Bicarbonate Buffer System

The Bicarbonate Buffer System

The bicarbonate buffer systemThe main buffer in ECF, involving HCO₃⁻ and H₂CO₃. is an important ECF buffer in the plasmaThe liquid component of blood. and in the IF. The primary role of the carbonic-acid-bicarbonate buffer system is to limit pH changes caused by metabolic and fixed acids.  Recall from the minilecture on pH management that

Your blood always has this collection of bicarbonate(HCO₃⁻) – A crucial buffer in blood that helps maintain pH balance; formed when carbon dioxide anions called the alkaline reserve.  This work great to keep the blood at the desired pH.  But, if the body is flooded with acids, these bicarbonate anions get used up.  If they are used faster than they are made, other organs must activate. They then buffer the blood pH.  

The enzyme carbonic anhydraseAn enzyme in red blood cells that helps convert carbon dioxide and water into carbonic acid. helps convert carbon dioxide (CO₂) and waterThe universal solvent essential for life. into carbonic acid(H₂CO₃) – A weak acid formed when carbon dioxide dissolves in water; plays a key role in blood (H₂CO₃). This acidA substance that releases hydrogen ions (H⁺) in solution. then dissociates into bicarbonate (HCO₃⁻) and hydrogen ionsCharged atoms or molecules. (H⁺).

Bicarb Compensation

I always remember how your lungs control pH by looking at two different issues: airway obstruction or whooping cough.

If a child has whooping cough, the excess coughing will blow off carbon dioxide at a high rate.  This will cause the child’s pH to increase.  If a patient is in acidosisA condition where blood pH falls below 7.35., we want to increase their pH.  We can get them to breathe faster and more shallowly to bring their pH up.

 If a child swallows an item, they will not be able to expire carbon dioxide as needed.  Too much of it will build up in the blood, making the child acidicA solution with a pH below 7, having a higher concentration of H⁺ ions..  Similarly, if a patient’s pH is too high, have them breathe into a bag. This will recirculate their own carbon dioxide and decrease their pH.

The dorsalRelating to the back side of the body. and ventralRelating to the front or belly side of the body. respiratory groups are triggered here.  They can adjust your breathing to compensate for pH changes that exhaust the blood buffersSubstances that resist changes in pH..  Sometimes, these are employed without the patient really realizing it.  In our case study, our patients has 22 respirations per minute listed in their initial signsObjective clinical findings observable by a provider (e.g., edema, fever)..  That’s a lot.  That is also Mr. Wu’s lungs trying really hard to do the job of his failed kidneys.

Only the kidney can remove ketone bodies, uric and lactic acids.  All of these things make your blood more acidic.  Ketone bodies can result from the metabolismThe sum of all chemical reactions in the body. of lipidsOrganic molecules including fats, oils, and steroids. in a diabetic. Uric and lactic acid can build up in muscles with overuse. This can also happen with certain diseases such as gouty arthritis.

In addition the kidney can manage the bicarbonate anionA negatively charged ion..  All parts of the nephronThe functional unit of the kidney that filters blood and produces urine. except for the loop can manage bicarbonate.  Bicarbonate can be conserved and reabsorbed by the PCT, DCT, and collecting ductA duct in the nephron that collects urine from multiple nephrons and adjusts water reabsorption..  The collecting duct can even make new bicarbonate ions.  You rarely want to do this, but the collecting duct can secrete bicarbonate is you are too alkaline.

Arterial Blood Gases

Let’s look at one of the main tests that can assess your pH management.  When you go to get blood drawn, that is a venipuncture where we are taking blood from the veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge.  We expect this blood to be carbon dioxide rich and oxygen poor.  If I want to know what’s in the arterial side of your systemic circuitThe part of the circulatory system that carries oxygenated blood from the heart to the body and retu, I must perform an arterial blood gas. It’s necessary to discover that information.  An arterial blood gas measures for pH, carbon dioxide, oxygen, and bicarbonate anions.  We can use these measurements to determine if our patient is in one of these four conditions. These conditions are named for the system that is causing them.  Respiratory acidosis occurs when there is low pH due to the respiratory systemThe organ system responsible for gas exchange (oxygen and carbon dioxide).. Respiratory alkalosisA condition where blood pH rises above 7.45. occurs when there is high pH due to the respiratory system.  Same with renal acidosis and alkalosis.  An arterial blood gas can help us determine if the lungs or the kidney is the culprit.  A blood gas adds the benefit of revealing if acidosis or alkalosis is compensatedWhen a buffer or system (respiratory or renal) has corrected pH despite ongoing imbalance.. It can also reveal if the body is at least trying to counteract the condition.  For example, in the case study, our patient is in renal acidosis. His lungs are making a strong effort to compensate.

Arterial Blood Gas Steps

Personally, I dislike arterial blood gases because I can never remember the steps.  And, different resources will having you doing different steps to make a diagnosis.  My only suggestion is to find a reference and stick to it.  I can always identify an ABG that indicates either renal acidosis or alkalosis.  In renal acidosis, all items on an ABG are decreased and in renal alkalosis they are all increased.  With respiratory acidosisLow pH due to CO₂ retention (e.g., hypoventilation). and alkalosis the pH changes in opposite directions as the bicarb and carbon dioxide.  Now, the tricky part is determining if the condition is being compensated and if the compensation is partial or complete.  This can be a matter of opinion, especially which it comes to determining partial or complete compensation.  But, the key is to remember this. Once you reach the pH range, whatever organ system is compensating will stop.  It doesn’t bring you into the middle of a range, just inside the upper or lower limit.