Gases Involved. Oxygen and carbon dioxide are the primary gases involved in respiration. Under special circumstances, nitrogen may also be of concern.
Pressure Gradients. A gas moves from an area where its pressure is greater to an area where its pressure is less. Thus, the movement of gases depends upon such pressure gradients.
External Respiration. At the alveoli, gases are exchanged between the air inside and the blood in the adjacent capillaries.
Internal Respiration. Within the body, gases are exchanged between the blood of the capillaries and the individual cells of the body.
Transportation of Gases. The gases are transported between the alveoli and the individual cells by the cardiovascular system.
Some of the gases are dissolved directly in the plasma of the blood.
However, in humans, the greater percentage of the gases is carried within the substance of the RBCs (red blood cells, erythrocytes). The RBC, found in great numbers in the blood, is specially constructed for transporting the gases.
Hemoglobin, a substance found within RBCs, has a great affinity for oxygen. Yet, the hemoglobin can readily give up the oxygen wherever it is needed.