Transport of Gases in Human Bodily Fluids

Transport of Gases in Human Bodily Fluids

Learning Objectives

  • By the end of this section, students will be able to:
    • Describe how oxygen is bound to hemoglobin and transported to body tissues.
    • Explain how carbon dioxide is transported from body tissues to the lungs.

Gas Exchange Process

  • Oxygen Diffusion: Once oxygen diffuses across the alveoli, it enters the bloodstream and is transported to tissues.
  • Carbon Dioxide Diffusion: Carbon dioxide diffuses out of the blood and into the alveoli to be expelled from the body.
  • Although gas exchange is a continuous process, oxygen and carbon dioxide are transported by different mechanisms.

Transport of Oxygen in the Blood

  • Dissolved Oxygen: Although oxygen dissolves in blood, only a small amount (1.5%) is transported this way.
  • Hemoglobin Binding: Most oxygen (98.5%) is bound to a protein called hemoglobin and carried to the tissues.
Hemoglobin (Hb)
  • Hemoglobin is a protein molecule found in red blood cells (erythrocytes).
  • Structure:
    • Composed of four subunits:
    • Two alpha subunits
    • Two beta subunits
    • Each subunit surrounds a central heme group containing iron.
    • Each heme group binds one oxygen molecule; thus, each hemoglobin molecule can bind four oxygen molecules.
  • Color Change: Molecules with more oxygen bound are brighter red, leading to the distinction:
    • Oxygenated arterial blood: Bright red (Hb carrying four oxygen molecules).
    • Deoxygenated venous blood: Darker red.

Oxygen Binding Dynamics

  • Shape Change: Binding of oxygen to hemoglobin induces a conformational change in the hemoglobin molecule.
  • The binding process is not equal:
    • It is easier for the second and third oxygen molecules to bind than the first.
    • The fourth oxygen molecule is more difficult to bind.
  • Oxygen Dissociation Curve: The relationship between partial pressure of oxygen and hemoglobin-oxygen saturation forms a sigmoidal or S-shaped curve.
    • Higher partial pressure leads to increased hemoglobin saturation.

Factors Affecting Oxygen Binding

  • The oxygen-carrying capacity of hemoglobin varies due to:
    • Carbon Dioxide Levels: An increase in carbon dioxide reacts with water to form bicarbonate (HCO3−HCO_3^-) and hydrogen ions (H+H^+).
    • Higher carbon dioxide levels lead to increased hydrogen ions, decreased pH, and reduced hemoglobin affinity for oxygen (shifts curve to the right).
    • Body Temperature: Increased temperature (as seen in active skeletal muscles) reduces hemoglobin affinity for oxygen.
    • Diseases: Conditions like sickle cell anemia and thalassemia decrease oxygen delivery capacity:
    • Sickle Cell Anemia: Crescent-shaped and stiff red blood cells reduce the ability to deliver oxygen.
    • Thalassemia: Defect in either alpha or beta subunit leading to lower hemoglobin levels despite higher quantities of red blood cells.

Transport of Carbon Dioxide in the Blood

  • Carbon Dioxide Transport Methods: Carbon dioxide is transported from body tissues to the lungs via three methods:
    1. Dissolution: 5 to 7% of carbon dioxide dissolves directly into plasma.
    2. Carbaminohemoglobin: 10% of carbon dioxide binds to hemoglobin forming carbaminohemoglobin; this binding is reversible.
    3. Bicarbonate Ion: 85% is transported as bicarbonate ions.
Bicarbonate Buffer System
  • Mechanism:
    • Carbon dioxide diffuses into red blood cells, where carbonic anhydrase (CA) converts it into carbonic acid:
      extCO2+extH2extO<br/>ightleftharpoonsextH2extCO3ext(carbonicacid)ext{CO}_2 + ext{H}_2 ext{O} <br /> ightleftharpoons ext{H}_2 ext{CO}_3 ext{ (carbonic acid)}
      extH2extCO3<br/>ightleftharpoonsextHCO3−+extH+ext(bicarbonate)ext{H}_2 ext{CO}_3 <br /> ightleftharpoons ext{HCO}_3^- + ext{H}^+ ext{ (bicarbonate)}
  • Importance: This reaction ensures continuous carbon dioxide uptake and limits shifts in blood pH as hemoglobin binds free hydrogen ions.
  • Chloride Shift: Bicarbonate ions leave the red blood cell in exchange for chloride ions (extCl−ext{Cl}^-).
  • On reaching the lungs, bicarbonate is converted back to carbon dioxide for exhalation.
pH Regulation and Altitude Adaptation
  • The bicarbonate buffer system is critical to maintain stable pH, which is vital for bodily functions; a small pH change can lead to severe injuries or death.
  • This system also supports adaptation to high altitudes.

Carbon Monoxide Poisoning

  • Affinity Differences: Carbon monoxide (CO) binds to hemoglobin with greater affinity than oxygen, inhibiting oxygen transport.
  • Symptoms of CO Poisoning: Headaches, confusion, nausea; prolonged exposure can cause brain damage or death.
  • Treatment: Administering 100 percent oxygen accelerates the separation of carbon monoxide from hemoglobin, aiding in recovery.