|
In human beings, the respiratory organs are the lungs.
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 this 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 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.
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.
Structure
The respiratory system starts with the nose that encloses the nasal cavity. The nasal cavity opens 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 more air whenever required and also allows passage of air in case the nose is blocked. Pharynx continues into glottis.
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 cords. They extend from the posterior end of the pharynx to the end of larynx. When air passes through the larynx, these cords vibrate and produce sound.
The co-ordinated movement of the lips, cheeks, tongue and the jaws produce 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 also known as 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.
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.
The composition of air that we breathe in is:
Nitrogen - 78%
Oxygen - 21%
Carbon dioxide - 0.03 - 0.04%
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 into 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 repiratory tract and also trap any fine particles of dust or bacteria that have escaped the hairs of the nasal cavity. The cilia beat with an upward motion such that the foreign particles along with 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.
The capillaries lining the alveoli have impure blood which has low concentration of oxygen. So, the oxygen from the air easily diffuses into the blood through the thin barrier of the alveolus wall. Similarly since the concentration of carbon dioxide is quite high in the blood, the gas easily diffuses out into the alveolar space. From here, the air that has comparatively more concentration of carbon dioxide than the air that entered it, leaves the lungs.
| Emphysema is condition wherein the area for gaseous exchange in lungs get reduced. This occurs commonly in heavy smokers. The walls separating alveoli breakdown resulting in abnormal alveoli with lesser area. Due to this condition the heart has to pump more blood. This may lead to a strain on the heart causing heart failure.
|
| 
