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Theory Notes

Normal Acid-Base Homeostasis and Role of Lungs

Systemic arterial pH is maintained between 7.35 and 7.45 by extracellular and intracellular chemical buffering together with respiratory and renal regulatory mechanisms. The control of arterial CO2 tension (paCO2) by the central nervous system and respiratory systems; and the control of the plasma bicarbonate by the kidneys stabilize the arterial pH by excretion or retention of acid or alkali.

The metabolic (bicarbonate) and respiratory components (carbonic acid) that regulate systemic pH are described by the Henderson-Hassel Balch equation:

pH = 6.1 + log (HCO3/ H2 CO3)

H2 CO3 = PCO2 (mm Hg) X 0.03

Under most circumstances, CO2 production and excretion are matched, and the usual steady-state paCO2 is maintained at 40 mm Hg. Under excretion of CO2 produces hypercapnia, and over excretion causes hypocapnia. Nevertheless, production and excretion are again matched at a new steady-state paCO2. Therefore, the PaCO2 is regulated primarily by neural respiratory factors and is not subject to regulation by the rate of CO2 production. Hypercapnia is usually the result of hypoventilation rather than of increased CO2 production. Increases or decreases in paCO2 represent derangements of neural respiratory control or are due to compensatory changes in response to a primary alteration in the plasma [HCO3].

In conditions of low plasma [HCO3] due to acidity in the medium (high H+ concentration), medullary chemo receptors  are stimulated with the resultant hyperventilation and elimination of H2CO3(CO2), the ratio of HCO3/ H2 CO3 is restored back to normal , pH is also restored back to normal.

Reverse occurs in conditions of high plasma bicarbonate concentration (low H +), the medullary chemo receptors are depressed with the resultant hypoventilation and retention of CO2 (H2CO3). The ratio is restored, bringing  pH also back to normal.

 Effect of pCO2

↑pCO2 → ↑Ventilation →Eliminates CO2 → Reduces [H+]  and ↑pH

↓pCO2 → ↓Ventilation → ↑CO2 → ↑ [H+] & ↓ pH

 Doubling the ventilation → ↑pH

 ¼ of normal ventilation → ↓ pH

 Effect of [H+]

– ↑ [H+] → ↑Alveolar Ventilation →↓CO2

– ↓pH (from 7.4-7.0) → ↑Alveolar Ventilation by 4 times normal.

– ↑pH → ↓Alveolar Ventilation

Respiratory Mechanism has effectiveness between 50-75% and  is 1-2 times as great as the buffering power of all other chemical buffers in ECF. The lungs should be healthy for these compensatory changes.  

Role of Kidney in maintaining acid base homeostasis

Acids are added daily to the body fluids. These acids first are buffered by the HCO3 /H2 CO3 system as follows:

H2 SO4 + 2NaHCO3 «Na2 SO4 + 2H2 CO3 «2H2O +2 CO2

The net result is buffering of a strong acid (H2 SO4) by 2 molecules of HCO3 and production of a weak acid (H2 CO3), which minimizes the change in pH. The lungs excrete the CO2 produced, and the kidneys replace the consumed HCO3 , to prevent progressive HCO3 loss and metabolic acidosis, (principally by H+ secretion in the collecting duct).

To maintain normal pH, the kidneys must perform 2 physiological functions.

The first is to reabsorb all the filtered HCO3 (any loss of HCO3 is equal to the addition of an equimolar amount of H+), a function principally of the proximal tubule.

The second is to excrete the daily H+ load (loss of H+ is equal to addition of an equimolar amount of HCO3 ), a function of the collecting duct.

HCO3 re-absorption

With a serum HCO3 concentration of 24 mEq/L, the daily glomerular ultra filtrate of 180 L, in a healthy subject, contains 4300 mEq of HCO3 , all of which has to be reabsorbed. Approximately 90% of the filtered HCO3 is reabsorbed in the proximal tubule, and the remainder is reabsorbed in the thick ascending limb and the medullary collecting duct (figure-1).

