Acid-Base Balance and Physiological Disorders

Fundamental Concepts of Acids and Bases

• pH Determination: The pH of a solution is determined by its concentration of hydrogen ions ($[H^+]$).

• Definition of an Acid: An acid is a molecule that releases $H^+$ into a solution, functioning as a proton donor.     * Strong Acid: A strong acid ionizes freely and gives up most of its hydrogen ions.         * Example: Hydrochloric acid ($HCl$).         * Reaction: $HCl \rightarrow H^+ + Cl^-$     * Weak Acid: A weak acid only ionizes partially and keeps some hydrogen ions bound.         * Example: Carbonic acid ($H_2CO_3$).         * Equilibrium Example: $10\,H_2CO_3 \rightleftharpoons 5\,H_2CO_3 + 5\,H^+ + 5\,HCO_3^-$

• The Carbonic Acid and $CO_2$ Relationship:     * Reaction: $H_2CO_3 \rightleftharpoons H_2O + CO_2$     * Most carbon dioxide ($CO_2$) in body solutions is converted into carbonic acid ($H_2CO_3$).     * $H_2CO_3$ dissociates into $H^+$ and bicarbonate ($HCO_3^-$).     * Consequently, $CO_2$ and pH are inversely related: as $PCO_2$ increases, pH decreases.     * Full Sequence: $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

• Definition of a Base: A base is a molecule that accepts $H^+$ions, functioning as a proton acceptor.     * Strong Base: A strong base has a strong affinity for $H^+$ and exerts a significant effect on pH.         * Example: Sodium hydroxide ($NaOH$).         * Reaction with acid: $NaOH + HCl \rightarrow NaCl + H_2O$     * Weak Base: A weak base takes up less $H^+$ due to a weaker affinity, resulting in less effect on pH.         * Example: Ammonia ($NH_3$).         * Reaction: $NH_3 + H^+ \rightleftharpoons NH_4^+$

Buffering Systems

• Buffers: Buffers are mechanisms that protect the body against changes in pH.

• Chemical Buffers: These are molecules that bind $H^+$ or release $H^+$ to restore the desired pH.     * Bicarbonate System: The primary buffer of the Extracellular Fluid (ECF).     * Phosphate System: Important internal cellular buffering.     * Protein Buffer System: Proteins can act as buffers due to their chemical structure.

• Physiological Buffering Systems: Organ systems that regulate pH by controlling the elimination of acids or bases.     * Respiratory System: Controls $CO_2$ levels.     * Renal System: Controls the excretion and reabsorption of $H^+$ and $HCO_3^-$

• Amphoteric Molecules: A single protein can act as either a base or an acid depending upon the pH of the environment.

Blood pH Homeostasis and Disorders

• Normal Blood pH Range: $7.35-7.45$

• Bicarbonate to Carbonic Acid Ratio: The ECF maintains a $20:1$ ratio of bicarbonate ($HCO_3^-$) to carbonic acid/carbon dioxide ($H_2CO_3/CO_2$).

• Acidosis: Defined as a blood pH less than $7.35$.     * Cellular Mechanism: As $[H^+]$ increases in the ECF, $H^+$ diffuses into cells, and potassium ($K^+$) diffuses out to maintain electrical neutrality.     * Intracellular Buffering: The incoming $H^+$ is buffered by proteins in the Intracellular Fluid (ICF).     * Net Result: The loss of positive charge (due to $K^+$ exit) within the cell leads to hyperpolarization.     * Physiological Effects: Hyperpolarization depresses the Central Nervous System (CNS) and muscles.     * Lethality: A pH below $6.9$ can lead to coma and death.

• Alkalosis: Defined as a blood pH greater than $7.45$.     * Cellular Mechanism: As $[H^+]$ decreases in the ECF, $H^+$ diffuses out of cells, and potassium ($K^+$) diffuses in.     * Net Result: A net gain of positive charge in the ICF brings cells closer to the threshold.     * Physiological Effects: Cells become partially depolarized and hyperexcitable. This can lead to muscle spasms, tetany, convulsions, and respiratory paralysis.     * Lethality: A pH above $7.8$ can be fatal.

Compensatory Mechanisms

• Restoration of Ratio: The respiratory and renal systems attempt to restore the $20:1$ ratio of base ($HCO_3^-$) to acid ($H_2CO_3$).

