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Human Blood Bicarbonate Buffer System

    In this human blood bicarbonate buffer system post we have briefly explained about bicarbonate buffer system of blood, chloride shift, regulation by respiratory mechanism and regulation by renal mechanism.  

    Bicarbonate Buffer System of Blood

    Human blood contains a buffer of carbonic acid (H2CO3) and bicarbonate anion (HCO3-) in order to maintain blood pH between 7.35 and 7.45.

    Bicarbonate buffer system of blood is defined as a solution which resists the change in pH that will occur on addition of small quantities of acid or base to the solution. Buffers are mixtures of weak acid and its salt or weak base and its salt.

    The pH of the solution is defined as the negative logarithm of hydrogen ion concentration. The pH of buffers is determined by Henderson Haselbach equation.

    When there is a deviation of blood pH away from that of normal value, the two systems in body are actuated for restoring the equilibrium.

    The respiratory system modifies the rate of respiration that will in turn alter the concentration of carbon dioxide in the blood; the renal system modifies the reabsorption/and or production of bicarbonate/ hydrogen ions. This balance is known as “compensation”.

    Blood Buffer system

    By the process of metabolism, carbon di oxide is produced in the tissues and enters the blood. This CO2 will be hydrated forming H2CO3 and this gets ionized yielding H+ and HCO3– . When the oxygen tension is reduced in the tissues, oxy-hemoglobin will dissociate, thereby producing oxygen and hence reduced hemoglobin is formed.

    In lungs, oxy-hemoglobin (strong acid) is formed from reduced hemoglobin releasing hydrogen ions that reacts with bicarbonate and yield carbonic acid. Owing to lower CO2 tension in lung, the shift of equilibrium towards the production of carbon di oxide is enabled which in turn will be continuously eliminated during exhalation.

    Reduced hemoglobin will act as anion and accepts the H+ ions to produce acid reduced hemoglobin (HHb). By the production of weak acid, the arrivals of H+ ions are buffered thereby causing little change in pH.

    On the return of blood to the lungs, owing to the formation of oxy-hemoglobin (stronger acid) as mentioned above, these H+ ions are released. Immediately these released H+ ions are neutralized by HCO3– . This is inevitable for the lungs to release CO2.

    Human Blood Bicarbonate Buffer System

    Bicarbonate Buffer System of Blood

    Chloride Shift

    In RBCs CO2 reacts with water forming carbonic acid in a reaction that is catalysed by carbonic anhydrase. The formed carbonic acid is buffered by phosphate buffer as well as haemoglobin buffers.

    Bicarbonate returns to plasma and gets exchanged with chloride ion that enters into (shifts into) the cell if the tension of CO2 increases in the blood. On the contrary, if the CO2 tension is reduced, chloride will exit the cell and enters plasma.

    In general, red blood corpuscles are impermeable to sodium or potassium whereas permeable to hydrogen, bicarbonate and chloride ions. Potassium which is a cation is available to the plasma by anionic (chloride) exchange. This results in added CO2 being carried by plasma as sodium bicarbonate.

    The cycle continues as CO2 enters and passes to the red blood cells and forms carbonic acid (partial amount of which returns to plasma) by carbonic anhydrase. The remaining carbonic acid then reacts with hemoglobin buffers yielding bicarbonate that travels to the plasma in exchange of chloride and is transported.

    The reactions mentioned so far are reversible. In lung tissue, chloride shifts back into plasma when blood becomes arterial. This eventually releases intracellular potassium to buffer the oxy-hemoglobin. In plasma, it neutralizes sodium.


    Respiratory Mechanism

    We have seen that the carbonic acid will dissociate into CO2 and H2O. If there is more of H+ within the blood more of CO2 elimination will be carried out by lungs.

    If there is more of HCO3 , the lungs will ensure low respiratory rate thus enhancing the retention of CO2 so that it can be useful for forming carbonic acid that can buffer the excess of bicarbonate.

    The alveoli and bronchioles of lungs do perform such functions effectively. A phospholipoprotein molecule is secreted by a particular type of cells of the lung lining the alveoli and bronchioles.

    This surfactant regulates (lowers) the surface tension of the alveolar membranes thereby protecting the alveoli during exhalation and inhalation.

    Renal Mechanism

    The major functions of kidneys are regulation of water and electrolyte balance. This is done by excretion of waste substances in urine. The formation of urine involves three stages (i) filtration, (ii) reabsorption, (iii) secretion.

    The regulatory role of renal organ is achieved by the buffering capacity of the organ. For this function membrane performs key role. Passive and active transports are involved in addition to osmosis and pinocytosis.

    Movement of substances across the renal tubular membrane that aid in urine formation (by filtration, reabsorption, secretion) by kidneys in bicarbonate buffer system of blood.

    Human Blood Bicarbonate Buffer System

    Bicarbonate buffer system of blood

    Further Readings