7: Gas Exchange in Humans

Question 1

Why is gas exchange necessary?

Answer 1

It ensures a continuous supply of oxygen for the respiration in cells, and the removal of carbon dioxide produced in the process.

Question 2

How does gas exchange take place?

Answer 2

It takes place by diffusion across a moist respiratory surface.

Question 3

Explain why large multicellular organisms need a breathing system.

Answer 3

The surface area to volume ratio is smaller than that of unicellular organisms, and the diffusion of gases across the body surface cannot satisfy the needs of these organisms. They have a breathing system for efficient gas exchange.

Question 4

State the structure through which the lungs are connected with the external environment.

Answer 4

Respiratory tract

Question 5

Describe the airflow when we inhale.

Answer 5

Air flows into the nasal cavity, the pharynx, the larynx, the trachea, the bronchi, the bronchioles, and reaches the air sacs in the lungs.

Question 6

Describe the airflow when we exhale.

Answer 6

Air flows from the air sacs in the lungs, to the bronchioles, the bronchi, the trachea, the larynx, the pharynx, and to the nasal cavity before exiting our body through the nostrils.

Question 7

Describe the function of our nose.

Answer 7

It has features to clean, moisten, and warm the inhaled air.

Question 8

State the function of hairs inside the nostrils.

Answer 8

They filter larger dust particles from the inhaled air.

Question 9

State the function of mucus-secreting cells in the lining of the nasal cavity.

Answer 9

They secrete mucus, which traps dust particles and microorganisms in the inhaled air and moistens the inhaled air to prevent the moist respiratory surface inside the lungs from drying out.

Question 10

State the functions of the ciliated epithelial cells in the lining of the nasal cavity.

Answer 10

Cilia of ciliated epithelial cells in the lining beat to sweep the mucus towards the pharynx. The mucus is then coughed up, or swallowed into the stomach where most of the trapped microorganisms will be killed by hydrochloric acid in gastric juice.

Question 11

State the functions of the blood vessels in the lining of the nasal cavity.

Answer 11

The lining of the nasal cavity has a rich supply of blood vessels. The blood in the capillaries warms the inhaled air, which avoids irritation of the lungs.

Question 12

State the material making up the larynx.

Answer 12

Cartilage

Question 13

State the importance of cartilage in the larynx.

Answer 13

It prevents the larynx from collapsing due to the pressure changes during breathing.

Question 14

State the importance of the shape of the cartilage of the trachea.

Answer 14

The trachea has a C-shaped cartilage, with the open part of the cartilage lying next to the oesophagus. This allows the oesophagus to expand when food is swallowed.

Question 15

State the functions of the inner walls of the trachea and bronchi.

Answer 15

The inner walls are lined with mucus-secreting cells and ciliated epithelial cells, and they also have a rich supply of blood vessels. These features can clean, moisten, and warm the inhaled air.

Question 16

State the importance of smooth muscles and elastic fibres in the walls of the trachea and bronchi.

Answer 16

The smooth muscles can contract and relax to help regulate the diameter of the airway and hence the airflow.

Question 17

Compare the composition of walls of bronchi and bronchioles.

Answer 17

The wall of bronchioles have no cartilage unlike bronchi. Their walls are mostly made up of smooth muscles and elastic fibres. Larger bronchioles are lined with mucus-secreting cells and ciliated epithelial cells like the bronchi.

Question 18

Compare the cartilage content in walls of the trachea, bronchi and bronchioles.

Answer 18

The wall of the trachea has C-shaped cartilage, while the walls of bronchi have cartilage plates surrounding it. There is no cartilage in the bronchioles.

Question 19

State the number of lobes in each half of the lung.

Answer 19

The right lung has two lobes, while the left lung has three lobes.

Question 20

Compare the sizes of the left lung and right lung.

Answer 20

The left lung is smaller in size than the right lung, which leaves space for the heart.

Question 21

Explain the colour of the lungs.

Answer 21

The lungs are pink in colour due to the presence of blood capillaries.

Question 22

Explain the texture of the lungs.

Answer 22

It is spongy and elastic since is has a large amount of air-filled space and elastic fibres.

Question 23

State the name of the chamber enclosed by the rib cage.

Answer 23

The thoracic cavity

Question 24

State the composition of the wall of an air sac.

Answer 24

Its wall is made up of an epithelium which is one-cell thick, and it secretes watery fluid which keeps the inner surface of the air sac moist.

Question 25

State the significance of the elasticity of the lungs.

Answer 25

When we inhale, the elastic fibres in our lungs **stretch** to allow the lungs to **inflate**. When we exhale, the lungs **recoil** due to the elasticity of its tissue, which forces air out of the lungs.

Question 26

State the surfaces which are adhered to by the pleural membranes.

Answer 26

The inner pleural membrane adheres to the **outer surface of the lungs**, while the outer pleural membrane adheres to the **inner wall of the thoracic cavity** and the **upper surface of the diaphragm**.