The 3Na+ -2K+ «ATPase (sodium-potassium «adenosine triphosphatase) provides the energy for this process, which maintains a low intracellular Na+ concentration and a relative negative intracellular potential. The low Na+ concentration indirectly provides energy for the apical Na+/H+ exchanger, which transports H+ into the tubular lumen. H+ in the tubular lumen combines with filtered HCO3 in the following reaction:

HCO3 + H+ « H2 CO3 « H2 O + CO2

Carbonic Anhydrase (CA IV isoform) present in the brush border of the first 2 segments of the proximal tubule accelerates the dissociation of H2 CO3 into H2O + CO2, which shifts the reaction shown above to the right and keeps the luminal concentration of H+ low. CO2 diffuses into the proximal tubular cell perhaps via the aquaporin-1 water channel, where carbonic anhydrase (CA II isoform) combines CO2 and water to form HCO3 and H+. The HCO3 formed intracellularly returns to the pericellular space and then to the circulation via the basolateral Na+/3HCO3 co transporter.

In essence, the filtered HCO3 is converted to CO2 in the lumen, which diffuses into the proximal tubular cell and is then converted back to HCO3 to be returned to the systemic circulation, thus reclaiming the filtered HCO3

 Bicarbonate reabsorption

Figure-1- Re-absorption of HCO3

 Acid excretion

Excretion of the daily acid load (50-100 mEq of H+) occurs principally through H+ secretion by the apical H+ «ATPase in A-type intercalated cells of the collecting duct.

HCO3 formed intracellularly is returned to the systemic circulation via the basolateral Cl/HCO3 exchanger, and H+ enters the tubular lumen via 1 of 2 apical proton pumps, H+ «ATPase or H+ -K+ «ATPase. The secretion of H+ in these segments is influenced by Na+ reabsorption in the adjacent principal cells of the collecting duct. Hydrogen ions secreted by the kidneys can be excreted as free ions but, at the lowest achievable urine pH of 5.0 (equal to free H+ concentration of 10 µEq/L), would require excretion of 5000-10,000 L of urine a day. Urine pH cannot be lowered much below 5.0 because the gradient against which H+ «ATPase has to pump protons (intracellular pH 7.5 to luminal pH 5) becomes too steep. Maximally acidified urine, even with a volume of 3 L, would thus contain a mere 30 µEq of free H+. Instead, more than 99.9% of the H+ load is excreted buffered by the weak bases NH3 or phosphate.

 Titratable acidity

The amount of secreted H+ that is buffered by filtered weak acids is called titratable acidity. Phosphate as HPO4 2- is the main buffer in this system(figure-2) but other urine buffers include uric acid and creatinine.

H2 PO4 «H+ + HPO4 2-

The amount of phosphate filtered is limited and relatively fixed, and only a fraction of the secreted H+ can be buffered by HPO4 2-.

 Phosphate mechanism

Figure-2- showing the buffering of secreted H+ by HPO4

Ammonia mechanism

A more important urine-buffering system for secreted H+ than phosphate, ammonia (NH3) buffering occurs via the following reaction:

NH3 + H+ «NH4 +

Ammonia is produced in the proximal tubule from the amino acid glutamine, and this reaction is enhanced by an acid load and by hypokalemia. Ammonia is converted to ammonium (NH4 +) by intracellular H+ and is secreted into the proximal tubular lumen by the apical Na+/H+ (NH4 +) antiporter. It can be secreted as such also and can later combine with H+ in the lumen to form NH4+.

 NH4 + is trapped in the lumen and excreted as the Cl salt, and every H+ ion buffered is an HCO3 gained to the systemic circulation (figure-3)

The kidneys can adjust the amount of NH3 synthesized to meet demand, making this a powerful system to buffer secreted H+ in the urine.

