gas exchange in lungs Flashcards
Describe how oxygen gets from the atmosphere to the cells
- Oxygen is inhaled from the atmosphere into the alveoli within the lungs
- Oxygen diffuses from alveoli into the blood within pulmonary capillaries
- Oxygen transported in blood predominantly bound to haemoglobin
- Oxygen diffuses into cells to be used in aerobic respiration
Carbon dioxide diffuses from respiring tissues to blood and into lungs for gas exchange
Describe in DETAIL the passage of oxygen through the alveoli into the blood
• Oxygen enters alveolar airspace from the atmosphere
• Oxygen dissolves in the alveolar lining fluid
• Oxygen diffuses through the alveolar epithelium, basement membrane and capillary endothelium
Oxygen then dissolves in the blood plasma within the lung capillary and binds to Hb molecule
What is gas exchange and what is the rate of diffusion determined by?
Gas exchange is the diffusion of gases between air and blood
The rate of diffusion is determined by:
1. The partial pressure gradient between the two areas: between PA (alveolar air) and PC (capillary blood)
2. The size of diffusion distance: depth of the layers between the alveoli and capillary
3. The surface area: alveolar surface area
Rate of diffusion = (Surface area/distance^2 ) x PA-PC
MAXIMUM DIFFUSION RATE REQUIREMENTS?
- High partial pressure gradient
- High surface area
Low distance (small barrier thickness)
List some diseases that affect the rate of diffusion and state how they affect it.
- Hypoventilation (type II respiratory failure) = lowers Pa
- Hypoperfusion (type I respiratory failure) = lowers Pc
- Emphysema = lowers surface area
- Fibrosis = increases basement membrane thickness (increases diffusion distance)
- Pulmonary oedema = increases thickness of fluid layer (increases diffusion distance)
Describe how ventilation affects the gaseous contents within the alveoli
High ventilation = higher PO2
Low ventilation = higher PCO2
How are the pressure gradients between the alveoli and the blood maintained?
They are maintained by adequate ventilation and adequate perfusion.
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 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.
What relationship does V/Q describe?
The V/Q ratio describes the relationship between pulmonary perfusion (Q) and alveolar ventilation (V)
VQ SHOULD = 1
>1 means hypoperfusion
<1 means hypoventilation
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 hyperventilation 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 particular metabolic substrate being used
RER for:
Carbohydrates = 1
Fatty acids = 0.7
How do you calculate the alveolar oxygen content?
O2 inspired - O2 consumed
