How the body maintains an acid - base balance

by henriette195 (student)

How the body maintains the correct acid base balance

Our bodies are very sensitive to pH levels, so strong mechanisms are in place to in place to regulate it. If pH rises or falls outside acceptable ranges for a specific part of the body, proteins and enzymes may become denatured and lose their ability to function. This could lead to serious damage or death. That is why is absolutely essential that our body has a homeostatic mechanism to regulate the correct acid-base balance.

Blood pH is strictly regulated at a pH of 7.4. A variety of buffers are used to maintain this. Several buffers reversibly bind to or release H+ ions and inflict any changes on pH that need to be made.

As we know, the higher the hydrogen ion concentration, the lower the ph (more acidic)

Extracellular buffers (buffers found in the fluid outside of cells) include bicarbonate and ammonia.

A very common buffering system is the bicarbonate buffering system. It’s one of 3 major buffer systems in acid-base homeostasis.

This is how it works: Carbon dioxide (CO2) combines with water (H2O) to form carbonic acid (H2CO3) which quickly splits into a hydrogen ion (H+) and a bicarbonate (HCO3-) .

This can be shown in the equation below

If there is a disturbance of the system, it will be compensated for by a shift in the chemical equilibrium. For example if here was a sudden rush of H+ ions into the blood and the bloods acidity was increased, the equation would work going to the left. Most of the excess H+ ions would combine with bicarbonate to form carbonic acid - the result would be a much smaller increase of acidity than would have otherwise been.

Another way that the body would decrease the number of H+ ...

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This can be shown in the equation below

Another way that the body would decrease the number of H+ ions would be an increase inbreathing. This would remove some CO2 from circulation, driving the equilibrium to the left. This process would continue until all the excess acid is removed.

The Next buffer system is the Phosphate buffer. It consists of two ions, dihydrogen phosphate (2 hydrogen atoms and a phosphate) and mono hydrogen phosphate ions (1 hydrogen atom and a phosphate). They work in the following equilibrium equation to take up or release H+ ions.

H2PO4(-) + H2O <=H3PO4 => HPO4(2-) + H+

When the bloodstream (or any other extracellular fluid) is in basic conditions (low H+ concentration) The Dihydrogen phosphate would combine with water to form phosphoric acid which would split into monohydrogen phosphate and H+. This would increase the acidity of the blood to bring it back to its correct pH. The same reaction would work goint to the left if the blood were in acidic conditions (H+ ions would be accepted to reform dihydrogen phosphate)

Finally there is the protein buffer system . As most proteins are found within cells, the protein buffer system is responsible for maintaining pH in intracellular fluid. Heamoglobin (Hb) is a globular protein that makes a good buffer because of its ability to bind with and release H+ ions. For example, when red blood cells are near the lungs, Hemoglobin binds with oxygen, releasing the CO2 and H+ ions. The H+ ions combine with bicarbonate (HCO3) ions to form carbonic acid (H2CO3). The H2CO3 breaks down to form water (H2O) and carbon dioxide (CO2) which are excreted via expiration through the lungs. This is how a neutral pH is maintained

The lungs regulate blood pH by increasing the amount of CO2 that is released from the body. A decrease of CO2 means that less carbonic acid is form, which dissociates into H+ and bicarbonate. This will raise the pH level.

The kidneys are also involved in the regulation of pH. They do this by 2 methods:

Through the reasbsorbtion of bicarbonate (after it has been filtered)
Excreting acids associated with H+ from the body

We know that H+ ions are constantly being produced by cells through cellular respiration and we know that when these H+ ions are attached bicarbonate and form carbonic acid, they dissociate into water and CO2. Water and carbon dioxide pose no threat to blood pH.

At the kidneys, carbon dioxide and water (from the cells that needed to rid themselves of H+) are made into carbonic acid and breaks down into bicarbonate and H+ again. The acid-secreting cells contain carbonic anhydrase, which facilitates this reaction to occur so quickly. Now because this occurs at the kidneys, the H+ ions can be transported out of the body.

What happens to the bicarbonate?

The most of the bicarbonate is reabsorbed by the proximal tubule, to return to the blood stream and repeat this reaction in more cells (by associating with their H+_ from ongoing cell respiration)> the rest is reabsorbed in the Distal tubule and collecting duct.

The reabsorption of one molecule of HCO3 and one molecule of Na+ from the tubular lumen into the blood stream for each molecule of H+ secreted. Na+ absorption is also vital in reabsorbing bicarbonate.

What happens to the H+?

Free hydrogen ions cannot simply leave the body in that form.

The cells of the proximal tubules are responsible in producing ammonia which binds to the free H+ ions to form ammonium. Ammonium can easily be released in the urine.

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