Regulation of [H+]

Acid-Base Physiology.

pH vs [H+]

Why is it important to regulate [H+]?

H2OH+ + 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.• [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+]?

• 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

• 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

• 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

• 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