The acid base balance is vital for normal bodily functions. When this equilibrium is disrupted, it can lead to severe symptoms such as arrhythmias and seizures. Therefore, this balance is tightly regulated. In this article, we will look at the buffering system, responses of the respiratory system and relevant clinical conditions.
Blood has the ability to be resistant to small changes in pH, which is a characteristic known as “buffering”. This is due to the basal levels of bicarbonate and hydrogen ions in blood. The chemical reaction is given by:
This reaction can be used to control pH, as will be discussed in the next section. For example, in metabolically active tissues, there is an increase in hydrogen ions. These can then react with bicarbonate in the red blood cells to form carbon dioxide which can then be exhaled by the lungs. The compensatory systems of the body rely on this equation. This will be discussed in more detail later.
The Henderson-Hassalbalch equation relates the pH to the ratio between the concentration of bicarbonate and the partial pressure of carbon dioxide. It is given by:
This shows that the ratio between bicarbonate production and partial pressure of carbon dioxide drive the pH levels of the blood. By increasing bicarbonate levels, the pH will rise and turn more alkaline, and by increasing the partial pressure of carbon dioxide the pH of blood will fall and turn acidic. The usual range of blood pH is from 7.35 to 7.45. When pH levels drop below 7.35, it is said to be acidotic, and when pH levels rise above 7.45 it is said to be alkalotic.
How is Balance Restored?
When blood pH deviates from the normal range, there are two body systems which are activated to restore equilibrium. The respiratory system alters the respiratory rate, to change the concentration of carbon dioxide in the blood, whilst the urinary system changes the reabsorption or production of bicarbonate or hydrogen ions. This is known as “compensation”.
Information on the response of the urinary system can be found here.
There is a complex regulatory mechanism for changing the respiratory rate. Chemoreceptors detect the levels of certain molecules in the blood, and alter the respiratory rate accordingly. Peripheral chemoreceptors, in the carotid sinus and aortic arch, signal to the brain stem via cranial nerves to alter the respiratory rate. Central chemoreceptors function via a different method. When there is a rise in carbon dioxide in the blood, it can diffuse into the cerebrospinal fluid as it is a small molecule. An enzyme called Carbonic Anhydrase can then turn carbon dioxide and water into bicarbonate and hydrogen ions. Hydrogen ions are then sensed by chemical chemoreceptors which alter the respiratory rate directly.
Further information on the role of chemoreceptors can be found here.
Clinical Relevance – Metabolic acidosis
Metabolic acidosis can result from numerous causes. This can occur due to an increase of hydrogen ions being produced, such as in diabetic ketoacidosis, or can be due to a disorder of the kidneys themselves such as in chronic kidney disease, resulting in a decreased bicarbonate production. The respiratory system attempts to compensate by increasing respiration rate. However, the main correction must be accomplished by the kidneys which increase hydrogen excretion and bicarbonate reabsorption.
Clinical Relevance – Metabolic alkalosis
Metabolic alkalosis is caused by a massive loss of hydrogen ions, such as in vomiting or an increase of bicarbonate in the blood, such as in Milk-Alkali syndrome. Similarly to metabolic acidosis, metabolic alkalosis is compensated by the respiratory system by decreasing respiration rate. This in turn would increase the partial pressure of carbon dioxide and drop the pH. However, this is naturally limited due to the fact that the respiratory rate can only decrease so much before there are fatal repercussions. Additionally, bicarbonate reabsorption is quickly saturated which can lead to bicarbonate being easily excreted.