A perfusion system comprising: a heart lung machine; a blood reservoir (56; 82; 92; 100; 114; 150) adapted to hold a fluid, the blood reservoir (56; 82; 92; 100; 114; 150) having a capacity determined by a geometric configuration, wherein the blood reservoir (56; 82; 92; 100; 114; 150) includes an information device storing information about the geometric configuration of the blood reservoir (56; 82; 92; 100; 114;150), the information about the geometric configuration of the blood reservoir comprising known cross-sectional area of the blood reservoir at various heights throughout the reservoir, wherein the information device is configured for communicating said information about the geometric configuration of the blood reservoir (56; 82; 92; 100; 114;150); an ultrasonic sensor (58; 86; 96; 104, 106; 118; 160) coupled to the blood reservoir (56; 82; 92; 100; 114; 150) and configured to determine a fluid level within the blood reservoir (56; 82; 92; 100; 114; 150); a receiver configured to communicate with the information device to receive the information about the geometric configuration of the blood reservoir (56; 82; 92; 100; 114; 150); a controller coupled to the heart lung machine and to the receiver, the controller configured to receive the fluid level from the ultrasonic sensor (58; 86; 96; 104, 106; 118; 160) and to receive the information about the geometric configuration of the blood reservoir (56; 82; 92; 100; 114; 150) from the receiver, the controller configured to calculate a blood volume contained within the blood reservoir (56; 82; 92; 100; 114;150) by integrating the fluid level in the blood reservoir (56; 82; 92; 100; 114; 150) against the known cross-sectional area of the blood reservoir at various heights throughout the reservoir (56; 82; 92; 100; 114;150);; and a display coupled to the heart lung machine, the display configured to display the calculated blood volume; wherein the heart lung machine is configured to adjust an operating parameter based on the calculated blood volume. a heart lung machine; a blood reservoir (56; 82; 92; 100; 114; 150) adapted to hold a fluid, the blood reservoir (56; 82; 92; 100; 114; 150) having a capacity determined by a geometric configuration, wherein the blood reservoir (56; 82; 92; 100; 114; 150) includes an information device storing information about the geometric configuration of the blood reservoir (56; 82; 92; 100; 114;150), the information about the geometric configuration of the blood reservoir comprising known cross-sectional area of the blood reservoir at various heights throughout the reservoir, wherein the information device is configured for communicating said information about the geometric configuration of the blood reservoir (56; 82; 92; 100; 114;150); an ultrasonic sensor (58; 86; 96; 104, 106; 118; 160) coupled to the blood reservoir (56; 82; 92; 100; 114; 150) and configured to determine a fluid level within the blood reservoir (56; 82; 92; 100; 114; 150); a receiver configured to communicate with the information device to receive the information about the geometric configuration of the blood reservoir (56; 82; 92; 100; 114; 150); a controller coupled to the heart lung machine and to the receiver, the controller configured to receive the fluid level from the ultrasonic sensor (58; 86; 96; 104, 106; 118; 160) and to receive the information about the geometric configuration of the blood reservoir (56; 82; 92; 100; 114; 150) from the receiver, the controller configured to calculate a blood volume contained within the blood reservoir (56; 82; 92; 100; 114;150) by integrating the fluid level in the blood reservoir (56; 82; 92; 100; 114; 150) against the known cross-sectional area of the blood reservoir at various heights throughout the reservoir (56; 82; 92; 100; 114;150);; and a display coupled to the heart lung machine, the display configured to display the calculated blood volume; wherein the heart lung machine is configured to adjust an operating parameter based on the calculated blood volume.