Erythrocytes which is also known as red blood cells,(RBCs) carry oxygen and carbon dioxide in the blood. Hemoglobin, a protein in RBCs, binds to oxygen and carbon dioxide. 

Oxygen: RBCs pick up oxygen in the lungs, where the partial pressure of oxygen is high. The hemoglobin becomes saturated with oxygen, forming oxyhemoglobin. The oxygen-rich RBCs then travel to tissues throughout the body. In the tissues, the oxyhemoglobin releases oxygen, which is used to produce energy.

Carbon dioxide: RBCs carry carbon dioxide away from tissues back to the lungs. Some carbon dioxide binds directly to hemoglobin. RBCs also carry an enzyme that converts carbon dioxide into bicarbonate, which is transported to the lungs. In the lungs, the bicarbonate is converted back into carbon dioxide and released.