Saturation vs. content: two different questions
Only a tiny fraction of oxygen dissolves freely in plasma; over 98% is carried clipped onto hemoglobin inside red blood cells. So two questions matter. First, how full are the hemoglobin seats? — that is oxygen saturation (SaO2), a percentage. Second, how many seats are there in total, and how full? — that is oxygen content, the actual amount of oxygen per unit of blood.
This distinction is life-saving to grasp. Someone with severe anemia can have a perfect 99% saturation yet dangerously low oxygen content, simply because there are too few hemoglobin seats. Saturation tells you the percentage full; content tells you the total cargo delivered.
Oxygen content, simplified (CaO2):
CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2)
|__ hemoglobin-bound __| |_ dissolved _|
(tiny)
Worked example — healthy adult:
Hb = 15 g/dL, SaO2 = 0.98, PaO2 = 100 mmHg
bound = 1.34 x 15 x 0.98 = 19.7 mL O2/dL
dissolved = 0.003 x 100 = 0.3 mL O2/dL
CaO2 = ~20.0 mL O2/dL
Same person, anemic (Hb = 7.5):
bound = 1.34 x 7.5 x 0.98 = 9.8 mL O2/dL
CaO2 = ~10.1 mL O2/dL <- half the oxygen,
yet SaO2 still 98%!Reading the dissociation curve
The oxygen–hemoglobin dissociation curve plots saturation against the blood oxygen pressure (PaO2). Its S-shape is not an accident; it has a flat top and a steep middle, and each part serves a purpose. The relationship between the percentage figure your finger probe shows and the underlying pressure is curved, not straight.
- The flat top (high PaO2, in the lungs) protects loading. Even if alveolar oxygen pressure dips a bit, saturation stays near 100% — hemoglobin still fills up reliably.
- The steep middle (lower PaO2, in the tissues) favors unloading. A small drop in pressure releases a lot of oxygen exactly where hard-working tissues need it.
- The curve shifts. Heat, acid, high CO2, and exercise shift it right — hemoglobin lets oxygen go more easily, helping busy muscles. Cold and alkalosis shift it left, holding oxygen tighter.
The finger probe, and carbon dioxide’s ride home
A pulse oximeter clipped to a finger estimates saturation by shining light through tissue — a wonderful, painless tool. But it has honest limits. It cannot tell you oxygen content (it does not know the hemoglobin level), it can read falsely with carbon monoxide poisoning, and it grows unreliable with poor circulation, dark nail polish, or motion. A normal reading does not rule out a person being short of breath, and visible cyanosis — a bluish tinge — appears only once a fair amount of hemoglobin is already oxygen-poor.
Carbon dioxide makes the return trip in three forms: a small part dissolved, a small part bound to hemoglobin, and the great majority — about 70% — converted into bicarbonate, the body’s main buffer. We will pick up bicarbonate and pH in any acid–base discussion; for now, just hold the picture that blood carries oxygen out and carbon dioxide back, each in its own clever way.