Why plasma pH must be tightly regulated
Plasma pH is kept between 7.35 and 7.45 because proteins such as haemoglobin (the oxygen-carrying protein in red blood cells) work only within a narrow pH range.
Remember
Proteins such as haemoglobin and enzymes only work properly within a narrow pH range, so a drift in plasma pH disturbs how haemoglobin binds and releases oxygen.
Proteins fold into precise three-dimensional shapes, and the shape is what lets them work. A change in pH alters the electric charges on a protein, so it works badly or can even lose its shape.
Haemoglobin is the oxygen-carrying protein in red blood cells, and it works best at a pH of about 7.4. That is why plasma pH is kept between 7.35 and 7.45, which allows adequate oxygen transport. When the pH drifts, it changes how well haemoglobin picks up oxygen in the lungs and gives it up to the tissues.
The Bohr effect is a good example of this. A lower, more acidic pH makes haemoglobin release oxygen more readily, whereas a higher, more alkaline pH makes it hold on to oxygen more tightly.
Enzymes (proteins that speed up chemical reactions) depend on pH too. At extremes of pH they can denature, which means they lose their shape and cannot work. That would disturb processes such as aerobic respiration (making energy using oxygen) and gluconeogenesis (making glucose from other molecules).
A pH above about 7.6 also leaves less calcium free, because more of it binds to proteins, so tetany (involuntary muscle spasm) can occur. In clinical practice, an arterial blood gas (ABG, a test on blood taken from an artery) measures plasma pH, and understanding the physiology behind pH control is important for interpreting one.
Key facts
- Normal range
- plasma pH is kept between 7.35 and 7.45.
- Haemoglobin
- it functions best at a pH of about 7.4.
- Shape and charge
- pH changes the charge and shape of proteins such as haemoglobin and enzymes.
- Lower pH
- a more acidic pH makes haemoglobin release oxygen more readily.
- Higher pH
- a more alkaline pH makes haemoglobin hold on to oxygen.
- Oxygen carried
- each intact haemoglobin molecule can carry four oxygen molecules; physiological pH changes alter how readily haemoglobin binds and releases oxygen, not how many it can carry.
Exam traps
- A more acidic pH does not make haemoglobin hold on to oxygen more tightly: it makes haemoglobin release oxygen more readily (the Bohr effect), and a more alkaline pH is what makes it hold on more tightly.
- Tight pH control is not about how well oxygen dissolves in plasma: most of the oxygen in blood is carried bound to haemoglobin, so the amount dissolved in plasma is small, and the reason for tight control is the effect of pH on haemoglobin and other proteins.
- A physiological change in pH does not change how many oxygen molecules an intact haemoglobin molecule can carry, because each can carry four; it changes how readily haemoglobin binds and releases oxygen.
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