Transport of gases

About 70 per cent of $CO_2$ is carried as bicarbonate ions, 20–25 per cent as carbamino-haemoglobin and about 7 per cent dissolved in plasma.

Carbon dioxide is transported in the blood in three forms.
About 70 per cent is carried as bicarbonate ($HCO_3^-$).
About 20–25 per cent is carried by RBCs as carbamino-haemoglobin.
About 7 per cent is carried in a dissolved state in the plasma.
RBCs contain a very high concentration of the enzyme carbonic anhydrase (minute quantities are also present in plasma), which catalyses $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons HCO_3^- + H^+$
Hence the bulk of $CO_2$ is present as bicarbonate in blood plasma and RBCs.

Every 100 mL of oxygenated blood carries about 20 mL of $O_2$ and delivers only about 5 mL of it to the tissues under normal physiological conditions.
So the tissues use only about 25% of the oxygen; venous blood is still about 75% saturated with oxygen.
During muscular exercise the tissues have a lower $pO_2$, higher $pCO_2$, higher $H^+$ concentration and higher temperature, which make oxyhaemoglobin dissociate more.
The extra oxygen needed is then drawn from this unused portion, which therefore serves as a reserve.
Hence the unused $O_2$ acts as a reserve during muscular exercise.

About 97 per cent of $O_2$ is transported by RBCs in the blood, bound to haemoglobin as oxyhaemoglobin; the remaining 3 per cent is carried in a dissolved state in the plasma.
So the figures 99.5 per cent and 80 per cent are both wrong.
Nearly 20–25 per cent of $CO_2$ is transported by RBCs as carbamino-haemoglobin.
About 70 per cent of $CO_2$ is carried as bicarbonate and about 7 per cent in a dissolved state in the plasma.
Hence the true statement is that RBCs carry about 20–25 percent of $CO_2$.