Life Processes · Lesson 8 of 13
Respiration in Human Beings
“Millions of tiny air sacs keep every cell supplied with oxygen.”
• Trace air through the human respiratory system. • Explain how air is filtered and passages remain open. • Describe inhalation and exhalation. • Relate alveolar structure to rapid gas exchange. • Explain transport of oxygen and carbon dioxide. • Describe how tobacco damages respiratory health.
Every cell needs a steady oxygen supply, yet oxygen enters the body only at the lungs. The respiratory system provides a protected route for air, an enormous exchange surface and a close partnership with the circulatory system.
Passage And Preparation Of Air
Air enters through the nostrils. Fine hairs trap larger particles, while mucus captures dust and microbes and moistens the air. Air then passes through the throat into branching airways. Cartilage rings prevent the main air passage from collapsing when pressure changes during breathing.
Breathing Mechanism
During inhalation, ribs lift and the diaphragm flattens. The chest cavity becomes larger, pressure inside falls, and air moves into the lungs. During exhalation, the ribs return and the diaphragm relaxes upward, reducing chest volume and pushing air out. A residual volume remains, allowing gas exchange to continue between breaths.
| Feature | Inhalation | Exhalation |
|---|---|---|
| Ribs | Move upward and outward | Move downward and inward |
| Diaphragm | Contracts and flattens | Relaxes and becomes dome-shaped |
| Chest volume | Increases | Decreases |
| Air movement | Into lungs | Out of lungs |
Alveoli And Gas Exchange
Alveoli are microscopic, thin-walled air sacs in the lungs that provide a large surface for exchange of oxygen and carbon dioxide with blood.
Each branching airway ends in many alveoli. Their walls are extremely thin, moist and surrounded by dense capillary networks. Their combined surface is very large. Oxygen diffuses from alveolar air into blood, while carbon dioxide diffuses from blood into the alveoli. A steep concentration difference, thin barrier and large area make this exchange rapid.
| Adaptation | Effect on gas exchange |
|---|---|
| Very large number of alveoli | Produces a vast surface area |
| One-cell-thick walls | Creates a short diffusion distance |
| Moist lining | Allows gases to dissolve before diffusion |
| Dense capillary supply | Maintains concentration differences and carries gases away |
| Residual air | Provides time and continuity for exchange |
Transport Of Respiratory Gases
Diffusion alone would be far too slow to carry oxygen from lungs to distant tissues. Haemoglobin in red blood corpuscles binds oxygen where its concentration is high and releases it in tissues where oxygen is lower. Carbon dioxide is more soluble in water than oxygen and is transported mainly in dissolved chemical forms in blood.
Haemoglobin is the iron-containing respiratory pigment in red blood corpuscles that binds and transports oxygen.
Problem
Why do lungs contain millions of small alveoli rather than two simple hollow sacs?
- 1.Dividing the space into many tiny sacs greatly increases total surface area.
- 2.More air can contact the thin exchange surface at the same time.
- 3.A larger area permits more oxygen and carbon dioxide to diffuse per unit time.
- 4.Therefore, numerous alveoli make gas exchange far more efficient.
Problem
Predict the effect of a significant haemoglobin deficiency.
- 1.The lungs may still fill with oxygen normally.
- 2.However, blood has less capacity to bind and carry oxygen.
- 3.Tissues receive less oxygen for aerobic respiration.
- 4.Less ATP may become available during activity.
- 5.The person may experience tiredness and breathlessness.
Problem
A disease thickens alveolar walls without changing breathing rate. Explain why oxygen delivery may fall.
- 1.Oxygen must diffuse across a greater distance.
- 2.Diffusion becomes slower even if air reaches the alveoli.
- 3.Less oxygen enters capillary blood per unit time.
- 4.Haemoglobin leaving the lungs may carry less oxygen.
- 5.Tissues may then receive inadequate oxygen.
Tobacco And Respiratory Health
Tobacco smoke contains harmful substances that damage cilia, irritate air passages and harm lung tissue. Damaged cilia remove dust and microbes less effectively, increasing infection risk. Tobacco use also raises the risk of cancers, heart disease, stroke and chronic respiratory disease. Smokeless tobacco is not a safe alternative.
Lungs do not actively push oxygen into blood. Oxygen and carbon dioxide diffuse because their concentrations differ across the thin alveolar-capillary surface.
Quiz
What prevents the main air passage from collapsing?
What happens to the diaphragm during inhalation?
Which alveolar feature shortens diffusion distance?
Where is haemoglobin found?
Why can smoking increase respiratory infections?
Practice Problems
- Trace a molecule of oxygen from a nostril to a muscle cell.
- Explain pressure changes during inhalation using rib and diaphragm movement.
- Describe four structural adaptations of alveoli and connect each to its function.
- Explain why normal lungs cannot fully compensate for severe haemoglobin deficiency.
- Compare the transport of oxygen and carbon dioxide in human blood.
Key Takeaways
• Air is filtered, moistened and conducted through protected passages. • Rib and diaphragm movements change chest volume and air pressure. • Alveoli provide a large, thin, moist and well-supplied exchange surface. • Oxygen and carbon dioxide diffuse in opposite directions across alveolar walls. • Haemoglobin carries most oxygen. • Carbon dioxide is transported mainly in dissolved chemical forms. • Tobacco damages respiratory defences and increases serious disease risk.