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The regulation of the ratio of the cardiopulmonary blood volume to stroke volume in sheep.

Volume expansion with dextran was used in unanaesthetised sheep to investigate the mechanisms which are responsible for keeping the ratio of the cardiopulmonary blood volume to stroke volume (CPFI) constant under physiological conditions. For animals with heart rates lower than 80, active pulmonary vasoconstriction was mainly responsible for the regulation obtained with volume expansion, while the Starling mechanism was responsible for keeping the ratio constant for animals with heart rates above 79. The involvement of a regulatory mechanism was shown by the absence of large changes in the CPFI after inducing mechanical stress on the cardiopulmonary system as a result of experimental hypertension. Results obtained with receptor blockade showed that active pulmonary vasoconstriction may be the result of a reflex mechanism with parasympathetic efferents. It is suggested that the receptors for this reflex are situated in the pulmonary arteries and are activated by the pressure changes as a result of heart contractions.