Respiratory System in Humans

(1) Nose: There is a pair of external nostrils at the lower end of nose. The air enters through the nostrils. The air passing through nostrils is filtered by fine hairs that lines the passage. The passage is lined with mucus which help in this process.

(2) Throat or Pharynx: The nasal passage open behind into pharynx. It is a common passage for food and air. It leads into an air tube called trachea through a slit called glottis. The glottis always remain open except during swallowing. The glottis bear a flap called epiglottis. During swallowing epiglottis closes the glottis to avoid entry of food into air pipe.

(3) Trachea: It is a thin walled tube that extends downwards through the neck and divides into 2 major bronchi. One major bronchi enters right lung and other left lung.

(4) Lungs: In humans, the respiratory organs are lungs which provide the surface for the exchange of gases. The lungs lie in the thoracic cavity on the sides of the heart.

The thoracic cavity is separated from abdominal cavity by muscular portion called diaphragm.

Each lung is enclosed in two membrane, the pleura.

(5) Bronchi and Bronchioles: Within the lungs the major bronchi further divide into secondary bronchi which sub-divide into small tertiary bronchi and finally into bronchioles.

Each bronchioles divide into alveolar duct which enters the alveolar sac or alveoli. The alveoli are made up of non-ciliated squamous epithelium cells. It is surrounded by network of blood capillaries.

(i) When we breathe in, chest cavity become large due to lifting of our ribs and flattening of diaphragm. Because of this, air is sucked into lungs and fills the alveoli. The blood brings carbon dioxide from rest of the body and release into the alveoli and oxygen in the alveolar air is taken up by blood and transported to various cells in our body.