pH + pOH = 14 for aqueous solutions at 298 K.
From household cleaners to orange juice, almost all daily items can, in some fashion, be classified as an acid or a base. Most of what is consumed as food or beverage is considered moderately acidic. Blood, on the other hand, needs to be slightly on the basic side of the pH scale. With that in mind, you can see how important it is for our bodies to have an effective—if not infallible—method for controlling blood pH to within a narrow, survivable range.
Chemicals are classified as acids or bases in a variety of ways, the most common of which is how they affect the hydrogen ion concentration, designated as [H+], in water when they dissociate or break up into their component cations and anions. Chemicals that increase the [H+] are called acids (e.g., vinegar, citrus juice [grapefruit, orange, etc.], and the hydrochloric acid in our stomachs). Chemicals that decrease the [H+] in water when they dissociate are referred to as bases (e.g., soap, baking soda [sodium bicarbonate], and bleach). The relative strength of each of these chemicals to increase or decrease the [H+] in water can be shown with the pH scale.
The pH scale is a logarithmic scale centered around the [H+] in pure water. A discussion of how the scale was derived is not included except to say that pure water has a pH (which stands for parts hydrogen ion) of 7. Acids increase the [H+], which results in the pH decreasing from 7 to 0, whereas bases decrease the [H+] and therefore have a pH from >7 to a maximum value of 14.
| [H+] | pH | Solution |
|---|---|---|
| High | Low, <7 | Acidic |
| Low | High, >7 | Basic |
Although the pH scale is concerned with acidity and alkalinity relative to pure water, for biological purposes, acidity and alkalinity relative to the typical range of the pH of blood is the primary focus. Blood has a normal pH range 7.35–7.45. A pH that deviates far outside this range can be lethal. If a person has blood pH <7.35, the person is said to be acidotic or have acidosis. When pH >7.45, the person has alkalosis or is alkalotic.
Because the body requires a very narrow pH range in which to function optimally, deviations outside this range can have far-reaching effects. Here, the focus is on understanding the most important and often confusing buffer system and chemical reaction in the body: the bicarbonate buffer system. A buffer system, or buffer, minimizes the impact on a system’s pH from the onslaught of an acid or a base. Buffers are reactions in a dynamic equilibrium that can neutralize bases and acids without meaningfully affecting the pH.
Central to this reaction is carbonic acid, H2CO3. You should be very familiar with this chemical if you have ever had any kind of carbonated beverage or soda. What happens to soda as soon as you pour it into a glass? It almost immediately starts to bubble and foam up, sometimes over the edge of the glass. Sometimes, even before pouring, you can see bubbles forming on the inside of the bottle. This reaction is the side shaded in blue, going from the green box and moving right. It is spontaneous in that direction and forms the respiratory side of this buffer system.
In the yellow shaded area, you have what often is referred to as the renal component of the reaction. It illustrates an alternative option for breaking down the carbonic acid in the green section. It could be broken down into a hydrogen ion and a bicarbonate ion. Carbon dioxide (CO2) is most commonly transported in the bloodstream as the bicarbonate ion (HCO3-). Bubbles of CO2 do not effervesce in a person’s blood.
Keep in mind the following during further analysis of these reactions: Both the blue and yellow shaded sides can move left to right through the arrow, or right to left, depending on the needs of the body. Furthermore, you will seldom see this reaction with the intermediate, carbonic acid, present. Yet this illustration hopefully shows why this reaction is possible; it is simply a rearrangement of atoms in the molecules. In the body, enzymes catalyze reactions without stopping at the carbonic acid step. From this point on, the only reaction that will be referred to is the bicarbonate buffer system.
This reaction is always happening regardless of whether any species are added or removed on either side of the double-headed arrow. This is what is known as dynamic equilibrium and is illustrated with a double-headed arrow separating the reactants (left side) from the products (right side).
A principle in chemistry dictates in which direction an equilibrium reaction, such as a buffer reaction, will go based on adding or removing pieces from either side of the arrow. If this system is stressed, by, say, adding H+ to the system, the reaction would go to the right to eliminate the newly introduced and excess H+. The reaction would similarly move from left to right if CO2 is removed from the system; the reaction would move to replace it to maintain equilibrium.
Because this reaction can move in both directions (indicated by the double-headed arrow), if excess CO2 is added to the system, the reaction will move to the left to relieve the stress. Finally, if it were somehow possible to remove HCO3- from the system, the reaction would proceed in the direction that replenishes it (i.e., the left), as shown in Figure 1.5. You can play the equilibrium game by adding or removing any of the 4 species—but only these 4 species. Let’s take this into the world of the paramedics, shall we?
