Chapter 18: Gas Exchange and Transport: Human Physiology

Gas Exchange and Transport

Overview of Pulmonary Circulation

The pulmonary circulation is responsible for the exchange of gases between the lungs and the blood. Arterial blood levels of oxygen (O2) and carbon dioxide (CO2) are maintained within narrow limits to support cellular metabolism.

• At rest:

  • O2 consumption: 250 mL/min

  • CO2 production: 200 mL/min

• Ventilation: Average adult inhales 6000 mL/min; about 4200 mL/min reaches the alveoli (alveolar ventilation).

Structure of the Respiratory Membrane

The respiratory membrane is the site of gas exchange between alveolar air and blood in pulmonary capillaries.

• Composed of alveolar epithelium, fused basement membranes, and capillary endothelium.

• Thin barrier (0.1–1.5 μm) facilitates rapid diffusion of gases.

Diffusion and Partial Pressure of Gases

Dalton's Law and Partial Pressures

Dalton's law states that the total pressure exerted by a mixture of gases is equal to the sum of the pressures exerted by each individual gas.

• Equation: $P_{total} = P_1 + P_2 + P_3 + ... + P_n$

• Partial pressure of a gas: $P_{gas} = \%_{gas} \times P_{total}$

Composition of dry air:

• Nitrogen (N2): 79% ($P_{N_2} = 0.79 \times 760 = 600$ mm Hg)

• Oxygen (O2): 21% ($P_{O_2} = 0.21 \times 760 = 160$ mm Hg)

• Carbon dioxide (CO2): 0.03% ($P_{CO_2} = 0.0003 \times 760 = 0.23$ mm Hg)

Effect of humidity: Water vapor reduces the partial pressures of other gases in humid air.

Gas	Dry Air (mm Hg)	Humid Air (mm Hg)
O2	160	150
CO2	0.23	0.21
Water vapor	0	47

Solubility of Gases in Liquids

Gas molecules can exist in the gas phase or dissolved in a liquid. The amount of gas dissolved is proportional to:

• Pressure gradient of the gas

• Solubility of the gas

• Temperature (relatively constant in humans)

CO2 is about 20 times more soluble in plasma than O2.

Gas Exchange in the Lungs and Tissues

Exchange of Oxygen and Carbon Dioxide

Gas exchange occurs by diffusion down partial pressure gradients:

• O2 diffuses from alveoli (high PO2) to blood (low PO2).

• CO2 diffuses from blood (high PCO2) to alveoli (low PCO2).

Alveolar gas pressures differ from atmospheric pressures due to mixing with dead space air and humidification.

Gas Exchange in Respiring Tissue

• PO2 in cells ≤ 40 mm Hg; PO2 in systemic arteries = 100 mm Hg → O2 diffuses from blood to cells.

• PCO2 in cells ≥ 46 mm Hg; PCO2 in systemic arteries = 40 mm Hg → CO2 diffuses from cells to blood.

Mixed venous blood: Reflects the average O2 and CO2 content returning to the right heart. Values vary with tissue metabolism.

• PO2 = 40 mm Hg

• PCO2 = 46 mm Hg

	Arterial	Venous
PO2	95 mm Hg (85–100)	40 mm Hg
PCO2	40 mm Hg (35–45)	46 mm Hg
pH	7.4 (7.38–7.42)	7.37

Determinants of Alveolar Partial Pressures

• PO2 and PCO2 of inspired air (affected by altitude)

• Alveolar ventilation (volume of fresh air reaching alveoli per minute)

• Rate and depth of breathing

• Airway resistance

• Lung compliance

• Gas diffusion between alveoli and blood (surface area, barrier thickness, diffusion distance, amount of fluid)

• Adequate perfusion of alveoli

Causes of Low Alveolar PO2

• Inadequate alveolar ventilation (hypoventilation):

  • Decreased lung compliance

  • Increased airway resistance

  • CNS depression (e.g., drug overdose)

Pathologies Affecting Gas Exchange

Condition	Effect on Gas Exchange
Normal Lung	Normal PO2
Emphysema	Destruction of alveoli reduces surface area → low PO2
Pulmonary Edema	Fluid in interstitial space increases diffusion distance → low PO2

Local Regulation of Ventilation and Perfusion

Matching Ventilation to Perfusion

Efficient gas exchange requires matching of alveolar ventilation (airflow) to perfusion (blood flow).

