A novel method to obtain pressure-diameter relationship of human brachial artery noninvasively in a wide pressure range was proposed. An airtight box was installed on the forearm and its inner pressure was changed from -100 to +150 mmHg to change transmural pressure of the artery. Inflation and deflation of the artery due to transmural pressure change was monitored with an ultrasound probe attached in the box. When the artery was collapsed by pressurizing the box and then returned to the initial state, its diameter in the physiological state increased, while it decreased following temporal overload to the artery, i.e., temporal negative pressure application in the box. These changes may be caused by the active response of the smooth muscle cells in the media, i.e., Bayliss effect, and are not observed in excised arteries. In vivo measurement of mechanical properties of the arteries is important.