The maintenance of acid-base balance is crucial for survival. The body is relatively adroit at compensating for variations in pH by employing the acid-base buffer reaction discussed previously.
| Condition | Causes | Symptoms |
|---|---|---|
| Respiratory acidosis |
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| Respiratory alkalosis |
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| Metabolic acidosis |
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| Metabolic alkalosis |
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Respiratory acidosis is a decrease in blood pH that primarily has a respiratory component. The human body has a necessarily efficient way of removing CO2 from the body so that this does not happen, but let’s take a moment to understand what happens if that fails. Looking at the reaction in Figure 1.5, where might respirations come into play? With the CO2, of course. Now, in which direction would the reaction need to proceed to relieve the stress of excess CO2? To the left, exactly! When the body’s CO2 level increases, the reaction proceeds to the left to relieve the stress, which has the unfortunate outcome of increasing the [H+], decreasing the body’s pH or acidosis. Therefore, this condition results from a systemic increase in CO2 level.
A variety of causes underlie this condition; however, broadly, it always results from decreased respiratory efficiency or hypoventilation. Any condition that causes CO2 retention can cause respiratory acidosis, including cardiac or respiratory arrest, airway obstruction, asphyxia, or a head injury. As respiratory acidosis progresses, more problems can be seen. To compensate for the acidosis, potassium ions are released from cells, which frequently results in cardiac dysrhythmias. Calcium ions are also released from muscles and cause a decreasing level of consciousness and delayed nerve signal transit, resulting in sluggish pupils and delayed responses to painful stimuli.
Respiratory alkalosis is an increase in pH, or a decrease in [H+], with a primarily respiratory component. If you’re thinking that since acidosis is caused by a retention of CO2, then alkalosis must be caused by an increase in exhalation of CO2, you would be correct. An increase in respiratory rate does, indeed, lead to respiratory alkalosis and a resultant decrease in CO2 in the blood. Looking at the reaction again, if CO2 is removed from the system, the reaction will work to replace it, thus stripping the body of its hydrogen ion. A decrease in [H+] results in a higher or more basic pH, the hallmark of alkalosis.
Fever, anxiety, or excessive artificial ventilation are common causes of respiratory alkalosis. As alkalosis progresses, hydrogen ions leave the cells in an attempt to replenish what has been lost from the high respiratory rate. To compensate for this, calcium ions move into the cells, resulting in a state of hypocalcemia, which is responsible for the symptoms that can be seen in many alkalotic patients, including carpal-pedal spasms, tingling in the lips and face, and dizziness.
Metabolic acidosis is a type of acidosis that results primarily from a metabolic disorder or ingestion and typically does not contain a respiratory component; that is, metabolic acidosis does not need hypoventilation to occur. In this case, the ingestion of a poison or an intentional overdose, or the body’s usual production of acids through normal processes, overwhelms the body’s ability to remove the acids that are present. Refer back to the reaction in bicarbonate buffer system (Figure 1.5); the body’s primary means of relieving a stress of excess acid is to have that reaction proceed to the right, producing more CO2. The person will then breathe faster and more deeply to try and exhale the CO2 being produced.
Diabetic ketoacidosis (DKA) heads our list of common causes of metabolic acidosis. DKA occurs in patients who do not take any or enough insulin, which causes cells to shift to using fatty acids for fuel in lieu of glucose. A more detailed treatment of this condition can be found in the diabetic emergencies section of chapter 5. Lactic acidosis, the presumed cause of death in the movie A Few Good Men, occurs when the cells are not getting enough oxygen (O2) and shift to anaerobic respiration, which leaves behind a lot of acids. Aspirin overdoses also cause metabolic acidosis because this is a direct ingestion of an acid: salicylic acid. Signs of metabolic acidosis include Kussmaul respirations (deep, rapid respirations), hot flushed skin, and bounding pulses.
Metabolic alkalosis is perhaps the rarest of acid-base disturbances and occurs when the system loses an excessive amount of acid. Look at the reaction in Figure 1.5 again. You can see that if acid is removed from the system, the reaction will proceed to the left in an attempt to replenish it. It will do this by intentionally retaining CO2 and thus reducing respirations.
The most likely cause of this in the emergency setting is prolonged vomiting. The elimination of acid in this manner is the fastest way to cause the necessary reduction in circulating hydrogen ions. Excessive intake of acid-neutralizing medication, such as over-the-counter antacids, also could account for metabolic alkalosis.