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, determined by the arterial partial pressure of oxygen (PO2) and the solubility coefficient of oxygen in plasma.

• Hemoglobin-Bound Oxygen: Typical carrying capacity with hemoglobin is about 20.1 mL O2 per 100 mL blood.

Equation:

Example: At normal arterial PO2, the majority of oxygen in blood is carried by hemoglobin, not dissolved in plasma.

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. For example, when CO2 is bound, hemoglobin's affinity for O2 decreases.

Example: The Bohr effect describes how increased CO2 and H+ (lower pH) reduce hemoglobin's oxygen affinity, facilitating oxygen release in tissues.

Oxygen-Hemoglobin Dissociation Curves

Oxygen Affinity and Carrying Capacity

The oxygen-hemoglobin dissociation curve illustrates the relationship between the partial pressure of oxygen (PO2) and the percent saturation of hemoglobin.

• Oxygen Affinity (P50): The PO2 at which hemoglobin is 50% saturated with oxygen. Lower P50 indicates higher affinity.

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

Example: At a PO2 of about 100 mm Hg (arterial blood), hemoglobin is nearly fully saturated; at 40 mm Hg (venous blood), saturation drops, allowing oxygen release to tissues.

Factors Affecting Oxygen-Hemoglobin Binding

Bohr Effect, Temperature, and Organic Compounds

Several physiological factors influence hemoglobin's affinity for oxygen, shifting the dissociation curve and affecting oxygen delivery.

• Bohr Effect: A decrease in pH (increase in H+) and an increase in CO2 lower hemoglobin's oxygen affinity, shifting the curve to the right.

• Equation:

• Temperature: Higher temperatures reduce hemoglobin's oxygen affinity, also shifting the curve to the right.

• 2,3-Diphosphoglyceric Acid (2,3-DPG): Increased levels of 2,3-DPG in red blood cells decrease hemoglobin's oxygen affinity, facilitating oxygen release in tissues.

Example: During exercise, increased CO2, temperature, and 2,3-DPG all promote oxygen unloading in active muscles.

Oxygen Equilibrium Curves: Table

The following table summarizes the effects of various factors on the oxygen-hemoglobin dissociation curve:

Long-Term Response to High Altitude

Physiological Adaptations

At high altitude, the body experiences lower atmospheric oxygen pressure, leading to several compensatory mechanisms to maintain oxygen delivery.

• Increased Ventilation: Breathing rate increases to enhance oxygen uptake.

• Increased Red Blood Cell Production: More hemoglobin is produced to increase oxygen carrying capacity.

• Increased 2,3-DPG Production: Shifts the dissociation curve to the right, promoting oxygen release to tissues.

Example: People living at high altitudes often have higher hematocrit and hemoglobin levels than those at sea level.

Additional info: The notes also mention upcoming topics such as diving mammals, which relate to adaptations in oxygen storage and use in different species.