Gas Exchange and Gas Transport in the Blood

Gas exchange is the diffusion of oxygen into the blood and carbon dioxide out of it across the walls of the alveoli and capillaries, and gas transport is the carrying of these gases around the body in the blood. In brief: oxygen is carried mainly by hemoglobin in red blood cells as oxyhemoglobin, and carbon dioxide is carried mostly as dissolved hydrogencarbonate ions in plasma. In the lungs, oxygen diffuses into the blood and carbon dioxide diffuses out, because of differences in partial pressure.

This exchange takes place continuously in the lungs and in every tissue. This guide explains partial pressure and diffusion gradients, how oxygen and carbon dioxide are carried, why the oxygen dissociation curve is S-shaped, how to compare inhaled and exhaled air, a worked example and common mistakes.

How does gas exchange occur in the lungs?

Gases move by diffusion, from a region of higher concentration to one of lower concentration. For gas mixtures, the “concentration” is described by partial pressure, the share of the total pressure caused by one gas. Atmospheric pressure is about 101 kPa, and since oxygen makes up about 21 percent of air, the partial pressure of oxygen is roughly 21 kPa. Alveolar air has a lower value, about 13 kPa, because it is mixed with air already in the lungs and loses oxygen to the blood.

Blood arriving at the lung capillaries from the body has a low partial pressure of oxygen, about 5 kPa, and a high partial pressure of carbon dioxide, about 6 kPa. The alveolar air has about 5 kPa of carbon dioxide, which is slightly lower. Oxygen therefore diffuses from the alveolus into the blood, and carbon dioxide diffuses the other way. Carbon dioxide diffuses easily because it is much more soluble in water than oxygen. Blood leaving the lungs has a partial pressure of oxygen of about 13 kPa, nearly equal to that of the alveoli.

Gas exchange is efficient because of the thin walls, large surface and constant blood flow and ventilation that maintain steep gradients, as explained in the article on alveoli.

How is oxygen transported?

Oxygen is not very soluble in water. Plasma can dissolve only about 1.5 percent of the oxygen carried in blood, so almost all of it is carried by hemoglobin in red blood cells (about 98 percent). Each hemoglobin molecule has four heme groups, each with an iron atom that can bind one oxygen molecule, giving a maximum of four.

The equation is: Hb + 4O₂ ⇌ Hb(O₂)₄, in which Hb(O₂)₄ is oxyhemoglobin. The reaction is reversible, so hemoglobin picks up oxygen where the partial pressure is high (lungs) and releases it where the partial pressure is low (tissues).

The oxygen dissociation curve

A graph of the percentage saturation of hemoglobin against the partial pressure of oxygen is S-shaped. At low partial pressures the curve is shallow, then it rises steeply, and at high pressures it levels off. The shape arises because binding of the first oxygen makes it easier for the next ones to bind. In the lungs, at around 13 kPa, hemoglobin is about 97 to 98 percent saturated. In resting tissues, at about 5 kPa, it is about 75 percent saturated, so about a quarter of the oxygen is given up. In active muscle with a very low partial pressure, much more is released.

The curve moves to the right when carbon dioxide is high or pH is low, which is called the Bohr effect. Hemoglobin then releases oxygen more readily, which is useful in exercising tissues. Fetal hemoglobin has a higher affinity for oxygen, so it can take oxygen from the mother’s blood across the placenta. Myoglobin in muscle stores oxygen and has an even higher affinity.

How is carbon dioxide transported?

Carbon dioxide is made by cells in respiration and is carried to the lungs in three ways.

• As hydrogencarbonate ions (about 70 percent): in red blood cells, the enzyme carbonic anhydrase speeds up the reaction CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. The hydrogencarbonate ions diffuse into plasma, and hemoglobin takes up the hydrogen ions and acts as a buffer.
• Bound to hemoglobin (about 20 to 25 percent): carbon dioxide attaches to the protein part of hemoglobin as carbaminohemoglobin.
• Dissolved in plasma (about 5 to 10 percent): it is more soluble than oxygen.

In the lungs the reactions run in reverse, and carbon dioxide is released and diffuses into the alveoli to be exhaled. The percentages are approximate and differ slightly between sources.

What happens in the tissues?

In body tissues, the partial pressure of oxygen is low because cells use oxygen constantly, and the partial pressure of carbon dioxide is high. Oxyhemoglobin releases oxygen, which diffuses into the tissue fluid and into cells. Carbon dioxide diffuses from cells into the blood. This exchange occurs in capillaries and relies on the same principles as in the lungs, but in the opposite direction.

Composition of inhaled and exhaled air

Values are approximate. The nitrogen percentage includes small amounts of argon and other gases, and exhaled air also contains the same inert gases. Nitrogen is not used by the body, so it remains unchanged.

Worked example: reading partial pressures

The table gives typical partial pressures. Decide the direction of diffusion in each place.

1. Lungs, oxygen: alveolar 13 kPa, blood 5 kPa. Gradient of 8 kPa. Oxygen diffuses from the alveolus into the blood.
2. Lungs, carbon dioxide: blood 6 kPa, alveolar 5 kPa. Gradient of 1 kPa. Carbon dioxide diffuses from the blood into the alveolus.
3. Muscle, oxygen: blood arriving from the lungs has a partial pressure of oxygen of 13 kPa, while resting muscle has 5 kPa. The gradient of 8 kPa makes oxygen diffuse out of the blood into the tissue.
4. Muscle, carbon dioxide: tissue has 6 kPa, and blood arriving from the lungs has 5 kPa. Carbon dioxide diffuses from the tissue into the blood.

The gradient for carbon dioxide is small, but the gas still moves quickly because it dissolves easily in water, so about the same amount of carbon dioxide is exchanged as oxygen is taken up. The calculations rely on typical values and ignore differences between people.

How was gas transport discovered?

In the 1860s Felix Hoppe-Seyler crystallized hemoglobin and showed that it binds oxygen. Christian Bohr, the father of the physicist Niels Bohr, described the effect of carbon dioxide on oxygen binding in 1904. Max Perutz determined the three-dimensional structure of hemoglobin using X-rays in the late 1950s and shared the 1962 Nobel Prize in Chemistry for this work with John Kendrew.

Which mistakes are common?

Mistake 1: Saying that oxygen is carried mainly dissolved in plasma

Almost all is bound to hemoglobin. Dissolved oxygen is a minor part.

Mistake 2: Saying that carbon dioxide is carried mainly by hemoglobin

Most is carried as hydrogencarbonate ions in plasma.

Mistake 3: Forgetting that exhaled air still contains much oxygen

Exhaled air contains about 16 percent oxygen, enough to support mouth-to-mouth resuscitation.

Mistake 4: Treating the dissociation curve as a straight line

The S shape means that small changes in the partial pressure in tissues cause large changes in the amount of oxygen released.