O2 & CO2 Transport Vocabulary

Gas Transport: Refers to the movement of gases, primarily oxygen (O₂) and carbon dioxide (CO₂), between the alveoli in the lungs and systemic tissues through diffusion and convection. This process is essential for maintaining cellular respiration and metabolic homeostasis throughout the body.

Oxygen Transport:

• Approximately 98.5% of oxygen is transported bound to hemoglobin in red blood cells (RBCs), forming oxyhemoglobin, which enhances the oxygen-carrying capacity of the blood.

• The remaining 1.5% is dissolved in plasma, playing a minor role compared to bound oxygen.

• Factors influencing oxygen transport include the partial pressure of oxygen (PO₂), pH, temperature, and the presence of certain metabolites.

Carbon Dioxide Transport:

• About 90% of carbon dioxide is converted into carbonic acid (H₂CO₃) through a reaction with water, facilitated by the enzyme carbonic anhydrase, where it dissociates into bicarbonate ions (HCO₃⁻) and hydrogen ions (H⁺).

• Approximately 5% of CO₂ binds to proteins in the blood, forming carbamino compounds with hemoglobin, while another 5% is transported in dissolved form in plasma.

• Notably, 75% of carbon dioxide is carried within red blood cells, whereas 25% exists in plasma, highlighting the predominance of intracellular processing in gas transport.

Hemoglobin Structure and Function:

• Hemoglobin (Hb) is a red pigment found in RBCs composed of four polypeptide chains (two alpha and two beta chains) and four heme groups, each containing an iron atom that binds one oxygen molecule, forming oxyhemoglobin.

• Hemoglobin exhibits cooperative binding, meaning that the binding of one oxygen molecule increases the affinity for additional oxygen molecules, optimizing oxygen uptake in the lungs and release in tissues.

• When oxygen is released from hemoglobin, it is referred to as deoxyhemoglobin, which can also bind hydrogen ions and carbon dioxide, facilitating CO₂ transport back to the lungs.

Hemoglobin Concentration:

• The oxygen-carrying capacity of blood is significantly dependent on hemoglobin concentration; levels can vary with physiological conditions.

• Anemia indicates low hemoglobin concentration, leading to reduced oxygen transport capacity.

• Polycythemia refers to an elevated RBC count, consequently resulting in increased hemoglobin concentrations, which may occur in response to chronic hypoxia or other stimuli.

• Erythropoietin is a hormone produced by kidneys in response to tissue hypoxia, stimulating increased production of hemoglobin and RBCs in the bone marrow, thus enhancing oxygen delivery.

Oxygen Saturation and Transport:

• Percent Oxy-Hemoglobin Saturation is a crucial measurement, representing the ratio of oxyhemoglobin to total hemoglobin in blood; normal resting arterial Oxy-Hb concentration is around 97% (range 96-99%). Values below 90% can indicate tissue hypoxia, prompting clinical attention.

• Oxygen Association and Dissociation processes are influenced by the partial pressure of oxygen (PO₂) and the affinity between hemoglobin and O₂. In the lungs, high PO₂ promotes oxygen loading (association), while in body tissues, low PO₂ promotes oxygen unloading (dissociation).

Oxygen-Hemoglobin Dissociation Curve:

• The curve is sigmoidal (S-shaped), indicating that at high PO₂ levels, small changes in PO₂ result in minimal variations in oxygen binding, ensuring robust oxygen delivery.

• The steep region of the curve signifies that small decreases in PO₂ can lead to substantial reductions in hemoglobin saturation, highlighting the critical response of the body during hypoxic conditions.

• Factors influencing hemoglobin's affinity for oxygen include:

  • Decreased Affinity (shifting to the right) can occur due to increased hydrogen ions (lower pH), elevated CO₂ levels, increased temperature, and elevated levels of 2,3-BPG (a byproduct of glucose metabolism in RBCs). These conditions facilitate enhanced oxygen delivery to actively respiring tissues.

Carbon Dioxide Transport:

• CO₂ is transported in three main forms:

  1. Dissolved in plasma (7-10%), which is much more soluble than oxygen, contributing to the partial pressure of CO₂ in blood.

  2. As carbamino-hemoglobin, where CO₂ binds to hemoglobin (20-30%), influencing the Bohr Effect and altering hemoglobin's affinity for oxygen.

  3. As bicarbonate ions (HCO₃⁻) (60-70%), the predominant form of CO₂ transport.

• Bicarbonate is formed from the dissociation of carbonic acid and plays a crucial role in buffering blood pH and transporting CO₂ out of tissues.

• Bicarbonate ions move out of RBCs into plasma via an exchanger with chloride ions (the Chloride Shift), maintaining ionic balance during gas transport.

Significance of H⁺ and CO₂ Binding:

• The Bohr Effect describes the physiological phenomenon where the binding of H⁺ to hemoglobin decreases its affinity for O₂, facilitating O₂ release in tissues, particularly under conditions of elevated CO₂ and lower pH.

• This interaction also increases the blood's capacity to carry CO₂, promoting the transport of waste gases from tissues back to the lungs for exhalation.

• High partial pressure of oxygen in the lungs leads to a higher affinity for oxygen, favoring the production of oxyhemoglobin and supporting the dissociation of H⁺ ions, thereby converting bicarbonate back to carbonic acid.

Sickle Cell Disease:

• Sickle cell disease is caused by a recessive gene affecting 8-11% of the African American population.

• It results from a single nucleotide mutation in the hematopoietic stem cells, leading to the production of hemoglobin S (Hb-S). The presence of Hb-S causes red blood cells to become rigid and sickle-shaped at low PO₂ levels.

• These sickle-shaped RBCs can obstruct blood flow, leading to vaso-occlusive crises, hemolysis, and subsequent hemolytic anemia, significantly affecting the individual’s overall health and oxygen transport efficiency.

Myoglobin:

• Myoglobin is a red pigment found predominantly in striated muscle cells, particularly within slow-twitch aerobic skeletal muscle and cardiac muscle, which are designed for endurance activities.

• Unlike hemoglobin, myoglobin contains only one heme group and possesses a significantly higher affinity for O₂, allowing it to effectively capture and store oxygen at very low PO₂ levels.

• This characteristic enables myoglobin to function as a vital shuttle for oxygen from the blood to the mitochondria within muscle cells, supporting aerobic respiration during prolonged physical activities.

Summary of Gas Exchange at Tissues and Lungs:

• At Tissues: Carbon dioxide is produced as a metabolic waste product, diffusing into blood, while oxygen is utilized for cellular respiration and energy production. The resulting changes in partial pressures drive gas exchange.

• At Lungs (Alveoli): Oxygen is taken up from inhaled air, enriching blood oxygen levels, while carbon dioxide is expelled from the blood into the alveoli to be exhaled. This continual process maintains homeostasis in gas exchange, ensuring optimal oxygen delivery and carbon dioxide removal from the body.