Blood volume changes in outdoor exercise of 8-10 hour duration.

1. Blood volume was measured with carbon monoxide on six hill-walkers before and immediately after a 28-mile walk on two occasions. The subjects had free access to food and fluid.2. Blood volume increased and haematocrit fell significantly on both occasions compared with the control observations. The mean increase of blood volume was 204 ml. or 3.9% (P < 0.01) and the mean reduction of haematocrit was 2.1% (P < 0.02). Plasma volume calculated from these results increased by 233 ml. or 7.3%. There was no significant change in red cell volume or plasma protein concentration.3. These changes are the opposite of those taking place in short-term exercise and suggest that in the absence of dehydration a compensatory adjustment of blood volume takes place during exercise of many hours duration.