Gravitational forces increase blood flow down the lung so that it is unevenly distributed. The lung has been described as being made up by a number of zones defined by the driving forces for blood flow (.Hu..g..h,e.s...J..9.9..1. ; N.y.Q..Q 19.9.3l). An upper lung region may exist where alveolar pressure exceeds arterial and venous pressures, and the alveoli compress the alveolar capillaries and obstruct their blood flow (zone I, no blood flow zone). Although the alveolar capillaries are closed and blood flow has ceased through them, a persistent tiny blood flow in zone I has been demonstrated. Histological studies of excised lung tissue have shown that this blood passes through corner vessels, i.e. vessels located in the junctions between alveolar septa. These vessels appear to be subject to forces other than those acting on the alveolar capillaries and they are kept patent even when in zone I. By using the multiple inert gas elimination technique (see below) it has also been demonstrated that this corner vessel blood flow participates in gas exchange, creating a distinct mode known as the high VA/Q mode.
Further down the lung arterial pressure increases sufficiently to exceed alveolar pressure, although the latter is still higher than venous pressure. In this region (zone II) the driving pressure equals arterial pressure minus alveolar pressure. The effect on blood flow in this zone has variously been called the 'sluice', the 'waterfall phenomenon', or the 'Starling resistor'. In a simple model it can be seen that a constriction will develop at the downstream end of a collapsible tube surrounded by a pressure that is higher than venous pressure. The pressure inside the tube at the collapse point will then be equal to the external pressure. When the tube is completely collapsed, the higher arterial pressure will be transmitted to the collapse point and reopen the vessel. This may result in an unstable and fluttering vascular wall. Because of an increasing arterial pressure down this zone and a maintained alveolar pressure, blood flow increases down the zone.
Still further down the lung both arterial and venous pressures exceed alveolar pressure. Thus the driving force is arterial pressure minus venous pressure, similar to that in the systemic circulation. The pressure is constant down this zone (zone III), with hydrostatic pressure adding equally to both arterial and venous pressures. However, there is still some increase in blood flow down this zone, indicating a decrease in vascular resistance, presumably as a consequence of increasing dilatation of the alveolar capillaries. In the lowermost region of the lung, blood flow may be reduced (zone IV). This may be attributable to compression of extra-alveolar vessels brought about by increasing interstitial pressure.
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