Oxygen Transport in the Body

Overview of Oxygen Carrying Capacity

Oxygen transport is a critical physiological process that ensures tissues receive adequate oxygen for cellular metabolism. The blood's oxygen carrying capacity is determined by both dissolved oxygen and oxygen bound to respiratory pigments, primarily hemoglobin.

• Oxygen Carrying Capacity: The total amount of oxygen that can be carried by the blood, including both dissolved oxygen and that bound to hemoglobin.

• Respiratory Pigments: Molecules that increase the oxygen-carrying capacity of blood by binding oxygen reversibly.

Respiratory Pigments

Types and Functions

Respiratory pigments are specialized proteins that bind oxygen, facilitating its transport in the blood or tissues. The main types in animals include hemoglobin, myoglobin, and hemocyanin.

• Hemoglobin (in RBCs):

  • Composed of four heme groups, each capable of binding one oxygen molecule.

  • Allows a ~70-fold increase in oxygen concentration in blood compared to dissolved oxygen alone.

• Myoglobin (in muscles):

  • Contains a single heme group.

  • Functions as an oxygen reserve in muscle tissue.

• Hemocyanin:

  • Contains copper instead of iron as the oxygen-binding element.

  • Found in some invertebrates, such as the horseshoe crab.

Example: The horseshoe crab uses hemocyanin, which gives its blood a blue color due to the presence of copper.

Why Do We Need Hemoglobin?

Oxygen Content and Carrying Capacity

Hemoglobin is essential because dissolved oxygen alone is insufficient to meet the metabolic demands of most animals, including humans.

• Dissolved Oxygen: Only about 0.3 mL O2 per 100 mL blood can be carried in dissolved form.

• Hemoglobin-Bound Oxygen: Increases the carrying capacity to approximately 20.1 mL O2 per 100 mL blood.

• Determining Factors:

  • Dissolved oxygen is determined by arterial partial pressure of oxygen () and the solubility coefficient of oxygen in plasma.

  • Hemoglobin-bound oxygen is determined by and the oxygen equilibrium curve of hemoglobin.

Equation:

Oxygen Transport by Respiratory Pigments

Hemoglobin Binding and Allosteric Modifiers

Hemoglobin can bind oxygen and other ligands, and its affinity for oxygen is modulated by various factors.

• Ligands: Hemoglobin can bind H+, CO2, and other molecules in addition to O2.

• Allosteric Modifiers: Molecules that bind to hemoglobin and alter its affinity for oxygen.

• Example: When CO2 is bound to hemoglobin, its affinity for O2 decreases (Bohr effect).

Oxygen-Hemoglobin Dissociation (Equilibrium) Curves

Understanding Oxygen Affinity and Carrying Capacity

The oxygen-hemoglobin dissociation curve describes the relationship between the partial pressure of oxygen () and the percentage saturation of hemoglobin.

• Oxygen Affinity (): The at which hemoglobin is 50% saturated with oxygen. Lower indicates higher affinity.

• Carrying Capacity: The maximum amount of oxygen that can be carried by the blood, dependent on hemoglobin content.

Example: A typical for human hemoglobin is about 26-27 mm Hg.

Factors Affecting Oxygen-Hemoglobin Affinity

Bohr Effect, Temperature, and Organic Compounds

Several physiological factors can shift the oxygen-hemoglobin dissociation curve, altering oxygen delivery to tissues.

• Bohr Effect: A decrease in pH (increase in H+) or an increase in CO2 reduces hemoglobin's affinity for oxygen, facilitating oxygen release in tissues.

  • Equation:

• Temperature: Increased temperature reduces hemoglobin's affinity for oxygen, promoting oxygen unloading in active tissues.

• 2,3-Diphosphoglyceric Acid (2,3-DPG): Produced by red blood cells, 2,3-DPG binds to hemoglobin and decreases its affinity for oxygen (increases ), enhancing oxygen delivery to tissues.

Example: During exercise or at high altitude, increased 2,3-DPG production helps release more oxygen to tissues.

Long-Term Response to High Altitude

Physiological Adaptations

At high altitude, the body undergoes several adaptations to compensate for reduced atmospheric oxygen pressure.

• Increased Red Blood Cell Production: Enhances oxygen-carrying capacity.

• Increased 2,3-DPG Levels: Promotes oxygen unloading in tissues.

• Ventilatory Adjustments: Increased breathing rate to improve oxygen uptake.

Example: People living at high altitudes often have higher hematocrit and hemoglobin concentrations.

Summary Table: Comparison of Respiratory Pigments