RESP: Gas exchange Flashcards
How does oxygen get from the atmosphere to cells?
- Oxygen is inhaled from the atmosphere into alveoli within lungs
- Oxygen diffuses from alveoli into blood within pulmonary capillaries
- Oxygen is transported in blood, predominantly bound to haemoglobin
- Oxygen diffuses into cells/tissues for use in aerobic respiration
- Carbon dioxide diffuses from respiring tissues to blood - exchanged at lungs
What is the composition of gases in air?
Air consists of a mixture of gases, which behave in accordance with their partial pressure, rather than concentration
Nitrogen - 78%
Oxygen - 21%
Carbon dioxide - 3%
Water (vapour) - Variable
How is partial pressure calculated?
By multiplying total pressure by mole fraction
P total is calculated by adding the partial pressure of water vapour together with the sum of the partial pressures of the constituent cases.
The partial pressure of the individual gases can be calculated by subtracting the atmospheric pressure by water vapour pressure, and multiplying that figure by the mole fraction of the gas.
What determines how much gas dissolves in a liquid?
The concentration of a gas dissolved within a liquid is determined by the partial pressure and solubility of the gas:
Concentration = Partial pressure x Solubility
Solubility = Ability of gas to dissolve in solvent to form a solution at a specific temperature
The partial pressure of gas dissolved in a liquid reflects the amount of gas that would dissolve (at equilibrium) if the liquid was placed in contact with a gas phase of equivalent partial pressure.
What are the structures and mediums involved in the diffusion of blood gases? Describe the movement of oxygen through these mediums
- Airspace
- Alveolar lining fluid
- Alveolar epithelial layer
- Basement membrane + interstitial fluid
- Capillary endothelial layer
- Blood plasma
- Lung capillary
O2 enters the alveolar airspace from the atmosphere
O2 dissolves in alveolar lining fluid
O2 diffuses through alveolar epithelium, basement membrane and capillary endothelial cells
O2 dissolves in blood plasma
O2 binds to Haemoglobin molecule
What are the properties of alveoli?
- Large SA (both individually and cumulatively)
- Thin outer structure (typically 1 cell thick)
- Richly innervated by capillaries.
All of these help to maximise the rate of diffusion
When must oxygenation of blood occur?
During the brief time it takes for RBCs to flow through pulmonary capillaries
What is rate of diffusion determined by?
- Alveolar surface area
- Partial pressure gradient between alveolar air (PA) and capillary blood (Pc)
- Epithelial and endothelial cell thickness + basement membrane thickness + fluid layer depth
Rate of diffusion ∝ (SA/ Distance squared) x (Pa - Pc)
Pa - Alveolar partial pressure
Pc- Capillary partial pressure
What are some defects at gas exchange surfaces that can impact maximum diffusion?
For maximum diffusion:
- ⬆️Partial pressure gradient - Hypoventilation (type II respiratory failure, this decreases the gradient)
- ⬆️Functional surface area - Emphysema- decreases SA; Acute lung injury - decreases functional SA
- ⬇️Distance (barrier thickness) - Fibrosis - Increases basement membrane thickness; Pulmonary oedema (pneumonia etc.) - Increases thickness of fluid layer/oedema
It typically takes approximately 0.75s for anRBC to pass through a pulmonary capillary, and it’s during this time oxygenation must take place. During intense exercise, pulmonary blood flow increases, so this time can decrease to as much as 0.25s
What are the adaptations of alveoli that allows it to maximise the rate of gas exchange?
- Large SA - Lungs have high SA:V ratio due to 3D structure
- Wall = 1 cell layer thick + basement membrane fused with blood vessel
- Richly innervated by capillaries (adequate blood supply)
How are pressure gradients between alveoli and blood maintained?
By adequate ventilation:
⬆️Ventilation = ⬆️Partial pressure gradient (b/w alveoli + blood) = ⬆️Gas exchange
Changing metabolic demands (due to exercise, infection, injury etc.) require varying levels of gas exchange to supply oxygen and remove CO2. Changing the rate of alveolar ventilation (Vol of fresh air reaching alveoli per unit of time) modulates the partial pressure gradients b/w alveoli and blood.
What is perfusion?
Supply of blood flow (to organs/ lymphatics)
What does efficient gas exchange require?
Matching of ventilation (V) to perfusion (Q) - This is VQ coupling
Blood travelling to the parts of the lung must be adequately oxygenated, therefore the lungs must be appropriately ventilated in order to ensure this.
