Structure of Thorax

Thorax is the chest cavity, which is the part of the body between the neck and the abdomen. Important organs for gas exchange, such as the lungs and air passages, are located in the thorax.

• Ribs: Ribs are the bones that surround the chest cavity and protect internal organs, such as the lungs.
• Intercoastal muscles: Intercoastal muscles are located between the ribs and help to expand and shrink the size of the chest cavity to facilitate breathing.
• Diaphragm: Diaphragm is a muscle at the base of the thorax. It contracts and flattens when we breathe in and out.
• Trachea: Trachea is the passageway where air flows into the thorax.
• Bronchi: The thorax branches into two bronchi, one for each lung.
• Bronchioles: Each bronchus further splits into smaller tubes called bronchioles. 
• Alveoli: At the end of each bronchiole is an alveolus, which is a tiny air sac where gas exchange actually occurs.
• Pleural membrane: Pleural membrane is a thin, moist and slippery layer that covers the lungs. It provides lubrication to the lungs so that the lungs can move easily with minimal friction between other organs.

The Role of Intercoastal Muscles and Diaphragm During Ventilation

When we breathe in (inhalation):

• The external intercoastal muscles contract, pulling the ribcage upwards.
• The diaphragm contracts downwards.
• The volume of thorax cavity increases, and the pressure inside the thorax cavity decreases.
• Air moves from outside the body (higher pressure) to the lungs (lower pressure).

• The external intercoastal muscles relax, pulling the ribcage downwards.
• The diaphragm relaxes upwards.
• The volume of thorax cavity decreases, and the pressure inside the thorax cavity increases.
• Air moves from the lungs (higher pressure) to outside the body (lower pressure.)

How the Structure of Alveoli is Adapted for Gas Exchange

Gas exchange happens between the alveoli in the lungs and capillaries that are located in the walls of alveoli. 

• As deoxygenated blood (high concentration of carbon dioxide and low concentration of oxygen) flows along the alveoli, carbon dioxide diffuses from the capillary to the alveoli. The carbon dioxide is then exhaled out of the body.
• The inhaled oxygen reaches the alveoli and diffuses from the alveoli to the capillary. Oxygenated blood (high concentration of oxygen and low concentration of carbon dioxide) is transported through the whole body.

Adaptations of the alveoli structure to facilitate gas exchange:

• The walls of alveoli are only one cell thick, which shortens the distance for diffusion of carbon dioxide and oxygen.
• Alveoli are folded to have a very large surface area for diffusion.
• Alveoli have a moist surface so that gases can easily dissolve. 

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