Rib cage moves down and inward. Lungs recoil due to elastic tissue within. External and Internal Respiration External respiration is exchange of gases between air in alveoli and blood in pulmonary capillaries. Internal respiration is exchange of gases between blood in systemic capillaries and tissue fluid. Diffusion alone governs direction of gas flow.
Oxygen diffuses into the capillary because there is a higher concentration of it in the alveolus.
High altitude planes pressurize cabins to maintain this gas pressure. Internal Respiration Cleanses Tissue Fluid a. Internal respiration is exchange of gases between blood in systemic capillaries and surrounding tissue fluid. Blood entering systemic capillaries is bright red due to oxyhemoglobin. The lower PO 2 of the tissues is due to tissues constantly using up oxygen in aerobic cellular respiration.