Lungs

Lungs

The pair of lungs are the actual organs of respiration. Each lung is conical in shape. The base of the lung rests on the diaphragm. The right lung is larger than the left and it weighs around 620g. The left lung weighs 560g. The right lung has three lobes and the left lung has two.

The lungs are placed within the thoracic cavity. Each lung is surrounded by separate pleural membrane. The region inside the pleural membrane is named as the pleural cavity. This cavity is filled with pleural fluid.

The region in between the two lungs is named as the mediastinum. It is a midline partition, being occupied by the heart, trachea and oesophagus

Structures such as the primary bronchi, blood vessels, nerves and lympatic vessels enter or exit the lungs at a specific region on the inner margin of the lungs. This region is known as the hilum. All structures passing through the hilum are referred to as the root of the lung.

The primary bronchi on entering into each lung divide further into secondary bronchi. There are two secondary bronchi in the left lung and three in the right lung. The secondary bronchi inturn give rise to tertiary bronchi. They divide still further and finally give rise to bronchioles. The diameter of the bronchioles is less than 1 mm. These bronchioles divide several times to become still smaller terminal bronchioles.

Like the trachea, the primary bronchi are supported by ‘C’. shaped cartilages and smooth muscles. As the bronchi become smaller the cartilages are replaced by smooth muscles.

The terminal branchioles end in small air filled chambers called alveoli. The alveoli are thin walled pouches. They collectivelly provide the respiratory surface for gaseous exchange. The wall of the alveolus is very thin providing a minimal barrier to gaseous exchange between air and blood. The thickness of the wall of the alveolus is as little as 0.05m. Studies have shown that in human lungs there are about 300 million alveoli. They provide a mean total alveolar surface area value of 143 m2 .