COMPONENTS OF HUMAN AIR PASSAGEWAY

Air Passageway of Human Respiratory System

In humans and other higher animals the exchange of gases is carried out by the respiratory system. We can divide the respiratory system into two parts i.e. the air passageway and the lungs.

Air Passageway:

The air passageway consists of the parts through which the outside air comes in the lungs and after the exchange of gases it goes out. Air passage ways consist of nostrils, nasal cavities, pharynx, larynx, trachea, bronchi, bronchioles and alveolar ducts which ultimately lead into the alveolar sac. Air passageway opens to exterior via opening called nostrils. Nasal cavities are lined with mucous membrane of ciliated epithelium. Each nasal cavity is subdivided into three passage ways by the projection of bones from the walls of the internal nose. Nasal cavities perform the following functions:

• Air enters the nasal cavity through nostril.
• The large dust particles are, trapped by the hair present in the nasal cavities.
• Air while passing through the nasal cavity, becomes moist, warm and filtered of smaller foreign particles by mucous membrane.

Pharynx:

The nasal cavity leads into the throat or pharynx by two internal openings. The pharynx is a muscular passage lined with mucous membrane and is common to both food and air. It extends to the opening of esophagus. In pharynx there are several openings, the air is channelized from the pharynx into the larynx.

Larynx:

The larynx or voice box is a Complex cartilaginous structure surrounding the upper end of the trachea. One of the cartilages, the epiglottis** has a muscularly controlled, hinge like action and serves as a lid which automatically covers the opening of the larynx during the act of swallowing so as to prevent the entry of food or liquids into the larynx.

Glottis:

The opening of larynx is called glottis, the part of the larynx consisting of the vocal cords. Glottis is also lined by mucous membrane. The glottis is guarded by a flap of tissue called epiglottis.

** The commonly held belief that the epiglottis closes downward upon the larynx when food is swallowed is not quite true. The closure is probably never complete; the degree of closure is determined partly by the backward movement of the tongue during swallowing (which forces epiglottis into a more or less horizontal position) and partly by the upward movement of the larynx (which brings it up under the epiglottis). Food does not enter the partly open larynx and obstruct breathing primarily because the epiglottis diverts the food mass to one side of the opening and safely down the esophagus.

Vocal cords:

In the glottis the Mucous membrane is stretched across into two thin edged fibrous bands called vocal cords, which help in voice production, when vibrated by air. The vibrations in vocal cords and the movements of lips, cheeks, tongue and 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 superior to all.

Trachea or wind pipe:

Trachea is large membranous tube strengthened by rings of cartilage, extending from the larynx to the bronchial tubes. The trachea or wind pipe is a tubular structure about 12 cm lying ventral to the esophagus and extends to the chest cavity or thorax where it is divided into right and left bronchi. In the wall of trachea there are a series of C shaped cartilage rings which prevent the trachea from collapsing and keep the passage of air open.

Bronchi and Bronchioles:

Each bronchus on entering the lung divides and subdivides progressively into smaller bronchi. When the smaller bronchi attain the diameter of one mm or less then they are called bronchioles. Bronchi have the same cartilage rings as the trachea, but the rings are progressively replaced by irregularly distributed cartilage plates and the bronchioles totally lack cartilages. Bronchioles are mainly made up of smooth muscles which are arranged in circular pattern.

Air Sacs and Alveoli:

The bronchioles continue to divide and subdivide deep into the lungs and finally open into a large number of air-sacs. In fact air-sacs are the functional unit of lungs. Each air-sac is like a cluster of grapes and consists of several microscopic single layered structures called alveoli. The alveoli are surrounded by a rich network of blood capillaries to produce an excellent site for the exchange of gases. There are about 700 million alveoli present in the lungs of a mammal. There are many factors which control exchange of gases in alveolar capillaries.

Lungs:

The lungs are closed sacs that are connected to the outside by the way of the trachea and then nostrils. Lungs are spongy because of the presence of million of alveoli. Lungs are placed in the chest cavity. Chest cavity is bounded by ribs and muscles on the sides. The floor of the chest cavity is called diaphragm which is sheet of skeletal muscles. Lungs are covered with double layered thin membranous sacs called pleura. Outer layer is called outer pleural membrane while inner layer is called inner pleural membrane. Pleura membranes enclose a fluid which provides lubrication for the free expanding and contracting of the lungs. The left lung is slightly smaller and has two lobes while the right lung is bigger with three lobes. The lungs have blood vessels that are the branches of the pulmonary arteries and veins.

Following diagram shows the passage of air in respiratory system:

Flowchart of movement of oxygen in Respiratory System