Respiratory Physiology Quiz

Ventilation/Perfusion

• Ventilation (Vo) and perfusion (Q) are mismatched in extreme cases, resulting in either wasted ventilation (V/Q = ∞) or venous admixture (V/Q = 0)
• The V/Q distribution throughout the normal lung is not homogeneous, with some units being overventilated and others underventilated

Uptake of O2 by Pulmonary Blood

• PO2 rises to equal alveolar pressure by the time blood has moved 1/3rd of the distance through the capillary, ensuring a safety factor
• During exercise, diffusion capacity for O2 increases almost 3-folds due to increased surface area of capillaries participating in diffusion and nearly ideal V/Q ratio in the upper lungs

Transport of O2 in Arterial Blood

• 98% of blood that enters the left atrium becomes oxygenated, with a PO2 of 104 mm Hg
• 2% of blood, from bronchial circulation, bypasses gas exchange areas, resulting in a PO2 of 40 mm Hg (like venous blood), known as venous admixture of blood
• The combined blood in the pulmonary veins has a PO2 of 95 mm Hg

Diffusion of O2 and CO2

• 1-3 mmHg of pressure is enough for full support of cellular metabolism, with a safety factor of 23 mm Hg
• Diffusion of CO2 from tissue fluid into peripheral capillaries and from pulmonary blood into the alveolus occurs efficiently

Control of Ventilation and Perfusion

• O2 content affects blood flow, while CO2 content affects airflow
• V/Q ratio increases with increased pulmonary blood flow and alveolar ventilation

Diffusing Capacity

• Diffusing capacity for oxygen: 21 ml/min/mm Hg, with a mean oxygen ∆P during quiet breathing of 11 mm Hg
• During strenuous exercise, diffusing capacity increases to 65 ml/min/mm Hg due to opening up of dormant pulmonary capillaries and extradilation of already open capillaries
• Diffusing capacity for CO2: 20 times that of O2, with values of 400-450 ml/min/mm Hg at rest and 1200-1300 ml/min/mm Hg during exercise

Gravity Effects

• Gravity affects venous return, cardiac output, and systemic blood volume distribution
• Gravity has a greater effect on pulmonary circulation than systemic circulation due to lower vascular pressures
• There are three zones of blood flow in the lung, depending on pulmonary arterial, venous, and alveolar pressures.

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