What is the pH of arterial blood?
The pH scale is a measure of the acidity or alkalinity of any solution or substance. With human blood, particularly arterial blood, this level is approximately 7.40. The pH scale ranges from 0 to 14, with a reading of 7.0 indicating a neutral level. Technically a level of 7.40 means that arterial blood is slightly alkaline but to a very slight degree. The body maintains arterial blood within a very narrow range between 7.36 to 7.44. The pH within a cell (intracellular pH) is about 7.0 (neutral) although it can fluctuate between 6.80 and 7.30.
Controlling the arterial blood pH is essential for the body to ensure that normal functioning of the body’s pH-sensitive enzymes is maintained.
The pH of blood is affected by a number of compounds – some as a result of metabolic activities, others related to gas perfusion (transportation of oxygen and carbon dioxide in the blood). If there are too many acidic compounds which the body cannot neutralise, the pH of the blood will fall thereby making the blood more acidic. Similarly if the body retains alkaline compounds (bases), the pH of the blood will rise (alkalinity).
The body maintains the blood pH around 7.40 by a number of mechanisms related to the lungs, kidneys and the buffer system. Variations which result in blood becoming more acidic or more alkaline will affect normal functioning and lead to serious medical conditions known as acidosis (when the blood is too acidic) or alkalosis (when the blood is too alkaline.
Lungs and Blood pH
Carbon dioxide is slightly acidic. If carbon dioxide is not exhaled at a suitable rate or to a sufficient degree, it will build up in the blood thereby making the blood more acidic. Efficient functioning of the lungs ensures that the body passes out this carbon dioxide but any lung disease can impact on this removal to varying degrees.
If the lungs fail to fulfil this function and the blood becomes acidic, then this is known as respiratory acidosis. In other situations, like hyperventilation, where the body passes out carbon dioxide too fast. the pH change in the blood is known as respiratory alkalosis. In hyperventilation related to anxiety disorders, this alkalosis, if any, is short term and does not pose any significant threat to the body.
Kidney and Blood pH
The kidney also plays an important part in maintaining the pH level of the blood. It does this by excreting acidic compounds in the blood, while reabsorbing alkaline compounds and those chemicals that can help neutralise certain acidic compounds. Failure to efficiently complete this function can result in acidosis. Acidic levels in the blood due to kidney dysfunction is known as renal tubular acidosis.
Buffer System and Blood pH
The body has a complex chemical system in place to regulate blood pH. This is known as the buffer system and involves an intricate play of carbonic acid, hydrogen ions and bicarbonate. An enzyme known as carbonic anhydrase regulates this system and works along with the kidneys and lungs to maintain the blood pH.
Acidosis and Alkalosis
Acidosis or a low pH level of the blood (acidic) and alkalosis where there is a high pH of the blood (alkaline) can occur for a number of reasons. Certain diseases, malfunctioning organs and disturbances in the regulating system can lead to these two conditions.
Metabolic acidosis is when the blood is too acidic as a result of an interruption within one or more systems – buffer system, urinary system (kidneys) and/or respiratory system (lungs). Metabolic alkalosis is when the blood is too alkaline due to disturbances within one or more of the abovementioned systems.