Lungs and Respiratory System



Lungs and respiration help us during inhalation and exhalation which allow breathing to take oxygen in and carbon dioxide out. The exchange of gases is called respiration.

Respiratory system is a network of organs and other tissues which help in breathing. The system consists of many parts, including nose, mouth, throat (pharynx), voice box (larynx), windpipe (trachea) lungs, bronchial tubes, bronchioles, alveoli (air sacs) and capillaries. There are many breathing muscles that aid the body in gases exchange, also part of the respiratory system.

According to Stanford Children’s Health

Respiratory rate mostly depends on age, for example in newborn babies the normal respiratory rate is about 40 to 60 times each minute, and when the baby is sleeping the rate may slow to 30 to 40 times per minute.

Functions of Respiratory System

By the side of inhalation and exhalation, there are many other functions performed by lungs:

  • Allow to talk and to smell
  • It also helps the body to maintain homeostasis
  • Protect airways from harmful substances.
  • Delivers oxygen to cells of the body and removes waste gases during exhalation.

gas exchange

Divisions of Respiratory System

Respiratory System is divided into two categories

  • Upper respiratory tract
  • Lower respiratory tract

Upper Respiratory Tract

URT includes nose, pharynx and larynx

  • Nose 

Nose is the only visible part of the respiratory system for air entry. Air enters into the lungs through mouth or nose. The air is warmed and humidified if it passes through nose openings called nostrils or nares. Inside the nose, sticky mucous membrane lining the nasal cavity traps dust particles. The cilia which is hair like tiny structures present in the nose, filtering out those dust particles.

There are nerve endings at the back of nasal cavities which are responsible for olfaction (smell). When there is any obstruction in nasal passage or sinuses (air-filled spaces) it affects nerve endings.

Three shell-like structures named as conchae or turbinates, increase the surface area for air flow.

  • Pharynx

Pharynx is a funnel-shaped tube which is about 12.5 cm in length. It is divided into 3 sections including nasopharynx, oropharynx and laryngopharynx.

There are two openings of the airway (nasal cavity and mouth) meet at the pharynx or throat. Below the pharynx, it divides into two pathways, one is an esophagus for digestive system and other one for air (respiratory system).

  • Larynx

Larynx or voice box contains a pair of vocal cords which vibrate to make sounds. It is made up of cartilage pieces connected through ligaments. The largest of these, the thyroid, which is prominent in males known as Adam’s apple.

The continuation of airways below the larynx is trachea (windpipe). Trachea is also lined with cilia to sweep fluids or foreign particles out. The epiglottis which partly covers the opening of the larynx, acts as a trap door and separates two pathways of pharynx and larynx.

Lower Respiratory Tract

It serves as both conducting airways (tracheobronchial tree) and respiratory units (acinar).

Extend from the level of true vocal cords in the larynx to alveoli within lungs.

  1. Tracheobronchial tree ( conducting airways)

Consist of trachea, bronchi and bronchioles which help in gases exchange from environment to lungs.

  • Trachea

Trachea is the continuation of larynx, which arises from the lower border of cricoid cartilage in the neck. It bifurcates at a level of sternal angle to form the right and left main bronchi. Trachea is located anteriorly to the esophagus.

There is ciliated pseudostratified columnar epithelium, interspersed by goblet cells, lined the trachea and bronchi to secret mucus.

At bifurcation of primary bronchi, the carcina, hook-shaped process, located at the fifth thoracic vertebrae or sternal notch ridge of cartilage is the most sensitive part for triggering cough reflex.

  • Bronchi

when at sternal angle, trachea bifurcates into left and right main bronchi, it further form branches of secondary bronchi also called lobar bronchi to supply to lobes of lungs (3 in right lobe and 2 in left lobe of lung) then give numerous segmental bronchi (known as tertiary bronchi), it supplies a bronchopulmonary segment. Basically bronchopulmonary segment is the subdivision of lung lobes.

  • Bronchioles

Bronchioles are smaller airways branching from the segmental bronchi.

  1. Terminal respiratory unit (acinar)

The conducting airways terminate in gas exchange airways made up of respiratory bronchioles, alveolar ducts and alveoli.

Although conducting bronchioles just transport air and don’t take part in gas exchange, these conducting bronchioles end up with terminal bronchioles, they furthur branching into respiratory bronchioles which are distinguishable by the presence of alveoli.

gases exchange

  • Respiratory bronchioles 

Respiratory bronchioles is lined by two types of epithelial cells: ciliated columnar cells and club cells ( clara cells), which produce surfactant lipoprotein to prevent walls of the airways sticking together during exhalation.

  • Alveolar duct

Alveolar ducts are tiny duct which connect the respiratory bronchioles to alveolar sacs (each of which contain numerous alveoli)

  • Alveoli

Alveoli are the tiny air-filled pockets for gases exchange in lungs. There are about 300 million alveoli to provide a large surface area for adequate gases exchange. The functional unit of the lung is alveoli.


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