Acid base balance(summary)

Acid Base balanceAcid Base balanceAcid-base balance refers to the mechanisms the body Acid-base balance refers to the mechanisms the body

uses to keep its fluids close to neutral pH (that is, uses to keep its fluids close to neutral pH (that is, neither basic nor acidic) so that the body can function neither basic nor acidic) so that the body can function normally.normally.

Arterial blood pH is normally closely regulated to Arterial blood pH is normally closely regulated to between 7.35 and 7.45.between 7.35 and 7.45.

acidsacids ？？

basesbases ？？

Any ionic or molecular substance Any ionic or molecular substance that can act as a proton donor.that can act as a proton donor.

Strong acidStrong acid ：： HCl, HHCl, H22SOSO44, H, H33POPO44..Weak acidWeak acid ：： HH22COCO33, CH, CH33COOH.COOH.

Any ionic or molecular substance Any ionic or molecular substance that can act as a proton acceptor.that can act as a proton acceptor.Strong alkaliStrong alkali ：： NaOH, KOH.NaOH, KOH.Weak alkaliWeak alkali ：： NaHCONaHCO33, NH, NH33, , CHCH33COONa.COONa.

pH

• pH is the negative log of hydrogen ion concentration.

pH= -log[H+]

If [H+] is high, the solution is acidic; pH < 7 If [H+] is low, the solution is basic or alkaline ; pH > 7

pH of ECF is between 7.35 and 7.45pH of ECF is between 7.35 and 7.45.. Deviations, Deviations, outside this range affect membrane function, outside this range affect membrane function, alter protein function, etc.alter protein function, etc.

- You cannot survive with a pH <6.8 or >7.7- You cannot survive with a pH <6.8 or >7.7- Acidosis- below 7.35 Acidosis- below 7.35 Alkalosis- above 7.45Alkalosis- above 7.45

In normal individuals, pH is controlled by two major and related processes

pH regulation and pH compensation. Regulation is a function of the buffer

systems of the body in combination with the respiratory and renal systems,

Compensation requires further intervention of the respiratory and/or renal systems to restore normalcy.

Maintenance of Acid-Base balance

• Acid base balance is regulated by three main process,

• Buffer systems.• Respiratory system.• Renal function.

• Respiratory System: • Removal of CO2 by lungs – stabilizes the

ECF, has direct effect on Carbonic Acid – Bicarbonate Buffer System

• Renal System: • Removal of H+ ions by kidneys• Regulation By Buffers

Regulated by buffer system:

• BUFFER:• Ability of an acid-base mixture to resist sudden

changes in pH is called its buffer. • Three major chemical buffer systems in the body

are:• Carbonic acid-bicarbonate buffer system• Phosphate buffer system• Protein buffer system

Buffer Systems in Body Fluids

Figure 27.7

1) The Bicarbonate (HCO3-) Buffer

System

The bicarbonate buffer system is an important buffer system in the acid base homeostatis  of living organisms. As a buffer, it tends to maintain a relatively constant plasma.

Prevents changes in pH caused by organic acids and fixed acids in ECF

The CO2 is removed by the lungs.

The Bicarbonate (HCO3-) Buffer System (contd)

• Carbonic acid (H2CO3) is a weak acid and is therefore in equilibrium with bicarbonate (HCO3

-) in solution. When significant amounts of both carbonic acid and bicarbonate are present, a buffer is formed. This buffer system can be written as: 

CO2+H2O------>H2CO3------> HCO3+H• Under normal circumstances there is much more

bicarbonate present than carbonic acid (the ratio is approximately 20:1). As normal metabolism produces more acids than bases, this is consistent with the body's needs.  

• The blood, with its high base concentration, is able to neutralize the metabolic acids produced. Since relatively small amounts of metabolic bases are produced, the carbonic acid concentration in the blood can be lower.

• The optimal pH is the pKa for the acid in the system. In this case the acid is carbonic acid.

