###### Abstract
To avoid an unexceptional measurement error for quantifying the blood flow rate, it is necessary to replace the blood viscosity with another property (i.e., pressure or work). As blood pressure is proportional to blood viscosity, it can be used to monitor blood flow instead of blood viscosity. Additionally, blood pressure, which is not affected by RBCs, can be used to monitor the contribution of plasma. [PERSON] et al. introduced pressure sensing with trapped air-liquid compression in a glass microchannel [43]. A glass microchannel is replaced by glass capillaries inserted into the PDMS microchannel channels [44; 45]. [PERSON] et al. reported an on-chip pressure sensor in terms of color intensity resulting from PDMS channel deformation [46]. [PERSON] et al. suggested a liquid-metal pressure sensor integrated into a microfluidic channel [47]. Blood pressure is monitored with the intensity of RBCs within the big dead channel [48]. As the method is only demonstrated under periodic on-off blood flows, it still shows limitations in the continuous monitoring of blood pressure. In this study, a coflowing channel as a pressure sensor is suggested to measure blood pressure. Using a coflowing channel used for measuring the viscosity of fluid [49; 50; 51], interfacial location in the coflowing channel provides blood pressure. A simple fluidic circuit model is adopted to find the blood pressure formula expressed by three vital factors (i.e., interface, the flow rate of reference fluid, and viscosity of reference fluid). According to the mathematical model, each stream has an identical pressure and shear stress in a straight coflowing channel. Additionally, two work formulas on blood flows (i.e., pressure-unit volume work and shear stress-unit volume work) are suggested with variations of shear stress and pressure with respect to the interface. Four properties of blood flows (i.e., pressure, shear stress, and two works) are then used to monitor blood flows continuously over time. Compared with previous methods, the present method offers several advantages. First, the present method can measure blood pressure under continuous blood flow. As the fluid pressure is proportional to the flow rate multiplied by the viscosity, the blood pressure provides variations in the flow rate and viscosity of the blood flowing in the microchannel. Second, the present method obtains pressure or shear stress of test fluid (i.e., blood) by measuring the interface. Third, the present method does not require any information on blood flow (i.e., hematocrit, flow rate, and viscosity) in advance. Furthermore, the present method can be used effectively for monitoring the blood, in which the hematocrit or flow rate varies continuously over time. As an experimental demonstration, the present method was applied to quantify test fluids (i.e., glycerin, blood) under different flow patterns (i.e., constant flow rate,sinusoidal flow rate, and peristaltic flow rate). First, the flow of glycerin without including RBCs was quantified under constant flow rate and sinusoidal flow rate (i.e., Section 3.2). Second, blood flow with different hematocrit was monitored under sinusoidal flow rate (i.e., Section 3.3). At last, to mimic in vitro blood circulation, the blood flow of normal and aggregation-enhanced bloods was quantified under peristaltic flow rate (i.e., Section 3.4).