Question 27

State the significance of pleural fluid.

Answer 27

Pleural fluid **holds** the two layers of **pleural membranes** together. It also acts as a **lubricant** to reduce friction between the pleural membranes during breathing movement.

Question 28

Explain why the lung collapses when the pleural cavity is ruptured.

Answer 28

Air leaks into the pleural cavity and the **negative pressure** cannot be maintained. The lung collapses due to its own **elasticity**.

Question 29

State how oxygen is uptaken by the blood in the air sacs.

Answer 29

When air is inhaled, it flows into air sacs. Oxygen in the inhaled air dissolves in the **water film** lining the air sacs. Since the concentration of oxygen in the water film is higher than that of the deoxygenated blood, a **concentration gradient** is formed, and the dissolved oxygen in the water film **diffuses** across the walls of the air sacs and the capillary walls **into the blood**.

Question 30

State how carbon dioxide is removed from the blood in the air sacs.

Answer 30

Since the concentration of carbon dioxide in the deoxygenated blood is higher than that of the water film, a **concentration gradient** is formed, and the carbon dioxide in the blood **diffuses** across the capillary walls and the walls of the air sacs **into the blood**. Carbon dioxide is then removed from the body when we exhale.

Question 31

State the difference between the oxygen and carbon dioxide content before and after blood undergoes gas exchange.

Answer 31

After undergoing gas exchange, the **oxygen** content of the blood **increases** and the carbon dioxide content **decreases**. The blood becomes **oxygenated**.

Question 32

State 5 adaptive features of air sacs for gas exchange.

Answer 32

1. There are a **large number** of air sacs. 2. The walls of air sacs are **one-cell thick**. 3. The inner surfaces of air sacs are **moist**. 4. Air sacs have a rich supply of **blood capillaries**. 5. The walls of air sacs are **in close contact** with capillaries.

Question 33

Explain the advantage of there being a large number of air sacs.

Answer 33

This provides a very large **surface area** for the diffusion of gases.

Question 34

Explain the advantage of air sacs having one-cell thick walls.

Answer 34

This ensures that the epithelium of the walls of air sacs are **very thin**, which **reduces** the **diffusion distance** of gases.

Question 35

Explain the advantage of air sacs having a moist inner surface.

Answer 35

Oxygen in inhaled air **dissolves** in the water film lining the inner surfaces of the air sacs before diffusion takes place.

Question 36

Explain the advantage of air sacs having a rich supply of capillaries.

Answer 36

This allows **rapid transport** of gases to and away from the air sacs. Therefore, a **steep concentration gradient** of gases between the air sacs and the blood can be maintained, ensuring that there is a **high rate of diffusion** of gases

Question 37

Explain the advantage of the walls of air sacs being in close contact with capillaries.

Answer 37

This **reduces the diffusion distance** of the gases.

Question 38

State and explain the difference in concentration of oxygen in inhaled and exhaled air.

Answer 38

The oxygen concentration in inhaled air (21%) is higher than that in exhaled air (16%). This is because oxygen in inhaled air diffuses from the air sacs into the blood during gas exchange, and is then **used** by body cells for **respiration**.

Question 39

State and explain the difference in concentration of carbon dioxide in inhaled and exhaled air.

Answer 39

The carbon dioxide concentration in inhaled air (0.03%) is lower than that in exhaled air (4%). This is because carbon dioxide is **produced** by body cells in **respiration**, which diffuses from the blood into the air sacs during gas exchange, and is then removed from the body by exhalation.

Question 40

State and explain the difference in concentration of water vapour in inhaled and exhaled air.

Answer 40

The water vapour concentration in inhaled air is **variable**, while that in exhaled air is always **saturated**. This is because exhaled air is **moistened** by the water film lining the air sacs and the **mucus** lining in the respiratory tract.

Question 41

State and explain the difference in concentration of nitrogen in inhaled and exhaled air.

Answer 41

The concentrations of nitrogen in inhaled and exhaled air (78%) are **the same** since nitrogen gas is **not produced nor used** by the body cells.

Question 42

State and explain the difference in concentration of noble gases in inhaled and exhaled air.

Answer 42

The concentrations of noble gases in inhaled and exhaled air (~1%) are **the same** since noble gases are **not produced nor used** by the body cells.

Question 43

State and explain the difference in temperature of inhaled and exhaled air.

Answer 43

Exhaled air is **warmer** than inhaled air since it is warmed by **blood in the capillaries** near the inner surface of the respiratory tract.

Question 44

State how oxygen is transported in blood.

Answer 44

A small proportion of oxygen is transported in its dissolved from in **plasma** while most oxygen is transported in **red blood cells**.

Question 45

State the protein which greatly increases the oxygen-carrying capacity of blood.

Answer 45

Haemoglobin

Question 46

State the equations of the reversible binding of haemoglobin and oxygen.

Answer 46

Under **high** oxygen concentration, haemoglobin + oxygen -> oxyhaemoglobin Under **low** oxygen concentration, oxyhaemoglobin -> haemoglobin + oxygen

Question 47

State two adaptive features of red blood cells for transporting oxygen.

