Gas Exchange and Transport of Oxygen and Carbon Dioxide

Overview of Gas Exchange in Blood

Respiratory Tract Structure

  • Oxygen and carbon dioxide are exchanged in the blood primarily at the alveolus.
  • The process begins in the respiratory tract:
    • Nasal cavity → Trachea → Bronchi → Alveoli

Oxygen Transport in Blood

From Alveolus to Capillary

  • Exchange of oxygen occurs from alveolus into capillary.
  • Red blood cells (RBCs) in capillaries are central to oxygen transport.
    • Each RBC contains hemoglobin.

Hemoglobin Composition

  • Hemoglobin consists of:
    • Globin: a protein component.
    • Heme: a pigment that contains iron.
  • Each RBC has about 250,000,000 hemoglobins. An individual hemoglobin can carry up to four oxygen molecules.
  • When oxygen binds to iron in heme, it forms oxyhemoglobin.
  • Around 98.5% of oxygen enters RBCs, while 1.5% dissolves in plasma.

Transport to the Heart

  • Oxygen-rich blood travels to the left side of the heart:
    • Left atrium → Left ventricle → Aorta → Body tissues.

Delivery of Oxygen to Tissues

Process of Delivery

  • Oxygen is delivered to tissues through capillaries:
    • Separation of oxygen from hemoglobin occurs at the tissue level.
    • Each hemoglobin typically releases one oxygen molecule.
    • With many RBCs, the total oxygen delivery to tissues is substantial.

Oxygen Utilization

  • Cells use oxygen, producing carbon dioxide (CO₂) as a by-product.

Carbon Dioxide Transport in Blood

Three Mechanisms of Transport

  1. Dissolved in Plasma:
    • About 10% of CO₂ simply dissolves in the plasma.
  2. Attached to Hemoglobin:
    • CO₂ binds to the globin part of hemoglobin, forming carboaminohemoglobin.
  3. As Bicarbonate:
    • CO₂ reacts with water in red blood cells to form carbonic acid, which dissociates into bicarbonate ($HCO_3^-$) and hydrogen ions ($H^+$).
    • The hydrogen ions can bind to hemoglobin forming deoxyhemoglobin.
    • Bicarbonate is transported out of the cell in exchange for chloride ions (chloride shift).

Returning CO₂ to the Lungs

Process of Gas Exchange in the Lungs

  • Blood returns to the right side of the heart:
    • Right atrium → Right ventricle → Pulmonary trunk → Lungs.
  • In the alveoli, CO₂ must be converted back to a gaseous form for exhalation.

Reversing the Transport Mechanisms

  1. Dissolved CO₂:
    • Directly exhaled.
  2. Carboaminohemoglobin:
    • Separates into hemoglobin and CO₂, which is exhaled.
  3. Bicarbonate Conversion:
    • Bicarbonate re-enters the RBC, combining with hydrogen ions to form carbonic acid.
    • Carbonic acid converts back into water and CO₂, which is then exhaled.

Summary of Carbon Dioxide Transport

  • 70% of CO₂ is carried as bicarbonate, 20% as carboaminohemoglobin, and 10% dissolved in plasma.
  • Efficient recycling of components allows CO₂ to be transported back to the lungs effectively for exhalation.