Lungs

What are the lungs?
The lungs are the primary organs for respiration. They form the lower and the most important part of the respiratory tract. They are two in number and enclosed within the rib cage also called as the thorax.

Where are the lungs situated?
The lungs are situated in a special structure called as the rib cage or thorax. This is made of the ribs on either side of the chest joined by a strip of bone called the breastbone or sternum situated in between. The ribs on either side meet at the back at the spiny projections in the midline of the back called as vertebrae.

The two lungs envelop the heart structures in between themselves. Below the lungs lies the thick muscular layer called the diaphragm, which assists in breathing. The apex or the topmost part of the lung lies behind the collarbone or the clavicle. This is the area of the lung affected in diseases such as tuberculosis. The lungs are attached to the bronchi, which are the two divisions of the trachea, at the point of entry into each lung called as the hilum.

How can we describe the lungs?
The lungs are two spongy organs, brownish gray in color. As age progresses, due to the atmospheric influence, the lungs become mottled or spotted because of the deposition of inhaled carbon particles. The right lung is larger than the left lung. It is also heavier than the left lung weighing 700 gms. as compared to the left which weighs 600 gms.

Each lung is conical in shape, having a tapering are at the top called as the apex. It has a base, which rests on the diaphragm. The lung also has three borders; one in front, one the hind side and one at the bottom the lung surface.

Furthermore, the right lung is divided into three smaller divisions called as lobes by two dividing lines called as fissures. The left lung is divided into two lobes by a single obliquely placed fissure.

Internal structure of the lung:
The lungs are a continuation of the respiratory system. The trachea divides into two main divisions. Each division is called as the principal bronchus in the region of the thorax. The principal bronchus on the right side is shorter, thicker, wider and straighter than that on the left side. This structure makes the right principal bronchus and right lung more likely to get infected. Each bronchus further enters the lung through the hilum, the entry point of the lung.

As it enters the lung, the bronchus branches to each lobe of the respective lung. Thus it divides into three branches in the right lung and two branches in the left lung. These branches are called the lobar bronchi. The lobar bronchi further divide into 10 branches in the right lung and 8 branches in the left lung. These divisions are called as the segmental bronchi. They supply the corresponding number of divisions called ‘segments’ of the lung. The segmental bronchi are then divided into respiratory bronchioles. The part of the lung supplied by a single respiratory bronchiole is called as a pulmonary unit. The respiratory bronchioles further divide into the final structures of the lung viz. The alveolar ducts, the atria, air saccules and the alveolus. The gaseous exchange in the lung takes place in the alveolus.

Outer covering of the lung:
The lung is covered from the outside by two coats of thin tissue called as pleura. This is of two types; visceral and parietal. The visceral pleura is adherent to the lung itself and cannot be differentiated from the lung surface. The parietal pleura lines the lung on its outside, thus creating a space between the two layers of pleurae. This space is called the pleural cavity. This is where there is collection of fluid in case of severe lung infections.

The blood supply of the lung:
The main function of the lung is imparting oxygen to the blood. Blood from the heart flows to the lungs for oxygenation through blood vessels called as pulmonary arteries. Generally arteries carry pure blood but these are the only arteries in the body carrying impure or de-oxygenated blood. These are two in number and arise from the right ventricle that is the right bottom chamber of the heart.

The oxygen rich blood from the lungs enters the heart via the pulmonary veins into the left atrium or the left top chamber of the heart. These blood vessels are four in number.

Functions of the Lung

• The primary function of the lung is respiration. Respiration is a continuous process that is brought about by the need of oxygen for all the cells in the body. The process is controlled by the respiratory area in the brain and by the levels of oxygen in the blood.
• During inhalation negative pressure is created in the thoracic cavity which causes the atmospheric air to rush in. When the lungs are filled up there is increased positive pressure build up and air is exhaled automatically. In the meantime the oxygen from air travels from the alveoli in the lung to the blood vessels in the lung and the purified blood is sent to the heart for circulation in the whole body.
• The segmental division of the lungs prevents the spread of infection from one segment to the other thus protecting the other areas of the lung from infection.
• The lung does not inflate to its full capacity all the time we breathe. Hence there is always scope for expansion, which is brought about while performing strenuous work. At the same time the lung does not deflate fully on exhaling air. This keeps a reserve volume of air in the lungs at all times. This flexibility of the lung assists in performing any task without any discomfort and with minimal loss of energy.
• Speech is also a function of the lungs. Forced expiration of air from the lungs causes air to reach the larynx and vibrate the vocal cords. The force of the exhaled air during speech determines the loudness of voice.
• As the lung is a delicate organ in contact with the atmospheric air at all times, it is more prone to infection. Hence adequate care must be taken of any respiratory complaints and proper hygiene of the respiratory tract as a whole should be maintained.