ACID-BASE BALANCE
In a healthy person the PH of the extracellular fluid ranges from 7.35 to 7.45 Maintenance of this narrow range is essential for the survival and depends on three major mechanism :buffer systems respiration and kidney excretion
Buffer system ::
Most buffer systems of the body are weak acids and weak bases They include carbonic acid-bicarbonate system the phosphate system the hemoglobin -oxyhemoglobin system and the protein system
The carbonic acid-bicarbonate buffer system is primarily based on the carbonic acid and sodium bicarbonate The following equations illustrate the mechanism
HCl+NaHCO3--------> NaCl+H2CO3
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NaOH+H2CO3--------> NaHCO3+H2O
<--------
Normal body processes tend to acidify the blood rather than to make it more alkaline In other words body needs more bicarbonate salt than carbonic acid and when the extracellular pH is normal (7.4) bicarbonate molecules out number carbonic acid (20:1)
The phosphate buffer system has two components namely sodium dihydrogen phosphate and sodium monohydrogen phosphate acting as weak acid and weak base respectively
NaOH +NaH2PO4-------> H2O+Na2HPO4
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HCI+NaHPO4 ------------> NaCI+NaH2PO4
<-------
This phosphate buffer system is an important mechanism in kidney NaH2PO4 is formed when excess of H+ ion in the kidney combines with NaHPO4 The Na+ released from this reaction forms sodium bicarbonate and is passed into the blood The H+ ion that replaces Na+ becomes part of the NaH2PO4 and is passed into the urine Thus this reaction tends to reduce acidity of blood by increasing alkalies to urine
The haemoglobin -oxyhaemoglobin buffer system is an effective method for buffering carbonic acid in the blood
The Protein buffer system is the most abundant buffer in body cells and plasma The amino acids of the protein contain carboxyl group (COOH) and atleast one amine group (NH2) The carboxyl group acts like an acid and amine group as a base and thereby can react with excess hydroxide and hydrogen ion respectively
R R
/ /
NH2-C-COO-+H+ --------------> COOH-C-NH3+OH-
/ /
H H
Thus proteins act as both acidic and basic buffers
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