Chapter 7. Transport of Oxygen and Carbon Dioxide in the Blood

The reader understands how oxygen and carbon dioxide are transported to and from the tissues in the blood.

• States the relationship between the partial pressure of oxygen in the blood and the amount of oxygen physically dissolved in the blood.
• Describes the chemical combination of oxygen with hemoglobin and the “oxyhemoglobin dissociation curve.”
• Defines hemoglobin saturation, the oxygen-carrying capacity, and the oxygen content of blood.
• States the physiologic consequences of the shape of the oxyhemoglobin dissociation curve.
• Lists the physiologic factors that can influence the oxyhemoglobin dissociation curve, and predicts their effects on oxygen transport by the blood.
• States the relationship between the partial pressure of carbon dioxide in the blood and the amount of carbon dioxide physically dissolved in the blood.
• Describes the transport of carbon dioxide as carbamino compounds with blood proteins.
• Explains how most of the carbon dioxide in the blood is transported as bicarbonate.
• Describes the carbon dioxide dissociation curve for whole blood.
• Explains the Bohr and Haldane effects.

The final step in the exchange of gases between the external environment and the tissues is the transport of oxygen and carbon dioxide to and from the lung by the blood. Oxygen is carried both physically dissolved in the blood and chemically combined to hemoglobin. Carbon dioxide is carried physically dissolved in the blood, chemically combined to blood proteins as carbamino compounds, and as bicarbonate.

Oxygen is transported both physically dissolved in blood and chemically combined to the hemoglobin in the erythrocytes. Much more oxygen is normally transported combined with hemoglobin than is physically dissolved in the blood. Without hemoglobin, the cardiovascular system could not supply sufficient oxygen to meet tissue demands.

Physically Dissolved

At a temperature of 37°C, 1 mL of plasma contains 0.00003 mL O2/mm Hg . This corresponds to Henry’s law, as discussed in Chapter 6. Whole blood contains a similar amount of dissolved oxygen per milliliter because oxygen dissolves in the fluid of the erythrocytes in about the same amount. Therefore, normal arterial blood with a of approximately 100 mm Hg contains only about 0.003 mL O2/mL of blood, or 0.3 mL O2/100 mL of blood. (Blood oxygen content is conventionally expressed in milliliters of oxygen per 100 mL of blood, or volumes percent.)

A few simple calculations can demonstrate that the oxygen physically dissolved in the blood is not sufficient to fulfill the body’s oxygen demand (at normal Fio2 and barometric pressure). The resting oxygen consumption of an adult is approximately 250 to 300 mL O2/min. If the tissues were able to remove the entire 0.3 mL O2/100 mL of blood flow they receive, the cardiac output would have to be about 83.3 L/min to meet the tissue demand for oxygen at rest:

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