Cardiac output

Cardiac output (Q) is the volume of blood the heart ejects per minute: heart rate (HR, beats/min) × stroke volume (SV, mL/beat). At rest Q is 4–6 L/min, rising to 20–25 L/min in healthy adults and exceeding 40 L/min in elite endurance athletes. Oxygen delivery to working muscles depends directly on Q, making it the primary central limiter of VO2max — expressed as VO2max = peak Q × peak arteriovenous oxygen difference (a-vO2 diff). SV is shaped by ventricular preload, contractility, afterload, and venous return via the skeletal-muscle pump. With healthy aging both components decline: maximal HR falls approximately 1 beat/min/year; SV decreases due to impaired diastolic filling, reduced ventricular compliance, and blunted beta-adrenergic responsiveness. Ogawa 1992 (Circulation, n=148 men and women across four age decades) found lower SV accounted for nearly 50% of the 25–32% age-related VO2max decline, with lower maximal HR and reduced peripheral oxygen extraction explaining the remainder. Pandey 2020 (JACC Heart Failure, n=104 healthy volunteers) confirmed progressive age-related reductions in peak cardiac index and peak SV during upright exercise, independent of body size. Endurance training increases peak Q by expanding plasma volume and inducing eccentric ventricular remodelling. HERITAGE (Wilmore 2001, n=631 sedentary adults aged 17–65) showed significant Q and SV increases at standardised submaximal workloads after 20 weeks across sex, race, and age groups, though training-induced SV adaptation is markedly attenuated in older adults.