Oxygen (O2) and Carbon Dioxide (CO2) Transport

INTRODUCTION

• Tissues consume approximately 250 mL of oxygen per minute, referred to as VO2 (Oxygen Consumption).

• Oxygen is transported to tissues in the blood in two primary ways:

  1. Dissolved in Plasma

  • At body temperature (37 °C), 0.003 mL of oxygen will dissolve in 100 mL of blood for every 1 mmHg of partial pressure (P).

  • At a partial pressure of oxygen (PaO2) of 100 mmHg, only 0.3 mL of oxygen per 100 mL of blood will dissolve, which is insufficient to meet tissue oxygen demand.

  1. Bound to Hemoglobin (Hb)

  • The vast majority of oxygen is transported bound to hemoglobin (HbO2).

  • Oxygen (O2) moves freely through plasma in its gaseous state; however, only O2 dissolved in plasma contributes to the partial pressure of oxygen in the blood.

  • PaO2 is measured as the oxygen dissolved in plasma only.

• Calculating Oxygen Dissolved in Plasma:

  • Formula:
    \text{Oxygen Dissolved in Plasma} = \text{PaO2} \times 0.003

  • Example:

    • At PaO2 of 100 torr, the dissolved amount of oxygen is calculated as:
      100 \times 0.003 = 0.3 \text{ mL O2/dL blood}

UNITS OF MEASURE

• Vol% represents the volume of O2 in 100 mL (1 dL) of blood. Oxygen content can be expressed in several units:

  • Vol%

  • mL O2 / 100 mL

  • mL O2 / dL of blood

OXYGEN BOUND TO HEMOGLOBIN (HbO2)

• Only 0.3 mL of oxygen per dL of blood is insufficient to meet body tissue demands.

• The majority of oxygen in blood is carried bound to hemoglobin (as HbO2).

  • Oxyhemoglobin (HbO2): Combined hemoglobin with oxygen.

  • Deoxyhemoglobin (Hb): Reduced hemoglobin, devoid of oxygen.

• Normal hemoglobin concentration: approximately 12 to 15 g/dL.

• Each gram of Hb can carry 1.34 mL of oxygen.

  • Therefore, with a normal hemoglobin level (~15 g/dL), hemoglobin can carry roughly 20 mL of oxygen per 100 mL of blood.

OXYGEN CHEMICALLY BOUND TO HEMOGLOBIN

• Reaction:
  \text{Hb} + \text{O2} \rightleftharpoons \text{HbO2}

• Cooperativity in O2 binding explained through the interaction of hemoglobin molecules and how it influences their affinity for oxygen.

LOW HEMOGLOBIN AND ANEMIA

• A low hemoglobin level (anemia) decreases the blood's oxygen-carrying capacity.

  • Conceptualization: Hemoglobin is likened to a bus transporting oxygen; insufficient buses result in inadequate oxygen distribution.

• Treatment options for anemia include:

  • Iron supplements

  • EPOGEN (a drug to stimulate RBC production)

  • PRBC transfusion (packed red blood cells)

• Erythropoietin: A hormone that regulates erythropoiesis (red blood cell production).

OXYGEN BOUND TO HEMOGLOBIN CALCULATION

• Formula for oxygen bound to hemoglobin: \text{O2 bound to Hb} = 1.34 \times \text{Hb} \times \text{SaO2}

  • Example:

  • If Hb = 15 g/dL and SaO2 = 97%:

    • Calculation:
      1.34 \times 15 \times 0.97 = 19.5 \text{ mL O2/dL blood}

• Total Oxygen Content of Arterial Blood (CaO2):
  \text{CaO2} = (\text{Hb} \times 1.34 \times \text{SaO2}) + (\text{PaO2} \times 0.003)

• Total Content of Oxygen in Mixed Venous Blood (CvO2):
  \text{CvO2} = (\text{Hb} \times 1.34 \times \text{SvO2}) + (\text{PO2} \times 0.003)

• Oxygen Content in Pulmonary Capillary Blood (CcO2): \text{CcO2} = (\text{Hb} \times 1.34) + (\text{PAO2} \times 0.003)

  • Note: CcO2 is assumed to be 100% saturated since there’s no shunt before blood reaches systemic circulation.

