Gas Exchange and Transport in the Respiratory System

Gas Exchange and Transport in the Respiratory System

Overview

This section covers the mechanisms and factors influencing gas exchange and transport in the human respiratory system. It explains how oxygen (O2) and carbon dioxide (CO2) are exchanged between the lungs and blood, and how these gases are transported to and from tissues to meet the body's metabolic needs.

Learning Objectives

• Identify the factors determining how much oxygen participates in gas exchange.

• Describe the process of gas exchange and the factors influencing it between alveoli and blood.

• Explain how O2 is transported in the blood and how this is regulated according to the body's needs.

• Describe how CO2 is transported in the blood and how transport mechanisms function at the lungs and tissues.

Summary of Gas Exchange and Transport

• Gas exchange occurs at two main sites:

  • Between alveoli and pulmonary capillaries (lungs)

  • Between systemic capillaries and tissue cells (body tissues)

• Gas transport in the blood occurs by:

  • Dissolved in plasma

  • Bound to haemoglobin in red blood cells

• O2 is delivered to cells for use in cellular respiration (energy production).

• CO2 is removed from cells as a waste product of metabolism.

• Gas exchange occurs down a partial pressure gradient (from high to low pressure).

Factors Affecting Oxygen Reaching the Alveoli

• Ventilation (airflow into and out of the lungs)

  • Airway resistance (diameter of airways)

  • Lung compliance and elasticity (ability to stretch and recoil)

  • Rate and depth of breathing

• Amount of oxygen in the air

  • Lower at high altitude

• Amount of water in the air

  • Water vapor 'dilutes' air, reducing the partial pressure of O2

• Solubility of oxygen in water

  • Oxygen must dissolve in water lining the alveoli before diffusing into the blood

Partial Pressures and Dalton's Law

The total pressure of a gas mixture is the sum of the partial pressures of each individual gas. This is described by Dalton's Law:

$P_{\text{total}} = P_{1} + P_{2} + P_{3} + \ldots$

• Atmospheric pressure at sea level: $P_{\text{atm}} = 760$ mm Hg

• Air contains approximately 21% O2

• Partial pressure of O2 at sea level:

  • $P_{O_2} = 760 \times 0.21 = 160$ mm Hg

• At higher altitudes, atmospheric pressure is lower, so $P_{O_2}$ is also lower.

The Effect of Water Vapor on Partial Pressures

• Water vapor in the air reduces the partial pressure of other gases.

• At 100% humidity and 37°C, water vapor pressure ($P_{H_2O}$) is 47 mm Hg.

• To calculate the partial pressure of O2 in humid air:

  • Subtract water vapor pressure from atmospheric pressure: $760 - 47 = 713$ mm Hg

  • Then, $P_{O_2} = 713 \times 0.21 = 150$ mm Hg

The Solubility of Oxygen in Water and Henry's Law

Henry's Law states that at a given temperature, the amount of a gas that dissolves in a liquid is proportional to its partial pressure and its solubility in that liquid:

$C = kP$

• Where $C$ is the concentration of dissolved gas, $k$ is the solubility constant, and $P$ is the partial pressure of the gas.

• CO2 is about 20 times more soluble in water than O2.

• CO2 dissolves and undergoes gas exchange more readily than O2.

Gas Exchange at the Lungs and Tissues

• Gases diffuse down their partial pressure gradients.

• In the lungs:

  • O2 diffuses from alveoli (100 mm Hg) to pulmonary capillaries (40 mm Hg).

  • CO2 diffuses from pulmonary capillaries (46 mm Hg) to alveoli (40 mm Hg).

• At the tissues:

  • O2 diffuses from systemic capillaries (100 mm Hg) to cells (40 mm Hg).

  • CO2 diffuses from cells (46 mm Hg) to systemic capillaries (40 mm Hg).

Factors Influencing Gas Transfer Between Alveoli and Blood

• Oxygen reaching the alveoli (ventilation, airway resistance, compliance, etc.)

• Structural characteristics of the alveoli:

  • Thin alveolar and capillary walls (short diffusion distance)

  • Large surface area for gas exchange

  • Alveoli lined with water (gases must dissolve before diffusing)

• Matching of alveolar ventilation to pulmonary blood flow (perfusion)

Gas Transport in the Blood

Oxygen Transport

• Most O2 (>98%) is transported bound to haemoglobin (Hb) in red blood cells.

• A small amount is dissolved in plasma.

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

• O2 is picked up in the lungs (forming oxyhaemoglobin, HbO2) and released at tissues (deoxyhaemoglobin, Hb).

Factors Determining Oxygen-Haemoglobin Binding

• Partial pressure of O2 (higher pressure = more binding)

• Amount of haemoglobin available (affected by anaemia, blood loss, etc.)

• Number of red blood cells

Oxygen-Haemoglobin Dissociation Curve

• Shows the relationship between O2 partial pressure and Hb saturation.

• At alveolar PO2 (~100 mm Hg), Hb is nearly 100% saturated (4 O2 per Hb).

• At tissue PO2 (~40 mm Hg), Hb is about 75% saturated (3 O2 per Hb).

• During exercise, tissue PO2 drops further, and Hb releases more O2.

Factors Shifting the Dissociation Curve

• Right shift (lower affinity, more O2 released):

  • Lower pH (higher H+)

  • Higher CO2

  • Higher temperature

  • Higher 2,3-bisphosphoglycerate (BPG)

• Left shift (higher affinity):

  • Foetal haemoglobin has higher affinity for O2 than adult Hb, allowing O2 transfer from mother to foetus.

Carbon Dioxide Transport

• CO2 is transported in three main forms:

  • Dissolved in plasma (7%)

  • Bound to haemoglobin (23%) as carbaminohaemoglobin (HbCO2)

  • As bicarbonate ions (HCO3-) (70%)

• CO2 + H2O → HCO3- + H+ (catalyzed by carbonic anhydrase)

• Bicarbonate formation is important for acid-base regulation; high CO2 can cause acidosis.

Summary Table: Forms of Gas Transport in Blood

Gas	Transport Form	Percentage
Oxygen (O2)	Bound to haemoglobin	>98%
Oxygen (O2)	Dissolved in plasma	<2%
Carbon dioxide (CO2)	Dissolved in plasma	7%
Carbon dioxide (CO2)	Bound to haemoglobin (carbaminohaemoglobin)	23%
Carbon dioxide (CO2)	As bicarbonate ions (HCO3-)	70%

Additional info: The above notes synthesize and expand upon the provided lecture slides, including definitions, equations, and physiological context for exam preparation.