Gas Exchange In The Lungs Flashcards
Gas exchange involves diffusion of blood gases through multiple structures and mediums.
Describe how oxygen gets from the lungs to the blood.
1) O2 enters the alveolar airspace from the atmosphere.
2) O2 dissolves into the alveolar fluid lining.
3) O2 diffuses through the alveolar epithelium, the basement membrane and the capillary endothelial cells.
4) O2 dissolves in blood plasma.
5) O2 binds to the haemoglobin molecule.
What is the rate of diffusion determined by?
The rate of dissuion (on which gas exchange depends) is determined by the partial pressure gradient between the two areas, the size of the diffusion distance and the surface area.
List some diseases that affect the rate of diffusion, and how they affect it.
- hypoventilation (type II respiratory failure) = lowers PA
- hypoperfusion (type I respiratory failure) = lowers Pc
- emphysema = lowers surface area
- fibrosis = increase basement membrane thickness
- pulmonary oedema (eg. pneumonia) = increases thickness of fluid layer/ oedema
Alveoli have intricate structures and multiple adaptations to maximise the rate of gas exchange.
List a few of them.
- large surface area (lungs have a high surface-area to volume ratio due to their 3D structure)
- their walls are only one cell thick, and the basement membrane is fused with the blood vessel, which decreased the distance for diffusion
- they are richly innervated by capillaries (given an adequate blood supply to maintain the gradient)
How are the pressure gradients between the alveoli and the blood maintained?
They are maintained by adequate ventilation and adequate perfusion.
What are hypoventilation and hyperventilation defined by (in terms of CO2 levels)?
Hyperventilation is defined by excessive levels of CO2 within arterial blood (PaCO2 < 4.9 kPa).
Hypoventilation is defined by reduced levels of CO2 within arterial blood (PaCO2 > 4.9 kPa).
Increasing the rate of ventilation increases alveolar oxygen partial pressure (PAO2), and decreases alveolar carbon dioxide partial pressure (PACO2). Decreasing the rate of ventilation has the opposite effects.
Explain how adequate perfusion is required for maintaining the pressure gradients for diffusion.
Blood flow through the pulmonary capillaries (perfusion, Q) needs to be matched to alveolar ventilation (VA) to enable efficient gas exchange, as there is a maximum amount of O2 each unit of blood can carry.
What relationship does V/Q describe?
The V/Q ratio describes the relationship between pulmonary perfusion (Q) and alveolar ventilation (V).
1L of blood can carry about 200 ml of O2, and 1L of dry air contains about 210 ml of O2. Therefore, ideally, the V/Q value should be around 1.
At rest, ventilation and perfusion are both around 5L/min, so the V/Q should be around 1-0.8.
How is ventilation-perfusion coupling maintained?
Ventilation-perfusion coupling is maintained by hypoxic vasoconstriction.
Homeostatic mechanisms exist to reduce ventilation-perfusion mismatching. Hypoxic vasoconstriction of capillaries diverts blood flow from poor to well-ventilated alveoli.
1) Under normal conditions, blood flow and ventilation are matched.
2) If ventilation of specific alveoli decreases, PACO2 will rise and PAO2 will fall. Therefore, there is decreased oxygenation of blood flowing through the innervating capillaries.
3) The decreased PaO2 induces vasoconstriction, which decreases blood flow. The blood flow is then diverted to alveoli with increased ventilation.
What happens when there is a ventilation-perfusion mismatch (with reduced perfusion)?
The reduced perfusion of lung regions causes an increase in V/Q ratio. This could lead to:
- heart failure (cardiac arrest)
- blocked vessels (pulmonary embolism)
- loss/ damage to capillaries (emphysema)
The affected alveoli become physiological dead-space, as there is no/ reduced gas exchange.
What happens when there is a ventilation-perfusion mismatch (with reduced ventilation)?
Reduced ventilation of alveoli (or limits to diffusion) causes a decrease in V/Q ratio. This could lead to:
- asthma (COPD)
- pneumonia (fibrosis)
Blood returns to the left part of the heart from the right without taking part in gas exchange (shunt).
How can we determine the cause of hypoxaemia?
Clinical situations arise where it is useful to know alveolar oxygen pressure (PAO2), for example, when determining the cause of respiratory failure. We need to determine if it is due to hypventilation or poor oxygenation.
It is not practical in such cases to sample gas directly from the alveoli; however, PAO2 can easily be calculated from other measurements:
PAO2 = FIO2 x (P B - P H2O) - PaCO2/RER
FIO2 is the fraction of oxygen present in inspired gas, P B is the barometric pressure, P H2O is the water vapour pressure, PaCO2 is the arterial CO2 pressure and RER is the respiratory exchange ratio.
What is the RER (respiratory exchange ratio)?
RER = VCO2 produced/ VO2 consumed
The RER describes the relationship between the CO2 elimination and O2 consumption. It measured the difference between O2 and CO2 in inspired and expired air.
The main determinant of RER is the paticular metabolic substrate being used (eg. for fat/ carb, the RER for the modern diet is about 0.8).
What is the RER for the oxidation of carbohydrates, and for the oxidation of fatty acids?
Oxidation of CARBOHYDRATES = 1
Oxidation of FATTY ACIDS = 0.7
