PULMONARY CAPILLARY RECRUITMENT IN NEONATAL LAMBS

In species having lungs large enough to develop hydrostatic perfusion zones, increased pulmonary arterial pressure causes blood flow to be r edistributed from the lower to the upper lung. The blood flow increase in the upper lung recruits capillaries and increases gas exchange sur face area. There is disagreement, however, about whether such capillar y recruitment occurs in young animals with small lungs. To investigate this issue, we used in vivo microscopy to directly study capillary pe rfusion in individual alveolar walls in the upper lungs of neonatal la mbs and in older lambs with larger lungs. Pulmonary arterial pressure was elevated by airway hypoxia. In neonatal lambs (<10 d old; n = 7), hypoxia increased pulmonary arterial pressure by 55% but did not cause capillary recruitment. In older lambs (20-61 d old; n = 6), hypoxia i ncreased pulmonary arterial pressure by 40% and caused a 46% increase in recruited capillaries. These results support the hypothesis that ca pillary recruitment does not occur in newborn lambs when pulmonary art erial pressure increases and implies that there is limited gas exchang e reserve. In older lambs, however, pas exchange reserve develops thro ugh recruitable capillaries as the lungs mature.