Acid-Base Balance and Lung Function

Acid-Base Balance: Role of the Lungs

• Acid-base balance refers to the regulation of free hydrogen-ion (H+) concentration in the body fluids.

Sources of Acidity

• CO2 + H2O → H2CO3 → H+ + HCO3- (source of acidity)

Regulation of H+ Balance

• The Henderson-Hasselbalch equation is used to calculate how changes in CO2 and HCO3− affect pH.
• pH = [HCO3-] controlled by kidney function / [CO2] controlled by lung function.

Defense Mechanisms against Changes in [H+]

• Three lines of defense operate to maintain [H+] at a nearly constant level (pH 7.4):
• Chemical buffer systems
• Respiratory mechanism of pH control
• Renal mechanism of pH control

Respiratory Mechanism of pH Control

• Ventilation can compensate for pH disturbances.
• Increased H+ concentration stimulates respiration, which increases alveolar ventilation and decreases H+ concentration.

Relationship between Respiration and pH

• High concentrations of hydrogen trigger hyperventilation.
• Respiratory system is a negative feedback mechanism.
• In hypoventilation, we have increased CO2 levels.

Respiratory Acid-Base Regulation

• Acts at a moderate speed.
• Activated when chemical buffer systems alone are unable to minimize pH deviations.
• Requires a few minutes to be initiated.
• Respiratory system serves as the second line of defense.

Respiratory Compensation

• Peripheric chemoreceptors and central chemoreceptors are involved in respiratory compensation.

Respiratory Acidosis

• Causes:
• Impaired gas exchange
• Type B chronic obstructive pulmonary disease (COPD)
• Bacterial pneumonia
• Severe asthma episode
• Pulmonary edema
• Acute (adult) respiratory distress syndrome
• Impaired neuromuscular function
• Chest injury or surgery (pain limits ventilation)
• Hypokalemic respiratory muscle weakness
• Severe kyphoscoliosis
• Respiratory muscle fatigue
• Impaired respiratory control (brainstem)
• Respiratory depressant drugs (opioids, barbiturates)
• Signs and symptoms:
• Headache
• Tachycardia
• Cardiac dysrhythmias
• Neurologic abnormalities
• Blurred vision
• Tremors
• Vertigo
• Disorientation
• Lethargy
• Somnolence

Respiratory Alkalosis

• Causes:
• Hypoxemia
• Signs and symptoms:
• Increased neuromuscular excitability

Oxygen Pressure (pO2)

• Oxygen molecules dissolved in plasma impinge on the measuring oxygen electrode, reflecting the partial pressure of oxygen
• The number of O2 molecules dissolved in plasma determines how many molecules will bind to hemoglobin
• PaO2 only measures free oxygen molecules, not oxygen bound to hemoglobin

Oxygen Saturation (SaO2)

• Percentage of available heme binding sites saturated with oxygen
• Measured as the hemoglobin oxygen saturation

Oxygen Content (CaO2)

• Calculated by multiplying hemoglobin concentration, oxygen-carrying capacity of hemoglobin, and oxygen saturation
• Formula: CaO2 = Hb (gm/dl) x 1.34 ml O2/gm Hb x SaO2 + PaO2 x (.003 ml O2/mm Hg/dl)

Acclimatization to High Altitude

• The body adapts to high altitude through immediate and long-term acclimatization
• Short-term adaptation: increased breathing depth and rate (hyperpnea) due to carotid body sensing lack of oxygen
• Hyperpnea leads to respiratory alkalosis, which inhibits further respiratory rate enhancement

Oxygen Toxicity

• Breathing oxygen at high pressure (4 atm) causes seizures, coma, muscle twitchings, dizziness, and other symptoms
• Oxygen toxicity leads to formation of oxygen free radicals, damaging cell membranes and enzymes

Nitrogen Narcosis

• High pressure causes nitrogen to dissolve in fat tissues, including neuronal membranes, reducing neuronal excitability

Decompression Sickness

• When a diver surfaces too quickly, nitrogen bubbles form in body fluids, blocking blood vessels and causing tissue ischemia and death
• Symptoms include pain in joints and muscles, nervous system symptoms, and shortness of breath

Treatment of Decompression Sickness

• Gradual decompression in a pressurized tank to normal atmospheric pressure

Diving Problems

• Descending: compression of gases causes eardrum rupture, sinus damage, lung squeeze, and eye problems
• Ascending: rapid pressure change causes re-expansion of gases, leading to pulmonary barotrauma and air embolism if not exhaled properly

Henry's Law

• At constant temperature, the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas