After an increase in microvascular filtration rate, lung lymph may con
tain protein washed from the tissue spaces plus protein from the filtr
ate. If so, then the lymphatic protein concentration may be significan
tly higher than the filtrate protein concentration (C(f)). To test thi
s hypothesis, we decreased the plasma protein concentration from 5.1 /- 0.6 to 0.54 +/- 0.15 g/dl and increased the pulmonary microvascular
filtration rate in four dogs. We estimated C(f) to be 0.16 +/- 0.05 g
/dl after we reduced the plasma protein concentration, and the lymphat
ic protein concentration (0.43 +/- 0.04 g/dl) was significantly greate
r than C(f). Our results indicate that lung microvascular membrane ref
lection coefficients estimated from lung lymph data may be too low. Ho
wever, the amount of error caused by tissue protein washout is probabl
y small. To account for the protein washout error, we estimated the lu
ng microvascular membrane reflection coefficient to be approximately 0
.74-0.76 instead of the approximately 0.70 previously reported for dog
s (J. C. Gabel et al. J. Appl. Physiol. 55: 866-869, 1983).