Porth's Essentials of Pathophysiology, 4e

532

Respiratory Function

U N I T 6

Oxygen Transport

U N D E R S T A N D I N G

All body tissues rely on oxygen (O 2 ) that is transported in the blood to meet their metabolic needs. Oxygen is carried in two forms: dissolved and bound to hemoglobin. About 98% of O 2 is carried by hemoglobin and the remaining 2% is carried in the dissolved state. Dissolved oxygen is the only form that diffuses across cell membranes and produces a partial pressure (PO 2 ), which, in turn, drives diffusion.The transport of O 2 involves (1) transfer from the alveoli to the pulmonary capillaries in the lung, (2) hemoglobin binding and transport, and (3) the dissociation from hemoglobin in the tissue capillaries.

Lung

1

Alveoli-to-Capillary Transfer. In the lung, O 2 moves from the alveoli to the pulmonary capillar- ies as a dissolved gas. Its movement occurs along a concentration gradi- ent, moving from the alveoli, where the PO 2 is about 100 mm Hg, to the venous end of the pulmonary capil- laries with their lesser O 2 concentra- tion and lower PO 2 . The dissolved O 2 moves rapidly between the alve- oli and the pulmonary capillaries, such that the PO 2 at the arterial end of the capillary is almost if not the same as that in the alveoli.

Alveolus

O 2

O 2

Pulmonary capillary

Red blood cell

PO 2

HbO 2

2

O 2

Hemoglobin Binding and Transport. Oxygen, which is rel- atively insoluble in plasma, relies on hemoglobin for transport in the blood. Once oxygen has dif- fused into the pulmonary capillary, it moves rapidly into the red blood cells and reversibly binds to hemo- globin to form HbO 2 . The hemo- globin molecule contains four heme units, each capable of attaching an oxygen molecule. Hemoglobin is 100% saturated when all four units are occupied and is usually about 97% saturated in the sys- temic arterial blood. The capacity of the blood to carry O 2 is dependent both on hemoglobin levels and the ability of the lungs to oxygenate the hemoglobin.

β 1

β 2

Heme

α 1

α 2