 Ammonia mechanism

Figure –3- showing ammonia mechanism. Glutamine is first converted to glutamate and then to alpha keto glutarate

Renal glutaminase activity is increased in conditions of acidosis, to excrete out the excess acid load  whereas it is decreased in  conditions of alkalosis to conserve acids (H+) to maintain the acid base balance of the body.

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An acid is a substance that can donate hydrogen ions (H+), and a base is a substance that can accept H+ ions, regardless of the substance’s charge.

H2 CO3 (acid) « H+ + HCO3 (base)

Strong acids are those that are completely ionized in body fluids, and weak acids are those that are incompletely ionized in body fluids.

HCl « H+ + Cl

Hydrochloric acid (HCl) is considered a strong acid because it is present only in a completely ionized form in the body, whereas H2 CO3 is a weak acid because it is ionized incompletely, and, at equilibrium, all 3 reactants are present in body fluids.

H2 CO3 (acid) « H+ + HCO3 (base)

In body fluids, the concentration of hydrogen ions ([H+]) is maintained within very narrow limits, with the normal physiologic concentration being 40nEq/L. The concentration of HCO3– (24mEq/L) is 600,000 times that of [H+]. The tight regulation of [H+] at this low concentration is crucial for normal cellular activities.

Significance of pH

1) Specific tautomeric forms exist at physiologic pH. This helps in proper hydrogen bonding between the complementary base pairs in the structure of DNA.

2) The solubility and biologic activity of a protein depends upon its 3D structure and that depends upon net charge on protein for the maintenance of hydrogen and ionic interactions. The net charge depends upon the pH of the medium.

3) The movement of ions across the membrane depends upon their net charge as determined by the pH.

4) Ionic state of the nucleic acids, lipids and mucopolysaccharides is also determined by the physiological pH

5) All enzymes function best within an optimum pH range.

6) Nerve conduction and muscle contractions are also pH dependent

7) All metabolic processes are pH dependent.

8) Oxygen and CO2 transport, release or gaseous exchange is pH dependent.

Maintenance of pH is important for proper physiological functioning of cells and tissues. Any changes in pH can alter enzyme activity, cellular uptake, incorporation and use of minerals and metabolites, uptake and release of oxygen, and the formation of biological structural components.

Normal plasma pH = 7.40 (±0.05). The pH range that is compatible with life is from 6.8 to 7.8. The body can comfortably tolerate a shift in pH of about 0.04. Most cells of the body have a pH = 7.0, but RBC’s boast a pH of 7.2. The pH of the body affects its acid-base balance and the pH of blood has the greatest effect.

Sources for pH disturbances

1) Organic acids- The most common sources for pH disturbances are the body’s production of organic acids (acetic, acetoacetate, propionic, butyric, lactic, etc.), which are the major sources of hydrogen ion.

2) Carbonic acid is the chief acid (volatile acid) produced in the body by the metabolic processes in the body. Approximately 300 litres of CO2 are produced and eliminated daily in the body of an adult.

3) Sulphuric acid- it is produced during the oxidation of sulphur-containing amino acids and vitamins.

4) Phosphoric acid- is produced from the metabolism of dietary phosphoproteins, phospholipids, nucleic acids and hydrolysis of phosphoesters.

Mechanism of maintenance of Physiological pH

Under normal conditions, acids and, to a lesser extent, bases are being added constantly to the extracellular fluid compartment but still a physiologic [H+] of 40 nEq/L is maintained and the following 3 processes must take place:

  • Buffering by extracellular and intracellular buffers
  • Alveolar ventilation, which controls PaCO2
  • Renal H+ excretion, which controls plasma [HCO3 ]

Buffers

Buffers are weak acids or bases that are able to minimize changes in pH by taking up or releasing H+. Phosphate is an example of an effective buffer, as in the following reaction:

HPO4 2- + (H+) « H2 PO4

Upon addition of an H+ to extracellular fluids, the monohydrogen phosphate binds H+ to form dihydrogen phosphate, minimizing the change in pH. Similarly, when [H+] is decreased, the reaction is shifted to the left. Thus, buffers work as a first-line of defense to blunt the changes in pH that would otherwise result from the constant daily addition of acids and bases to body fluids. With the constant pouring in of H+, the concentration of the monohydrogen phosphate will ultimately diminish and the pH will start falling.

