Oxygen and Carbon Dioxide Respiration

Oxygen and Carbon Dioxide Respiration

Oxygen and Carbon Dioxide Blood Transport

Compare and Contrast Oxygen and Carbon Dioxide Respiration

Human aerobic respiration involves the oxidation of glucose to form carbon dioxide (CO2), or glycolysis, and the reduction of oxygen (O2) to water. This process produces the primary carrier of cellular energy, ATP (adenosine triphospate). Oxygen is therefore required to sustain life and carbon dioxide is a waste product that must be eliminated from the body to maintain the correct pH. This essay will review the physiology of gas exchange across the most important membranes during respiration in a normal healthy adult at sea level.

Alveolar Membrane

The partial pressures of O2 and CO2 in bronchial alveoli are 13.5 and 5.3 kPa, respectively (Table 1). Although the partial pressure of O2 in air is much higher at 21 kPa, the increased temperature inside the lungs, increased partial pressure of water vapor, and the mixing between the different gases during inhalation and exhalation reduce its effective pressure. By contrast, humans are essentially CO2 factories when we exhale a partial pressure of about 3.5 kPa into atmospheric air with a CO2 partial pressure near zero.

Table 1: Partial Pressures (kPa) by Anatomic Location

Membrane Boundary

PO2

PCO2

Alveolar Space

13.5

5.3

Alveolar Capillaries

13.3

5.3

Arterial Blood

12.5

5.3

Venous Blood

5.3

6.1

Tissue

< 5.3

> 6.1

Oxygen-depleted venous blood has a PO2 of about 5.3 kPa. At the interface between the thin alveolar bronchial and capillary membranes, the higher partial pressure of O2 inside the alveolar space leads to a rapid transfer of the gas across these membranes down a steep concentration gradient. Based on Henry's Law, the PO2 of the alveolar capillary blood as it exits the alveoli should be 13.5 kPa, but due to mixing with venous blood, the actual PO2 of arterial blood is about 12.5 kPa.

At the same time that oxygen is being transferred into alveolar capillaries, the metabolic waste product CO2 is being transferred out. Although the partial pressure gradient is much lower for this gas compared to that of oxygen, the much higher water solubility of CO2 compensates for the smaller partial pressure gradient.

Arterial Blood

The PO2 of arterial blood is about 12.5 kPa, which represents only about 3 ml/L of dissolved gas. This amount is insufficient to sustain human life. The vast majority of oxygen is transported inside erythrocytes by the hemoglobin protein, which consists of four subunits. Each subunit contains a heme or iron-porphyrin complex, which binds the oxygen molecule reversibly. Therefore, each hemoglobin molecule can bind four oxygen molecules. In a healthy adult, the concentration of hemoglobin in blood is 150 g/L. Importantly, 1.34 ml of O2 can bind 1 g of hemoglobin. Each liter of blood can therefore carry 3 ml of dissolved CO2 and 201 ml hemoglobin-bound O2. The hemoglobin bound O2 does not contribute to arterial PO2.

By contrast, CO2 is transported by the blood in three different forms. Since CO2 is very soluble in water, about 10% is transported as a dissolved gas. Another 60% is transported in the form of bicarbonate. Carbonic acid will also form, but at a ratio of 1:20 with bicarbonate. This represents the primary pH buffering system in the human body. Another 30% of CO2 will be carried by amine residues on hemoglobin and plasma proteins, with hemoglobin carrying the most CO2 by far.

Oxygen therefore requires a special mechanism to transport sufficient quantities to sustain human life, which is the hemoglobin protein resident within circulating erythrocytes. This mechanism includes cooperative binding of O2 to hemoglobin (Fig. 1). An unbound hemoglobin protein has a low affinity to oxygen, but the binding of oxygen to one heme group induces a conformational change that converts the other CO2 binding sites to high-affinity. This results in a sigmoidal O2 binding curve. By comparison, the myoglobin protein, which has only one heme group, produces a hyperbolic binding curve (Fig. 1, top panel).

Figure 1: Saturation/Dissociation Curves for O

2

and CO

2

. Top panel is O

2

binding to myoglobin. Middle panel is O

2

binding to hemoglobin. Bottom panel is CO

2

saturation curves for blood. Fully oxygenated blood in a healthy adult has 204 ml of O2 per liter. By comparison, venous blood, which is only 75% saturated, contains about 150 ml/L of O2. The hemoglobin molecules in venous blood would therefore be in a high-affinity conformation as they enter the alveolar capillaries (Fig. 1, middle panel) and could rapidly absorb O2.