Blood spends about 0.75 sec in a pulmonary capillary; and under normal conditions, all of the gas exchange in the vessel occurs early. Because equilibrium is reached so rapidly (less than 0.25 sec for both O2 and CO2), blood at the end of a pulmonary capillary has the same P02 and PCO2 as alveolar gas, and gas ex­change is said to be "perfusion limited." Although increasing the cardiac output decreases the transit time of blood through the capillaries, only the most strenu­ous forms of exercise (cardiac output increases of 5 to 6 times above normal) result in a few mm Hg differ­ence between alveolar and end-capillary gas tensions. Gas exchange at a high altitude or anywhere in a normal lung (apex to base) remains perfusion limited.