Question
▶️Answer/Explanation
The alveolus is the microscopic air sac in the lungs where gas exchange occurs. It contains three main types of cells, each with a distinct function:
Type I Pneumocyte
- These are thin, flat epithelial cells that form the majority of the alveolar surface.
- Their primary function is gas exchange.
- Due to their thinness, they allow oxygen to diffuse from the alveolus into the blood, and carbon dioxide to diffuse from the blood into the alveolus efficiently.
Type II Pneumocyte
- These are cuboidal cells and are fewer in number compared to Type I cells.
- Their main role is the secretion of pulmonary surfactant.
- Surfactant is a lipid-protein substance that reduces surface tension within the alveoli, preventing their collapse during exhalation and helping maintain their shape for efficient gas exchange.
Phagocyte
- These are immune cells, typically macrophages, found within the alveolus.
- Their main function is the ingestion and destruction of pathogens, such as bacteria or dust particles that enter the lungs.
- They help in defending the lungs from infection and keeping the alveolar space clean.
Question
What causes the expansion of the thorax during inspiration?
A. Air entering the lungs
B. An increase in tidal volume
C. An increase in pressure inside the lungs
D. The contraction of the diaphragm and external intercostal muscles
▶️Answer/Explanation
Answer: D. The contraction of the diaphragm and external intercostal muscles
Explanation:
What causes the thorax to expand during inspiration?
Inspiration (inhalation) is the process of drawing air into the lungs. For this to happen, the volume of the thoracic cavity (chest) must increase. This expansion is caused by the contraction of specific muscles, which pulls the rib cage upward and outward and flattens the diaphragm, reducing pressure inside the lungs and allowing air to flow in.
Now evaluate each option:
Option A. Incorrect – Air entering the lungs
Air enters the lungs after the thorax expands, and pressure drops. It’s a result, not the cause, of thoracic expansion.
Option B. Incorrect – An increase in tidal volume
Tidal volume is the amount of air moved in or out of the lungs during a normal breath. It increases because of thoracic expansion, not the other way around.
Option C. Incorrect – An increase in pressure inside the lungs
Inspiration happens when pressure in the lungs decreases below atmospheric pressure. An increase in pressure would push air out, not pull it in.
Option D. Correct – The contraction of the diaphragm and external intercostal muscles
This is the correct cause. The diaphragm contracts and flattens, and the external intercostal muscles raise the rib cage, increasing thoracic volume and allowing air to flow into the lungs.
Question
Which type of cell is specialized to facilitate gas exchange?
A. Type I pneumocytes
B. Type II pneumocytes
C. Internal intercostal muscle fibres
D. External intercostal muscle fibres
▶️Answer/Explanation
Answer: A. Type I pneumocytes
Explanation:
Which type of cell is specialized for gas exchange?
The main cells involved in gas exchange in the lungs are pneumocytes. These cells are located in the alveoli (air sacs in the lungs) and help facilitate the exchange of gases (oxygen and carbon dioxide) between the air and blood.
Now, let’s evaluate each option:
Option A. Correct – Type I pneumocytes
Type I pneumocytes are thin, flat cells that make up the majority of the alveolar walls. Their thin structure allows for efficient gas exchange because oxygen and carbon dioxide can easily diffuse through them.
Option B. Incorrect – Type II pneumocytes
Type II pneumocytes are cuboidal cells that secrete surfactant, a substance that reduces surface tension in the alveoli, preventing them from collapsing. They do not play a direct role in gas exchange.
Option C. Incorrect – Internal intercostal muscle fibers
Internal intercostal muscles are responsible for forced exhalation, not gas exchange. They contract to decrease the volume of the thoracic cavity, pushing air out of the lungs.
Option D. Incorrect – External intercostal muscle fibers
External intercostal muscles help with inhalation by raising the ribs and increasing the volume of the thoracic cavity, but they do not directly facilitate gas exchange.
Question
Which process results in the exchange of gases across the membrane of pneumocytes?
A. Active transport
B. Simple diffusion
C. Facilitated diffusion
D. Mass flow
▶️Answer/Explanation
Answer: B. Simple diffusion
Explanation:
What process facilitates the exchange of gases across the membrane of pneumocytes?
The exchange of gases (oxygen and carbon dioxide) across the membrane of pneumocytes (specifically Type I pneumocytes) occurs through a passive process that does not require energy. The gases move from areas of high concentration to areas of low concentration.
Now, let’s evaluate each option:
Option A. Incorrect – Active transport
Active transport requires energy to move substances against their concentration gradient (from low to high concentration). Gas exchange occurs along concentration gradients, so active transport is not involved.
Option B. Correct – Simple diffusion
Gas exchange across the membrane of Type I pneumocytes occurs by simple diffusion. Oxygen moves from the alveoli (where its concentration is high) into the blood (where its concentration is low), and carbon dioxide moves in the opposite direction. This process is driven by differences in concentration, requiring no energy.
Option C. Incorrect – Facilitated diffusion
Facilitated diffusion requires the help of specific carrier proteins to move molecules across a membrane. Gas exchange in the alveoli happens via simple diffusion, not facilitated diffusion, as oxygen and carbon dioxide can directly diffuse across the lipid bilayer of the pneumocyte membranes.
Option D. Incorrect – Mass flow
Mass flow refers to the movement of substances due to a pressure gradient, which is not the mechanism for gas exchange in the alveoli. Gas exchange occurs by diffusion, not mass flow.
Question
The diagram shows a section through an alveolus.
What are the functions of the following labelled structures?
Answer/Explanation
Answer: C
Explanation:
Type I Pneumocyte
- Flat, thin cells that form the majority of the alveolar surface.
- Function = Gas exchange.
- So, both A and C say “Gas exchange” – this part is correct in both.
Type II Pneumocyte
- These are smaller, rounded cells.
- Function = Production of surfactant to reduce surface tension.
- A and C both say “Produces surfactant” – both are correct here too.
Capillary
In gas exchange, carbon dioxide leaves the blood and goes into the alveoli, and oxygen enters the blood from the alveoli. But:
- Carbon dioxide is transported TO the alveolus from blood.
- So, capillary’s role here = brings CO₂ to alveolus.
- Option C says: Capillary → Transports carbon dioxide to alveolus ← That is scientifically correct.