• Local control mechanisms adjust airflow and blood flow to optimize gas exchange.

• Decreased ventilation to an alveolus leads to increased PCO2 and decreased PO2 in that region.

• Increased PCO2 in bronchioles → bronchodilation (increases ventilation)

• Decreased PO2 in pulmonary arterioles → vasoconstriction (decreases perfusion)

Stimulus	Bronchioles	Pulmonary Arterioles
↑ PCO2	Dilate	No effect
↓ PO2	No effect	Constrict

Transport of Gases in the Blood

Oxygen Transport

Oxygen is transported in blood in two forms:

• Dissolved in plasma (very small amount, ~3 mL O2/L blood)

• Bound to hemoglobin (majority, ~197 mL O2/L blood)

Hemoglobin (Hb): Each molecule can bind up to four O2 molecules, forming oxyhemoglobin (HbO2).

• Equation: $Hb + O_2 \rightleftharpoons HbO_2$

• Binding is cooperative: binding of one O2 increases affinity for the next.

Hemoglobin Saturation and Dissociation Curve

Hemoglobin saturation refers to the percentage of binding sites occupied by O2:

• Arterial blood (PO2 = 100 mm Hg): ~98.5% saturated

• Venous blood (PO2 = 40 mm Hg): ~75% saturated

The hemoglobin-oxygen dissociation curve is sigmoidal due to cooperative binding.

Factors Affecting Hemoglobin-Oxygen Affinity

• Right shift (decreased affinity): More O2 unloading in tissues

  • Increased temperature

  • Decreased pH (Bohr effect)

  • Increased PCO2

  • Increased 2,3-BPG (not shown in detail)

• Left shift (increased affinity): Less O2 unloading

Effects of Temperature and pH

• Higher temperature (active tissues): right shift, more O2 unloading

• Lower pH (more acidic, active tissues): right shift, more O2 unloading (Bohr effect)

Effects of CO2 and Carbon Monoxide

• CO2 binds to hemoglobin, forming carbaminohemoglobin (HbCO2), which has lower affinity for O2.

• Increased CO2 in tissues promotes O2 unloading.

• Carbon monoxide (CO) binds hemoglobin with much higher affinity than O2, preventing O2 transport.

Carbon Dioxide Transport

• 7% dissolved in plasma

• 23% bound to hemoglobin (carbaminohemoglobin)

• 70% converted to bicarbonate (HCO3-) in erythrocytes by carbonic anhydrase

Equation:

$CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

Law of mass action: increased CO2 increases HCO3- and H+ production.

Neural Control of Breathing

Motor Neurons and Respiratory Muscles

• Respiratory muscles are skeletal muscles controlled by motor neurons.

• Inspiration: phrenic nerve (diaphragm), external intercostal nerve (external intercostals)

• Expiration: internal intercostal nerve (internal intercostals)

Respiratory Centers in the Brainstem

• Located in the medulla and pons

• Medulla: ventral respiratory group (VRG) and dorsal respiratory group (DRG)

• VRG: contains inspiratory and expiratory neurons

• DRG: primarily inspiratory neurons

Control of Ventilation by Chemoreceptors

Peripheral Chemoreceptors

• Located in carotid bodies near the carotid sinus

• Respond to decreases in arterial PO2, decreases in pH, or increases in PCO2

• Afferent signals sent to medullary respiratory centers

Central Chemoreceptors

• Located on the ventral surface of the medulla oblongata

• Respond to changes in pH of cerebrospinal fluid (CSF), which reflects CO2 levels

• Not directly responsive to O2

Summary Table: Effects of Arterial Blood Gases on Ventilation

Parameter	Normal Value	Effect of Deviation
PO2	85–100 mm Hg	↓ PO2 increases ventilation
PCO2	35–45 mm Hg	↑ PCO2 increases ventilation
pH	7.38–7.42	↓ pH (acidosis) increases ventilation

Example:

At high altitude, atmospheric pressure and PO2 decrease, leading to hypoxemia and increased ventilation via chemoreceptor activation.

Additional info: The above notes integrate textbook-style explanations, diagrams, and tables to provide a comprehensive overview of gas exchange and transport as covered in a college-level Anatomy & Physiology course.