1. Wall: The wall of alveolus is very thin. 0.2 micrometers thick. It is composed of squamous cells. There are two types of cells in the wall. Type-1 cells and type-2 cells. It contains collagen and elastin fibers. Collagen is elastic and facilitates contraction-expansion. The capillary network arises from the pulmonary (pulmonary) arteries. Capillary networks extend into the walls of the alveoli and later join to form the pulmonary veins. The walls of the alveoli contain phagocytic macrophages. Macrophages destroy microorganisms and foreign particles.
2. Chamber: Alveolus is a special bubble-like air cavity. It stores O2 and releases CO2. The volume of alveoli increases or decreases according to the amount of gas.
3. Surfactant: Type-2 cells in the alveolar wall contain detergent-like chemicals. These chemicals are called surfactants. Detergents are chemicals like phospholipids. It reduces the surfactant tension of the alveolar wall. Prevents alveolus from collapsing. Contracts and expands the lungs with less effort. Facilitates rapid exchange of CO2 and O2. Destroys germs (bacteria). Surfactant secretion begins in the 23-week-old human fetus.

Alveolus is the structural and functional unit of the lung. The alveoli are the chambers in the lungs that are composed of squamous cells and filled with capillary networks that carry out gaseous exchange. Its diameter is 200-300 micrometers. Human lungs have about 480 million (48 crore) alveoli. These create 525 square feet, the equivalent of a tennis court, and about 11,800 square centimeters of breathing space.

1. Respiratory Organs: Lungs act as the main respiratory organs of animals.
2. Gaseous exchange: The alveoli of the lungs carry out gas exchange.
3. CO2 emission: It emits CO2.
4. Synthesis of chemical substances: Synthesis of proteins, sugars and fats takes place in the lungs.
5. Release chemicals: Pulmonary canals synthesize and release bradykinin and prostaglandins.
6. Sound production: Sound is produced through the larynx.
7. Chemical storage: Pulmonary banana stores and releases histamine and serotonin.
8. Inactivation of hormones: It inactivates adrenaline and noradrenaline hormones.
9. Water and heat balance: It regulates body temperature and water balance. About 400-600 milliliters of water is released from the body every day through breathing.
10. Chemical secretion: It secretes immunoglobulin and angiotensin-1 to angiotensin-2.
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11. Homeostasis: It maintains the internal environment or homeostasis state of the body.
12. Volatile gas: It emits chloroform, ether, ammonia etc.
13. Preventing the entry of harmful substances: Air germs and harmful substances prevent the entry into the body.
14. Acid-base balance: Lungs maintain acid-base balance in the body.
15. Energy production: The absorbed O2 is used in cellular growth and produces energy.

The light pink colored large and soft organ like sponge in human chest is called lung. Human body has two lungs. Right and left lung. The two lungs together weigh 2.9 pounds. The right lung is large and weighs 625 grams. It is divided into three lobes by horizontal fissure and oblique fissure. Superior lobe, middle lobe and inferior lobe. Left lung small weight 565 grams. It is divided into two lobes by the oblique fissure. Superior Lobe and Inferior Lobe. There are 10 lobules in the right lung and 8 in the left lung. Each lobule is composed of alveoli. The space through which the bronchi, blood vessels and lymphatic channels enter the lungs is called Alveoli. The bronchi, blood vessels and alveolar ducts are surrounded by connective tissue and form the root of the lungs.
Each lung is covered by a two-layered thin membrane called Pleura. The outer layer is called Parietal and the inner layer is called Visceral layer. Serous fluid is between the two layers. Serous fluid protects the lungs from frictional injury.

The end of the trachea divides and forms two branches. Right and left branches. Both branches are called bronchus. The point where the trachea divides is called the carina. The right bronchus is surrounded by 9-12 and the left bronchus by 6-8 cartilaginous rings. The right branch divides into 3 parts and enters the right lung and the left branch divides into 2 parts and enters the left lung. Each branch divides repeatedly to form numerous bronchioles. Each bronchiole contains numerous tiny air chambers. These are called alveoli. Alveolus is the structural and functional unit in the lungs. The entire trachea looks like an inverted tree and is called the respiratory tree.
Function: Air enters the lungs from the trachea through the bronchus and air containing CO2 leaves the lungs through the trachea.

The part from the vocal cord to the fifth thoracic vertebra is called the trachea. It is a hollow tube. Its length is 12 cm and diameter is 2 cm. It is also known as Wind pipe. It is composed of 16-20 hyaline cartilage rings in a C (semi ring) shape. The cartilage is called the tracheal ring. Because the rings are connected by fibrous tissue, the trachea does not collapse. The inner wall of the trachea is covered by ciliated mucous membrane and the outer wall by connective tissue.
Function: Moves air through the trachea. The cilia of the mucous membrane prevent the entry of foreign matter.

The larynx is a small cartilaginous organ located below the pharynx. It produces voice so it is called Voice organ. It is not silent as it contains cricoid, thyroid, arytenoid, cuneiform, corniculate and epigøtis cartilages. It opens through an opening called Glottis. At the top of the larynx is a small flap called the Epiglottis. Thyroid cartilage is the largest of the laryngeal cartilages and is known as Adam’s apple. Inside the larynx are mucous membranes and 6 elastic vocal cords. When under tension the vocal cords are vibrated by air and produce sound.
Function: Epigypsy prevents food from entering the larynx. Vocal cords produce vowels or sounds.

The part from posterior nostril to larynx is called nasopharynx. It is an identical part of esophagus and trachea.

The opening through which the nostrils open into the nares is called Choana.
Function: Air enters the nostrils through this orifice.

The part after vestibule is called nasal cavity. Its inner wall consists of ciliated epithelium, mucous membranes, olfactory cells, blood vessels, nerve endings, oil glands and numerous hairs. Olfactory cells help in receiving olfactory stimuli.
Function: It moistens dry air and cools hot air. Ciliated cells and mucous membranes trap dust and germs. Olfactory cells help in receiving olfactory stimuli.