Acid Base Equilibrium, Clinical Concepts and Acid Base

Disorders

HomeostasisThe Interstitial Fluid is the environment of the cells,

and life depends on the constancy of this internal sea.

Homeostatic Mechanisms : Maintain within a narrow range.

Tonicity VolumeSpecific ion concentrationDefence of Tonicity –(280-295mOsm/L)Vasopressin secretion Thirst Mechanism

Increased Osmolality of ECF

Thirst IncreasedVasopressin

SecretionIncreased Water Intake Water Retention

Dilution of ECF

Inhibitory

HomeostasisDefence of Volume:ECF Na+ - Most

importantRenin-Angiotensin-

Aldosterone SystemVasopressin Secretion:

Volume stimuli override osmotic regulation

ANP & BNP

Angiotensinogen Renin

Angiotensin I ACE

Angiotensin II

Aldosterone Vasopressin

AdrenalCortex Brain

KidneyNa RetentionWater Retention

Blood VesselVasoconstriction

Thirst

HomeostasisDefence of Specific Ionic Concentration:GlucoseNa+ & K+ Ca++ - Mainly by Parathyroid & CalcitoninMg++ - Incompletely understood mechanisms

Also dependent on H+ ion

pH is maintained within a narrow range.

Acid Base EquilibriumWhat is Acid Base Equilibrium About?

?Buffer

s? Fixed Cation

?

Base Excess/ Deficit? Anion

Gap?

Acid Base EquilibriumAcid Base Equilibrium is all about Maintenance of H+ ion concentration of the ECF.

Source of H+ ion in Body:CO2 from metabolis

m

H+ load from AA

metabolism

Strenuous Exercise Lactic Acid

Diabetic KA

Ingestion of NH4Cl, CaCl2

Failure of Kidneys to Excrete PO4--, SO4--

H+ ion

12500 mEq/d 50 - 100 mEq/d

Some Basic ChemistryDefinitions:Arrhenius:

Acid: H+ Donor in SolutionBase: OH- donor in Solution

Browsted and Lowry:Acid: Proton DonorBase: Proton Acceptor

H20 can be both

Some Basic ChemistrySimple Rule of Thumb:Acid Higher conc. Of H+ ionBase Lower conc. Of H+ ionStrong Acid/Base Dissociates completely and irreversiblyWeak Acid/Base Dissociates partially and reversibly

Strong Electrolyte: Dissociates completely in solution at physiological pH

Eg: NaCl, KCl

Weak Electrolyte: Dissociates incompletely in solution at physiological pH

Eg: CO2 – HCO3- System, Proteins

Some Basic ChemistrypH (Puissant of Hydrogen):Negative logarithm of H+ ion concentration to the

base of 10

Why pH?Normal H+ ion conc: 0.00004meq/L or 40nEq/L or

4x10-9 mol/LpH converts to decimal numbers & takes away

negative sign.Normal pH: 7.35-7.45Normal H+ Conc: 0.00002mEq/L – 0.0001 mEq/L

Some Basic ChemistryPitfalls: Non-linear Negative Logarithmic scale

pH Decreases as [H+] increases.Each unit change in pH from 7 represents 10 fold

change in H+ ion conc. Eg: At pH 4, there are 10 times as much H+ than at pH 5, &

100 times as at pH 6Same numeric change in different portions of the pH

scale implies vastly different nanomolar change in H+ ions Eg: pH 56 => 100 times greater change in ionic conc than

when pH 7 8Body H+ ion conc is not as tightly controlled as the

other ion, though the pH scale implies so.

Thank You