Acids and Bases

A brief recap of the basics

What are acids and bases?

An acid is a substance that produces H+ ions when dissolved in water. For example, hydrochloric acid (HCl) dissolves in water to form H+ and Cl- ions. The predominant acid in the blood is carbonic acid, formed by the dissolution of carbon dioxide.

A base is a substance that accepts H+ ions, often by forming hydroxyl (OH-) ions when dissolved in water. For example, sodium hydroxide (NaOH) dissolves in water to form Na+ and OH- ions - these OH- ions can then 'accept' H+ ions to form H2O. The predominant base is bicarbonate, which is a byproduct of metabolism.

pH

Normal range: 7.35 - 7.45

The pH indicates the acidity or alkalinity of the blood, and is inversely proportional to the number of hydrogen (H+) ions. It is kept within a tight range to maintain normal physiological function.

pH scale with colours representing those shown by a universal indicator
pH scale with colours representing those shown by a universal indicator

A decrease in pH below the normal range is called acidaemia, with the process leading to the drop in pH termed acidosis.

An increase in pH is called alkalaemia, and the process causing the increase is alkalosis.

Buffering

Carbonic acid and bicarbonate work in tandem to create a buffering system, meaning the concentrations will increase or decrease to neutralise excess acids or bases in the blood.

The chemical equation for this buffer is:

H+ + HCO3- ⇋ H2CO3 ⇋ H2O + CO2

The buffer likes to exist in equilibrium – a state of balance where the forward reaction occurs at the same rate as the reverse reaction, and the ratio of reactants (on one side of the equation) to products (on the other) will remain steady.

Think of it as two beakers joined by a channel at the base of each. If we pour water into one beaker, it will travel through the channel until a balance is reached on both sides.

Adding more water to the first beaker increases the volume in the second, but the ratio remains the same
Adding more water to the first beaker increases the volume in the second, but the ratio remains the same

If we add more water to the first beaker (symbolising the reactants), the volume will also increase in the second beaker (the products), but the ratio of reactants:products remains the same.

The buffering system works in the same way. If we were to add more acid (which dissociates into H+ ions) the result would be an increase in H2O and CO2. If we were to add more CO2 the result would be an increase in H+ and HCO3-.

When this system is overwhelmed (i.e. too much acid or base), the blood cannot regulate it’s pH effectively and will become more acidic or alkaline. This is known as an acid-base disorder.