Gaseous Exchange in Humans and Plants
Senior 2 · Biology
Gaseous Exchange in Humans and Plants
Lesson 4 of 5
25 min
Let's begin
Coughs, colds and chest infections are among the most common reasons people visit a health centre. Some respiratory diseases pass from person to person in the tiny droplets we breathe out; others are caused by what we breathe in, such as dust, cooking smoke and tobacco smoke. Knowing the cause of each one tells you how to prevent it.
An infectious disease is caused by a pathogen (a disease-causing microorganism such as a bacterium or virus) and can spread from person to person. A non-infectious disease cannot be caught from someone else.
| Disease | Cause | Spread | Main signs | Prevention and treatment |
|---|---|---|---|---|
| Tuberculosis (TB) | bacterium (Mycobacterium tuberculosis) | droplets from coughing and sneezing | cough lasting more than two weeks, sometimes with blood; fever; night sweats; weight loss | BCG vaccine; well-ventilated rooms; a full course of antibiotics, usually for six months |
| Pneumonia | bacteria or viruses infecting the alveoli, which fill with fluid | droplets | fever, cough, fast or difficult breathing, chest pain | vaccination; antibiotics for bacterial pneumonia |
| Bronchitis | inflammation of the bronchi: usually a virus (short-term), or smoke and dust (long-term) | droplets, if caused by an infection | cough with mucus, tight chest | avoid smoke and dust; rest and fluids |
| Whooping cough (pertussis) | bacterium (Bordetella pertussis) | droplets | violent coughing fits ending in a "whoop"; very dangerous for babies | the DTP vaccine given to infants; antibiotics |
| Common cold | viruses | droplets and touching contaminated hands or surfaces | runny nose, sneezing, sore throat | hand washing; rest and fluids (antibiotics do not work) |
| Influenza (flu) | influenza virus | droplets | fever, aching muscles, cough, tiredness | flu vaccine; rest and fluids |
| COVID-19 | virus (SARS-CoV-2) | droplets and tiny airborne particles | fever, cough, loss of taste or smell, difficult breathing | vaccination; ventilation; hand washing; masks in crowded places |
| Asthma (not infectious) | the bronchioles narrow when triggered by dust, pollen, smoke, cold air or exercise; their muscles tighten, their lining swells and makes extra mucus | not spread | wheezing, breathlessness, tight chest | avoid triggers; reliever inhalers widen the airways |
Worked example
A doctor prescribes antibiotics for TB but not for a cold. Why? Antibiotics kill bacteria, or stop them growing, but they have no effect on viruses. TB is caused by a bacterium, so antibiotics cure it. A cold is caused by a virus, so antibiotics would not help, and using them unnecessarily helps bacteria become resistant.
Worked example
Why does pneumonia make a patient breathless even at rest? Reasoning: fluid fills some alveoli, so those alveoli hold no air and cannot exchange gases, and where fluid lines the walls the diffusion distance is longer. Less oxygen enters the blood, so the patient breathes faster to make up for it.
Protect yourself and others: cough or sneeze into a tissue or your elbow, wash your hands with soap, keep rooms ventilated, complete vaccinations, and go to a health centre early if a cough lasts more than two weeks.
| Substance in tobacco smoke | Effect |
|---|---|
| Tar | a sticky mixture that coats the airways, paralyses and destroys the cilia, and contains carcinogens (substances that cause cancer) |
| Carbon monoxide | a poisonous gas that combines with haemoglobin more strongly than oxygen does, so the blood carries less oxygen |
| Nicotine | an addictive drug that raises heart rate and blood pressure and narrows blood vessels |
| Smoke particles | irritate the airways and make them produce extra mucus |
The results build up over years:
Breathing other people's smoke (passive smoking) causes similar harm, which is why Rwanda's law bans smoking in public places. Smoke from cooking over wood or charcoal in a closed room also damages the lungs, so good ventilation and improved stoves protect families.
Tip
Bacteria → antibiotics can help (TB, bacterial pneumonia, whooping cough). Viruses → antibiotics are useless (cold, flu, COVID-19).
For smoking, remember one effect per substance: tar → cilia and cancer; carbon monoxide → less oxygen carried; nicotine → addiction and heart.
Name two diseases in the table that can be prevented by vaccination.
Solution: Any two of: tuberculosis (BCG), whooping cough (DTP), influenza, COVID-19, pneumonia.
Why is asthma not described as an infectious disease?
Solution: It is not caused by a pathogen and cannot be passed from one person to another. The airways narrow in response to triggers such as dust or smoke.
Explain why a heavy smoker becomes breathless when climbing a hill.
Solution: Carbon monoxide takes up some of the haemoglobin, so the blood carries less oxygen; and damage such as emphysema reduces the surface area of the alveoli, so less oxygen enters the blood.
A student has had a cough for three weeks with night sweats and weight loss. What should the student do, and why?
Solution: Visit a health centre to be tested for TB. These are its typical signs, TB is curable with a full course of antibiotics, and early treatment also stops it spreading to others.
Remember
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