Respiratory System: Study Notes

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Human Respiratory Tract

The human respiratory system is a complex network of structures that allows for the intake of oxygen and removal of carbon dioxide, essential for cellular respiration and energy production. The process begins as air enters the body, passing through various structures that filter, warm, and direct it towards the lungs, where gas exchange occurs. Here is the step-by-step pathway of air through the respiratory tract, highlighting each structure and its role:

1. Nose

• The respiratory process begins at the nose, where air enters through a pair of external nares.
• These nasal passages are lined by a mucous membrane that traps dust particles and humidifies the air.

2. Pharynx

• Air then moves into the pharynx, a shared pathway for both food and air.

3. Larynx

• The air is directed from the pharynx into the larynx through an opening called the glottis, which is guarded by the epiglottis to prevent food from entering the airway.
• The larynx, or voice box, contains vocal cords that enable sound production.

4. Trachea

• From the larynx, air passes into the trachea, a tube lined with ciliated epithelium that moves trapped dust and mucus upwards.
• Goblet cells in the tracheal lining produce mucus to help trap debris.

5. Bronchi

• The trachea divides into two bronchi, each leading air into one lung (right or left).

6. Bronchioles

• Within each lung, the bronchi further divide into smaller tubes called bronchioles.
• These bronchioles continue branching and eventually end in clusters of tiny air sacs.

7. Alveoli

• At the end of the bronchioles are alveoli, where gas exchange occurs.
• Each alveolus is lined with a thin fluid layer containing surfactant, which reduces surface tension and prevents collapse during gas exchange.

8. Lungs

• The lungs, where the alveoli are housed, are soft and spongy structures.
• The right lung has three lobes, and the left lung has two lobes, each surrounded by pleural membranes with a fluid-filled pleural cavity to facilitate smooth lung movement.

Mechanism of Breathing: Inspiration vs. Expiration

Inspiration (Active Process)

• Muscle Action: External intercostal muscles contract; internal intercostal muscles relax.
• Diaphragm: Contracts and flattens to increase thoracic volume.
• Rib Cage: Moves up and out, increasing thoracic volume.
• Thoracic Volume: Increases.
• Thoracic Pressure: Decreases below atmospheric pressure, creating a pressure differential.
• Air Movement: Air is drawn into the lungs (negative-pressure breathing).
• Alveoli: Inflate as air enters until lung pressure equals atmospheric pressure.

Expiration (Passive Process)

• Muscle Action: External intercostal muscles relax; internal intercostal muscles contract.
• Diaphragm: Relaxes and becomes dome-shaped, reducing thoracic volume.
• Rib Cage: Drops due to its weight, reducing thoracic volume.
• Thoracic Volume: Decreases.
• Thoracic Pressure: Increases above atmospheric pressure.
• Air Movement: Air is expelled from the lungs.
• Alveoli: Air leaves the lungs as thoracic pressure rises.

TRANSPORT OF OXYGEN

• Since the alveolar air contains a higher concentration of oxygenthan the blood, oxygen dissolves in the moisture lining thealveolar walls and then diffuses into the blood capillaries.
• Oxygen combines with the haemoglobin in red blood cellsto form oxyhaemoglobin. This reaction isreversible.
• Each hemoglobin binds with 4 oxygen molecules.
• When the blood passes through oxygen-poor tissues, the oxyhaemoglobin releases oxygen, which will then diffuse through the walls of the blood capillaries into the cells of the tissues.

TRANSPORT OF CARBON DIOXIDE

• Tissue cells produce a large amount of carbon dioxide as a result of aerobic respiration.
• As blood passes through these tissues via blood capillaries, carbon dioxide diffuses into the blood and enters the red blood cells.
• The carbon dioxide then reacts with water in the red blood cells to form carbonic acid. This reaction is catalysed by the enzyme carbonic anhydrase which is present in red blood cells.
• The carbonic acid is then converted into hydrogencarbonate ions which diffuse out of the red blood cells.
• 70% of the carbon dioxideis carried as hydrogen carbonate ions in the bloodplasma.
• 20% is carried as carbaminohemoglobin.
• 5% is carried by plasma proteins in blood.

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