Oxygen Transport and Content

• The transport of oxygen between the lungs and the body's cells hinges on the functionality of both blood and heart.
• Oxygen travels in the blood in two primary forms: dissolved within the blood plasma and chemically attached to hemoglobin.

Oxygen Transport

• Total oxygen content is the sum of oxygen bound to hemoglobin and oxygen dissolved in plasma.
• Oxygen bound to hemoglobin can be calculated using the equation: 1.34 mL O2 X Hb.
• The result is then multiplied by the SaO2 to determine the actual amount of oxygen bound.
• Oxygen dissolved in plasma is determined by the equation: PaO2 X 0.003.

Total Oxygen Content

• CaO2, representing the oxygen content of arterial blood, reflects the overall oxygen-carrying capacity of arterial blood, with a normal range of 16-20%.
• The formula to calculate CaO2 is: (Hb × 1.34 × SaO2) + (PaO2 × 0.003).
• CvO2, representing the oxygen content of mixed venous blood, reflects the overall oxygen level in the blood returning to the right heart, with a normal value of 12-15 vol%.
• The formula for CvO2 is: (Hb x 1.34 x SvO2) + (PvO x 0.003). Factors like low Hb, decreased cardiac output, and increased metabolic rate can decrease CvO2.
• Pulmonary end-capillary blood oxygen content reflects the optimal oxygen-carrying capacity of the cardiopulmonary system.
• Low hemoglobin levels can significantly lower the end-capillary oxygen content, calculated as (Hb × 1.34) + (PAO2 × 0.003).

Oxyhemoglobin Dissociation Curve

• Factors that shift the oxygen dissociation curve include temperature, PCO2, 2,3-DPG, and pH.
• A shift to the left increases O2 affinity, while a shift to the right decreases affinity, promoting more unloading into the tissues, which requires higher pressure.
• A normal PaO2 of 60 torr results in plasma PO2 of approximately 60 torr, and Hb saturation of approximately 90%.
• The 40-50-60/70-80-90 PaO2/SaO2 rule facilitates estimation of blood gas levels using pulse oximetry.
• A left shift of the curve, due to a pH of 7.6, at a PAO2 of 60 torr results in a hemoglobin saturation of 95%.

Oxygen Delivery

• Total oxygen delivery, or the amount of oxygen delivered or transported to peripheral tissues, depends on the body's ability to oxygenate blood, hemoglobin concentration, and cardiac output.
• The calculation for total oxygen delivery (DO2) is: DO2 = QT x (CaO2 x 10).
• The normal oxygen content difference between arterial and venous blood is about 5 mL/dL.
• Approximately 25% of available oxygen is used for tissue metabolism, meaning hemoglobin returning to the lungs is normally about 75% saturated with oxygen.
• Arterial-venous oxygen content difference is normally 5 vol%, as calculated by C(a-v)O2 = CaO2 – CvO2, where CaO2 is typically 20 vol% and CvO2 is 15 vol%.

Oxygen Extraction Ratio

• Oxygen Extraction Ratio (O2ER) measures the portion of oxygen used in peripheral tissue versus the oxygen delivered.
• A normal hemoglobin saturation returning to the alveoli is approximately 75%.
• Factors that can increase O2ER: increased cardiac output and hypothermia.
• Factors that can decrease O2ER: decreased cardiac output and exercise
• O2ER is calculated as: (CaO2 – CvO2) / CaO2.

Pulmonary Shunting

• Pulmonary shunting is defined as the portion of cardiac output that bypasses the lungs, moving from the right to the left side of the heart without being exposed to alveolar oxygen (PAo2).
• Absolute shunts are also called true shunts being unresponsive to oxygen therapy.
• Anatomic shunt: blood flows from the right side of the heart to the left without contacting an alveolus for gas exchange; normally about 3% of cardiac output.
• Relative Shunt occurs when pulmonary capillary perfusion exceeds alveolar ventilation.
• The severity is classified where <10% is normal, 10-20% usually indicates intrapulmonary, 20-30% means significant intrapulmonary diseases and >30% is life threatening.

Hypoxemia and Hypoxia

• Normal PaO2 levels range from 80-100.
• Mild hypoxemia is classified as 60-79.
• Moderate hypoxemia is between 40-59.
• Severe hypoxemia is defined as less than 40.

Types of Hypoxia

• Hypoxic hypoxia is inadequate oxygen at the tissue cells caused by low arterial oxygen tension (PaO2).
• Anemic hypoxia: PaO2 is normal, but the oxygen-carrying capacity of hemoglobin is inadequate.
• Circulatory hypoxia (stagnant or hypoperfusion hypoxia): blood flow to tissue cells is inadequate, resulting in oxygen not meeting tissue needs.
• Histotoxic hypoxia is an impaired ability of tissue cells to metabolize oxygen.

Carbon Dioxide Transport

• Metabolizing tissue cells consume around 250 mL of oxygen and produce approximately 200 mL of carbon dioxide (CO2) each minute.
• Carbon dioxide is transported from tissue cells to the lungs via six ways: three in the plasma and three in the red blood cells.
• In plasma, CO2 travels as carbamino compound (bound to protein), bicarbonate, and dissolved CO2.
• Within red blood cells, CO2 travels as dissolved CO2, carbamino-Hb, and bicarbonate.
• The majority of CO2, about 63%, is transported in the form of HCO3

Cyanosis

• Cyanosis is the bluish or purplish discoloration of the skin and mucous membranes due to the presence of at least 5g/dL of reduced hemoglobin in the blood.