Lesson 2 of 11 · 6 min
Human respiratory system
NCERT §14.1.1
The gun hasn't fired yet. Meher takes one slow breath before the start, and that single breath is a guided tour of the whole human airway.
The lesson in notes
In short
Air enters through a pair of external nostrils above the upper lip, runs through the nasal passage into the nasal chamber, then opens into the pharynx, a passage shared by food and air.
From the pharynx, air passes through the larynx into the trachea. The larynx, a box made of cartilage, produces sound, which is why it is known as the sound box.
During swallowing, the glottis (the opening of the larynx) can be covered by the epiglottis, a thin elastic cartilaginous flap, so food does not enter the larynx.
The trachea is a straight tube running down to the mid-thoracic cavity, where at the level of the 5th thoracic vertebra it splits into right and left primary bronchi.
Each primary bronchus branches repeatedly into secondary and tertiary bronchi and then bronchioles, ending in very thin terminal bronchioles.
The trachea, the primary, secondary and tertiary bronchi and the initial bronchioles are held open by incomplete cartilaginous rings.
Every terminal bronchiole ends in a cluster of alveoli, thin, irregular-walled sacs richly supplied with blood vessels. Together, the branching bronchi, bronchioles and alveoli form the lungs.
A double-layered pleura covers each lung, and the pleural fluid between its layers lowers friction on the lung surface. Of the two pleural membranes, the outer is in contact with the thoracic lining and the inner with the lung surface.
The conducting part (external nostrils up to terminal bronchioles) carries air to the alveoli, clears it of foreign particles, humidifies it and brings it to body temperature. The respiratory or exchange part is the alveoli and their ducts, where O₂ and CO₂ actually diffuse between blood and air.
The lungs lie in the thoracic chamber, which is air-tight. Its walls are the vertebral column at the back (dorsally), the sternum in front (ventrally), the ribs on the sides and the dome-shaped diaphragm below. Whatever change occurs in thoracic volume is transmitted to the lung (pulmonary) cavity, so we can breathe even though lung volume cannot be changed directly.