# Exchange of gases: Partial pressure, Diffusion, Sites of exchange of gases, Practice Problems and FAQs

We all cannot lead a normal life without respiration in this world. It is an essential life process, because all living organisms need energy for carrying out their different life processes and they get this energy by the oxidation of food. Breathing is the first step of this process of respiration. It involves inspiration (inflow of air) and expiration (outflow of air). Cellular respiration is the second major part which involves the burning up of food substances within the tissues to form carbon dioxide, water and energy.

GIF: Breathing

But do you know, when we breathe in how the gases are transported within the respiratory system and what is the science behind the exchange of gases? The oxygen and carbon dioxide are the two main gases that are involved in breathing and cellular respiration. Exchange of gases by simple diffusion is mainly based on pressure differences or concentration gradient. Partial pressure for oxygen is represented as pO2 and for carbon dioxide it is represented as pCO2 . The diffusion of gases takes place from a region of their higher partial pressure to a region of their lower partial pressure. Let’s understand about the exchange of gases in depth in this article.

## Exchange of gases

Exchange of gases occurs inside lungs and inside tissues. Here in these sites, the oxygen and carbon dioxide are exchanged by the process of simple diffusion.

## Factors affecting exchange of gases

The exchange of gases depends on the following factors:

• Partial pressure of gases
• Diffusion
• Solubility of gases
• The thickness of the respiratory membrane

### Partial pressure of gases

It is the pressure contributed by an individual gas in a mixture of gases. Gases diffuse downhill according to their partial pressure. Partial pressure is represented by the letter ‘p’ (small letter) and the symbol of the gas. For example, the partial pressure for oxygen is represented as pO2 . There is 21% oxygen and 0.04% of carbon dioxide in the atmosphere. Let’s calculate the partial pressure in mm Hg for oxygen and carbon dioxide.

The unit of pressure in mercury is mmHg and 1 atm = 760 mmHg.

So the partial pressure created by oxygen in atmosphere will be represented in the following way

100

So it is 159 mm Hg in the atmosphere.

Partial pressure of carbon dioxide is represented as pCO2. It can also be calculated using the same formula as follows: Partial pressure of carbon dioxide is

Hence it is 0.3 mm Hg in the atmosphere.

Fig: Partial pressure of oxygen and carbon dioxide at different parts involved in the diffusion

### Diffusion

Diffusion is the movement of a substance from an area of high pressure or concentration to an area of low pressure or concentration. Diffusion happens in liquid and gases. With the same process oxygen and carbon dioxide are exchanged in the body.

GIF: Diffusion

The rate of diffusion is proportional to the concentration gradient of the associated substances. There exists a concentration gradient for oxygen from alveoli to blood and blood to tissues. Similarly a concentration gradient is present for carbon dioxide in the opposite direction, that is from tissues to blood and blood to alveoli.

Fig: Direction of diffusion of oxygen

Fig: Direction of diffusion of carbon dioxide

### Solubility of gases

A gas having high solubility, diffuses at a faster rate than the gas having lower solubility. For example, the solubility of CO2 is 20 - 25 times higher than that of O2, hence the amount of CO2 that diffuses across diffusion membranes is much higher than that of O2.

### Thickness of the respiratory membrane

It is the surface where gaseous exchange between alveoli and blood in the lungs occurs. There will be a low rate of diffusion, if the thickness of the membrane is more. Thinner the membrane more is the diffusion as the distance travelled by the substance will be less.

## Sites of exchange of gases

Gaseous exchange occurs in two main areas in the human body as follows:

• In lungs
• In tissues

### Exchange of gases between lung alveoli and blood

Exchange of gases between the lung alveoli and the blood capillaries is also called external respiration. The wall of alveoli is very thin and has a rich network of blood capillaries. Due to this network, the alveolar wall looks like a sheet of flowing blood and is called diffusion or respiratory or alveolar-capillary membrane.

Fig: Exchange of gases between alveoli and blood

#### Respiratory membrane (Diffusion membrane)

It is the surface where gaseous exchange between alveoli and blood in the lungs takes place. The diffusion membrane’s total thickness is less than a millimetre.

Respiratory membrane is made up of three layers as follows:

• The thin squamous epithelium of the alveoli.
• The endothelial lining of the alveolar capillaries which is made of squamous epithelium.
• Basement membrane present in between thin squamous epithelium of alveolar walls and endothelium of alveolar capillaries.

Fig: Respiratory membrane (Diffusion membrane)

#### Partial pressure (mm Hg) of oxygen and carbon dioxide at different areas involved in diffusion

Fig: Partial pressure of oxygen and carbon dioxide at different parts involved in the diffusion

The partial pressure of oxygen in the atmosphere is 159 mm Hg. In the alveoli the partial pressure of oxygen is 104 mm Hg. Hence exchange of oxygen takes place between air and lungs. The partial pressure of carbon dioxide in the atmosphere is 0.3 mm Hg and in the alveoli is 40 mm Hg. So the carbon dioxide moves outside from the lungs.

The capillaries bring deoxygenated blood from the tissues which have low oxygen concentration. As the concentration of oxygen is low, the partial pressure is also at a lower side (40mmHg).

