Anatomy and Physiology / Unit 9: The Respiratory and Digestive Systems
9.3 Gas Exchange and Transport
Notes
Gases move by diffusion down their partial pressure gradients.
- External respiration: in the lungs, oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli. Internal respiration: in tissues, oxygen diffuses from blood to cells and carbon dioxide from cells to blood.
- Oxygen transport: about 98.5% bound to hemoglobin (each can bind four O2); the rest dissolved. Carbon dioxide is carried mostly (about 70%) as bicarbonate ions in plasma, some bound to hemoglobin, and some dissolved.
- Conversion: carbon dioxide + water forms carbonic acid, which splits into hydrogen and bicarbonate ions. This helps buffer blood pH.
- Carbon monoxide binds hemoglobin much more strongly than oxygen and causes poisoning.
- Oxygen unloads more readily in active tissues (low pH, high CO2, higher temperature).
- Example: a muscle working hard has lower oxygen and higher CO2, so hemoglobin releases more oxygen there.
Practice
In which direction does oxygen diffuse at the alveoli?
Oxygen partial pressure is higher in alveolar air than in the blood arriving from the body.
In what form is most carbon dioxide transported in the blood?
About 70% is carried as bicarbonate.
Why is carbon monoxide poisoning dangerous?
CO occupies binding sites on hemoglobin and blocks oxygen transport.
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