When we consume food, cells in our body burn these foods for energy and they end up with Carbon Dioxide (Co2) as a waste product. i.e. Co2 is formed by the metabolism of nutrients combined with Oxygen in the tissues to produce energy in the form of Adenosine triphosphate (ATP). On the other hand lungs in our body will take care of Co2 by expelling it out of our body. But Co2 is much more than just waste.
Co2 plays a critical role in regulating blood pH. Approximately 80% – 90% of Co2 dissolves in water in our body, 5% – 10% dissolves in the plasma and 5% – 10% is bound to Hemoglobin and eliminates from the body through respiration.
When Co2 dissolves in water it forms a weak acid known as Carbonic Acid (H2Co3) with the help of an enzyme called Carbonic Anhydrase. This Carbonic Acid further dissociates into a Hydrogen ion (H+) and Bicarbonate ion (HCo3ˉ). Most of the Co2 In blood is found in the form of bicarbonate. When Co2 levels are higher in the blood, an increase in the concentration of H+ decreases the pH resulting in an acidic environment. On the contrary, when Co2 levels are low, the decrease in the concentration of H+ increases the pH resulting in the alkaline environment of the blood. 
The carbonic acid neutralizes the pH when the alkaline environment is present because of decreased H+ concentration, whereas bicarbonate ion neutralizes the blood pH when the acidic environment is observed because of increased H+ concentration both resulting in normalizing the blood pH.
A dialysis patient is said to be an acid accumulator for about 44 hrs (Interdialytic period) followed by 4 hrs effective period of retitration (Intradialytic period) which can be accompanied by various degrees of hypoxemia (decreased oxygen in the blood). Hypoxemia occurs because during dialysis Co2 tends to diffuse across the dialyser membrane resulting in decreased Co2 concentration in the blood (pCo2) causing hypoventilation (slow breathing rate) in dialysis patients.
Acetate Dialysis
Co2 removal by Renal Replacement Therapy can be enhanced by using acetate as dialysate. Uremic blood cells in the dialyser generate Co2 when exposed to low bicarbonate present in the acetate dialysate. This enhances Co2 removal, this removal of Co2 depends on the volume of ultrafiltration and plasma bicarbonate concentration.
The removal of Co2 causes fall in pCo2 to less than 20 mmHg (Normal pCo2: 35 – 45 mmHg), this causes mild to moderate hypoventilation with or without breathing irregularities to normalize the pCo2.
Bicarbonate Dialysis
During bicarbonate dialysis i.e. using bicarbonate based dialysate, there is a gain of Co2. When the bicarbonate solution (Part B) mixes with an acid solution (Part A) just before flushing into the dialyser the Co2 generates causing pCo2 in the dialysate range between 80 – 100 mmHg. Whereas dialysis patient arterial pCo2 commonly is bellow 40 mmHg. This results in high Co2 dialysance and thus an increase in pCo2 in the blood returning from the dialyser into the patient.
This hypercapnia (high Co2 in the blood) lowers pH despite high bicarbonate concentration in the blood. This added Co2 is quickly removed by respiration and systemic Co2 remains unaffected.
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