Tissue pressure and plasma oncotic pressure during exercise.

Six healthy male subjects exercised on a cycle ergometer for 3 min for assessment of forces involved in transvascular fluid shift during intense exercise. The work load was at 105% of peak O2 uptake of the subjects. This caused a 17.2 +/- 1.2% reduction in plasma volume. The plasma volume loss was associated with an increase in plasma sodium, from 142.6 +/- 0.5 to 148.1 +/- 1.0 meq X 1(-1) (P less than 0.005); chloride, from 101.8 +/- 0.6 to 104.6 +/- 0.9 meq X 1(-1) (P less than 0.005); lactate, from 1.4 +/- 0.2 to 14.0 +/- 1.5 meq X 1(-1) (P less than 0.005); and osmolality, from 283 +/- 2 to 299 +/- 3 mosmol X kg-1 H2O (P less than 0.005) within 2 min after cessation of exercise. Plasma protein increased from 7.0 +/- 0.2 to 8.1 +/- 0.3 g X dl-1 (P less than 0.005), and plasma colloid osmotic pressure from 25.1 +/- 0.6 to 30.6 +/- 1.4 mmHg (P less than 0.005) after exercise. Interstitial fluid pressure in the exercising vastus lateralis muscle increased from a base-line value (SE) of -1.0 +/- 0.9 to + 1.5 +/- 1.1 cmH2O, 14 min after the end of exercise (P less than 0.05). Interstitial fluid pressure of the triceps brachii (inactive) did not change significantly after exercise. Our data suggest that increased transvascular colloid osmotic pressure and elevation of interstitial fluid pressure become increasingly important in preventing loss of plasma volume during maximal exercise.