The major Buffer system of the body

(1) HCO3 /H2 CO3 buffering system HCO/CO2 (bicarbonate/carbon dioxide)

(2) HPO42―/H2PO4 (phosphate),

(3) Organic Phosphate Esters, and

(4) Proteins.

Proteins with side chains that contain more carboxyl terminal groups than amino terminal groups promote an acidic environment. Proteins with side chains that contain more amino terminal groups than carboxyl terminal groups promote an alkaline environment. Protein with side chains containing equal numbers of amino and carboxyl side groups are neutral, not affecting the pH.

Details of Buffers

(1) HCO3 /H2 CO3 buffering system

In the ECF bicarbonate buffer is the most important buffer. Its function is illustrated by the following reactions:

H2 O + CO2 «H2 CO3 « H+ + HCO3

When an acid load (H+) is added to the body fluids, it results in consumption of HCO3 by the added H+. Carbonic acid thus formed, in turn, forms water and CO2. CO2 concentration is maintained within a narrow range via the respiratory drive, which eliminates accumulating CO2. The kidneys regenerate the HCO3 consumed during this reaction.

Put simply, whereas simple buffers rapidly become ineffective as the association of the hydrogen ion and the weak anion of the weak acid reaches equilibrium, the bicarbonate system keeps working because the carbonic acid is removed as CO2. The limit to the effectiveness of the bicarbonate system is the initial concentration of bicarbonate. The acid base status of the patient is assessed by the bicarbonate concentration in the plasma. The association of hydrogen ion with bicarbonate occurs rapidly but the dissociation of H2CO3 to CO2 and H2O is slow. This process is accelerated by the enzyme Carbonic anhydrase, which is present in the erythrocytes and in the kidney whenever this reaction is needed. Buffering at the expense of bicarbonate effectively removes hydrogen ions from ECF. CO2 is removed from the lungs and water assimilates in the ECF without producing any change in p H. The ECF contains a large amount of bicarbonate to the extent of 24 mmol/L, when the H+ concentration increases the bicarbonate concentration comes down since it is used up during the process of buffering.

This reaction continues to move to the left as long as CO2 is constantly eliminated or until HCO3 is significantly depleted, making less HCO3 available to bind H+. Since HCO3 and PaCO2 can be managed independently (kidneys and lungs, respectively) that makes this a very effective buffering system. One of the major factors that make this system very effective is the ability to control PaCO2 by changes in ventilation. As can be noted from this reaction, increased carbon dioxide (CO2) concentration drives the reaction to the right, whereas a decrease in CO2 concentration drives it to the left.

Assessing acid base status

An indication of the acid base status of the patient can be determined by measuring the components of the bicarbonate system.

The Henderson-Hassel Balch equation describes the relationship between blood pH and the components of the H2 CO3 buffering system.  

pH = 6.1 + log (HCO3/ H2 CO3)

Bicarbonate (HCO3) is in equilibrium with the metabolic components.

  • Bicarbonate production in the kidney
  • Acid production from endogenous or exogenous sources

Carbonic acid (H2 CO3) is in equilibrium with the respiratory component, as shown by the below equation:

H2 CO3 = PCO2 (mm Hg) X 0.03

Note that changes in pH or [H+] are a result of relative changes in the ratio of PaCO2 to [HCO3 ] rather than to absolute change in either one. In other words, if both PaCO2 and [HCO3 ] change in the same direction, the ratio stays the same and the pH or [H+] remains relatively stable. To diminish the alteration in pH that occurs when either HCO3 or PaCO2 changes, the body, within certain limits, changes the other variable in the same direction.

