Module 2 - Exchange and Transport

CO2 is released from respiring tissues. It must be removed from these tissues and transported to the lungs. CO2 in the blood is transported in three ways:

• About 5% is dissolved directly in the plasma.
• About 10% is combined directly with haemoglobin to form a compound called carbaminohaemoglobin.
• About 85% is transported in the form of hydrogencarbonate ions (HCO3-).

As carbon dioxide diffuses into the blood, some of it enters the red blood cells. It combines with water to form a weak acid called carbonic acid. This is catalysed by the enzyme carbonic anhydrase.

CO2 + H2O --- H2CO3

This carbonic acid dissociates to release hydrogen ions and hydrogencarbonate ions.

H2CO3 --- HCO3- + H+

The hydrogencarbonatte ions diffuse out of the red blood cell into the plasma. The charge inside the red blood cell is maintained by the movement of chloride ions (Cl-) from the plasma into the red blood cell. This is called the chloride shift.

The hydrogen ions could cause the contents of the red blood cell to become very acidic. To prevent this, the hydrogen ions are taken up by haemoglobin to produce haemoglobinic acid. The haemoglobin is acting as a buffer (a compound that can maintain a constant pH)

Dissociation curves show how efficient haemoglobin is at absorbing oxygen in the lungs and delivering oxygen to tissues. Samples of blood are exposed to different mixtures of oxygen and nitrogen, and shaken to ensure that haemoglobin absorbs as much oxygen as possible. Partial pressure (measured in kilopascals, kPa) is the pressure exerted by oxygen in this mixture. The percentage saturation is calculated as the percentage of the maximum quantity of oxygen that haemoglobin absorbs.

Lungs (at sea level) -

• PP of oxygen/KPa - 13.0
• Saturation of haemoglobin with oxygen/% - 98

Maternal blood in placenta -

• PP of oxygen/KPa - 4.0
• Saturation of haemoglobin with oxygen/% - 60

Fetal blood in placenta -

• PP of oxygen/KPa - 4.0
• Saturation of haemoglobin with oxygen/% - 80

• Haemoglobin is fully saturated at the partial pressure of oxygen in the lungs. It becomes fully loaded with O2 as it passes the gas exchange surface (the alveoli). When fully saturated, almost all the haemoglobin is in the form of oxyhaemoglobin.
• As it flows through the tissues, oxyhaemoglobin responds to low partial pressures of oxygen by dissociating (giving up some of its oxygen). Oxygen diffuses through capillary walls and tissue fluid to respiring cells.
• As tissues use up oxygen during exercise, oxyhaemoglobin dissociates even more.
• The S-shape of the curve shows that oxyhaemoglobin responds to small decreases in oxygen concentration in the tissues by giving up a lot of oxygen.
• Fetal blood has a higher affinity for oxygen than adult blood. This means that oxygen diffuses from maternal blood to fetal blood across the placenta, even though the partial pressures are similar.

As the blood enters respiring tissues, the haemoglobin is carrying oxygen in the form of oxyhaemoglobin. The oxygen tension of the respiring tissues is lower than that in the lungs because oxygen has been used in respiration. As a result, the oxyhaemoglobin begins to dissociate and releases oxygen to the tissues.

The hydrogen ions released from the dissociation of carbonic acid compete for the space taken up by oxygen on the haemoglobin molecule. So when carbon dioxide is present, the hydrogen ions displace the oxygen on the haemoglobin. As a result, the oxyhaemoglobin releases more oxygen to the tissues.

Where tissues (such as contracting muscles) are respiring more, there will be more carbon dioxide. As a result there will be more hydrogen ions produced in the red blood cells. This makes the oxyhaemoglobin release more oxygen. This it the Bohr effect. At any particular oxygen tension, the haemoglobin releases more oxygen when more carbon dioxide is present. So when more carbon dioxide is present, haemoglobin dissociation curve shift downwards and to the right (the Bohr effect).

The Bohr effect results in oxygen being more readily released where more carbon dioxide is produced from respiration. This is just what the muscles need for aerobic respiration to continue.

When cells respire, they produce carbon dioxide. The graph on the next card shows what happens when carbon dioxide is added to the gas mixture.

When there is more CO2 in the mixture, haemoglobin is less saturated with oxygen. (On the graph) - the second line is to the right of the first line. The effect of carbon dioxide on the dissociation curve is the Bohr shift or Bohr effect.

CO2 interacts with haemoglobin to cause it to give up its O2. This is good news, because it means that haemoglobin delivers more oxygen to those tissues that are respiring quickly, such as muscles during exercise. This happens because an enzyme in red blood cells, carbonic anhydrase, catalyses the reaction between H2O and CO2.

Haemoglobin absorbs the hydrogen ions that form inside the red blood cells, which causes them to lose the oxygen molecules they are carrying.