Which of the following statements are true of external respiration and internal respiration? (Read carefully and select all of the correct statements.)

A)The P in PO₂ means presence of oxygen.
B)In the alveoli, PCO₂ is low and PO₂ is high.
C)In the tissues, PO₂ is high and PCO₂ is low.
D)Internal respiration takes place in the pulmonary capillaries.
E)External respiration takes place in the nasal cavities as the air is warmed
F)The pressure gradient of a gas is much the same as a concentration gradient.