What does it mean to have a cardiopulmonary reserve?

Cardiac reserve: Let's make an analogy to 'army reserve'. These army men are just standby without active duty. But when there is a war, these people will be involved. Similarly, you don't use cardiopulmonary reserve during normal activities. During stress or illness, you want your lungs to take as much air/oxygen as possible and increase heart rate to maintain adequate blood flow (cardiac output) to tissues.