1. Impedance to venous return by distention of a balloon in the inferior vena cava (IVCB) was utilized in 10 patients, during diagnostic procedures, to reduce blood flow through the lungs at baseline and to buffer its changes during stimulation of the adrenergic system, which was obtained with mental arithmetic (AT) and cold pressor (CPT) tests.

2. When venous return was unimpeded, cardiac output rose by 2.06 1/min during the AT and remained steady during the CPT, and arteriolar resistance in the pulmonary circuit was significantly reduced and slightly raised, respectively.

3. During IVCB, baseline cardiac output decreased by 710 ml/min, it rose by 925 ml/min during the AT and again remained steady during the CPT; pulmonary arteriolar resistance was unchanged from before obstruction at baseline and was significantly augmented by both tests. In particular, AT became a clear vasoconstrictor stimulus, having originally produced vasodilatation.

4. These observations support the view that lung blood vessels in man are sensitive to adrenergic influences and that vasoconstriction can be elicited in circumstances in which the flow through the lungs is restrained. The physiological importance of the neural regulation of the pulmonary circulation in man remains undefined.

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