Movement of Water within the Body of Sheep fed at Maintenance under Thermoneutral Conditions

Two mature Border Leicester x Merino wethers were maintained with continuous feeding under thermoneutral conditions. Their water balance was recorded for 2 weeks; at the beginning of each week they were given a dose of tritiated water (TOH) into the pulmonary artery or the rumen and samples were taken from both the pulmonary artery and the rumen. A four-compartment model was developed which simultaneously fitted the balance and tracer data. The half-time of TOH in body water was 6.7 days for one sheep and 7.6 days for the other; TOH space was about 55% of liveweight in both sheep. The bidirectional flux of water between the plasma-accessible compartment and 'bound' intracellular water averaged 45 litres per hour and that between plasma-accessible water and rumen water averaged 3.2 litres per hour. The mean residence times of a water molecule in the rumen were, for the two sheep, 60 and 63 min and the time constants for flow from the rumen were 12.3 and 13.4 h. Consideration of the rumen water balance suggested that there was net movement of water from the rumen to the plasma at about 200 ml/h; diffusion accounted for 86% of the influx to and 92% of the efflux from the rumen.