Answer 47

1. Mature red blood cells have **no nucleus**. 2. Red blood cells have a **biconcave disc shape**.

Question 48

Explain the advantage of mature red blood cells having no nucleus.

Answer 48

The absence of a nucleus provides **space for more haemoglobin**, which increases the **oxygen-carrying capacity** of blood.

Question 49

Explain the advantage of mature red blood cells having a biconcave disc shape.

Answer 49

The biconcave disc shape provides a **large surface area to volume ratio** and a **short distance of diffusion** for oxygen. This allows oxygen to **reach and leave** haemoglobin in red blood cells rapidly.

Question 50

Describe the process which oxygen is uptaken and transported to the body cells.

Answer 50

In air sacs, oxygen concentration is **high** due to the **continuous replacement** of air from the external environment. Most of oxygen diffuses into **red blood cells**, where is binds to haemoglobin to form **oxyhaemoglobin**. The blood becomes **oxygenated** and is carried via the pulmonary veins to the heart. The heart then pumps the blood to different body tissues.

Question 51

Describe the process which oxygen is released to the body cells.

Answer 51

In body tissues, oxygen concentration is **low** because oxygen is consumed continuously by body cells during **respiration**. **Oxyhaemoglobin** in blood **breaks down into haemoglobin and oxygen**, and the oxygen **diffuses** into the body ells. The blood becomes **deoxygenated** and is carried back to the heart, and then to the lungs via the pulmonary artery.

Question 52

State the colour difference between oxygenated and deoxygenated blood.

Answer 52

Oxygenated blood is bright red due to the colour of oxyhaemoglobin and deoxygenated blood is purplish red due to the colour of haemoglobin.

Question 53

Describe the process in which carbon dioxide is uptake and transported from the body cells.

Answer 53

In body tissues, carbon dioxide concentration is **high** because it is produced continuously by body cells during **respiration**. Most of the carbon dioxide diffuses into **red blood cells**, where it reacts with water to form **hydrogencarbonate ions** and hydrogen ions catalysed by an enzyme. The hydrogencarbonate ions then diffuse out of the red blood cells are carried by **plasma** to the air sacs of the lungs.

Question 54

Describe the process in which carbon dioxide is transported and released to the air sacs.

Answer 54

In the air sacs of the lungs, carbon dioxide concentration is **low** because carbon dioxide is removed continuously when we exhale. Hydrogencarboate ions in plasma **diffuse** into red blood cells, where hydrogencarbonate ions and hydrogen ions react to form **carbon dioxide** and water catalysed by an enzyme. The carbon dioxide diffuses into plasma and then into the air sac for removal.

Question 55

State the equations of the reversible reaction of carbon dioxide and water in red blood cells.

Answer 55

High carbon dioxide concentration: carbon dioxide + water - enzyme -> hydrogen ion + hydrogencarbonate ion Low carbon dioxide concentrations: hydrogencarbonate ion + hydrogen ion - enzyme -> carbon dioxide + water

Question 56

State the significance of ventilation.

Answer 56

It helps maintain a **steep concentration gradient** between air in the air sacs and the blood in surrounding capillaries, as air in air sacs is **continuously replaced** from the environment.

Question 57

Describe the process of inhalation.

Answer 57

**Intercostal muscle contracts** to result in **upward and outward movement** of rib cage. **Diaphragm muscle contracts** to flatten the diaphragm; both actions **increase the volume of the thoracic cavity**. This causes an increase in the **volume of lungs** because the inner wall of the thoracic cavity and the outer surfaces of the lungs are held together by pleural membranes and pleural fluid. As a result, the **air pressure inside the lungs drops below atmospheric pressure**, and air is forced into the lungs through the respiratory tract.

Question 58

Describe the process of exhalation.

Answer 58

**Intercostal muscles relax** to result in **downward and inward movement** of rib cage. **Diaphragm muscles relax** and the diaphragm returns to its **dome shape**; both actions **decrease the volume of the thoracic cavity**. This causes a decrease in the **volume of lungs** due to the **elasticity** of its fibres. As a result, the **air pressure inside the lungs increases above atmospheric pressure**, and air is forced out of the lungs through the respiratory tract.

Question 59

State two models which can demonstrate the mechanisms of inhalation of exhalation.

Answer 59

Rib-cage model and bell-jar model

Question 60

State the instant where the volume of the lungs is the greatest.

Answer 60

Just after inhalation and just before exhalation

Question 61

State the instant where the air pressure inside the lungs is decreasing and equal to the atmospheric pressure.

Answer 61

Just after exhalation and just before inhalation

Question 62

State the instant where the air pressure inside the lungs is increasing and equal to the atmospheric pressure.

Answer 62

Just after inhalation and just before exhalation

Question 63

State the instant where the volume of the lungs is the smallest.

Answer 63

Just after exhalation and just before inhalation