Blood : a study in general physiology / by Lawrence J. Henderson.

  • Lawrence Joseph Henderson
Date:
1928
Catalogue details

Licence: In copyright

Credit: Blood : a study in general physiology / by Lawrence J. Henderson. Source: Wellcome Collection.

    260/434 page 232
    “The last question to be raised in respect to the under¬ lying assumptions in our calculations concerns the exten¬ sion of results, based on the theoretical properties of blood in a given capillary, to cover the lung as a whole or the tissues as a whole. The problem of the tissues is beset with difficulties. In the first place the oxygen pressure of the tissues cannot be accurately determined. Secondly, the oxygen pressure in the tissues along a capillary is probably not uniform, but higher at the end which re¬ ceives the arterial blood. Thirdly, the mixed venous blood, derived from various organs in which p02 may be very different, offers scant information in respect to the pre¬ cise prevailing conditions. However, marked variations, characteristic of the tissue circulation as a whole or of a significant portion of it, will be reflected in the composi¬ tion of the mixed venous blood. Thus comparative data may be obtained, and for this purpose it may be useful to select the arbitrary value p02 = 0 as a reference stand¬ ard for the oxygen pressure of the tissues. “In contrast to the diversity of conditions presented by the rest of the body, the lung offers a condition of relative uniformity. The oxygen pressure of the alveolar air can be obtained with considerable accuracy, and this pressure is presumably uniform along the length of a capillary. In the lung there is practically only one case which cannot be accurately described from data on the mixed arterial blood; namely, a condition of partial stagnation. After a quantity of blood has nearly reached equilibrium with al¬ veolar air, it may travel through any additional length of capillary without further detectable change in composi¬ tion. If a considerable quantity of such blood mixes with blood which is not in equilibrium with alveolar air, the mixed blood will not reveal the existence of the super¬ fluous length of capillary through which the first portion has passed. The values of Ap\ as determined by the data on such mixed arterial blood, will be too high, and con¬ sequently the values of the diffusing capacity will be too
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