(a) (i) Complete Table 3.1 by writing in the percentages of carbon dioxide and oxygen in inspired air and in expired air.
(ii) A scientist measured the number of dust particles in inspired air and in expired air. They found fewer dust particles in expired air. Suggest a reason for their observation.
(b) Fig. 3.1 is a diagram of alveoli and associated blood vessels.
(i) Explain how the structure of a capillary is related to its function.
(ii) State the name of the component of blood that transports oxygen.
(iii) State the name of the blood vessel that transports blood from the heart towards the capillaries in the lungs.
(iv) State the location and function of cartilage in the breathing system.
(c) A student measured the rate and depth of breathing of an athlete for 30 seconds at rest. The data are shown in Fig. 3.2.
(i) Using the information in Fig. 3.2, calculate the rate of breathing at rest.
The measurements were repeated while the athlete was running on a treadmill. The data are shown in Fig. 3.3.
(ii) Using the information in Fig. 3.3, calculate the volume of air inspired in one breath from 25 seconds.
(iii) Explain the effect of exercise on the rate and depth of breathing shown in Fig. 3.2 and Fig. 3.3.
▶️ Answer/Explanation
(a)(i)
Explanation: Inspired air contains approximately 21% oxygen and only 0.04% carbon dioxide, which is the normal atmospheric concentration. During respiration, oxygen is used up and carbon dioxide is produced, so expired air contains less oxygen (about 16%) and more carbon dioxide (about 4%).
(a)(ii) Dust particles are trapped in nose hairs or mucus in the respiratory tract.
Explanation: The respiratory system has several defense mechanisms to prevent dust and other particles from reaching the lungs. Nose hairs act as a physical barrier, while mucus produced by the lining of the respiratory tract traps particles. Cilia then move this mucus upwards to be swallowed or coughed out, resulting in fewer particles being exhaled.
(b)(i) Capillaries have thin walls (one cell thick) for efficient diffusion of gases; small lumen to slow blood flow allowing more time for gas exchange; and are numerous creating a large surface area for exchange.
Explanation: The structure of capillaries is perfectly adapted for their function of gas exchange. Their extremely thin walls (just one cell thick) minimize the diffusion distance for oxygen and carbon dioxide. The narrow lumen slows down blood flow, maximizing the time available for gas exchange. Their extensive network throughout the body ensures a large surface area for this vital process.
(b)(ii) Red blood cells.
Explanation: Red blood cells contain hemoglobin, a protein that binds oxygen in the lungs and releases it to tissues throughout the body. Each red blood cell can carry about a billion oxygen molecules.
(b)(iii) Pulmonary artery.
Explanation: The pulmonary artery carries deoxygenated blood from the right ventricle of the heart to the lungs for oxygenation. It’s the only artery in the body that carries deoxygenated blood.
(b)(iv) Location: trachea; Function: prevents airway collapse and keeps airway open.
Explanation: Cartilage rings are found in the trachea and bronchi. These C-shaped rings provide structural support to prevent the airways from collapsing during breathing, while still allowing some flexibility for movement. The open part of the C-shape faces the esophagus, allowing food to pass easily during swallowing.
(c)(i) 12 breaths per minute.
Explanation: From the graph, we can count that the athlete takes about 6 breaths in 30 seconds. To calculate breaths per minute, we multiply by 2 (since there are two 30-second periods in a minute).
(c)(ii) 2.5 dm³.
Explanation: At 25 seconds on the graph during exercise, the volume change between inhalation and exhalation is about 2.5 dm³, which represents the volume of air inspired in one breath.
(c)(iii) Exercise increases both the rate and depth of breathing. More oxygen is needed for aerobic respiration in muscles, and more carbon dioxide needs to be removed. The brain detects increased CO₂ levels and sends impulses to respiratory muscles to contract more frequently and forcefully.
Explanation: During exercise, muscles require more energy, which is produced through increased aerobic respiration. This requires more oxygen and produces more carbon dioxide. The brain’s respiratory center detects the rising CO₂ levels (or the accompanying drop in pH) and responds by increasing both the rate (number of breaths per minute) and depth (tidal volume) of breathing. This ensures that more oxygen is delivered to the muscles and more CO₂ is removed from the body. Additionally, adrenaline released during exercise stimulates faster breathing.
(a) Fig. 2.1 is a diagram of the gas exchange system in humans.
(i) State the names of the parts labelled X, Y and Z in Fig. 2.1.
(ii) The wall of the trachea contains rings of tissue.
State the name of this tissue and describe its function.
(iii) State the names of two types of cells responsible for protecting the breathing system from particles.
(b) A scientist estimated the pressure and volume in the thorax during one breath.
Fig. 2.2 shows a graph of the results.
Describe and explain the changes in the thorax that occur during section A only in Fig. 2.2.
(c) Complete the sentences to describe the effect of carbon dioxide concentration on breathing.
During physical activity, the carbon dioxide concentration of the blood ……….. .
This is detected by the ……….. .
This results in an increased rate and greater ……….. of breathing.
▶️ Answer/Explanation
(a)(i)
X: External intercostal muscle (helps in rib movement during breathing).
Y: Bronchiole (smaller airway branching from bronchi).
Z: Diaphragm (contracts/flattens during inhalation).
(a)(ii)
Name: Cartilage (a strong, flexible connective tissue).
Function: Prevents tracheal collapse during pressure changes.
(a)(iii)
1: Ciliated cells (sweep mucus and trapped particles away).
2: Goblet cells (secrete mucus to trap foreign particles).
(b)
Section A represents inhalation:
– Thoracic volume increases due to diaphragm contraction and rib movement.
– Pressure decreases as air rushes into the lungs (Boyle’s Law: \( P \propto \frac{1}{V} \)).
(c)
During activity, CO2 concentration increases.
Detected by chemoreceptors in the brain.
Results in faster and deeper breathing to expel CO2.