Ventilation and perfusion relationships Medical Assignment Help

For efficient gas exchange it is important that there is a match between ventilation of the alveoli (VA) and their perfusion (Q). There is a wide variation in the VA/Q ratio throughout both normal and diseased lung. In the normal lung the extreme relationships between alveolar ventilation and perfusion are:
• Ventilation but no perfusion (physiological dead space)
• Perfusion but no ventilation (physiological shunting)
These and the ‘ideal’ match are illustrated. In normal lungs there is a tendency for ventilation not to be matched by perfusion towards the apices, with the reverse occurring at the bases.
An increased physiological shunt results in arterial hypoxaemia. The effects of an increased physiological dead space can usually be overcome by a compensatory increase in the ventilation of normally perfused alveoli. In advanced disease this compensation cannot occur, leading to increased alveolar and arterial Pc02, together with hypoxaemia.
Hypoxaemia occurs more readily than hypercapnia because of the different ways in which oxygen and carbon dioxide are carried in the blood. Carbon dioxide can be considered to be in simple solution in the plasma, the volume carried being proportional to the partial pressure. Oxygen is carried in chemical combination with haemoglobin in the red blood cell, and the relationship between the volume carried and the partial pressure is not linear. Alveolar hyperventilation resulting in a low alveolar Pc02 and a high alveolar PO, will therefore lead to a marked reduction in the carbon dioxide content of the resulting blood but no increase in the oxygen content.
The hypoxaemia of even a small amount of physiological shunting cannot therefore be compensated for by hyperventilation. The Pa02 and PaCOZ of some individuals who have mild disease of the lung causing slight VA/Q mismatch may still be normal. Increasing the requirements for gas exchange by exercise will widen the VIQ mismatch and the PaOZ will fall. VIQ mismatch is by far the commonest cause of arterial hypoxaemia.

Relationships between

Relationships between

Hypoxaemia occurs more readily than hypercapnia because of the different ways in which oxygen and carbon dioxide are carried in the blood. Carbon dioxide can be considered to be in simple solution in the plasma, the volume carried being proportional to the partial pressure. Oxygen is carried in chemical combination with haemoglobin in the red blood cell, and the relationship between the volume carried and the partial pressure is not linear . Alveolar hyperventilation resulting in a low alveolar Pc02 and a high alveolar PO, will therefore lead to a marked reduction in the carbon dioxide content of the resulting blood but no increase in the oxygen content.
The hypoxaemia of even a small amount of physiological shunting cannot therefore be compensated for by hyperventilation. The Pa02 and PaCOZ of some individuals who have mild disease of the lung causing slight VA/Q mismatch may still be normal. Increasing the requirements for gas exchange by exercise will widen the VIQ mismatch and the PaOZ will fall. VIQ mismatch is by far the commonest cause of arterial hypoxaemia.

Alveolar stability

The alveoli of the lung are essentially hollow spheres. Surface tension acting at the curved internal surface tends to cause the sphere to decrease in size. The surface tension within the alveoli would make the lungs extremely difficult to distend were it not for the presence of surfactant. The type II cells within the alveolus secrete an insoluble lipoprotein largely consisting of dipalmitoyl lecithin, which forms a thin monomolecular layer at the air-fluid interface. Surfactant reduces surface tension so that alveoli remain stable.
Fluid surfaces covered with surfactant exhibit a phenomenon known as hysteresis, i.e. the surfacetension- lowering effect of the surfactant can be improved by a transient increase in the size of the surface area of the alveoli. During quiet breathing, small areas of the lung undergo collapse, but it is possible to re-expand these rapidly by a deep breath, hence the importance of sighs or deep breaths as a feature of normal breathing. Failure of such a mechanism, which can occur, for example, in patients with fractured ribs, gives rise to patchy basal lung collapse. Surfactant levels may be reduced in a number of diseases that cause damage to the lung (e.g. pneumonia), and may playa central role in the respiratory distress syndrome of the newborn. Severe reduction in perfusion of the lung causes impairment of surfactant activity and may well account for the characteristic areas of collapse associated with pulmonary embolism.

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