External Respiration:

Diffusion of oxygen into blood and carbon dioxide into alveoli (external respiration)  is the diffusion of oxygen from air in  the alveoli of lungs to blood  in the pulmonary capillaries and diffusion of carbon dioxide in  the opposite direction.

Transport of Oxygen and Carbon Dioxide:

Transport of oxygen and carbon dioxide in the blood and body fluids to and from the cells. 98.5 per cent oxygen bound  to haemoglobin in  the RBC and  1.5 per  cent of oxygen dissolved in the blood plasma  is transported  to the body cells.
Whereas carbon dioxide from the tissues is diffused  into the b1,ood and transported to the  lungs in  the form of bicarbonate ions (78 per cent), as carbamino compounds (13 per cent) and a small percentage  (9 per oent) dissolved in blood plasma. From the  lungs carbon dioxide is removed in  the exhalation. 

Internal Respiration

This occurs in  tissues throughout the body the exchange of oxygen and carbon dioxide between the  systemic capillaries and tissue cells  is called the  internal respiration. Partial pressure of oxygen (PO₂)  in the systemic capillaries is higher (100 mtnHg) than the PO₂  in tissues cells in  the tissue cells (40 mmHg) at rest. Due to the pressure difference, oxygen diffises out of the capillaries into the tissue cells and blood PO₂  drops to 40 rnm Hg. Tissues are constantly using oxygen and producing carbon dioxide. Partial Pressure of  carbon dioxide (PCO₂) of cells is higher  (45 mrnHg) than that of systemic capillary blood  (40 mmHg). As a result carbon dioxide diffise  fiom tissue cells into the systemic capillaries until the PCO₂  increases to 45 mm Hg.