Fig and GIF: Exchange of gases at a capillary level of alveoli

The partial pressure of oxygen is represented by pO2. The pO2 is higher in the alveoli (104mmHg) than the deoxygenated blood (40mmHg). As a result there is diffusion of O2 from alveoli to the blood.

Fig: Transportation of O2 from alveoli to the blood

This makes the deoxygenated blood as oxygenated blood. Oxygenated blood has higher concentration of oxygen. The pO2 in oxygenated blood is 95mmHg.

Fig: Transportation of O2 from alveoli to the heart

The oxygenated blood is then transported away from the lungs to the heart. The heart will pump this blood to all parts of the body, so that it can be used by various tissues.

The partial pressure of carbon dioxide is higher in deoxygenated blood (45 mm Hg) than in alveoli (40 mm Hg). Therefore carbon dioxide passes from the blood to the alveoli.

### Exchange of gases between blood and tissue

Exchange of gases between the blood and the tissues is also called internal respiration. The pO2 of oxygenated blood is 95mmHg, which is the same as it left the lungs. The pO2 of the tissues is much lower (40mmHg), so the oxygen diffuses from the blood to the tissues.

Fig: Transportation of O2 from blood to the tissues

Thus, it becomes deoxygenated, and the pO2 of blood drops to 40mmHg.

Fig: pO2 of tissues increases

Now, the oxygen is consumed in cellular respiration. Food is oxidised with the help of oxygen to release energy, water and carbon dioxide. This occurs mainly by glycolysis and Kreb’s cycle.

Fig: Cellular respiration

The O2 is consumed and CO2 is released out. The pCO2 is higher in the tissues (45mmHg) compared to the oxygenated blood (40mmHg), so the CO2 moves from tissues to the blood.

Fig: Transportation of O2 from blood to the tissues and CO2 from tissues to the blood

This along with the removal of oxygen makes the blood deoxygenated. Now the pCO2 changes to 45mm Hg.

Fig: pCO2 of tissues decreases

### Summary of exchange of gases

The entire process of exchange of gases in the respiration in human body can be summarised in the following way:

Fig: Summary of exchange of gases

## Practice Problems

Q1. The rate of diffusion does not depend on which factor?

A. Solubility of the gas
B. Thickness of respiratory membrane
C. Size of the particle
D. Energy

Solution: Diffusion is referred to as the movement of substances from the region of high concentration to the region of low concentration according to the concentration gradient. The rate of diffusion is proportional to the concentration gradient of the associated substances. Factors affecting the rate of diffusion are as follows:

• Solubility of the gases
• Thickness of the respiratory membrane
• Size of the particle
• Partial pressure of the gases
• Concentration of the gases

Diffusion is considered as a passive process which means that it does not require energy. Hence, the correct option is d.

Q2. Which molecule is formed when carbon dioxide is reversibly combined with the haemoglobin?

A. Carboxyhaemoglobin
B. Carbaminohaemoglobin
C. Oxyhaemoglobin
D. Bicarbonate ions

Solution: The reversible combination of carbon dioxide and haemoglobin produces carbaminohaemoglobin. It is generated at the cellular level when the partial pressure of oxygen is low. Hence, the correct option is b.

Q3. What will be the partial pressure exerted by the oxygen gas, which makes up 21% of our atmosphere?

A. 158 mmHg
B. 159 mmHg
C. 160 mmHg
D. 173 mmHg

Solution: Partial pressure is the pressure exerted by an individual gas when a mixture of non-reactive gases exerts pressure on a system. A gas will flow from a region with a higher partial pressure to one with a lower partial pressure. We all know that 1 atmosphere is equal to 760 mmHg. For 21% of the oxygen gas, the partial pressure will be calculated as follows:

So it is 159 mm Hg in the atmosphere. Hence, the correct option is b.

Q4. Which process occurs when gases are transferred between blood and alveoli?

A. Osmosis
B. Diffusion
C. Active transport
D. Facilitated diffusion

Solution: Diffusion is referred to as the movement of substances from the region of high concentration to the region of low concentration according to the concentration gradient. The rate of diffusion is proportional to the concentration gradient of the associated substances. Factors affecting the rate of diffusion are as follows:

• Solubility of the gases
• Thickness of the respiratory membrane
• Size of the particle
• Partial pressure of the gases
• Concentration of the gases

Hence, the correct option is b.

## FAQs

Q1. Which are the different steps involved in gaseous exchange?
Answer: There are four steps involved in gaseous exchange that are listed below:

• Pulmonary ventilation
• Pulmonary gas exchange
• Respiratory gas transport
• Peripheral gas exchange

Q2. Which system is responsible for gaseous exchange in the human body and what parts are involved in it?
Answer: In the human body, the respiratory system is responsible for gas exchange. The main structures of the human respiratory system are the nose, nasal cavity, trachea, bronchi, alveoli and the lungs.

Q3. How does oxygen exchange occur in the lungs?
Answer: The oxygen transport from alveoli of the lungs to the blood capillaries through the process of diffusion. Then the oxygen is carried out to the rest of the body through blood.

Q4. Which molecule helps in the transportation of oxygen and carbon dioxide in the human body?
Answer: Haemoglobin is the respiratory pigment in the red blood cells. It is the molecule that helps in the transportation of oxygen and carbon dioxide in the human body mainly.

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