Acid-Base Balance & pH

• Normal blood pH ranges from 7.35 to 7.45 (average 7.4)
• Maintenance of blood pH within the normal range relies on the blood buffer, respiratory, and renal systems

Acids

• Acids donate hydrogen ions (H+) in solutions

Bases (Alkaline)

• Bases accept H+ ions and donate OH- ions

pH

• pH is the negative logarithm of H+ ion concentration
• pH is inversely proportional to H+ concentration

Henderson-Hasselbalch Equation

• pH = pKa + Log(Base/Acid), where pKa is 6.1 and the Base:Acid ratio should be 20:1

Acidosis

• Acidosis has a pH lower than 7.35
• Acidosis has increased H+ or decreased HCO3- levels

Alkalosis

• Alkalosis has a pH higher than 7.45
• Alkalosis has decreased H+ or increased HCO3- levels

Control of Acid-Base Balance

• Blood buffer system (chemical buffer)
• Respiratory control (physiological buffer)
• Renal control (physiological buffer)

Blood Buffer System

• A mixture of a weakly dissociated acid and its salt that serves as the first line of defense against acid-base disturbances
• Buffers include Carbonic acid/Bicarbonate, Hemoglobin (Hb), Phosphate, and Plasma protein systems

Carbonic Acid/Bicarbonate Buffer System

• The most important buffer system in body fluids
• Carbonic acid (H2CO3) originates from H+ (from cell metabolism) + HCO3- in cells
• Carbonic acid also originates from CO2 (from cell metabolism) + H2O in the lungs, kidneys, and RBCs via carbonic anhydrase

Role of Carbonic Anhydrase

• Carbonic anhydrase is present in the lungs to control CO2 concentration, erythrocytes and kidneys
• Normal blood pH around 7.4 means that the normal bicarbonate to acid ratio is 20:1, which determines plasma pH

Ventilation and CO2

• CO2 is mainly controlled by ventilation
• Abnormal ventilation patterns lead to CO2 and subsequent H2CO3 abnormalities

Respiratory Control of Blood CO2 Levels

Hypoventilation

• Hypoventilation involves deep and slow respiration
• Hypoventilation causes CO2 accumulation and increased H2CO3
• Hypoventilation may lead to respiratory acidosis from pulmonary diseases like pneumonia and pulmonary edema

Hyperventilation

• Hyperventilation involves shallow and rapid respiration
• Hyperventilation causes decreased CO2 accumulation and decreased H2CO3
• Hyperventilation may lead to respiratory alkalosis from respiratory center stimulator drugs

Non-Respiratory Control of Serum Bicarbonate

• Bicarbonate concentration is influenced by non-respiratory mechanisms such as renal and GIT function, as well as tissue metabolism
• Increased serum bicarbonate levels may lead to metabolic alkalosis

Metabolic Alkalosis

• Metabolic Alkalosis can be caused by reabsorption of HCO3 by renal tubules
• Metabolic Alkalosis can be caused by decreased acid sources, like vomiting, and cause decreased serum bicarbonate levels, leading to metabolic acidosis

Metabolic Acidosis

• Metabolic Acidosis can be caused by diarrhea

Respiratory Control

• The respiratory center in the medulla oblongata adjusts breathing based on pH and pCO2 levels to address acidosis or alkalosis

Acidosis

• Acidosis causes increased blood acid for reasons other than respiration, which lowers blood pH

Response to Acidosis

• To compensate for lower blood pH and increased blood acid, the respiratory rate is increased (hyperventilation)
• Hyperventilation results in decreased CO2 and carbonic acid levels in the blood

Alkalosis

• Alkalosis decreases blood acid for reasons other than respiration, which increases blood pH

Response to Alkalosis

• To compensate for higher pH by reduced respiratory rate (hypoventilation)
• Hypoventilation results in increased CO2 and carbonic acid levels in the blood

Renal Control

• Renal system is the most powerful system in regulating acid base balance because it controls H+ and HCO3- concentrations
• Response time for the renal system can range from hours to days
• Accumulation of nonvolatile acids (H+) reduces bicarbonate ion content
• The kidney secretes alkaline urine to lessen alkalosis

Role of the Kidney during Acidosis

• During acidosis, H+ + HCO3- produce H2CO3 in the tubule lumen, resulting in acidic urine
• During acidosis, Na + HCO3- accumulates in blood
• In renal tubules, ammonia (NH3) + H+ form ammonium (NH4+), which combines with Cl- to form ammonium chloride (NH4Cl) and passes into the urine and leads to hypochloridemia

Role of the Kidney during Alkalosis

• During alkalosis, kidneys increase bicarbonate excretion and H+ reabsorption

Acid-Base Imbalance

• Occurs when the balance between acid production and elimination is disrupted

Acidosis

• Characterized by increased H+ or decreased HCO3-
• Divided into metabolic (decreased HCO3 or increased acids) and respiratory (increased pCO2) types

Alkalosis

• Characterized by decreased H+ or increased HCO3
• Divided into metabolic (increased HCO3- or decreased acids) and respiratory (decreased pCO2) types

Compensation

Compensated By

• Respiratory tract
• Kidneys
• If the kidneys are the source of imbalance, the problem is outside the renal system

Mechanism

• The body alters PCO2 to counterbalance the primary imbalance
• The body partially restores pH (but not fully)
• The body changes the excretion or retention of H+ and bicarbonate

Definitions

Normal pH

• Normal pH ranges from 7.35 to 7.45

Acidemia

• Blood pH is less than 7.35

Acidosis (process)

• Blood pH is altered and may or may not be associated with acidemia

Alkalemia

• Blood pH is more than 7.45

Alkalosis (process)

