1.0 The Respiratory System: The Air Passage
Respiration is the process of inhaling oxygen and exhaling carbon dioxide, alongside the oxidation of food to release energy. The human respiratory tract is a series of organs that filter, warm, and transport air to the lungs.
1.1 The Respiratory Pathway
Air travels through a specific sequence of organs. Understanding this order is essential for "Sequence the following" questions in exams.
| Organ | Key Features & Function |
|---|---|
| Nose & Nasal Cavity | Lined with hair and Ciliated Epithelium to filter dust and germs. |
| Pharynx | A common passage for both food and air. |
| Larynx (Voice Box) | Houses vocal cords. Protected by the Epiglottis during swallowing. |
| Trachea (Windpipe) | Supported by C-shaped cartilaginous rings to prevent collapse. |
1.2 The Protective Lining
The entire respiratory tract (except the finest parts) is lined with specialized cells that keep the lungs clean.
- Mucus: Traps dust particles and bacteria.
- Cilia: Tiny hair-like structures that move in a rhythmic "flicking" motion to push mucus towards the throat to be swallowed or coughed out.
In "Give Reason" questions, you are often asked why we shouldn't talk while eating. The Epiglottis is a leaf-like flap that closes the windpipe during swallowing. Talking may cause the epiglottis to stay open, allowing food to enter the trachea and causing choking.
The Adam's Apple is a prominent part of the Larynx (thyroid cartilage) that is usually more visible in males than females due to hormonal changes during puberty.
2.0 The Lungs: The Center of Exchange
The lungs are a pair of spongy, elastic organs located in the thoracic cavity, protected by the rib cage. The right lung is slightly larger and has three lobes, while the left lung has only two lobes to make space for the heart.
2.1 The Pleura
Each lung is enclosed by a double-walled membrane called the Pleura.
- Pleural Fluid: The space between the two membranes is filled with pleural fluid.
- Function: It acts as a lubricant, reducing friction between the lungs and the chest wall during breathing.
2.2 Alveoli: The Functional Units
The bronchioles eventually terminate in clusters of tiny air sacs called Alveoli. There are roughly 300 to 700 million alveoli in the human lungs.
| Feature of Alveoli | Importance for Gas Exchange |
|---|---|
| Enormous Surface Area | Increases the total area available for O2 and CO2 diffusion. |
| Thin Epithelium | The walls are only one-cell thick, allowing for rapid diffusion. |
| Rich Blood Supply | Surrounded by a dense network of capillaries for immediate transport. |
| Moist Surface | Gases must dissolve in moisture before they can diffuse. |
In respiratory system diagrams, look for the dome-shaped muscular sheet at the base of the chest cavity. This is the Diaphragm. It separates the thoracic (chest) cavity from the abdominal cavity and is the primary muscle for breathing.
The Trachea divides into two Bronchi (singular: Bronchus). Inside the lungs, these divide into smaller and smaller tubes called Bronchioles, which eventually lead to the alveoli.
3.0 How We Breathe: The Physical Process
Breathing is a mechanical process involving two phases: Inspiration (breathing in) and Expiration (breathing out). It is driven by changes in the volume and pressure of the thoracic cavity.
3.1 Inspiration vs. Expiration
| Feature | Inspiration (Inhalation) | Expiration (Exhalation) |
|---|---|---|
| Diaphragm | Contracts and moves downward (becomes flat). | Relaxes and moves upward (becomes dome-shaped). |
| Rib Cage | Moves upward and outward. | Moves downward and inward. |
| Thoracic Volume | Increases. | Decreases. |
| Air Pressure | Decreases inside lungs (Air rushes in). | Increases inside lungs (Air pushed out). |
3.2 Composition of Inspired vs. Expired Air
The air we breathe out is not just "Carbon Dioxide"; it is a mixture with shifted percentages compared to what we breathe in.
| Gas | Inspired Air | Expired Air |
|---|---|---|
| Oxygen | ~21% | ~16.4% |
| Carbon Dioxide | 0.04% | ~4.4% |
| Nitrogen | 78% | 78% (Unchanged) |
In "Name the following" questions, remember that the muscles between the ribs are called Intercostal Muscles. During inspiration, the External intercostals contract to lift the ribs.
As air passes through the respiratory tract, it absorbs body heat. Exhaled air is usually at body temperature (37°C) and is saturated with water vapor.
4.0 The Chemistry of Respiration
Once air reaches the alveoli, the process shifts from moving air to moving molecules. This happens through Diffusion—the movement of gases from an area of higher concentration to lower concentration.
4.1 Transport of Oxygen
Oxygen does not simply dissolve in the blood; it needs a carrier. That carrier is Haemoglobin, an iron-containing pigment found in Red Blood Cells (RBCs).
Haemoglobin + Oxygen ↔ Oxyhaemoglobin
This is an unstable compound, allowing oxygen to be easily released when it reaches the body cells.
4.2 Transport of Carbon Dioxide
CO2 is more soluble in water than oxygen and is transported in three main ways:
| Method | Form / Description | Approx. Amount |
|---|---|---|
| As Bicarbonates | Dissolved in the blood plasma as Sodium/Potassium Bicarbonate. | 70% |
| With Haemoglobin | Forms Carbamino-haemoglobin. | 23% |
| In Solution | Dissolved directly in the plasma. | 7% |
Haemoglobin has a much higher affinity (about 250 times) for Carbon Monoxide (CO) than for oxygen. It forms a stable compound called Carboxyhaemoglobin, which prevents the transport of oxygen and can lead to death. This is a very common "Explain why" question!
Inside the cells, glucose is broken down in the presence of oxygen to release energy. This happens in the Mitochondria and is summarized as:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (38 ATP)
5.0 Lung Capacities & Disorders
The amount of air our lungs can hold varies based on our activity level and physical health. These specific measurements are known as Pulmonary Volumes.
5.1 Vital Capacities and Volumes
| Term | Definition | Avg. Value |
|---|---|---|
| Tidal Volume | Air breathed in or out during normal quiet breathing. | 500 mL |
| Complemental Air | Extra air that can be inspired forcibly after a normal inspiration. | 3000 mL |
| Residual Volume | Air that always remains in the lungs, even after maximum expiration. | 1500 mL |
| Vital Capacity | The maximum air one can exhale after a maximum inspiration. | 4500 mL |
5.2 Common Respiratory Disorders
Diseases of the respiratory system can be caused by infections, allergies, or long-term exposure to pollutants.
Inflammation of the lining of the Bronchial tubes, leading to persistent coughing and mucus production.
A condition where the airways narrow and swell, often triggered by allergens like pollen or dust.
Breakdown of the Alveolar walls, usually caused by smoking, reducing the surface area for gas exchange.
Hypoxia is a condition where the body tissues do not get enough oxygen. Asphyxiation is a more extreme state of suffocation caused by a lack of oxygen and an excess of CO2 in the blood. Know the difference for "Define the term" questions!
- Can you trace the path of air from nostrils to alveoli?
- Do you understand why the trachea has cartilaginous rings?
- Can you explain the role of the diaphragm in inspiration?
- Do you know the difference between Vital Capacity and Tidal Volume?