• Physiologic pH Restoration: Restoring a normal or near-normal ratio helps restore physiologic pH even if the absolute concentrations of the molecules remain unchanged.

• Ketoacidosis Example: A person with ketoacidosis can increase their respirations to lower $CO_2$ levels. This raises the pH closer to normal without actually affecting the level of ketones in the body.

• Core Limitation: Compensatory mechanisms do not fix the underlying problem causing the imbalance.

Types of Acid-Base Disorders

1. Respiratory Acidosis

• Prevalence: The most common acid-base disorder.

• Mechanism: A failure to excrete adequate amounts of $CO_2$.

• Relationship: As $CO_2$ increases, pH decreases ($CO_2 + H_2O \rightarrow H_2CO_3$).

• Common Causes:     * Chronic Obstructive Pulmonary Disease (COPD).     * Impaired ventilatory movement.     * Barbiturate overdose.

• Compensation: Must come from the kidneys.     * Kidneys secrete more $H^+$ into the urine.     * This process generates and places more $HCO_3^-$ into the blood (via the chloride shift).     * This involves the regeneration of bicarbonate by the cells of the Proximal Convoluted Tubule (PCT).

2. Metabolic Acidosis

• Prevalence: The second most common acid-base disorder.

• Mechanism: Imbalance caused by the accumulation of acids other than $H_2CO_3$ ($CO_2$).

• Buffering Impact: Increased acids exceed the body’s chemical buffering capacity.

• Bicarbonate Levels: $HCO_3^-$ levels will be low because they are used up neutralizing the excess acid.

• Common Causes:     * Renal disease.     * Ketosis: Resulting from diabetes mellitus, starvation, or a ketogenic diet.     * Lactic acidosis: Resulting from near-drowning or other extended highly anaerobic conditions.

• Compensation: Must come from the respiratory system (because the kidneys are either the cause or cannot keep up).     * Mechanism: Hyperventilation to decrease $CO_2$ levels, which raises the pH.

3. Respiratory Alkalosis

• Mechanism: Excessive excretion of $CO_2$ due to hyperventilation.

• Causes:     * Anxiety.     * Brain tumor or brain injury.     * Certain drugs that stimulate the respiratory center.

• Compensation: Orchestrated by the kidneys.     * The kidneys increase the renal excretion of $HCO_3^-$ (base) and retain $H^+$.

4. Metabolic Alkalosis

• Mechanism: Increased $HCO_3^-$ levels which increase the pH.

• Causes:     * Vomiting (loss of stomach acid).     * Excess antacid consumption.     * Certain diuretics.

• Compensation: Managed by the respiratory system, though this is very limited.     * Strategy: The body needs to retain $CO_2$.     * Limitation: Hypoventilation is limited by the resulting decrease in $PO_2$ levels.

Clinical Diagnosis and Interpretations

Diagnostic Steps

1. Determine Status: Identify if the patient is in acidosis or alkalosis.

2. Evaluate $PCO_2$: Determine if the disorder is respiratory.     * Check if the $PCO_2$ fits the expected "teeter-totter" relationship (opposite of pH) for a respiratory disorder.     * If it fits, it is respiratory; if it does not, it is metabolic.

3. Assess Compensation: Determine if the patient is attempting to compensate.     * For a respiratory disorder, look at the kidney/bicarbonate ($HCO_3^-$) levels.     * For a metabolic disorder, look at the respiratory/$PCO_2$ levels.

The ROME Mnemonic

• Respiratory Opposite:     * Acidosis: pH is low and $PCO_2$ is high.     * Alkalosis: pH is high and $PCO_2$ is low.

• Metabolic Equal:     * Acidosis: pH is low and $HCO_3^-$ is low.     * Alkalosis: pH is high and $HCO_3^-$ is high.

Normal Reference Values

• pH: $7.35-7.45$

• $PCO_2$: $35-45\,mmHg$

• $HCO_3^-$: $22-26\,mEq/L$

Case Study: Patient 1

• Values: pH = $7.0$, $PCO_2 = 60\,mmHg$, $HCO_3^- = 28\,mEq/L$

• Interpretation:     * The pH is below $7.35$, indicating acidosis.     * The $PCO_2$ is high ($60\,mmHg$), which is opposite to the low pH, indicating Respiratory Acidosis.     * The $HCO_3^-$ is slightly elevated ($28\,mEq/L$), suggesting the kidneys are starting to compensate by retaining base.