OXYHEMOGLOBIN DISSOCIATION CURVE

• Illustrates hemoglobin's cooperative binding of oxygen: As PaO2 increases, more oxygen binds to hemoglobin, leading to a steep initial rise in the curve from 0 to 60 mmHg.

• Above 60 mmHg, the curve flattens, indicating that hemoglobin achieves 90% saturation, and further increases in PO2 have less impact on oxygen content.

HEMOGLOBIN'S AFFINITY FOR OXYGEN

• PO2 < 60 mmHg:

  • Hemoglobin has decreased affinity for oxygen (steep portion of the curve).

• PO2 > 60 mmHg:

  • Hemoglobin has increased affinity for oxygen (flat portion of the curve).

• 60-90 Rule:

  • A saturation of 90% correlates with a PO2 of 60 mmHg.

  • For instance,

    • PO2 50 mmHg correlates with SaO2 of 80%.

    • PO2 40 mmHg correlates with SaO2 of 70%.

SHIFTING OF OXYHEMOGLOBIN DISSOCIATION CURVE

• Influenced by various factors that can cause either left or right shifts in the curve.

  • Left Shift:

  • Characterized by increased hemoglobin’s affinity for O2, resulting in a higher SaO2% for a given PO2.

  • Right Shift:

  • Characterized by decreased hemoglobin’s affinity for O2, resulting in a lower SaO2% for a given PO2.

BOHR EFFECT

• Describes the decreased affinity of hemoglobin for O2 due to high PCO2 (rightward shift).

• CO2 interacts with hemoglobin producing carbaminohemoglobin (HbCO2).

• Haldane Effect:

• Signifies that decreased oxygen increases hemoglobin's affinity for CO2, particularly at the tissue level where oxygen is released and CO2 is collected by hemoglobin.

A-V OXYGEN CONTENT DIFFERENCE

• Reflects the amount of oxygen in arterial blood versus oxygen remaining in mixed venous blood post-oxygen extraction by tissues.

• Normal A-V Content Difference: 5 mL O2/dL of blood.

OXYGEN EXTRACTION RATIO

• Represents the percentage of oxygen extracted by tissues from the blood.

• TOTAL OXYGEN DELIVERY (DO2):

• Refers to the total amount of oxygen delivered to tissues, typically around 1000 mL of oxygen per minute.

DO2 & VO2

• Evaluation of SUPPLY and DEMAND:

  • Normal DO2: approximately 1000 mL/min.

  • Normal VO2 (Oxygen Consumption): 250 mL of O2/min.

  • VO2 usually exceeds tissue oxygen demand under normal circumstances.

• Factors Increasing VO2:

  • Exercise

  • Seizures

  • Shivering

  • Hyperthermia

• Factors Decreasing VO2:

  • Skeletal Muscle Relaxation

  • Hypothermia

NORMAL ADULT VALUES

MIXED VENOUS OXYGEN SATURATION (SvO2)

• Monitoring SvO2 is crucial, as it does not require calculations.

• Importance:

  • Tissues need adequate oxygen for energy production.

  • When oxygen fails to meet demand:

  1. Cardiac output increases (often through heart rate) to boost O2 delivery.

  2. Tissues extract more oxygen from arterial blood, resulting in decreased SvO2.

  3. Anaerobic metabolism begins as a last resort: results in lactic acidosis due to waste generation.

• SvO2 monitoring can detect oxygenation issues before they lead to anaerobic metabolism.

HYPOXEMIA VS. HYPOXIA

• Hypoxemia: Low levels of oxygen in blood (denoted by low PaO2 or SpO2).

  • Classification based on PaO2:

  • 80 – 100 mmHg: Normal Oxygenation

  • 60 – 79 mmHg: Mild Hypoxemia

  • 40 – 59 mmHg: Moderate Hypoxemia (Administer oxygen!)

