L10 physiology

Diffusion and Transport of Gases

• Learning Objectives: List mechanisms of oxygen and carbon dioxide transport, describe the reaction between hemoglobin and oxygen, explain oxygen dissociation curves, describe the Bohr effect, describe chloride shift, explain carbon dioxide dissociation curves, and describe the Haldane effect.

Gas Exchange in Lungs & Tissues

• Exchange Mechanism: Exchange of gases between lungs and blood, and at tissue level, occurs passively via diffusion, determined by pressure gradients.
• Alveolar Membrane: High surface area (70 sq.m) and thinness (0.5µm) make it ideal for diffusion.
• Pressure Gradients: Gradients for O2 (100-40 mmHg) and CO2 (46-40 mmHg) drive gas movement. Specific pressure values are given for inspired air, trachea, venous blood, arterial blood, pulmonary capillaries, and tissue capillaries.

Gas Exchange in the Lungs

• PO2 and PCO2: PO2 in alveoli is 100 mmHg, in pulmonary capillaries is 40 mmHg. This drives oxygen into the capillaries. PCO2 in pulmonary capillaries is 46 mmHg.
• Average Arterial Blood Gases: PO2 is 100 mmHg, PCO2 is 40 mmHg.
• Gas Transfer Capacity: May be impaired by membrane thickening or reduced surface area.

Transport of O2 in the Blood

• Dissolved Form: 0.3 ml/100 ml of blood, establishes PO2 of blood, regulates breathing, and determines hemoglobin loading.
• Oxyhemoglobin: Each hemoglobin molecule carries four oxygen molecules. Oxygen carrying capacity of 1 gram of hemoglobin is 1.34 ml of O2, in 100 ml of blood is 20ml oxygen.

Bohr Effect in the Tissues

• Unloading Conditions: Increased PCO2, temperature, 2,3-DPG, and decreased pH enhance oxygen unloading from hemoglobin.
• Oxygen Release: Helps oxygen unload from hemoglobin, particularly in actively contracting muscles.
• Myoglobin: Skeletal muscle myoglobin binds oxygen at low PO2 and releases it at very low PO2

Oxygen Hemoglobin Dissociation Curve

• Sigmoid Curve: The curve shows the relationship between PO2 and hemoglobin saturation.
• Plateau Region: High PO2 values show nearly complete saturation.
• Steep Region: Curve is steep between 60 and 20 mmHg PO2, critical for oxygen delivery.
• P50 Value: Pressure at which Hb is 50% saturated (normal range 24-28 mmHg).
• Arterial and Venous Blood: Arterial PO2 is 98 mmHg, saturation 95%; venous PO2 is 40 mmHg, saturation 75%.

P50

• Pressure for 50% Saturation: Indicates the pressure at which hemoglobin is 50% saturated with oxygen. Shift to left or right on the curve impacts this value. Factors such as increased temperature, increased DPG, increased CO2, and decreased pH shift the curve to the right and decrease the P50 value. Conversely, decreased temperature, decreased DPG, decreased CO2, and increased pH shift the curve to the left, increasing P50.

CO2 Transport

• Mechanisms: CO2 is transported as dissolved CO2 (5%), bicarbonate (70%), and carbaminohemoglobin (25%).
• Bicarbonate: The main form, plays a crucial role in acid-base balance and pH maintenance.

Chloride Shift in Tissue Capillaries

• CO2 Uptake: CO2 enters red blood cells and combines with water, forming carbonic acid, which then dissociates into bicarbonate and hydrogen ions.
• Bicarbonate Movement: Bicarbonate ions exit the red blood cell and chloride ions enter to maintain electrical neutrality.
• Chloride Shift: This exchange process is called the chloride shift or Hamburger's shift.

O2 Dissociation Curve in Anemia and CO Poisoning

• Anemia: Oxygen content in the blood is reduced, but PO2 and hemoglobin saturation may remain normal. The curve remains similar, but the overall oxygen-carrying capacity is decreased.
• CO Poisoning: Carbon monoxide binds to hemoglobin much more strongly than oxygen. The carboxyhemoglobin cannot release oxygen, reducing oxygen delivery. Significant shift to the left, making oxygen dissociation difficult.

CO2 Transport - Summary

• Chloride Shift Reversal: In the lungs, the chloride shift reverses. Chloride ions leave red blood cells and bicarbonate ions enter, facilitating CO2 release. CO2 is then eliminated from the body.

CO2 Dissociation Curve

• Linear Curve: The CO2 dissociation curve follows a fairly linear pattern in both venous and arterial blood.
• Venous CO2: Venous blood has a PCO2 of 46 mmHg and a volume of 52%.
• Arterial CO2: Arterial blood has a PCO2 of 40 mmHg and a volume of 48%.

Haldane Effect

• Oxygen Effect: Oxygenation of hemoglobin leads to a rightward shift of CO2 dissociation curve, facilitating CO2 release from the blood into the alveoli.
• Pressure Gradient: The higher PO2 in the lungs creates a pressure gradient for O2 uptake and CO2 release.
• Chloride shift: Reverse chloride shift facilitates CO2 release.

Learning Resources

• Text Book: Marieb EN. Human Anatomy and Physiology, 9th Edition, Pearson International Edition; 2014. ISBN-13: 978-1-2920-2649-7. Chapter 22, pp. 892-897.
• PowerPoint: Presentation materials available on Moodle.

This quiz explores the mechanisms of oxygen and carbon dioxide transport in the body. It covers critical concepts like hemoglobin reactions, oxygen dissociation curves, and the roles of the Bohr effect and chloride shift in gas exchange. Test your understanding of the diffusion process across alveolar membranes and the resulting pressure gradients in the lungs.