2) Phosphate buffer system (Na2HPO4/NaH2PO4)

The phosphate buffer system is directly linked up with kidney.

Upon addition of acid, the H+ is neutralized by the Na2 HPO4 component forming NaH2PO4 that is eliminated through the kidney without any change in pH.

Na2 HPO4 + HCl–>> NaH2PO4 + NaCl

Similarly upon addition of OH, the acid component reacts to form, Na2 HPO4 that can be eliminated as well through the kidney without any change in pH

NaH2PO4 + Na OH—>> Na2 HPO4 + H2O

In other words Phosphate buffer system works in conjunction with the kidney.

Chemically it is a very good buffer, as pKa is close to Physiological pH, but physiologically due to its less concentration (1.0 mmol/L as compared to bicarbonate 26-28 mmol/L) it is less efficient.

3) Role of Haemoglobin as a buffer

The buffering capacity of Hb is due to the presence of “Imidazole” nitrogen group of Histidine. Oxygenated Hb is a stronger acid than deoxygenated Hb. Acidity of the medium favors delivery of oxygen to the tissues. Alkalinity of the medium favors oxygenation of Hb. Sequence of events that occur in lungs and tissues is as follows;

  • In the lungs

The formation of oxy hemoglobin from deoxy hemoglobin, must release H+, which will react with HCO3– to form H2CO3. Due to the low CO2 tension in the lungs H2CO3, dissociates to form CO2 and H2O . CO2 is then eliminated in the expired air (Figure-1).

 Role of Hb as a buffer in lungs

Figure-1- Role of Hb as a buffer in the lungs.

  • In the tissues

Oxy Hb dissociates to give O2 to the tissues and the deoxy Hb (Reduced Hb) is formed. At the same time CO2 produced as a result of metabolism, is hydrated to for H2CO3, which ionizes to form H+ and HCO3-. Deoxy Hb acts as anion and accepts H+ to form acid reduced Hb (Figure-2)

Role of Hb as a buffer in tissues

Figure-2-  Role of Hb as a buffer at the tissue level.

4) Protein buffer system

Buffering capacity of plasma proteins is much less than Hb. In acidic medium protein acts a base and NH2 group takes up H+ forming NH3+, protein becomes positively charged.

 Reverse occurs in the alkaline medium. Acidic COOH to give H+ that neutralizes the OHforming H2O. Overall protein becomes negatively charged in the alkaline medium.

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Primary Disorder   Defect   Causes Effect on pH and Ratio of Bicarbonate: Carbonic acid  Compensatory Response
Metabolic Acidosis

HCO3-

Decreased

Gain in H+ or loss of HCO3-

 

(A) High anion gap (Acid gain)

1) Ketoacidosis

  • Diabetes
  • Chronic alcoholism
  • Under nutrition
  • Fasting

2) Lactic Acidosis

  • Shock
  • Primary hypoxia due to lung disorders
  • Seizures

3) Renal Failure

4) Toxins Metabolized to acids

  • Alcohol
  • Methanol (formate)
  • Ethylene glycol (oxalate)
  • Salicylates

B) Normal Anion Gap-Acidosis(Bicarbonate loss- Hyperchloremic acidosis)

1) GI HCO3 loss

  • Colostomy
  • Diarrhea
  • Enteric fistulas
  • Ileostomy

2)Urologic procedures

3) Renal HCO3 loss

  • Tubulointerstitial renal disease
  • Renal tubular acidosis

4) Ingestions

  • Acetazolamide
  • CaCl2
  • Mg sulfate (MgSO4)

 

pH –decreased,

Ratio- decreased

 

Respiratory Mechanism-Respiratory Alkalosis(Hyperventilation)

Pa CO2 Decreased 

Renal mechanisms

1) Increased excretion of H+ ions

2) Decreased excretion of K+ ions in the distal tubules

3) Decreased bicarbonate excretion

4)Increased ammonia formation

5) Increased acid phosphate excretion

 

