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Regulation of [H+]
Acid-Base Physiology.
pH vs
+
[H ]
Why is it important to regulate
[H+]?
H2OH+ + OH• Because of covalent bonding between O2
and H, water has little tendency to
dissociate the reaction is always far to the
left. At equilibrium the concentrations of H+
and OH- are very small.
Electrical Neutrality
• Essential to understanding A-B physiology.
• In a solution at equilibrium, Σ of + charges
always equal to Σ of – charges.
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[H+] = [OH-]
[H+]+[Na+] = [OH-]+[Cl-]
Strong Ions
• NaCl is a strong electrolyte.
NaCl Na+ + Cl• In solution we have Na+ and Cl- but no NaCl.
Strong Ion Difference (SID).
• The sum of all positive strong ions minus
the sum of all negative strong ions.
• SID = (Σ + ions) - (Σ - ions).
• SID = ([Na+]+[K+]+[Ca++]) - ([Cl-]+[Lact-])
• Negative SID  acid
• Positive SID  alkaline
Strong Ions and Carbon Dioxide
• Adding CO2 to a solution with a positive [SID] has a
dramatic effect on [H+].
• CO2 + H2O <--> H2CO3 <--> HCO3- + H+
• CO2 pushes this equation to the right and therefore increases
[H+] which decreases pH.
How does the body regulate [H+]?
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By controlling the independent variables.
Respiratory system controls PCO2 .
Renal system controls SID, mostly by controlling [Cl-].
Slight disturbances in pH can have serious
consequences.
Buffers
• Buffers resist changes in pH by converting
strong acids or bases to weak ones
Physiologic Buffers
• Body systems that control output of acids,
bases, or CO2 and thus stabilize pH
• Respiratory System
• HCO3- + H+ <--> H2CO3 <--> CO2
(expired) + H2O
• Urinary/Excretory System (can regulate H+
secretion into urine)
Buffer Systems = Chemical
Buffers
• Systems that quickly bind or release
hydrogen ions
Bicarbonate Buffer System
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CO2 + H2O <--> H2CO3 <--> HCO3- + H+
Carbonic Acid is a weak acid
Enzymes work best at pH of 6.1
Reaction to right lowers pH
Reaction to left raises pH
Phosphate Buffer System
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H2PO4- <--> HPO42- + H+
Reaction to right lowers pH
Reaction to left raises pH
Enzymes work ideally at pH of 6.8
Protein Buffer System – most
common buffer system
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Carboxyl groups on amino acids
--COOH --> --COO- + H+
Here it is acting like an acid as a proton donor
Amino groups on amino acids
--NH2 + H+ --> --NH3+
Here it is acting like a base, a proton acceptor