Gas exchange in the lungs Flashcards
How does oxygen get from the atmosphere to cells?
Oxygen diffuses from alveoli into the blood within pulmonary capillaries
Why is pressure used rather than concentration
• This is because gases react, dissolve, and diffuse more in accordance with their pressure than concentration. This is because pressure takes into account other factors that affect the properties/behaviour of a gas, such as temperature.
What is the equation for the total pressure of the gases by adding up the partial pressures?
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What does the partial pressure of a gas refer to, and what determines how much gas dissolves in a liquid?
The partial pressure of a gas dissolved in a liquid reflects the amount of gas that would dissolve in the liquid (if permitted to reach equilibrium) if the liquid was placed in contact with a gas phase of equivalent partial pressure . For example, if a sample of liquid was exposed to an atmosphere with PO2 = 20 kPa, the amount of oxygen dissolved in the liquid at equilibrium would equal 20 kPa.
The concentration of an individual gas dissolved in a liquid is determined by both partial pressure and solubility of the gas:
Concentration = Partial pressure x Solubility
For example, carbon dioxide has a water solubility of approximately 5.0 mL.L-1.kPa-1. This means there will be 5.0 mL of CO2 per litre of blood for each kPa of pressure. Therefore, if PaCO2 = 5.0 kPa, the concentration of CO2 dissolved in the plasma will be:
[CO2] = 5.0 mL.L-1.kPa-1 x 5 kPa = 25 mL
How are the alveoli adapted for there function?
They have a large surface area (both individually and cumulatively), have a thin outer structure (typically one cell thick), and are richly innervated by capillaries, all of which help to maximise the rate of diffusion.
Describe the barriers that oxygen passes through in order to enter the red blood cells?
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How long does it take for a red blood cell to pass through a pulmonary capillary?
In typical conditions, it takes approximately 0.75 seconds for a red blood cell to pass through a pulmonary capillary, during which time oxygenation must occur. During intensive exercise, where pulmonary blood flow is increased, this time may even decrease to as little as 0.25 seconds.
Whilst this shorter time available does not significantly impact on healthy individuals, it may limit the degree of oxygenation achieved in patients with abnormally reduced diffusion rates in their lungs due to disease.
What does the rate of diffusion depend on?
The gradient between the two areas, the size of the diffusion distance and the surface area.
What is the rate of diffusion proportional to?
Surface area/ Distance^2 (EPITHELIAL AND ENDOTHELIAL CELL THICKNESS AND BASEMENT MEMBRANE THICKNESS AND FLUID LAYER DEPTH) x (Pa - Pc) (PARTIAL PRESSURE GRADIENT BETWEEN ALVEOLAR AIR AND CAPILLARY BLOOD)
What factors are needed for maximal diffusion
Increase in partial pressure gradient
Increase in surface area
Decrease in distance (barrier thickness)
Describe the effect of specific pathologies on factors that affect the rate of diffusion
Hypoventilation (type 2 respiratory failure) = decreases Pa
Hypoperfusion (type 1 respiratory failure) decreases Pc
Emphysema decreases the surface area
Fibrosis increases the basement membrane thickness Pulmonary oedema (e.g pneumonia) which increases thickness of fluid layer/ oedema
In terms of gas exchange, how do we respond to an increase in metabolic demand
The role of ventilation in determining the level of gas exchange
Changing metabolic demands by the body (e.g. due to exercise, injury, or infection) require varying levels of gas exchange to supply oxygen and remove carbon dioxide. The method by which this is achieved is by changing the rate of alveolar ventilation (the volume of fresh air reaching alveoli per unit of time), in order to modulate the partial pressure gradients between the alveoli and blood.
How do we measure that sufficient gas exchange is taking place?
To ensure gas exchange takes place in an efficient manner, there must be sufficient blood (specifically haemoglobin binding sites) to absorb the quantity of oxygen arriving at the alveoli. Therefore the level of ventilation (supply of oxygen) and perfusion (supply of blood) need to be closely matched.
The degree to which they are matched is denoted by the ventilation/perfusion ratio. Ideally this should be close to 1. As described below, significant pathological mismatch of ventilation and perfusion to different regions of the lung results in reduced gas exchange and decreased oxygenation of blood, leading to hypoxaemia.
Describe what happens if perfusion is reduced (the dead space effect (ventilation without perfusion))
- If perfusion is reduced relative to ventilation, V/Q ratio will increase (V/Q >1) and the inspired oxygen will in effect be ‘wasted’ and not participate in gas exchange. This can occur due to reduced blood supply to specific regions of the lung
- The affected regions of the lung are referred to as ‘physiologic dead-space’ as they are effectively not participating in gas exchange
- In the case of a pulmonary embolism (a block of an artery in the lungs), the overall perfusion of the lungs as a whole may not decrease if blood is simply diverted through other pulmonary arteries/capillaries. In theory, increased ventilation of these areas of the lungs may compensate for the reduction in gas exchange in others. Otherwise, hypoxaemia and hypercapnia (increased PaCO2) will occur.
What medical conditions are created as a result of the dead space effect
- Heart failure
- Blocked vessels (pulmonary embolism)
- Loss and damage to capillaries (emphysema)
The effected alveoli = physiological dead-space and no/ reduced gas exchange occurs