• Ka=4.3 x 10-7, pKa=6.4

• Bicarbonate buffer system •   H2CO3 -----> H+ + HCO3

-

• When pH is rising: H2CO3 + H+ ---> HCO3

-

• When pH is falling: H2CO3 H+ <---- HCO3

-.

ADVANTAGES & DISADVANTAGE

• ADVANTAGE• Bicarbonate buffer system is quite efficient as compared

to other buffer system since it is present in very high concentration & it produce H2CO3 from which CO2 is exhaled out.

• DISADVANTAGE:• Bicarbonate is very weak buffer & hence pKa is very far

away from physiological pH.

• Cannot protect the ECF from pH changes due to increased or depressed CO2 levels.

2) The Phosphate Buffer System

• The phosphate buffer system (HPO42-/H2PO4

-) plays a role in plasma and erythrocytes.

• H2PO4- + H2O ↔ H3O+ + HPO4

2-

• Any acid reacts with monohydrogen phosphate to form dihydrogen phosphate

dihydrogen phosphate monohydrogen phosphate

H2PO4- + H2O ← HPO4

2- + H3O+ • The base is neutralized by dihydrogen phosphate dihydrogen phosphate monohydrogen phosphate

• H2PO4- + OH- → HPO4

2- + H3O+

• Their action is nearly identical to bicarbonate buffer• Mainly sodium salts of ;• Dihydrogen phosphate (H2PO4) a weak acid• Monohydrogen phosphate (HPO4) ,a weak base• The normal ratio in plasma is 4:1 and this is kept

constant by the help of kidneys for which phosphate buffer system is directly related to the kidneys.

. This dihydrogen phosphate is an efficient buffer in intracellular metabolism.

• Phosphate buffer system: • H2PO4

- ----> H+ + HPO4–

• When pH is rising: H2PO4----->H+ + HPO4

--;

• When pH is falling: H2PO4<------H+ + HPO4

ADVANTAGE & DISADVANTAGE

The phosphate buffer system has a pK of 6.8, which is not far from the normal pH of 7.4 in the body fluids; this allows the system to operate near its maximum buffering power.

Ka=6.2 x 10-8, pKa=7.2

DISADVANTAGE Provide only temporary solution to acid–base imbalance Do not eliminate H+ ions

3) The Protein Buffer System• The protein buffer system is part of body mechanism for

controlling blood H+ ion concentration both ECF & ICF More concentrated than either bicarbonate or phosphate

buffers Accounts for about three-quarters of all chemical

buffering ability of the body fluids,

AMINO ACIDS;

The carboxyl groups release H+ when pH rises and amino groups bind H+ when pH falls.

In acidic media protein act as base , NH2 group takes up H+ and formed NH3. Here, protein become positively charged.

• In alkaline media , protein act as acid and acidic group (COOH) dissociate in to COO- & H+. In this case protein become negatively charged.

NH2-CH2-CH2 CH2-CH2-COOH

H+ H+

Protein Buffers

• Carboxyl group gives up H+. • Amino Group accepts H+.

Protein Buffers• Proteins contain – COO- groups, which, like acetate ions

(CH3COO-) can act as proton donors.• Proteins also contain – NH3

+ groups, which, like ammonium ions (NH4

+), can accept protons.• If acid comes into blood, hydronium ions can be

neutralized by the – COO- groups• - COO- + H3O+ → - COOH + H2O• If base is added, it can be neutralized by the – NH3

+ groups

• - NH3+ + OH- → - NH2 + H2O

Hemoglobin & Plasma protein

• Protein buffers in blood include haemoglobin (150g/l) and plasma proteins (70g/l).

• Haemoglobin is quantitatively about 6 times more important then the plasma proteins as it is present in about twice the concentration and contains about three times the number of histidine residues per molecule.

• Hemoglobin buffer system H+ are buffered by hemoglobin

Acid-Base Imbalances

• pH< 7.35 acidosis• pH > 7.45 alkalosis• The body response to acid-base

imbalance is called compensation

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