in neutrophils
What is the overall function of the upper part of the airway?
To move the air down
Describe the structure of the bronchi
- Similar to trachea
- Progressively smaller network of tubes
- Dichotomous branching until terminal bronchioles (16 generations)
- Progressively reduced cartilage and increased smooth muscle
What is the conducting zone of the lungs?
- Mouth to bronchioles
- Do not take part in gas exchange and constitute dead space
How does surface area increase down the airway?
- Total number of tubes rises in a binary fashion
- So, total cross-sectional area rises greatly
What is the benefit of increasing surface area?
- Low resistance to air flow and a progressive reduction in linear velocity
- Most of the airway resistance is in the top part of the airway
What is a spirometer?
A breathing tube to measure volumes
What is functional residual capacity?
- Represents the volume of gas left in the lung at the end of a normal quiet expiration
- Neutral point from which inspiratory activity occurs
What is tidal volume?
- Builds on FRC
- Volume of each breath (half a litre usually)
- Empty under lung’s own capacity
- Builds on neutral point
vWhat is vital capacity?
Total volume of gas it is possible to inhale or exhale with a single breath
What is residual volume?
The volume of gas left in the lung at the end of maximum expiration
- Includes dead space but also gas left in alveoli
- Cannot completely empty alveoli when breathing out
What is the problem with assessing respiratory function?
- When trying to assess normal breathing, a patient may stop breathing normally
What is the expiratory reserve?
What can be breathed out
What is the calculation for minute volume?
Minute volume = tidal volume x rate
What is alveolar ventilation?
The amount of air passing through gas exchanging parts of the lung per minute
How do you calculate alveolar volume?
VA= (tidal volume - dead space) x rate/min
How can you increase alveolar ventilation?
Better to increase depth of breathing rather than rate
When might dead space increase?
Physiological dead space may increase in some disease states
- Clinically, care must be taken not to increase dead space by adding tubing
What is the relationship between alveolar ventilation and arterial PCO₂?
- If alveolar ventilation doubles, arterial PCO₂ will half
- If alveolar ventilation halves, arterial PCO₂ will double
How can PCO₂ be used to indicate ventilation?
- Raised arterial PCO₂- patient is hypo-ventilating (type 2 respiratory failure)
- Low arterial PCO₂- patient is hyperventilating
In a normal healthy lung, what average alveolar PO₂ equal to?
PaO₂ (almost)- arterial
- If not, this means that alveolar and arterial gases are not in equilibrium
What can cause alveolar PO₂ to be very different from arterial PO₂?
- Ventilation perfusion mismatch
- Shint
How long will a red blood cell spend in a lung and how many alveoli will it pass?
- 1 second
- 3 alveoli
Which parts of the lungs are better ventilated?
- Lower parts due to gravity
- Ensures lung bases are better perfused
Describe the mechanism by which airflow to the base areas of the lungs is increased
- Oxygen in the lung is a vasodilator
- So, blood vessels in well-ventilated areas tend to dilate
- These areas receive high blood flow- high local carbon dioxide pressure
- CO₂ is a bronchodilator, so airflow to these areas will be increased
Describe ‘Ventilation-Perfusion Mismatch’
- Reduced ventilation to certain parts of the lung
- Some pulmonary venous blood not fully oxygenated- passed through these parts
- Poorly saturated Hb
- Blood from well-ventilated area will have 100% Hb saturation
- Bloodstreams meet, final saturation is less than 100% and arterial PO₂ is reduced
- Body maintains normal PaCO₂ by over-ventilating areas in lung- so more CO₂ is blown off
