In conscious animals, the response to hemorrhage is biphasic. During phase 1, arterial pressure is maintained. Phase 2 is characterized by profound hypotension. Despite allied roles, less is known about the integrated cardiovascular and respiratory response to blood loss in conscious animals. We evaluated cardiorespiratory changes during hemorrhage to test the hypotheses that 1) respiratory rate (RR) and blood gases do not change during phase 1; 2) RR increases during phase 2; and 3) RR and

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... d gas changes during hemorrhage are similar in males and females. We measured mean arterial pressure, RR, and blood gases during hemorrhage in 16 conscious, chronically prepared, male and female New Zealand white rabbits. We removed venous blood until mean arterial pressure was Յ40 mmHg. Sex did not affect mean arterial pressure, heart rate, Pa O 2 , PaCO 2 , or pH during hemorrhage or the blood loss required to induce phase 2. Pa CO 2 decreased significantly from 37 Ϯ 1 to 33 Ϯ 1 and 29 Ϯ 1 mmHg (P Ͻ 0.001) during phase 1 and 2, respectively. Before hemorrhage, PaO 2 was 87 Ϯ 2 mmHg. PaO 2 was unchanged in phase 1 (92 Ϯ 2 mmHg) but increased in phase 2 (101 Ϯ 2 mmHg; P Ͻ 0.001). Body temperature, PvCO 2 (thoracic vena cava), and ventilation-perfusion mismatch (A-a gradient) were unchanged during phases 1 and 2. Neither sex increased RR during phase 1. While males doubled RR during phase 2, RR in females did not change (P Ͻ 0.001). Thus, while PaCO 2 decreases in phase 1 and phase 2, the decreases are achieved in different ways across the two phases and in the two sexes.