During a cardiopulmonary bypass (CPB) procedure using a heart-lung machine, the volume of blood inside the venous reservoir is a crucial piece of information for the perfusionist to make clinical decisions. In existing CPBs, venous reservoir blood volume can only be visually estimated by the perfusionists from the marks on the reservoirs. Such manual reading has poor resolution and it is impractical to continuously track the blood volume over a long period. In this work, we introduce a blood volume sensor that can automatically measure and monitor the blood volume inside the venous reservoir in real-time, with high accuracy and high resolution. Our sensor implements an optical scheme, in which the liquid level of blood is optically scanned by a contact image sensor (CIS) comprised of a linear array of CMOS sensor pixels. The liquid level is then converted into blood volume. For a changing blood volume, the net flow rate into the reservoir is also instantly calculated from the recorded volumes. The demonstrated sensor prototype has been calibrated for a specific commercial reservoir and tested in a simulated CPB experiment using bovine blood. The results showed that the blood volume inside the reservoir and the flow rate can both be reliably monitored using our sensor. The reading can cover the entire operable range of venous blood volume (250–4000 mL), and the resolution has reached better than 2 mL. Our blood volume sensor shows great promise for improving the monitoring and management of patient blood volume during CPB procedures.