Transport of oxygen and carbon dioxide in the blood

Oxygen transport

Oxygen is transported by the blood either combined with hemoglobin in the red blood cells(greater than 98%) or dissolved in the blood plasma(2%). Only about 3ml of oxygen are dissolved in each liter of plasma. Assuming a total plasma volume of 3 to 5L, only about 9 to 15ml of oxygen can be carried in the dissolved state. This limited amount of oxygen cannot adequately meet the needs of even resting body tissues, which generally require more than 250ml of oxygen per minute(depending on body size). However, hemoglobin, a protein contained within each of the body’s 4 to 6 billion red blood cells, allows the blood to transport nearly 70 times more oxygen than can be dissolved in plasma.

Hemoglobin saturation

As just noted, over 98% of oxygen is transported in the blood bound to hemoglobin. Each molecule of hemoglobin can carry four molecules of oxygen. When oxygen binds to hemoglobin, it forms oxyhemoglobin; hemoglobin that is not bound to oxygen is reffered to as deoxyhemoglobin. The binding of oxygen to hemoglobin depends on the PO₂ in the blood and the bonding strength, or affinity, between hemoglobin and oxygen. The curve in figure below is and oxygen-hemoglobin dissociation curve, which shows the amount of hemoglobin saturated with oxygen at different PO₂ values. The shape of the curve is extremely important for its function in the body. The relatively flat upper position means that, at high PO₂s such as in the lungs, large drops in PO₂ result in only small changes in hemoglobin saturation. This is called the “loading” portion of the curve. A high blood PO₂ results in almost complete hemoglobin saturation, which means that the maximal amount of oxygen is bound. But as the PO₂ decreases, so does hemoglobin saturation.

The steep portion of the curve coincides with PO₂ values typically found in the tissues of the body. Here, relatively small changes in PO₂ result in large changes in saturation. This is also advantageous because this is the “unloading” portion of the curve where hemoglobin loses its oxygen to the tissues.

Many factors determine the hemoglobin saturation. If, for example, the blood becomes more acidic, the dissociation curve shifts to the right. This indicates that more oxygen is being unloaded from the hemoglobin at the tissue level. This rightward shift of the curve(figure a), attributable to a decline in pH, is reffered to as the Bohr effect. The pH in the lungs is generally high, so hemoglobin passing through the lungs has a strong affinity for oxygen, encouraging high saturation. At the tissue level, especially during exercise, the pH is lower, causing oxygen to dissociate from hemoglobin, thereby supplying oxygen to the tissues. With exercise, the ability to unload oxygen to the muscles increases as the muscle pH decreases.

Blood temperature also affects oxygen dissociation. As shown in the figure b, increased blood temperature shifts the dissociation curve to the right, indicating that oxygen is unloaded from hemoglobin more readily at higher temperatures. Because of this, the hemoglobin unloads more oxygen when blood circulates through the metabolically heated active muscles.

Blood oxygen-carrying capacity

The oxygen-carrying capacity of blood is the maximal amount of oxygen the blood can transport. It depends primarily on the blood hemoglobin content. Each 100ml of blood contains an average of 14 to 18g or hemoglobin in men and 12 to 16g in women. Each gram of hemoglobin can combine with about 1.34ml of oxygen, so the oxygen-carrying capacity of blood is approximately 16 to 24ml per 100ml of blood when blood is fully saturated with oxygen. At rest, as the blood passes through the lungs, it is in contact with the alveolar air for approximately 0.75s. This is sufficient time for hemoglobin to become 98% to 99% saturated. At high intensities of exercise, the contact time is greatly reduced, which can reduce the binding of hemoglobin to oxygen and slightly decrease the saturation, although the unique S shape of the curve guards against large drops.

People with low hemoglobin concentrations, such as those with anemia, have reduced oxygen-carrying capacities. Depending on the severity of the condition, these people might feel few effects of anemia while they are at rest because their cardiovascular system can compensate for reduced blood oxygen content by increasing cardiac output. However, during activities in which oxygen delivery can become a limitation, such as highly intense aerobic effort, reduced blood oxygen content limits performance.

Carbon dioxide transport

Carbon dioxide also relies on the blood for transportation. Once carbon dioxide is released from the cells, it is carried in the blood primarily in three forms:

• As bicarbonate ions resulting from the dissociation of carbonic acid
• Dissolved in plasma
• Bound to hemoglobin(called carbaminohemoglobin).

Bicarbonate ion

The majority of carbon dioxide is carried in the form of bicarbonate ion. Bicarbonate accounts for the transport of 60% to 70% of the carbon dioxide in the blood. Carbon dioxide and water molecules combine to form carbonic acid(H₂CO₃). This reaction is catalyzed by the enzyme carbonic anhydrase, which is found in red blood cells. Carbonic acid is unstable and quickly dissociates, freeing a hydrogen ion(H⁺) and forming a bicarbonate ion(HCO3^-):

CO₂ + H₂O --> H₂CO₃ --> H⁺ + HCO3^-

The H⁺ subsequently binds to hemoglobin, and this binding triggers the Bohr effect, mentioned previously, which shifts the oxygen-hemoglobin dissociation curve to the right. The bicarbonate ion diffuses out of the red blood cell and into the plasma. In order to prevent electrical imbalance from the shift of the negatively charged bicarbonate ion into the plasma, a chloride ion diffuses from the plasma into the red blood cell. This is called the chloride shift.

Additionally, the formation of hydrogen ions through this reaction enhances oxygen unloading at the level of the tissue. Through this mechanism, hemoglobin acts as a buffer, binding and neutralizing the H+ and thus preventing any significant acidification of the blood.

When the blood enters the lungs, where the PCO₂ is lower, the H+ and bicarbonate ions rejoin to form carbonic acid, which then dissociates into carbon dioxide and water:

H⁺ + HCO3^- --> H₂CO₃ --> CO₂ + H₂O

The carbon dioxide that is thus re-formed can enter the alveoli and be exhaled.

Dissolved carbon dioxide

Part of the carbon dioxide released from the tissues is dissolved in plasma; but only a small amount, typically just 7% to 10%, is transported this way. This dissolved carbon dioxide comes out of solution where the PCO₂ is low, as in the lungs. There it diffuses from the pulmonary capillaries into the alveoli to be exhaled.

Carbaminohemoglobin

Carbon dioxide transport also can occur when the gas binds with hemoglobin, forming carbaminohemoglobin. The compound is so named because carbon dioxide binds with amino acids in the globin part of the hemoglobin molecule, rather than with the heme group as oxygen does. Because carbon dioxide binding occurs on a different part of the hemoglobin molecule than does oxygen binding, the two processes do not compete. However, carbon dioxide binding varies with the oxygenation of the hemoglobin(deoxyhemoglobin binds carbon dioxide more easily than oxyhemoglobin) and the partial pressure of CO₂. Carbon dioxide is released from hemoglobin when PCO₂ is low as it is in the lungs. Thus, carbon dioxide is readily released from the hemoglobin in the lungs, allowing it to enter the alveoli to be exhaled.