3.2Exchange and Transport
Human Gas Exchange — AQA A Level Biology 7402 Revision Notes & Topical Questions
10 min read 18 parts Lesson 10 of 48 Unit quiz
Human Gas Exchange System and Dissection — AQA A LEVEL BIOLOGY 7402 notes: Describe the structure of the human gas exchange system and how it, fish gills and insect tracheae are dissected and observed.The human gas exchange system is a series of organs that work together to enable gas exchange.
- bronchioles
- Insect tracheal system
- Worked example 1
- Worked example 2
- Worked example 3
- Respirometer
Section 1 of 18Structure of the human gas exchange system
The human gas exchange system is a series of organs that work together to enable gas exchange.
Section 2 of 18Dissection
Gas exchange system structure can be examined during dissection using equipment such as a scalpel (a sharp blade for precise cutting), scissors, forceps and a dissecting board. A lung dissection allows larger structures, such as the trachea, bronchi and bronchioles, to be examined. Take care and follow safety rules, wearing gloves and disposing of waste correctly.
Section 3 of 18Microscopy of gas exchange surfaces
Pre-prepared slides can be viewed with an optical microscope.
Section 4 of 18Investigating gas exchange: the respirometer
The volume of gas involved in gas exchange can be measured using a respirometer, which measures oxygen consumption in living organisms.
Section 5 of 18Worked examples
State the pathway taken by an oxygen molecule from the nose to the blood. [3]3 marks
Trachea, bronchus, bronchiole, alveolus, then diffuses across the alveolar epithelium and capillary endothelium into the blood.
Explain why soda lime is placed in a respirometer. [2]2 marks
It absorbs carbon dioxide produced, so the change in pressure is caused only by oxygen being used.
Describe how you would locate the gills of a fish in a dissection. [2]2 marks
Lift the operculum on the side of the head; the gills lie beneath it as red filaments on bony arches.
Section 6 of 18Alveolar epithelium as an exchange surface
Oxygen and carbon dioxide are exchanged by diffusion between the alveoli and the capillaries.
- and a steep concentration gradient maintained by ventilation and blood flow.
Section 7 of 18Ventilation
Oxygen diffuses down its concentration gradient from the air into the blood, and carbon dioxide diffuses from the blood into the air. Inhalation is an active process: the external intercostal muscles and diaphragm contract, the ribs move up and out and the diaphragm flattens, increasing the thorax volume and decreasing pressure below atmospheric, so air flows in.
Exhalation
- the diaphragm relaxes and curves upwards and the external intercostal muscles relax, the ribs fall, volume decreases and pressure rises above atmospheric, so air is forced out.
- Normal exhalation is passive.
- During forced exhalation it becomes active: the internal intercostal muscles contract to pull the ribs down and the abdominal muscles contract to push organs upwards against the diaphragm.
Section 8 of 18Measuring ventilation
Pulmonary ventilation rate is the volume of air an individual breathes per minute, an important measure of respiratory function, especially in exercise or illness. Tidal volume is the volume of air inhaled or exhaled in one normal breath; breathing rate is breaths per minute. Pulmonary ventilation rate = tidal volume × breathing rate.
Section 9 of 18Lung disease
Several conditions reduce lung function by affecting the structure of the alveolar walls or damaging the airways. Examples are lung cancer, where tumours develop due to uncontrolled cell division; fibrosis; asthma; and emphysema, in which alveolar walls are destroyed.
Worked explanation of emphysema symptoms
- patients have fewer or larger alveoli, so a reduced surface area;
- the rate of diffusion is reduced, so less oxygen enters the blood (causing breathlessness);
- there is a reduced rate of respiration, so less energy is released (causing fatigue).
Section 10 of 18Worked examples
Calculate pulmonary ventilation rate when tidal volume is 500 cm3 and breathing rate is 12 per minute. [1]1 mark
500 × 12 = 6000 cm3 min-1.
Explain why emphysema causes breathlessness and tiredness. [3]3 marks
Fewer, larger alveoli give a smaller surface area, slowing diffusion so less oxygen reaches the blood. Less aerobic respiration means less ATP, so fatigue.
Describe the changes that cause air to enter the lungs. [3]3 marks
The diaphragm contracts and flattens, and the external intercostal muscles contract to lift the ribs. Thorax volume increases, pressure drops below atmospheric and air flows in.
Section 11 of 18Pollution and smoking data
Exposure to air pollution and cigarette smoke has been linked to lung disease, for example through chemicals that affect the ability of the blood to transport oxygen. Exam questions may present data on lung disease and ask what can be concluded.
Section 12 of 18Evaluating a conclusion
Examplea student concluded from data that smoking causes lung cancer.
- so have cancer rates in men.
Section 13 of 18Risk factors
A risk factor is any factor linked to an increased probability of suffering from a particular condition, such as smoking or exposure to certain chemicals at work. You are not expected to know lung disease risk factors, but you must interpret data on their relationship to the incidence of lung disease.
Example (3 marks): analyse data on risk factors and worsening COPD. Being over 60 and having other chronic conditions are slightly associated with increased probability of worsening; being hospitalised in the past year has the strongest association; being male and being a current smoker are associated with a decreased probability of worsening.
The last result contradicts other data sources. Possible explanations are a small sample size, a lack of statistical testing, and failure to control other variables such as diet.
Section 14 of 18Study design and statistics
It would be unethical to run a controlled experiment asking one random group to smoke 10 cigarettes a day for 10 years and another to avoid smoking, so researchers rely on asking people about lifestyle and looking for patterns.
- and whether statistical tests were done.
Section 15 of 18Correlation and causation
A correlation is an association between variables.
Section 16 of 18Worked examples
A graph shows a positive correlation between pollution and COPD. State what can be concluded. [2]2 marks
There is an association between pollution level and COPD; higher pollution is correlated with higher COPD incidence. Causation cannot be concluded without further evidence.
Suggest why a study finds that smokers have lower risk of worsening COPD. [3]3 marks
Small sample size, no statistical test, and other variables (such as diet or age) not controlled.
Explain why a controlled experiment on smoking cannot be done. [1]1 mark
It would be unethical to expose people to a harmful risk.
Section 17 of 18Key terms
| Term | Meaning |
|---|---|
| Trachea | Windpipe |
| Bronchus | Airway from the trachea into a lung |
| Bronchiole | Narrow airway leading to alveoli |
| Operculum | Bony flap covering fish gills |
| Respirometer | Apparatus that measures oxygen consumption |
| Manometer | U-shaped tube measuring pressure difference |
| Term | Meaning |
|---|---|
| Alveolus | Air sac where gas exchange occurs |
| Tidal volume | Volume of one normal breath |
| Ventilation rate | Volume of air breathed per minute |
| Diaphragm | Muscle beneath the lungs that contracts to cause inhalation |
| Emphysema | Disease in which alveolar walls are destroyed |
| Term | Meaning |
|---|---|
| Correlation | Association between two variables |
| Causal relationship | One variable directly produces change in the other |
| Risk factor | Factor linked to a greater chance of disease |
| Statistical test | Test to see if a result is significant or due to chance |
| COPD | Chronic obstructive pulmonary disease |
Section 18 of 18Exam technique
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