Archive for the ‘ L2-Acid-Base Balance ’ Category

L2- Acid-Base Balance

Acid-Base Balance

•One of the homeostatic mechanisms in the body

•Acid-base balance is aimed at maintaining a balance between generation of H+ and its elimination

•The respiratory system is intimately involved in keeping this balance

Basic Concepts

•An acid:

is a molecule that liberates  H+ ion in a solution (proton or H+ donor).

  • 1) A strong acid;

is dissociated completely or almost to H & its corresponding base

HCl → H+ + Cl-

  • 2) A weak acid;

H2CO3 ↔ H+ + HCO3-

•A base:

a molecule that accepts H+ in a solution (proton acceptor), e.g. HCO3, HPO4 & NH3.

  1. A strong base; tightly binds to H+
  2. A weak base; weakly binds to H+

Buffering:

minimizing great changes in [H+] when a strong acid or base is added to a solution.

Buffer pair

HCl (strong acid) + NaHCO3 H2CO3 + NaCl.

NaOH (strong base) + H2CO3NaHCO3 + H2O.

What is a Buffer?

A buffer is any substance that can reversibly bind with H+

Buffer + H+ ↔ Buffer H

{ weak base     +     weak acid = Buffer Pair}

•The direction of the reaction depends on the conc. of H+ ;

  1. in case of ↑ [H+], the Buffer will bind more H+.
  2. in case of ↓ [H+] the weak acid dissociates into H+ & its base (buffer).

•Suppose HA is a weak acid

HA ↔ H+ + A-

•At equilibrium

[H+ ] [A-]     = K (Dissociation constant)

HA

K; is the [H] when concentration of HA (H+ donor) equals conc. of A (H+ acceptor)

Or

It is the [H+] when half of acid is dissociated & half is undissociated

Hendresson-Haselbalch Equation

pH= pK + Log Base or Acid

•pH = pK when acid conc. = base conc

•Or pK is the pH of a solution when concentration of buffer pair (acid & its base) is equal

•Or the point at which the effectiveness of buffer is maximal

Normal [H+] Concentration

[H+] Concentration can be expressed:

•Directly as [H+]

•Or indirectly as pH

•pH is –ve Log [H+]

•pH & [H+] are inversely related

•Range of [H+] that is compatible with life is

20 – 126 nmol/L : (pH = 7.7-6.9).

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What is the Source of Acid (H+) in the Body?

1.Volatile Carbonic Acid

1.Non-Volatile Acids (Fixed acids)

1- Volatile Carbonic Acid (H2CO3)

•Under normal conditions, the greatest source of H+ is CO2 produced from oxidation of glucose & fatty acids

•The daily body production of CO2 is ~ 13.000 mmol

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3

•This amount → 13.000 mmol H+

•If this H+ is not eliminated, [H+] will ↑ from the normal of 40nmol/L to 300 mmol/L.

2- Non- Volatile Fixed Acid

•Derived mainly from the metabolism of food stuffs:

Proteins:→ (H2SO4, HCl & H3PO4)

•Metabolism of exogenous proteins is the primary source for fixed acids ( ~ 235 mmol H+ daily)

Fat:→ (ketoacids)

•Oxidation of FA → Ketoacids

•An excess production of ketoacids occurs in Insulin-dependent Diabetes Mellitus (IDDM) & fasting → an increase in H+ (ketoacidosis).

Carbohydrates :→ Lactic acid

in the following conditions:

– ↓ O2 supply as in shock, hypoxia, cardiac failure; or ↑ O2 demand as in severe exercise (anaerobic glycolysis→ lactic acid)

-Liver disease (lactate oxidation occurs in liver).

-Vitamine B1 deficiency (needed for lactate oxidation).

-Vitamine B1 deficiency (needed for lactate oxidation).

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How The Body Keeps [H+] within Normal Range?

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3 Body Systems work to keep [H+] constant

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I- Buffers

•Bicarbonate Buffer System (BBS)

•Non-Carbonate buffer System:

  1. Protein
  2. Haemoglobin
  3. Phosphate (ICF)

Bicarbonate Buffer System (BBS)

•Bicarbonate buffer system is the most important because:

•Most powerful buffer in ECF buffering ~75% of fixed acids in plasma

•A strong ICF buffer binding most of excess H+ in ICF

•Its 2 components; HCO3, H2CO3 can be regulated in an independent way by kidney & respiratory systems

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II- Role of Respiratory System

•The buffering power of respiratory system in regulating pH is ~ 2 times that of all chemical buffers

•The effectiveness of respiratory system to return an altered pH to normal is ~ 50-75%

•This means that if the pH is suddenly lowered from 7.4 to 7, the respiratory system can raise it back to 7.2 or 7.3 within 3-12 min

•Acting through elimination or retention of CO2

•A decrease in pH (as a result of an increase in H2CO3) stimulates ventilation eliminating excess CO2

•However, can not eliminate fixed acids

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II- Role of the Kidney; Acidification of urine

•Normal urine is slightly acidic (pH ~6)

•Because:

  1. most of HCO3 filtered is reabsorbed by renal tubular epithelium
  2. H+ excretion

•Kidney can excrete H+ up to urine pH of 4.5

•When extra  H+ secreted, they are buffered by NH3 & HPO4

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Disturbance of Acid-Base Balance

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Primary cause:

  1. Respiratory; disturbance originating from changes in respiration
  2. Metabolic; disturbance originating from changes in cellular metabolism

Compensation:

•Body response to correct imbalance (normal pH & ratio)

•This response is brought about by lungs and/or kidneys

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1) Respiratory Disorders

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2) Metabolic Disorders

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Primary Acid-Base Disturbance in figures

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Compensation for Acid-Base disorders

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•Concentration of CO2 & HCO3 in different acid-base disorders

•For PCo2 & HCO3

  1. 2 arrows denote primary aetiology
  2. single arrow denotes compensatory change

Acid-Base Balance Disturbance & compensation

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Remember Some Normal Values

•pH : 7.35 – 7.45

•PaCO2 : 35 – 45 mmHg

•HCO3 : 22 – 26 mEq/L

•PaO2 : 80 – 100 mm Hg

•O2 saturation : 95 – 100 %


NOW, U should understand

•Basic concepts: acid, base, buffer, pH, Hendresson-Hasselbalch equation,

•why keeping acid-base balance is important

•Normal H+ concentration

•Source of H+ in the body

•Body systems regulating H+ conc.;

Buffers, Respiratory system & kidney

•Primary disturbances in Acid-Base balance; & its compensation