Gaseous Exchane in Humans1

There are a pair of lungs in the thoracic cavity – the left lung and the right lung. The left lung is slightly smaller (because of the heart which is slightly to the left of the body) and has two lobes and the right lung is bigger with three lobes. They are spongy and elastic organs that are broad at the bottom and taper at the top. They consist of air sacs, the alveoli. Many alveoli group together and open into a common space. From the space arise the alveolar ducts which join together to form bronchioles. The bronchioles connect them to the respiratory tract. The lungs also have blood vessels that are the branches of the pulmonary artery and veins.

Each lung is enclosed by two membranes called the outer pleural membrane and the inner pleural membrane. The membranes enclose a space called the pleural cavity that contains a fluid. The lungs are capable of expanding and contracting as they are elastic organs. Lubrication for their free movement is provided by the fluid in the pleural cavity.

The chest wall is made up of 12 pairs of ribs and the intercostal muscles that are attached to the ribs. A thick membranous structure, the diaphragm is present below the lungs and separates the thoracic from the abdominal cavity.

Respiratory Tract

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Apart from the lungs, there are several associated organs and structures which together form the respiratory system. The respiratory system is closely linked with the circulatory system as the transport of the gases takes place through blood.


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structure of human respiratory system

Respiratory System in Human Beings

The respiratory system starts with the nose that encloses the nasal cavity. The nasal cavity opens to the outside through the openings called the nostrils. The nasal cavity is divided into two portions by a cartilagenous septum and is lined by fine hairs that filter the dust particles from the air. The nasal cavity is separated from the mouth by hard and soft palate that form its floor. It opens into the region called the pharynx.

Pharynx is common to both food and air. This allows for taking in of more air whenever required and also allows passage of air in case the nose is blocked. Pharynx continues into glottis.

Pharyngeal areas behind the buccal cavity and larynx are called Oropharynx and Larynogopharnyx.

Glottis is the narrow opening into the larynx. It is guarded by a flap of tissue called the epiglottis. Several folds of elastic connective tissue are embedded into the posterior end of the glottis. They are called the vocal cords. These extend into the larynx.

Larynx is also called the voice box. The vocal cords stretch across the larynx and vibrate when the air passes through them. This vibration produces various sounds.

The respiratory tract also helps in the production of sound. The portion of the tract that carries out this function is the larynx or the voice box. It has several folds of elastic connective tissue called the vocal chords. They extend from the posterior end of the pharynx to the end of larynx. When air passes through the larynx, these chords vibrate and produce sound.

The coordinated movement of the lips, cheeks, tongue and the jaws produces specific sounds which result in speech. Speech is an ability that only humans are gifted with and this is one of the characteristics which has put human beings at the top of the evolutionary tree.

The larynx is held open with the help of cartilages. Adam’s apple is a prominent cartilage of larynx. Larynx continues as the trachea after the cords.

Trachea is also called the windpipe. The trachea are held open with the help of C-shaped cartilagenous rings. The open ends of the rings are towards the oesophagus, the foodpipe. The trachea is situated in front of the oesophagus. The cartilages keep the larynx and trachea from collapsing even when there is no air in them. The trachea then branch into two main branches called bronchi.

Each bronchus is also supported by the cartilagenous rings. The bronchus then branches into several bronchioles. The bronchioles progressively lose the cartilages as they become narrower. The bronchioles end as fine tubules called the alveolar ducts.

structure of bronchioles

Structure of Bronchioles

Each alveolar duct opens into an alveolar sac.

Alveolar sac is the extended region into which a group of alveoli or air sacs open.

Each alveolus is a sac-like structure lined by a single layer of epithelial cells. It is bound on the outside by a network of capillaries. All the alveoli on one side are enclosed by the membrane called the pleural membrane and constitute a lung.

The pulmonary artery from the heart containing impure blood enters the lungs and branches into minute capillaries which surround the alveoli. These then join together to form the pulmonary vein which, carries the purified blood back to the heart.

Path taken by inhaled air

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Composition of the air that we breathe in is:

Nitrogen – 78%, Oxygen – 21%, Carbon dioxide – .03 – .04%, Hydrogen – traces, noble gases – traces.

Thus the air naturally contains more oxygen than carbon dioxide. This oxygen-rich air is taken in by the nostrils. In the nasal cavity, it is filtered by the fine hair. The cavity also has a rich supply of blood vessels that keep the air warm. This air then enters the pharynx, then the larynx and then the trachea.

The trachea and the bronchi are lined with ciliated epithelial cells and secretory cells (goblet cells). The secretory cells secrete mucus which moistens the air as it passes through the respiratory tract and also traps any fine particles of dust or bacteria that have escaped the hairs of the nasal cavity. The cilia beat with an upward motion so that the foreign particles along the mucus is sent to the base of the buccal cavity from where it may be either swallowed or coughed out.

The air from the bronchus then enters the bronchioles and then the alveoli. The alveoli form the respiratory surface in the humans.


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