In the late 1860s, Adolf Fick, a professor of physiology in the Faculty of Medicine in Würzberg, Germany, developed one of the methods we use today for measuring how much blood your heart pumps in a minute. Your cardiac output as you read this sentence is probably about 7 L/min. At rest it is likely to be a bit under 6 L/min. If you are a trained marathon runner running a marathon, your cardiac output can be as high as 30 L/min.

Your cardiac output can be calculated with the following formula:

 y = Q / D,

where Q is the number of milliliters of CO2 you exhale in a minute and D is the difference between the CO2 concentration (ml/L) in the blood pumped to the lungs and the CO2 concentration in the blood returning from the lungs. With Q=233 ml/min and D=97-56=41 ml/L,

y = (233 ml/min) / (41 ml/L) ≈ 5.68 L/min,

fairly close to the 6 L/min that most people have at basal (resting) conditions.

(Data courtesy of J. Kenneth Herd, M.D., Quillan College of Medicine, East Tennessee State University.)

Suppose that when Q = 233 and D = 41, we also know that D is decreasing at the rate of 2 units a minute but that Q remains unchanged. What is happening to the cardiac output?