Oxygen and carbon dioxide transport across respiratory membranes

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.

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.

By comparison, the transport of CO2 seems to be more opportunistic. There are three saturation curves for PCO2, representing the three methods of CO2 transport. The curves for bicarbonate and amine-bound CO2 are hyperbolic, as was observed for O2 binding to myoglobin. In contrast, the dissolved CO2 has a linear dissociation curve. Importantly, the CO2 carrying capacity of blood is greater when partially deoxygenated. This is due to oxygen-bound hemoglobin inhibiting the binding of CO2 to amine residues in hemoglobin.

As the arterial blood passes through blood vessels in tissues, the partial pressure gradients again induce the transfer of O2 and CO2 across vascular and cellular membranes (Table 1). The blood PO2 is reduced to 5.3 kPa and PCO2 is increased to 6.1 kPa, thus converting arterial/oxygenated blood to dexoxygenated/venous blood.