 

Metabolic Alkalosis

HCO3- Increased

Gain in HCO3-or loss of H+ Chloride-responsive alkalosis

  • Loss of gastric secretions – Vomiting, NG suction
  • Loss of colonic secretions
  • Thiazides and loop diuretics (after discontinuation)
  • Cystic fibrosis( Due to loss of chloride in the sweat)
  • Ingestion of large doses of nonabsorbable antacids

Chloride-resistant alkalosis

  • Primary hyperaldosteronism
  • Cushing syndrome
  • Exogenous mineralocorticoids or glucocorticoids
  • Reno vascular hypertension
  • Renin- or deoxy corticosterone-secreting tumors
  • Current use of thiazides and loop diuretics
  • Hypomagnesaemia(Through Hypokalemia)
  • Milk Alkali Syndrome

 

pH increased,

Ratio increased

Respiratory Mechanism-

Respiratory Acidosis(Hypoventilation)

PaCO2 Increased.

Renal Mechanism

1) Decreased excretion of H+ ions

2) Increased excretion of K+ ions in the distal tubules

3) Increased bicarbonate excretion

4) Decreased ammonia formation

5) Decreased acid Phosphate excretion

 

Respiratory Acidosis

PaCO2

Increased

CO2 Retention A) Central

  • Drugs- Sedatives, Alcohol, General Anesthetic agents
  •  Infections
  • Injuries- head trauma
  • Diseases-  Intracranial tumor
  • Syndromes of sleep-disordered breathing, including the primary alveolar   and obesity-hypoventilation syndromes.

B)Airway obstruction

  • Severe asthma,
  • Anaphylaxis
  • Inhalational burn
  • Toxic injury
  • Laryngeal obstruction
  • End-stage obstructive lung disease.

C) Parenchymatous damage /Inflammation

  • Emphysema
  • Bronchitis
  • Adult Respiratory distress syndrome
  • Pleurisy
  • Barotrauma

D) Neuromuscular

  • Poliomyelitis
  • Kyphoscoliosis
  • Myasthenia gravis
  • Muscular dystrophies

E) Misc.

  • Certain congenital heart diseases
  • Mechanical ventilation
  • Rebreathing from a closed space

 

pH- decreased,

Ratio- decreased

Metabolic Alkalosis

HCO3-Increased.

Renal mechanisms 1) Increased excretion

of H+ ions

2) Decreased bicarbonate excretion

3) Increased ammonia formation

4) Increased acid phosphate excretion

 

Respiratory Alkalosis

PaCO2 Decreased

CO2 Washout A) Central nervous system

  • Pain
  • Hyperventilation syndrome
  • Anxiety
  • Psychosis
  • Fever
  • Cerebrovascular accident
  • Meningitis
  • Encephalitis
  • Tumor
  • Trauma
  • Hypoxia
    • High altitude
    • Severe anemia
    • Right-to-left shunts
  • Drugs
    • Progesterone
    • Methylxanthines
    • Salicylates
    • Catecholamines
    • Nicotine
  • Endocrine
    • Pregnancy
    • Hyperthyroidism
  • Pulmonary
    • Pneumothorax/hemothorax
    • Pneumonia
    • Pulmonary edema
    • Pulmonary embolism
    • Aspiration
    • Interstitial lung disease
    • Asthma
    • Emphysema
    • Chronic bronchitis
  • Miscellaneous
    • Sepsis
    • Hepatic failure
    • Mechanical ventilation
    • Heat exhaustion
    • Recovery phase of metabolic acidosis
    • Congestive heart failure

 

pH –Increased,

Ratio- Increased

Metabolic Acidosis

HCO3-Decreased

Renal Mechanism

1)Decreased excretion of H+ ions

2)Increased bicarbonate excretion

3)Decreased ammonia formation

4)Decreased phosphate excretion

 

 

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