Lesson 113- Transport of gases

Cardiovascular and Respiratory Systems Overview

Systems: Focus on gas transport in the body.Presented by: Hector Zerpa, St. George's University School of Veterinary Medicine, Grenada, West Indies.

Learning Outcomes

By the end of this session, students will be able to:

  • Define oxygen capacity and its clinical implications.

  • Determine factors influencing the sigmoid shape of the oxygen-hemoglobin dissociation curve and how it aids physiological function.

  • Explain advantages of the sigmoid shape in enhancing oxygen delivery during varying physical states (at rest vs. during exercise).

  • Analyze shifts in the oxygen-hemoglobin dissociation curve (to the right and left) and their physiological significance in different conditions.

  • Describe detailed carbon dioxide transport mechanisms from tissue cells to alveoli and the associated physiological processes.

  • Explain the chemical reaction forming bicarbonate, the central role of carbonic anhydrase, and its importance in pH regulation.

  • Discuss how chloride facilitates binding hydrogen ions in erythrocytes and its implications for carbon dioxide transport.

  • Explain the significance of carbamino bonding in erythrocytes and how it relates to overall gas exchange efficiency.

  • Outline carbon dioxide transport mechanisms in blood, including binding and conversion processes.

  • Define the Bohr effect, Haldane effect, and Hamburger Shift with relevance in exercise physiology and respiration.

  • Introduce biological and clinically relevant examples of gas transport, including the effects of high altitude on animals and conditions leading to compromised gas transport, such as anemia and carbon monoxide poisoning.

Oxygen Transport in Blood

Oxygen Capacity

The transport of O2 occurs in two main ways:

  1. Dissolved in Plasma: This method has a very low solubility; approximately ~3 mL of O2 can be carried per 1 L of blood, which is insufficient to meet physiological needs.

  2. Bound to Hemoglobin in Erythrocytes: Hemoglobin can carry around ~197 mL of O2 per liter at a concentration of 15.0 g/dL hemoglobin, showcasing its efficiency.

  • O2 Capacity: This term defines the maximum O2 that can be bound to hemoglobin per liter of blood.

  • Effects of Anemia: Anemia leads to reduced hemoglobin concentration, significantly affecting O2 capacity. The clinical signs of anemia can include pallor, lethargy, decreased exercise tolerance, and potential hypoxia.

  • Note: Only the fraction of oxygen that is physically dissolved in plasma is accessible to cells for metabolic processes, not the oxygen bound to hemoglobin.

Hemoglobin and O2 Transport

Hemoglobin (Hb):

  • Present in erythrocytes, hemoglobin is a protein that carries oxygen crucial for cellular respiration.

  • It consists of four polypeptide subunits, each associated with a heme group containing an iron center, which is essential for oxygen binding.

  • The reaction between oxygen and hemoglobin has implications for medical practices such as pulse oximetry, where oxygenated Hb appears bright red, while deoxygenated forms appear bluish.

  • Cooperative Binding: The binding of the first O2 molecule increases the affinity of hemoglobin for subsequent O2 molecules, allowing for efficient oxygen release in tissues as needed.

Oxygen-Hemoglobin Dissociation Curve

This curve reflects a non-linear relationship due to the cooperative binding effect:

  • At Rest: Arterial blood with a partial pressure of oxygen (PO2) of approximately 95 mmHg has around 95% saturation. In contrast, venous blood shows a PO2 of 40 mmHg with a saturation of about 75%.

    • Consequently, about 20% of the O2 is released to tissues under resting conditions.

  • During Exercise: As oxygen demand increases, a decreasing PO2 from 40 to 20 mmHg can result in an additional 50% O2 release to match metabolic needs, illustrating hemoglobin's adaptability.

Shifts in the Oxygen-Hemoglobin Dissociation Curve

  • Shift to the Right:

    • This shift eases the release of O2 to tissues, significantly influenced by factors such as low pH (more acidic), elevated CO2 pressure, increased body temperature, and high levels of 2,3-bisphosphoglycerate (2,3-BPG), which is produced during glycolysis.

  • Shift to the Left:

    • This shift enhances O2 binding to hemoglobin, induced by higher pH (less acidic, due to CO2 diffusion) and lower temperatures, favoring oxygen loading in the lungs.

Consequences of Shifts in the Curve

  • A leftward shift increases O2 binding affinity but can impede O2 delivery to tissues, while a rightward shift boosts O2 release, essential during exercise or metabolic stress.

Other Heme Group Proteins

  • Myoglobin: Myoglobin possesses a higher affinity for O2 compared to hemoglobin, saturating at lower O2 levels, making it vital for oxygen storage and transport in muscle tissues. Its dissociation curve is logarithmic, reflecting its unique function.

Carbon Monoxide Poisoning

  • Carbon monoxide competes with oxygen for binding sites on hemoglobin, impeding oxygen delivery. Understanding this condition involves comparing binding curves under normal conditions versus CO poisoning, with P50 values indicating differences in oxygen saturation due to CO interference.

High Altitude Adaptations

  • Animals residing at high altitudes exhibit a left-shifted O2 dissociation curve, optimizing oxygenation in environments with reduced O2 availability. Adaptations may include increased red blood cell production or alterations in hemoglobin affinity.

Carbon Dioxide Transport in Blood

Carbon dioxide (CO2) is significantly more soluble than O2:

  • Transport mechanisms include:

    • 7% dissolved in plasma.

    • 23% bound to hemoglobin in the form of carbaminohemoglobin.

    • 70% transported as bicarbonate (HCO3-), playing a pivotal role in maintaining acid-base balance in the body.

Role of Carbonic Anhydrase

  • This enzyme accelerates the conversion of CO2 and H2O into carbonic acid (H2CO3), which subsequently dissociates into bicarbonate and H+, facilitating rapid CO2 transport from tissues to alveoli and helping in the regulation of blood pH.

Transport Mechanisms of CO2

  • CO2 diffuses out of metabolically active cells into erythrocytes, where it undergoes conversion to bicarbonate and H+. The buffering of H+ ions prevents significant pH decreases in the plasma, maintaining homeostasis.

  • Bicarbonate and Chloride Exchange (Hamburger Shift): As bicarbonate ions exit the erythrocytes into the plasma, chloride ions move into the erythrocytes, facilitating a delicate balance in ion exchange that supports CO2 transport.

The Haldane and Bohr Effects

  • Haldane Effect: The loading of O2 onto hemoglobin in the lungs promotes the release of CO2 and H+ ions, enhancing the efficiency of gas exchange.

  • Bohr Effect: Increased concentrations of CO2 and H+ in tissues lower hemoglobin's affinity for O2, promoting effective delivery of oxygen where it is needed most, particularly during heightened metabolic activity.

Summary and Next Steps

The review of learning objectives emphasizes a comprehensive understanding of gas transport, the intricate functions of hemoglobin, and the physiological impacts of various conditions affecting oxygen and carbon dioxide transport. The next session will focus on the regulation of respiratory ventilation and related physiological responses.