30.15
Bicarbonate-Carbonic Acid Buffer
The carbonic acid-bicarbonate buffer system is critical for maintaining the body's pH balance. It operates on the equilibrium:
H2CO3 ⇋ H+ + HCO3-
In t…
The carbonic acid–bicarbonate buffer system works on the principle of bicarbonate ions acting as a weak base and carbonic acid functioning as a weak acid.
Bicarbonate ions effectively neutralize the excess hydrogen ions in acidic conditions by accepting them to form carbonic acid.
The newly formed carbonic acid, catalyzed by carbonic anhydrase, dissociates into carbon dioxide and water. The carbon dioxide formed is exhaled from the lungs.
If there is a decrease in the plasma concentration of hydrogen ions, carbonic acid compensates by dissociating and releasing more hydrogen ions.
The effectiveness of this buffer system depends on the concentrations of buffering substances.
When the system becomes overwhelmed by excess acid and exhausts all available bicarbonate ions, also called the alkaline reserve, the buffer system loses its effectiveness, resulting in changes in the blood pH.
However, this is rare because body fluids have abundant bicarbonate ions, which the kidneys can reabsorb when needed.
Because a steady supply of carbon dioxide is needed to form carbonic acid, this buffer system fails to protect against pH changes in cases of respiratory problems.
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Q1: How does the bicarbonate-carbonic acid buffer system neutralize excess hydrogen ions?
Bicarbonate ions act as weak bases and accept excess hydrogen ions to form carbonic acid. The enzyme carbonic anhydrase catalyzes the dissociation of carbonic acid into carbon dioxide and water. Carbon dioxide is then exhaled through the lungs, effectively removing hydrogen ions and lowering blood acidity.
Q2: What happens when plasma hydrogen ion concentration decreases?
When hydrogen ion levels drop, carbonic acid dissociates to release more hydrogen ions and bicarbonate ions, shifting the equilibrium rightward. This compensatory response replenishes hydrogen ions and stabilizes blood pH, maintaining the dynamic equilibrium necessary for normal physiological function and acid-base homeostasis.
Q3: Why does the bicarbonate-carbonic acid buffer system fail in respiratory problems?
This buffer system requires a steady supply of carbon dioxide to form carbonic acid. When respiratory problems impair CO2 excretion or intake, the system cannot maintain adequate carbonic acid levels, compromising its ability to buffer pH changes and protect against acidosis or alkalosis.
Q4: What is the alkaline reserve and how does it affect buffer effectiveness?
The alkaline reserve refers to available bicarbonate ions in body fluids. When excess acid overwhelms the system and depletes bicarbonate reserves, the buffer loses effectiveness and blood pH changes significantly. However, the kidneys can reabsorb bicarbonate when needed to maintain adequate reserves.
Q5: How does carbonic anhydrase contribute to the bicarbonate buffer system?
Carbonic anhydrase is an enzyme that catalyzes the rapid dissociation of carbonic acid into carbon dioxide and water. This acceleration enables efficient removal of carbon dioxide through the lungs, allowing the buffer system to respond quickly to changes in hydrogen ion concentration and maintain pH stability.
Q6: What role do bicarbonate ions play in maintaining acid-base homeostasis?
Bicarbonate ions function as weak bases within the buffer equilibrium, accepting hydrogen ions during acidic conditions and releasing them during alkaline conditions. This dynamic responsiveness to pH changes enables the bicarbonate-carbonic acid system to preserve the stable pH essential for vital physiological processes.
Q7: How does the buffer system respond when body fluids become too acidic?
When hydrogen ions increase and pH drops, the equilibrium shifts left as bicarbonate ions accept excess hydrogen ions to form carbonic acid. Carbonic anhydrase converts this carbonic acid to carbon dioxide and water, which is exhaled, effectively reducing hydrogen ion concentration and restoring normal pH.