  • < 40 mmHg: Severe Hypoxemia

• Hypoxia: Low oxygen levels at the tissue level.

  • Types of Tissue Hypoxia:

  1. Hypoxemic Hypoxia (Hypoxic Hypoxia):

     • Inadequate O2 in the blood due to hypoxemia (e.g., low PaO2).

  2. Anemic Hypoxia:

     • Reduced oxygen-carrying ability (e.g., anemia, CO poisoning).

  3. Circulatory Hypoxia:

     • Poor tissue perfusion (reduced blood flow) (e.g., decreased cardiac output).

  4. Histotoxic Hypoxia:

     • Poison inhibits tissue utilization of oxygen (e.g., cyanide poisoning).

CYANOSIS

• Occurs when deoxygenated hemoglobin (deoxyhemoglobin) releases oxygen, drastically changing shape and color, resulting in a deep purple hue.

• In severely hypoxemic patients, sufficient deoxygenated hemoglobin can cause bluish skin, lips, and mucous membranes, termed cyanosis.

  • Significant observation threshold: Average desaturated Hb concentration of 5 g/dL needed for visible cyanosis.

• Key Points:

  • An arterial hemoglobin saturation of about 83% produces ~5 g/dL of desaturated Hb, causing cyanosis.

  • Cyanosis can occur without hypoxemia: High Hb concentrations (polycythemia) can yield visual cyanosis without low oxygen content, whereas severe anemia can display hypoxemia without cyanosis due to insufficient total Hb concentration.

  • Carbon monoxide (CO) inhalation provides another instance of hypoxemia without cyanosis, as CO binds to hemoglobin, blocking oxygen binding sites, producing bright red blood.

CARBON DIOXIDE (CO2) TRANSPORT

• Understanding CO2 transport contributes to pulmonary physiology and blood gas interpretation.

• CO2 is produced by aerobic metabolism and is carried from tissues to the lungs for exhalation, aiding in acid-base balance.

• At rest, metabolizing tissue cells produce about 200 mL of CO2 (VCO2) while consuming 250 mL of O2 (VO2) each minute.

• Lungs typically eliminate CO2 at the same rate of production to maintain an average arterial CO2 of 40 mmHg.

CARBON DIOXIDE TRANSPORT MECHANISMS

• Carbon dioxide (CO2) is transported to lungs through three methods:

  1. Dissolved in Solution:

  • CO2 dissolved in plasma and inside RBCs; contributes to the partial pressure of CO2 (PaCO2).

  1. Bound to Hemoglobin (carbaminohemoglobin - HbCO2):

  • CO2 binds to hemoglobin, forming Carbamino-Hb, releasing O2 for tissue metabolism.

  1. As Bicarbonate (HCO3-):

  • The majority of CO2 is converted and transported as bicarbonate in blood.

DISSOLVED CARBON DIOXIDE

• Amount of dissolved CO2 in plasma impacts CO2 levels, exists in two forms:

  • Dissolved in plasma (measured as PaCO2).

  • Dissolved in the intracellular fluid of RBCs, which aids in O2 unloading.

CARBAMINO COMPOUNDS AND CARBAMINO-HB

• Carbamino Compounds in Plasma:

  • CO2 combines with plasma proteins, termed as carbamino compounds.

  • Within RBCs, CO2 binds to HbO2 to form Carbamino-Hb (HbCO2).

CO2 CONVERSION TO BICARBONATE IN PLASMA AND RBC

• CO2 combines with water to form carbonic acid (H2CO3), ionizing into HCO3- and H+.

• The process occurs slowly in plasma, whereas carbonic anhydrase rapidly catalyzes this reaction in RBCs, accelerating HCO3- formation.

• The majority of CO2 is transported as bicarbonate (HCO3-) in blood.

RECYCLE CO2 AT LUNGS

- When blood reaches the lungs, bicarbonate (HCO3-) is reconverted to CO2, enabling diffusion into the alveoli for exhalation.