Human Breathing System Structure

Structure of the Human Breathing System

• The lungs are enclosed in the thorax, an airtight compartment.
• Pleural membranes surround each lung and line the thorax.
• The pleural cavity sits between the pleural membranes and contains a few cm³ of pleural fluid, which acts as a lubricant.
• The pleural fluid prevents friction between the lungs and the inner wall of the thorax during ventilation.
• A dome-shaped sheet of muscle, the diaphragm, separates the thorax from the abdomen and sits at the base of the thorax.
• The ribs surround the thorax.
• Intercostal muscles are located between the ribs.
• The trachea is a flexible airway that brings air to the lungs.
• The two bronchi are branches of the trachea.
• The lungs consist of a branching network of tubes called bronchioles, which arise from the bronchi.
• Air sacs called alveoli sit at the ends of the bronchioles.
• Cartilage rings around the trachea are incomplete at the back to allow the oesophagus to bulge when swallowing a bolus of food.

Ventilation of the Lungs

• Mammals ventilate their lungs through negative pressure breathing, where the pressure inside the lungs must be below atmospheric pressure for air to enter.
• Breathing in(inspiration/inhalation): This is an active process because muscle contraction requires energy.
• The external intercostal muscles contract.
• The ribs are pulled upwards and outwards.
• The diaphragm muscles contract, so the diaphragm flattens at the same time as the above.
• The outer pleural membrane is attached to the thoracic cavity wall, so it is pulled up and out with the ribs and the lower part is pulled down with the diaphragm.
• The inner membrane follows, so the lungs expand, increasing the volume inside the alveoli.
• This reduces the pressure in the lungs.
• Atmospheric air pressure is now greater than the pressure in the lungs, so air is forced into the lungs.
• The internal intercostal muscles are antagonistic to the external intercostal muscles as they relax when the external intercostal muscles contract in inspiration and vice versa on expiration.