• Blood pH is altered and may or may not be associated with alkalemia

Respiratory Acidosis

• Respiratory acidosis is caused by increased pCO2, leading to increased carbonic acid

Causes

• Conditions or diseases that cause hypoventilation
• Pneumonia
• Pulmonary Edema
• Paralysis of respiratory muscles
• Morphine or barbiturate poisoning
• General anesthesia when CO2 removal is insufficient
• Emphysema
• Pneumothorax
• Respiratory center depression with some drugs

Uncompensated with respiratory acidosis

• Blood pH is less than 7.35
• pCO2 is increased, leading to increased plasma H2CO3
• Urine is acidic
• Plasma HCO3 is normal

Compensated with respiratory acidosis

• Achieved mainly by the kidneys

Mechanism

• Increased NaHCO3 reabsorption
• Increased H+ excretion, resulting in a more acidic urine
• Increased Cl- excretion, potentially leading to hypochloremia
• Blood pH is higher than in the uncompensated state
• pCO2 is increased
• Plasma HCO3 is increased

Respiratory Alkalosis

• Respiratory alkalosis is caused by decreased pCO2, which lowers carbonic acid

Causes

• Any condition or disease causing hyperventilation
• Respiratory center stimulation from certain drugs
• Hypoxia, such as at high altitudes
• Panting in animals such as dogs

Uncompensated respiratory alkalosis

• Blood pH is higher than 7.45
• pCO2 is decreased, leading to decreased plasma H2CO3
• Urine is alkaline
• Plasma bicarbonate is normal

Compensated respiratory Alkalosis

• Achieved mainly via the renal system

Mechanism

• Increased NaHCO3 excretion, which leads to a more alkaline urine
• Increased H+ reabsorption
• Increased Cl- reabsorption, resulting in normal to high levels of plasma Cl-
• Blood pH is over 7.45
• pCO2 is decreased
• Plasma bicarbonate is decreased

Metabolic Acidosis

• Metabolic acidosis has decreased HCO3 or increased acids other than H2CO3

Causes

• Excessive salivation, resulting in the loss of NaHCO3
• Extreme diarrhea, leading to loss of intestinal juices rich in NaHCO3
• Retention of organic acids (uremic acid) due to renal insufficiency titrating HCO3- and decreasing its levels
• Excessive muscular activity, leading to lactic acid accumulation
• Diabetes mellitus and starvation, leading to ketonic acid production and ketoacidosis
• Sequestration of intestinal contents

Uncompensated Metabolic Acidosis

• Blood pH is less than 7.35
• pCO2 is normal
• Plasma HCO3 is decreased
• Urine pH is acidic
• Serum K+ is increased

Compensated Metabolic Acidosis

Compensated Respiration

• Hyperventilation lowers pCO2 and H2CO3

Compensated Renally

• Increased NaHCO3 reabsorption
• Increased H+ excretion results in a more acidic urine
• Increased ammonia synthesis
• increased NH4Cl excretion causing hypochloridemia
• Blood pH is higher than 7.35
• PCO2is decrease
• Plasma HCO3 is decreased or normal
• Urine pH becomes more acidic

Metabolic Alkalosis

• Metabolic alkalosis has increased HCO3- or decreased acids other than H2CO3

Causes

• Hyperadrenocorticism (hyperaldosteronism) leading to increased aldosterone and excessive NaHCO3 reabsorption
• Vomiting in monogastric animals resulting in the loss of HCl
• Abomasum displacement leading to the pour of HCl into the rumen and HCL loss
• Potassium depletion (hypokalemia) causing movement of H+ into ICF resulting in loss of acid
• Alkaline

Uncompensated Metabolic Alkalosis

• Blood pH is greater than 7.45
• pCO2 is normal
• Plasma HCO3 is increased
• Urine pH is alkaline, and serum K+ & Cl- may be low

Compensated Metabolic Alkalosis

Compensated Respiration

• Hypoventilation increases pCO2 and H2CO3

Compensated Renally

• Increased NaHCO3 excretion resulting in alkaline urine
• H+ reabsorption
• Blood pH is greater than 7.45
• PCO2is increased
• Plasma HCO3 is increase
• Urine pH is alkaline

Metabolic Acidosis with an elevated Anion Gap

• Increased accumulation of metabolic acids that cannot be easily measured (unmeasurable anions)

Unmeasurable Anions

• Lactic acidosis
• Diabetes mellitus (ketoacidosis)
• Ketosis

Removal

• Decreased excretion of metabolic acids that cannot be measured, for example, renal failure leading to decreased excretion of uremic acids

Metabolic Acidosis with Normal Anion Gap

Increased Loss of HCO3

• Excessive salivation
• Chronic diarrhea
• Renal tubular acidosis

Hyperchloremia

• Cl- replaces the consumed HCO3 to maintain electrical neutrality

Paradoxical Aciduria

• In cases of alkalosis, the kidney secretes alkaline urine to normalize acid-base balance
• Kidneys are unable to correct alkalosis and secrete acidic urine, leading to further complications

Often Seen in

• Acute tubular necrosis
• Off food animal and excessive vomiting leading to Potassium deficiency (Hypokalemia)
• Abomasal displacement

ABG Analysis

Use

• Measure H+ directly from arterial blood
• Measure pCO2 directly from arterial blood
• Calculate HCO3 levels from arterial blood sample

Site

• Obtain samples from arterial blood

Anticoagulant

• Use heparin

Precautions

• Ensure no air in the syringe, and syringe immediately capped ready for transport

Important

• In the blood pH is the negative Log of H+ ion concentration

Reference Range

• pH = 7.35-7.45
• pCO2 = 38-42 mm Hg
• HCO3 = 22-28 mEq/L