Acid-Base Balance Disturbances

Normal bicarbonate (HCO3) to carbonic acid (H2CO3) ratio in the blood plasma is 20:1. 

In other words, for every H2CO3 produced in blood plasma, 20 HCO3 ions must be formed to maintain a 20:1 ratio (normal pH). 

Or, for every H2CO3 loss in the blood plasma, 20 HCO3 ions must be eliminated to maintain a normal pH. 

In other words, the H2CO3 is 20 times more powerful than the HCO3 ion in changing the blood pH.

Under normal conditions, the 20:1 acid-base balance in the body is automatically regulated by the:

Chemical buffer systems

Respiratory system

Renal system

However, these normal acid-base regulating systems have their limits. 

The bottom line is this:

The body's normal acid-base watchdog systems cannot adequately respond to sudden changes in H+ and HCO3 concentrations regardless of the cause.

For example:

Hypoventilation causes the partial pressure of the alveolar carbon dioxide (PACO2) to increase, which in turn causes the plasma PCO2, HCO3, and H2CO3 to all increase.

  1. This causes HCO3 to H2CO3 ratio to decrease, and the pH to fall.

 

fig 7-8.jpg

Fig. 7-8.  Alveolar hypoventilation causes the PACO2 and the plasma PCO2, H2CO3, and HCO3 to increase. This action decreases the HCO3/H2CO3 ratio, which in turn decreases the blood pH.

Or, when the PACO2 decreases, as a result of alveolar hyperventilation, the plasma PCO2, HCO3 and H2CO3 all decrease which in turn causes:

HCO3 to H2CO3 ratio to increase, and the pH to rise

 

fig 7-9.jpg

Fig. 7-9.  Alveolar hyperventilation causes the PACO2 and the plasma PCO2, H2CO3, and HCO3– to decrease.  This action increases the HCO3–/H2CO3 ratio, which in turn increases the blood pH.

Acid Excretion

Buffers are a temporary measure; if acids were not excreted, life-threatening acidosis would follow.

Lungs

Excrete CO2, which is in equilibrium with H2CO3

Crucial as body produces huge amounts of CO2 during aerobic metabolism (CO2 + H2O → H2CO3)

In addition, through HCO3 eliminate fixed acids indirectly as the byproducts are CO2 and H2O

Lungs remove ~24,000 mmol/L CO2 daily

The is inversely proportional to the PaCO2. As minute ventilation increases PaCO2 decreases; if minute ventilation decreases PaCO2 increases. Since minute ventilation is tidal volume multiplied by the respiratory rate (or frequency) then we can alter the PaCO2 by changing either or both of these components.

Kidneys

Physically remove H+ from body

Excrete <100 mEq fixed acid per day

Also control excretion or retention of HCO3

If blood is acidic, then more H+ are excreted and all the HCO3 is retained, vice versa

While lungs can alter [CO2] in seconds, the kidneys require hours to days change HCO3  and affect pH.

Role of urinary buffers in excretion of excess H+

Once H+ has reacted with all the available HCO3, the excess reacts with phosphate and ammonia.

If all urinary buffers are consumed, further H+ filtration ends when pH falls to 4.5.

Activation of ammonia buffer system enhances Cl loss and HCO3 gain.

 

Relationship between acute PCO2 changes, and the resultant pH and HCO3– changes that occur is graphically illustrated in the PCO2/HCO3–/pH nomogram

fig 7-10.jpg

 

PCO2/HCO3–/pH nomogram is an excellent clinical tool that can be used to identify a specific acid-base disturbance

Anion Gap

In the body electrical neutrality is maintained. Cations = Anions. The anion gap is a way to differentiate between a metabolic acidosis caused by HCO3- loss, or an increase in fixed acids. By looking at the difference between the measurable cations and ions we can evaluate the cause of a metabolic acidois.

The formula for calculating the anion gap is;

[Na+] - ([Cl-] + [HCO3-] = anion gap

Normal anion gap is 9 - 14 mEq/L

A gap of > 14 mEq/L is considered a metabolic acidosis due to the presence of fixed acids in the body

The following is taken from Mosby's online course for Egan's:

The anion gap clarifies whether the metabolic acidosis is caused by the accumulation of fixed acids or loss of HCO3.

When fixed acids accumulate, the anion gap increases, resulting in the following chain of events:

With HCO3 loss, the anion gap does not change. As a result: