In post 20.14, I mentioned the belief that acidic drinks could make our body fluids more acid; similarly, many people believe that eating alkaline foods will make their body fluids less acidic. To find out more about acids and alkalis, and the pH scale that is used to measure acidity /alkalinity, see posts 17.49, 17.50 and 18.1.
Like several other ideas that seem “obvious” (post 17.1) these ideas about acidity, alkalinity and diet are false.
The reason these ideas are false is that our bodies have mechanisms for controlling our chemical composition, including the concentration of hydrogen (H+) and hydroxide (OH–) ions that make our body fluids acidic or alkaline. However, the so-called “alkaline diet” does have health benefits because it involves eating more vegetables.
In post 19.1, we saw that when carbon dioxide dissolves in water, an equilibrium mixture of carbonic acid (H2CO3) and hydrogen carbonate (HCO3–, also called bicarbonate) and hydrogen (H+) ions is produced:
In post 17.49, we saw that there is a constant ratio between the molar concentrations (see post 17.48) of these components called the acid dissociation constant defined, for the carbon dioxide/hydrogen carbonate system, by
where [square brackets] represent concentration expressed as a molarity (post 17.48). An acid is simply a source of H+ ions (post 17.49). When we add an acid to the carbon dioxide/hydrogen carbonate system, the equilibrium moves to the left to remove excess H+ ions and so maintain a constant Ka. So excess H+ ions are converted into H2CO3 molecules. When we add an alkali to the system, we are adding OH– ions (post 17.50) that combine with H+ ions to form water (H2O) molecules. So adding an alkali removes H+ ions. But the equilibrium is restored, maintaining Ka at its constant value, by H2CO3 molecules dissociating into H+ and HCO3– ions. This ability for systems to return to their equilibrium position, when they are perturbed, is called Le Chatelier’s principle and has been well known to chemists, if not to people who give advice on nutrition, since the beginning of the twentieth century.
Why is this relevant to our body fluids? Because our body fluids are mostly water and dissolve carbon dioxide produced in respiration. Further control is provided by the ability of our bodies to control carbon dioxide by changing the rate at which we respire and, therefore, the rate at which we produce carbon dioxide. Also the levels of HCO3– and H+ ions in our body fluids can be adjusted by dialysis in our kidneys (post 18.27). For these mechanisms to work, we need cells in our bodies to sense HCO3– and H+ ion concentrations and for there to be mechanisms to convey the information to the lungs and kidneys. Many of the details of these mechanisms are unknown.
Why is control of pH important? Because we need to control pH values in our body fluids to enable the chemical reactions that keep us alive. And the pH values are different in different parts of our bodies – for example, saliva is alkaline but our stomach contents are acidic.
These natural control mechanisms are called homeostasis and there are homeostatic mechanisms for controlling the level of all aspects of the chemical composition of our bodies. I will give some examples below.
Our kidneys are also responsible for controlling the concentrations of other ions in our body fluids. Excess ions are removed by dialysis (post 18.27); ion pumps allow ions to be returned to the body fluids if their concentrations are too low. A pump can transport a specific ion – for example, sodium pumps pump sodium ions. The pumps move ions from areas of high concentration to areas of low concentration and so reverse the spontaneous process of diffusion. By analogy with reverse osmosis (post 18.29), ion pumps must do work and so need an energy source. This energy is provided by the conversion of ATP into ADP (post 16.33).
The kidneys also control the concentration of urine, so allowing the body to get rid of excess water or to retain water when necessary.
This post is a very brief introduction to a very complicated subject. The main thing to remember is that out bodies are not simply reservoirs that store whatever we eat, drink or breath. We have many complicated mechanisms for changing whatever we consume into something useful, to control the concentrations of the chemical components of our bodies and to get rid of waste products. As a simple example, let’s think about what we do with the fats that we consume (post 16.47). First of all we break them down into fatty acids (digestion) that we absorb. We can then use them to make ATP that provides the energy for our bodies. Any excess fatty acids can be converted into fats that we use for energy storage or that are required to build structures in our bodies, especially in the nervous system.
Homeostasis is not confined to controlling our chemical composition. In the next post, I will discuss homeostatic mechanisms for controlling temperature.