Introduction

Welcome to one of the most important chapters in your Human Biology course! We have already looked at how the thorax works and how gas exchange happens in the alveoli. Now, we are going to look at what happens when things go wrong. In this chapter, we explore how smoking damages the delicate structures of the respiratory system. Understanding this isn't just about passing your exams; it’s about understanding how chemicals can change the way our cells and organs function. Don't worry if the names of the diseases sound complicated—we will break them down step-by-step!

The "Big Three" Killers in Cigarette Smoke

Cigarette smoke contains over 4,000 chemicals, but for your IGCSE, we focus on the three main troublemakers that cause respiratory damage.

1. Tar

Tar is a thick, sticky, black substance that collects in the lungs as smoke cools down.
  • Cilia Paralysis: The trachea and bronchi are lined with tiny hairs called ciliated epithelial cells. Their job is to sweep mucus (which traps dirt and bacteria) up and out of the lungs. Tar coats these hairs and stops them from moving.
  • Mucus Buildup: Because the cilia can't move the mucus, it sits in the lungs. This leads to the "smoker's cough" as the body tries to force the mucus out.
  • Carcinogens: Tar contains chemicals that cause mutations in the DNA of lung cells, which can lead to lung cancer.

2. Nicotine

Nicotine is the chemical that makes smoking so difficult to quit.
  • Addiction: It is a stimulant drug that affects the brain, creating a physical dependency.
  • Circulatory Strain: While we are focusing on respiration, it's important to know that nicotine increases heart rate and blood pressure, which adds extra stress to a body already struggling to get enough oxygen.

3. Carbon Monoxide \( (CO) \)

Carbon monoxide is a poisonous gas that is very "greedy" for your blood cells.
  • Hemoglobin Binding: Usually, hemoglobin in your red blood cells carries oxygen \( (O_{2}) \). However, carbon monoxide binds to hemoglobin much more tightly than oxygen does, forming carboxyhemoglobin.
  • Reduced Oxygen: Because the hemoglobin is "busy" carrying \( CO \), it can carry much less \( O_{2} \). This is why smokers often get out of breath easily—their blood simply cannot carry enough oxygen to their muscles.

Major Respiratory Diseases Caused by Smoking

Smoking doesn't just make the lungs "dirty"; it physically changes their structure. Here are the three main conditions you need to know:

Chronic Bronchitis

When tar paralyzes the cilia, mucus builds up in the bronchi and bronchioles. This mucus becomes a breeding ground for bacteria, leading to painful infections and inflammation. Key symptom: A persistent, phlegm-filled cough and difficulty breathing.

Emphysema

This is a very serious condition where the smoke damages the walls of the alveoli.
  • The thin walls of the alveoli break down and fuse together.
  • Instead of having many tiny bubbles (which provide a huge surface area), the lungs end up with a few large, irregular "bags."
  • This massive reduction in surface area to volume ratio means diffusion of oxygen into the blood happens much more slowly.
Analogy: Imagine a sponge (lots of surface area to soak up water) turning into a hollow plastic cup. The cup can't "soak up" nearly as much!

Lung Cancer

As mentioned before, the carcinogens in tar cause cells to divide uncontrollably by mitosis. This creates a tumor that can block airways and spread to other parts of the body.
Quick Review: The Chain Reaction

Tar paralyzes Cilia \(\implies\) Mucus builds up \(\implies\) Bacteria grow \(\implies\) Bronchitis occurs.


Common Mistakes to Avoid

  • Mistake: Saying nicotine causes cancer. Correction: While nicotine is addictive and affects the heart, it is the tar that contains the carcinogens that primarily cause lung cancer.
  • Mistake: Thinking carbon monoxide makes you cough. Correction: Carbon monoxide is odorless and colorless; it affects oxygen transport in the blood, not the "feeling" in your throat. The cough is caused by tar and mucus.
  • Mistake: Confusing the trachea with the esophagus. Correction: Remember, the trachea is for breathing (respiratory), and the esophagus is for food (digestive). Smoking damages the trachea.

Did You Know?

If you could flatten out all the alveoli in a healthy pair of human lungs, they would cover the surface of an entire tennis court! This massive surface area is what allows us to absorb oxygen so efficiently. In a person with severe emphysema, that "tennis court" might shrink to the size of a small table.

Summary: Key Takeaways

1. Tar damages the ciliated epithelial cells and causes Bronchitis and Lung Cancer.
2. Nicotine is the addictive substance that also increases heart rate.
3. Carbon Monoxide binds to hemoglobin, reducing the blood's ability to carry oxygen.
4. Emphysema involves the destruction of alveoli walls, which reduces the surface area for gas exchange.


Note: To understand how these changes affect your ability to exercise, see the next chapter on "Exercise, fitness and oxygen debt."