V/Q is approx 1 to be considered a normal value
If it’s lower than 1, the likely cause is hypo perfusion (dead space effect)
If it’s higher than 1, the likely cause is hypoventilation (shunt)
Explain how ventilation-perfusion coupling is maintained
By hypoxic pulmonary vasoconstriction
Homeostatic mechanisms exist to reduce ventilation-perfusion mismatching. Hypoxic vasoconstriction of capillaries diverts blood flow from poor to well ventilated alveoli
Under normal conditions, blood flow and ventilation are matched
If ventilation of specific alveoli decreases, PaCO2 will rise and PaO2 will fall, therefore theres decreased oxygenation of blood flowing through innervating capillaries
Decreased PaO2 induces vasoconstriction, decreasing blood flow
Blood flow is diverted to alveoli with increased ventilation
Why can hypoxic vasoconstriction be problematic for patients with COPD?
The chronic hypoventilation that occurs in large sections of the lungs leads to prolonged and widespread pulmonary vasoconstriction. This increases resistance within the pulmonary vasculature, resulting in pulmonary hypertension. This can lead to right heart hypertrophy and eventually right heart failure.
What happens in situations where ventilation and perfusion to individual alveolar unit are not matched?
Not matched refers to the net result that there is insufficient ventilation to fully oxygenate whatever number of Hb molecules are perfusing each alveoli
When they’re not matched, the overall rate of gas exchange will be reduced
In most cases, increased PaCO2 will induce a reflex hyperventilation that clears the excess CO2 (but doesn’t increase PaO2), though in theory, V-Q inequality affects both O2 and CO2.
How does a pulmonary embolism result in VQ inequality?
Pulmonary embolisms block an artery in the lungs.
If blood is diverted through other pulmonary arteries/capillaries, overall perfusion of the lungs may not decrease.
Unless ventilation of these alveoli increases to match perfusion, hypoxaemia and hypercapnia will occur
What does reduced perfusion of lung regions cause?
Increase in V/Q ratio:
- Heart failure (cardiac arrest)
- Blocked vessels (pulmonary embolism)
- Loss/damage to capillaries (emphysema)
The affected alveoli - Physiological dead-space, as no/reduced gas exchange occurs.
What are the effects of perfusion without ventilation?
Reduced ventilation of alveoli or limits to diffusion cause a decrease in V/Q ratio:
- Cardiac shunts
- Pneumonia, acute lung injury, respiratory distress syndrome, atelectasis
Blood from the right heart to the left, without taking part in gas exchange (shunt)
What does the effect of O2 therapy on hypoxaemia depend on?
Nature of the pathology (overall V/Q inequality vs shunt)
Why does hypoxaemia caused by shunt not respond well to O2 therapy?
Shunt induced hypoxaemia has a limited response to O2 therapy, due to the fact that regardless of the degree of oxygenation in blood perfusing ventilated alveoli, it eventually mixes with deoxygenated blood returning from areas affected by shunt, so the overall partial pressure of O2 is reduced.
Administering supplemental O2 cannot increase oxygen saturation in well-ventilated regions to make up for the deoxygenated blood that eventually mixes.
Describe healthy lung function
In healthy lung function, ventilation and perfusion are approximately matched, allowing for adequate ventilation, so O2 saturation is approx 98%, returning to systemic circulation
Describe lung function with an acute lung injury - Induced shunt
Alveolar oedema and lung injury due to infection prevents ventilation in affected parts of the lung. This means there’s lower oxygen saturation of the blood, returning to systemic circulation
What is a pulmonary shunt or shunt-effect?
When alveolar ventilation is reduced, or limited diffusion occurs, resulting in a decrease in V:Q ratio.
Deoxygenated blood returns to the left side of the heart from the right, without having taken part in gas exchange.
Describe lung function with shunt + hyperventilation
Shunt - Alveolar oedema and lung injury due to infection prevents ventilation in affected parts of the lung
Hyperventilation - Increased ventilation of functional airways/alveoli supplies additional O2 to the alveoli. There’s only minimal extra Hb for O2 carrying capacity
Increased oxygenation of blood to hyper-ventilated, functional alveoli does not adequately compensate for O2 ‘lost’ due to poor oxygenation of blood in poorly ventilated alveoli
Blood saturation is therefore decreased
