Ch 18 A - Gas Exchange and Transport: Study Notes for Anatomy & Physiology

Gas Exchange and Transport

Overview of Chapter

This chapter explores the physiological mechanisms of gas exchange in the lungs and tissues, the transport of gases in the blood, and the regulation of ventilation. Understanding these processes is essential for comprehending how the body maintains homeostasis and responds to changes in oxygen and carbon dioxide levels.

• Gas exchange occurs in the lungs and tissues.

• Gas transport involves movement of oxygen and carbon dioxide in the blood.

• Regulation of ventilation ensures proper gas concentrations.

Sensors and Regulated Variables

Homeostatic Control of Blood Gases

To prevent hypoxia (low oxygen) and hypercapnia (high carbon dioxide), the body monitors three key variables:

• Oxygen (O2)

• Carbon dioxide (CO2)

• pH

Specialized sensors in the body detect changes in these variables and trigger compensatory responses.

Mechanisms of Gas Exchange

Pathways of Oxygen and Carbon Dioxide

Gas exchange occurs at the alveolar-capillary interface in the lungs and at the tissue-capillary interface in the systemic circulation.

• O2 enters alveoli from inspired air and diffuses into blood at the alveolar-capillary interface.

• O2 is transported in blood, either dissolved in plasma or bound to hemoglobin in red blood cells (RBCs).

• O2 diffuses into cells to support cellular respiration.

• CO2 produced by cells during metabolism diffuses out of cells into blood.

• CO2 is transported in blood, dissolved, bound to hemoglobin, or converted to bicarbonate.

• CO2 diffuses into alveoli and is exhaled.

Classification of Hypoxia

Types, Definitions, and Causes

Hypoxia refers to insufficient oxygen supply to tissues. It can be classified as follows:

Type	Definition	Typical Causes
Hypoxic hypoxia	Low arterial PO2	High altitude, alveolar hypoventilation, decreased lung diffusion capacity, abnormal ventilation-perfusion ratio
Anemic hypoxia	Decreased total amount of O2 bound to hemoglobin	Blood loss, anemia (low Hb or altered Hb), carbon monoxide poisoning
Ischemic hypoxia	Reduced blood flow	Heart failure, shock, thrombosis (hypoxia in a single organ)
Histotoxic hypoxia	Failure of cells to use O2 because cells have been poisoned	Cyanide and other metabolic poisons

Normal Blood Gas Values

Reference Ranges in Pulmonary Medicine

	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

Causes of Low Alveolar PO2

Factors Leading to Hypoxemia

• Low inspired oxygen content (e.g., high altitude)

• Inadequate alveolar ventilation (hypoventilation):

  • Decreased lung compliance

  • Increased airway resistance

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

Diffusion and Solubility

Determinants of Gas Exchange

Gas exchange across the alveolar membrane is influenced by:

• Surface area available for diffusion

• Barrier permeability (thickness and composition of alveolar-capillary membrane)

• Diffusion distance between alveoli and blood

• Concentration gradient (primary factor affecting gas exchange)

According to Fick's Law, the rate of diffusion is proportional to surface area and concentration gradient, and inversely proportional to diffusion distance.

Equation:

$ext{Rate of diffusion} \\ ext{(Fick's Law)} = rac{( ext{Surface Area}) imes ( ext{Concentration Gradient})}{( ext{Diffusion Distance})}$

Pathological Changes Affecting Gas Exchange

Diseases and Their Effects

• Emphysema: Decreased alveolar surface area due to destruction of alveoli.

• Fibrotic lung diseases: Increased thickness of alveolar membrane, reducing gas exchange efficiency.

• Pulmonary edema: Increased diffusion distance between alveoli and blood due to fluid accumulation.

• Asthma: Increased airway resistance due to bronchoconstriction.

All these conditions can lead to hypoxia by impairing oxygen transfer to the blood.

Movement of Gases

Physical Principles

• Pressure gradient: Gases move from areas of high to low partial pressure.

• Solubility: The ability of a gas to dissolve in a liquid (e.g., blood).

• Temperature: Higher temperatures decrease gas solubility.

Oxygen and carbon dioxide follow these principles during exchange and transport.

Gas Solubility in Liquids

Oxygen and Carbon Dioxide Solubility

• Oxygen: Poorly soluble in plasma; most is transported bound to hemoglobin.

• Carbon dioxide: More soluble in plasma than oxygen; transported dissolved, bound to hemoglobin, or as bicarbonate.

At equilibrium, partial pressures of gases in air and water are equal, but concentrations depend on solubility.

Gas Transport in the Blood

Oxygen Transport

• Fick equation: Oxygen consumption ($Q_{O_2}$) is calculated as: $Q_{O_2} = CO imes ( ext{Arterial } [O_2] - ext{Venous } [O_2])$ where CO = cardiac output.

• Hemoglobin (Hb): Binds 98% of oxygen, forming oxyhemoglobin (HbO2).

• Cooperative binding: Oxygen binding to Hb increases Hb's affinity for more oxygen.

• PO2: Determines the extent of oxygen binding to Hb.

Bohr Effect and Chronic Hypoxia

• Bohr effect: A decrease in pH (increase in H+) shifts the hemoglobin saturation curve to the right, reducing Hb's affinity for oxygen.

• Chronic hypoxia: Prolonged low oxygen increases RBC production of 2,3-diphosphoglycerate (2,3-DPG), which also decreases Hb's affinity for oxygen.

Factors Affecting Oxygen-Hemoglobin Binding

Determinants of Oxygen Content

• Plasma O2: Determines % saturation of Hb.

• Amount of hemoglobin: Determines total number of Hb binding sites (calculated from Hb content per RBC × number of RBCs).

• Other factors: pH, temperature, PCO2, and 2,3-DPG.

Oxyhemoglobin Dissociation Curve

The curve shows the relationship between PO2 and hemoglobin saturation. It is sigmoidal due to cooperative binding.

• Effect of pH: Lower pH shifts the curve right (Bohr effect).

• Effect of temperature: Higher temperature shifts the curve right.

• Effect of PCO2: Higher PCO2 shifts the curve right.

Carbon Dioxide Transport

Forms of CO2 in Blood

• Dissolved in plasma: 7%

• Converted to bicarbonate ion (HCO3-): 70% (via carbonic anhydrase and chloride shift)

• Bound to hemoglobin: 23% (as carbaminohemoglobin)

• Hemoglobin also binds H+ produced during CO2 conversion.

Key reactions:

$CO_2 + H_2O \xrightarrow{\text{carbonic anhydrase}} H_2CO_3 \rightarrow HCO_3^- + H^+$

The chloride shift exchanges HCO3- for Cl- across the RBC membrane.

Summary of O2 and CO2 Exchange and Transport

Integrated View

• Oxygen: Transported mainly bound to hemoglobin; small amount dissolved in plasma.

• Carbon dioxide: Transported as bicarbonate, bound to hemoglobin, and dissolved in plasma.

• Exchange: Driven by partial pressure gradients and facilitated by physiological adaptations.

Additional info:

• Animations referenced in the slides can be supplemented with textbook diagrams and interactive resources for deeper understanding.

• Clinical relevance: Disorders such as COPD, asthma, and pulmonary edema directly impact gas exchange and transport mechanisms.