Transport de l'Oxygène dans le Sang

Introduction

• The function of blood is to transport respiratory gases (O2 and CO2) between the lungs and tissues.
• This transport is mainly done by a protein called hemoglobin.
• In a liquid medium (like blood), gases exist in two forms: dissolved and combined.
• The dissolved portion of the gas is the only portion that directly participates in partial pressure.

Transport of Oxygen (O2) in the Blood

• General Overview: Oxygen transport is a collaborative process between the lungs and cardiovascular system to supply tissues with O2.

• Factors affecting tissue oxygen supply: Quantity of O2 inhaled, efficiency of gas exchange in the lungs, and blood flow to tissues.

• Two Forms of O2 Transport: Dissolved oxygen (a small portion) and oxygen bound to hemoglobin within erythrocytes. Reactions between these two forms are reversible, rapid, and dependent on the partial pressure of O2.

• Dissolved O2: The amount of dissolved O2 is directly proportional to the partial pressure of oxygen (PO2), as dictated by Henry's Law. In normal arterial blood, approximately 3 ml of oxygen are dissolved per liter of blood.

• Oxygen Bound to Hemoglobin: Hemoglobin in erythrocytes is responsible for most O2 transport (approximately 98.5%). Hemoglobin's ability to bind to O2 is determined by the partial pressure of oxygen (PO2).

Saturation of Hemoglobin with Oxygen

• Definition of Saturation: The percentage of hemoglobin molecules carrying oxygen. This is represented by the SaO2 or SO2.
• SO2: The level of SO2 in the blood is heavily influenced by the PO2.
• Effect of Different Factors on the Binding Curve: Various physiological factors (pH, temperature, and the 2,3-DPG concentration) affecting the oxygen-hemoglobin dissociation curve (sigmoidal curve).
• Normal values: At normal conditions, PO2 is 100 mm Hg, saturation is 97%. Normal venous blood has PO2 of 40 mm Hg and saturation 75%.

Factors Modifying the Oxygen-Hemoglobin Dissociation Curve

• Effect of PCO2 and H+ Concentration: Increased CO2 levels and hydrogen ion concentration shift the curve to the right, decreasing hemoglobin's affinity for oxygen (Bohr effect).

• This facilitates oxygen release to tissues with high metabolic needs.

• Effect of 2,3-DPG: Higher 2,3-DPG concentration shifts the curve to the right, promoting oxygen release.

• Effect of Temperature: Higher temperatures shift the curve to the right, increasing oxygen release to tissues.

Transport of Carbon Dioxide (CO2) in the Blood

• General Overview: The body continuously produces CO2 as a byproduct of cellular metabolism.
• Three Forms of CO2 Transport: Dissolved CO2, CO2 bound to hemoglobin (forming carbaminohemoglobin), and bicarbonate ions (HCO3-).
• Dissolved CO2: The amount of dissolved CO2 is proportional to its partial pressure (PCO2). CO2 is more soluble than the dissolved O2.
• CO2 bound to Hemoglobin (Carbaminohemoglobin): CO2 combines with hemoglobin to form carbaminohemoglobin.  This accounts for about 30% of total CO2.
• Bicarbonate Ions (HCO3-): The most significant form of CO2 transport, accounting for approximately 60% of total CO2. CO2 reacts with water in the presence of carbonic anhydrase to form carbonic acid. This acid then dissociates into bicarbonate and hydrogen ions.

Relation Between CO2 Content and Blood pH

• Overview: CO2 levels and blood pH are intricately connected.
• Blood Buffer Systems: Blood contains buffer systems, like the bicarbonate buffer system, to maintain a relatively stable pH, even as the body produces significant amounts of acid (such as CO2).
• pH, PCO2, and HCO3- interrelation: Changes in PCO2 significantly influence blood pH, playing a vital role in acid-base balance.
• Respiratory and Metabolic Imbalances: The body adjusts its respiratory rate to control CO2 levels and maintain proper blood pH. Kidneys also contribute to this regulation.

Acid-Base Imbalances

• Types of Imbalances: Acidosis (low pH) and alkalosis (high pH), classified into respiratory and metabolic types.
• Causes of Imbalances: Respiratory imbalances are often due to problems with breathing (e.g., hyperventilation). Metabolic imbalances result from metabolic processes which lead to increased or decreased acid or base production in the body.
• Compensation Mechanisms: The body employs compensation mechanisms (e.g., adjustments in respiration or kidney function).