Blood can be measured in several ways, depending on what aspect of blood you're interested in. This includes blood volume, blood pressure, blood alcohol content (BAC), cholesterol levels, and blood glucose levels.
Measuring Blood Volume
Blood volume, the total amount of blood in your body, is measured using a radioactive iodine tracer. A specific volume of this tracer is injected into a vein, and its distribution throughout the body is tracked to determine the total blood volume. Cleveland Clinic explains this process in detail.
Measuring Blood Pressure
Blood pressure, the force of blood against artery walls, is measured using a sphygmomanometer and stethoscope or an automated device. The measurement is expressed in millimeters of mercury (mmHg) as two numbers: systolic (higher number, pressure when the heart beats) and diastolic (lower number, pressure when the heart rests between beats). For example, a reading of 120/80 mmHg indicates a systolic pressure of 120 mmHg and a diastolic pressure of 80 mmHg. Accurate measurement requires proper technique, as detailed by the CDC and Mayo Clinic.
Measuring Blood Alcohol Content (BAC)
BAC is measured using a breathalyzer, which analyzes the alcohol concentration in a breath sample. The result is expressed as grams of alcohol per 210 liters of breath. This reflects the alcohol concentration in the blood because the ratio of breath alcohol to blood alcohol is approximately 2100:1. University of Toledo provides further details on BAC measurement and interpretation.
Measuring Cholesterol and Glucose Levels
Cholesterol and glucose levels are measured using blood samples analyzed in a laboratory. For cholesterol, medical devices called chemistry analyzers determine the amount of cholesterol in the sample by measuring the color change of a chemical reaction. NIST explains this process. Blood glucose levels are measured using a drop of blood applied to a chemically treated test strip inserted into a glucometer. NCBI describes this method.
