Transport of Gases in the Blood

Overview

The transport of oxygen and carbon dioxide in the blood is a fundamental aspect of human physiology, ensuring that tissues receive adequate oxygen for metabolism and that carbon dioxide, a metabolic waste product, is efficiently removed.

Blood and the Red Blood Cell (RBC)

Structure and Formation of RBCs

• Red blood cells (RBCs), or erythrocytes, are formed in the bone marrow.

• Their formation is stimulated by erythropoietin, a hormone produced primarily by the kidneys in response to hypoxia (low oxygen levels).

• RBCs are specialized for gas transport due to their biconcave shape and lack of nucleus in mature form.

Key Blood Parameters (Males)

Hemoglobin

Structure and Function

• Hemoglobin is a protein in RBCs responsible for binding and transporting oxygen.

• Each hemoglobin molecule consists of four polypeptide chains (globins), each with a heme group containing an iron atom.

• In adults, hemoglobin is composed of two alpha and two beta chains. Fetal hemoglobin contains two alpha and two gamma chains, giving it a higher affinity for oxygen.

• Each iron atom in the heme group can bind one molecule of oxygen.

• Most of the body's iron is found in hemoglobin.

Oxygen-Carrying Capacity

• The average adult has about 15 grams of hemoglobin per 100 mL of blood.

• Each gram of hemoglobin can bind approximately 1.34 mL of oxygen; thus, 15 grams can bind about 20.1 mL O2 per 100 mL blood when fully saturated.

• This defines the oxygen-carrying capacity of blood.

Iron States in Hemoglobin

• Iron in hemoglobin is normally in the ferrous state (Fe2+), which binds oxygen.

• If iron is oxidized to the ferric state (Fe3+), as in methemoglobin, it cannot bind oxygen.

Hemoglobin and Oxygen Binding

• Hemoglobin binds oxygen reversibly, following the Law of Mass Action:

• The amount of oxygen bound depends on the partial pressure of oxygen (PO2).

• 98% of oxygen in blood is transported bound to hemoglobin; only about 2% is dissolved in plasma.

Special Forms of Hemoglobin

• Carboxyhemoglobin: Formed when carbon monoxide (CO) binds to hemoglobin with much higher affinity than oxygen, preventing oxygen transport and causing CO poisoning.

• Carbaminohemoglobin: Formed when carbon dioxide binds to hemoglobin (at a different site than oxygen).

• Myoglobin: An oxygen-binding protein found in muscle, with a higher affinity for oxygen than adult hemoglobin; facilitates oxygen transfer to mitochondria in muscle cells.

Oxyhemoglobin Dissociation Curve

Basic Features

• The oxyhemoglobin dissociation curve describes the relationship between the partial pressure of oxygen (PO2) and hemoglobin saturation with oxygen.

• The curve is sigmoidal (S-shaped) due to cooperative binding: binding of each O2 molecule increases the affinity for the next.

• At high PO2 (e.g., in the lungs), hemoglobin is nearly 100% saturated; at lower PO2 (e.g., in tissues), hemoglobin releases oxygen.

Factors Shifting the Curve

• Right shift (reduced affinity, more O2 released): increased temperature, increased PCO2, increased 2,3-DPG, decreased pH (Bohr effect).

• Left shift (increased affinity, less O2 released): decreased temperature, decreased PCO2, decreased 2,3-DPG, increased pH.

• Bohr effect: The decrease in hemoglobin's oxygen affinity due to increased CO2 and decreased pH, facilitating oxygen release in metabolically active tissues.

Comparison: Adult, Fetal Hemoglobin, and Myoglobin

• Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin, allowing efficient transfer of oxygen from maternal to fetal blood.

• Myoglobin has an even higher affinity for oxygen than both adult and fetal hemoglobin, supporting oxygen storage in muscle.

Carbon Dioxide Transport by the Blood

Mechanisms of CO2 Transport

• CO2 is transported in three main forms:

  • Dissolved in plasma (~7%)

  • As bicarbonate ion (HCO3-) (~70%)

  • As carbaminohemoglobin (~23%)

• Conversion of CO2 to HCO3- in RBCs is catalyzed by the enzyme carbonic anhydrase:

• The chloride shift refers to the exchange of HCO3- and Cl- across the RBC membrane to maintain electrical neutrality.

• Transport of CO2 follows the Law of Mass Action, with reactions reversible depending on local concentrations.

Summary Table: Oxygen and Carbon Dioxide Transport

Key Points to Consider

• How is oxygen carried by the blood? (Mainly bound to hemoglobin)

• How is carbon dioxide carried by the blood? (Mainly as bicarbonate ion, also as carbaminohemoglobin and dissolved CO2)

• What are the basic features of hemoglobin? (Structure, oxygen binding, iron states)

• What is the difference between ferrous and ferric iron? (Fe2+ binds O2, Fe3+ does not)

• What are the features of carbaminohemoglobin, carboxyhemoglobin, and myoglobin?

• What are the basic features of a dissociation curve? (Shape, factors shifting the curve)

• How does carbon monoxide affect the oxygen-dissociation curve? (Shifts left, reduces O2 delivery)

• What is the Bohr effect? (Effect of CO2 and pH on O2 affinity)

• How do temperature, PCO2, 2,3-DPG, and pH affect the curve?

• How do the oxygen-dissociation curves compare for adult and fetal hemoglobin?

Additional info: Some explanations and context have been expanded for clarity and completeness based on standard academic knowledge in physiology.