HUMAN RESPIRATORY SYSTEM

The respiratory system of man consists of the following organs :

1. Nasal Cavity:

The nasal cavity is the first part of the respiratory system.

It opens to the exterior through nostrils.

Nasal cavity is divided into nasal chambers by the nasal septum. Each nasal chamber can be differentiated into three regions.

(i) Vestibular: First part starting from nostril, has oil glands and hair to prevent entry of large dust particles.

(ii) Respiratory part: Highly vascular and involved in conditioning of air as the air gets warmed and moist. Arising from the wall of each nasal chamber, there are 3 shallow bony ridges called nasal conchae. These are individually named as superior, middle and inferior. (Superior lies within the olfactory part; middle and inferior lie in the respiratory part). The conchae are covered with mucus membrane and greatly increase the surface of the nasal chambers.

(iii) Olfactory part: It is the upper part, lined by olfactory epithelium for smell.

2. Pharynx:

From the nasal cavity, the air enters the pharynx.

It serves as a common passage for both air and food.

The opening into the wind pipe or trachea is a narrow slit the glottis.The glottis is protected against the entrance of food by a triangular flap of tissue, the epiglottis.

3. Human Larynx:

Until puberty, there is little difference in the size of the larynx ('voice box') in male and female.

The larynx opens into the or opharynx by a slit like aperture, the glottis.

The larynx is composed of irregularly shaped cartilages attached to each other by ligaments and membranes.

The main cartilages are-

(i) Thyroid cartilage is the most prominent, C-shaped, incomplete dorsally, called Adam's apple as it is apple shaped and more prominent in male.

(ii) Cricoid cartilage lies below the thyroid cartila'ge, and its shape is like a signet ring.

(iii) Arytenoid cartilages are two roughly pyramid shaped cartilages forming posterior wall of the larynx.

(iv) Corniculate - These are two conical nodules of elastic fibro-cartilage which lie at the apices of arytenoid cartilages,

(v) Cuneiform - These are two small, elongated, club shaped nodules of elastic fibro-cartilage which lie above and anterior to corniculate cartilages. These connect epiglottis to arytenoid cartilage.

(vi) Epiglottis is a single leaf shaped cartilage which projects into the pharynx,

Thus a total of 9 cartilages -3 paired (iii, iv, v) and 3 unpaired (i, ii, vi).

The thyrohyoid membrane is a broad, flat membrane attached to the hyoid bone above and to the thyroid cartilage below.

Inside the larynx are present two pairs of vocal cords, one pair of false vocal cords which have little to do with sound production and the second inner pair or true vocal cords.

When air is forced through the larynx, it causes vibration of the true vocal cords and sound is produced.

The pitch of a sound is determined by the tension on the vocal cords-the greater the tension, the higher the pitch.

4. Trachea:

The trachea is a tube about 12 cm long with C shaped (dorsally incomplete) rings of hyaline cartilage in its walls.

These rings of cartilage make the wall non-collapsible.

It is internally lined by pseudostratified ciliated columnar epithelium.

Cilia beat to push out the mucus.

5. Bronchi:

The walls of the bronchi are likewise supported by cartilaginous rings.

Each bronohus divides and redivides into smaller and smaller bronchioles.

Epithelium gradually changes from pseudostratified ciliated columnar epithelium in the bronchi to ciliated simple cuboidal epithelium in the terminal bronchiole.

Incomplete rings of cartilage are gradually replaced by plates of cartilage that finally disappear in a distal bronchiole.

Terminal bronchiole is further subdivided into respiratory bronchioles. Respiratory bronchioles open into alveolar ducts ® atria ® alveolar sacs ® alveoli. From respiratory bronchioles onward the wall is very thin and is made up of simple squamous epithelium.

6. Lungs:

The lungs occupy the greater part of the thoracic cavity.

Surrounding each lung is a double walled sac within the walls of which lies, the pleural cavity.

The right lung is divided into three lobes and left into two.

Inside the lung each bronchus divides into numerous bronchioles, each of which terminates into an elongated saccule, the alveolar duct, which bears on its surface air sacs or alveoli.

The number of alveoli in the human lungs has been estimated to be approximately Respiratory organs of man 300 million.

The lungs are covered by a thin double layer of simple squamous epithelium called the pleura.

The outer or parietal pleuron remains attached to the wall of thoracic cavity.

The space between the two pleural membranes contains pleural fluid for reducing friction and makes the movement of lung easy.

Inflammation of the pleura causes a disease called 'pleurisy'.

Lungs are pink at birth.

They become dark grey and mottled in adults due to deposition of carbonaceous materials.

Darkening increases in smokers and persons exposed to pollutants.

The right lung is shorter by about 2.5 cm due to raised position of diaphragm on the right side to accomodate liver.

The left lung is longer. It is, however, narrower than the right lung because it contains a cardiac notch for accomodating asymmetrically placed heart. Left lung is divisible by an oblique fissure into two lobes, left superior and left inferior.

Right lung has two fissures, horizontal and oblique. They divide the right lung into 3 lobes -right superior, right middle and right inferior. The lobes are divided internally into segments and segments into lobules.

There are 8 segments in the left lung and 10 segments in the right lung. On the average an adult right lung weighs 625 g, while the left lung weighs 565 g.