Respiration refers to the movement of air into and out of the lungs . This process comprises of

• Inspiration is the process that causes air to enter the lungs, and
• Expiration is the process that causes air to leave the lungs  

Functions of the Respiratory system are :

• To provides oxygen to body tissues for cellular respiration,
• Removal of  the waste product carbon dioxide,
• To maintain acid–base balance .

 A respiratory cycle  is one sequence of inspiration and expiration .

Respiratory rates vary based on age.

The normal resting respiratory rate is as follows :

• for adults is 10–20 breaths per minute .
• For Children less  than one year is  30–60 breaths per minute,
• For children by the age is usually 18–30 .
• By adolescence, the respiratory rate is  12–18 breaths per minute (Much similar to adults ).
• The normal respiratory rates  decreases from birth to adolescence and often increase slightly over the age of sixty-five.

The Respiratory System

“Major Respiratory Organs” by OpenStax College via Wikimedia commons is licensed under CC BY 3.0

Nose and Nasal Cavity

The upper respiratory tract begins with the nose and ends with the larynx. The nose externally extends from the frontal bone to the maxilla. The nose consists of a central cartilage (the septum) with two lateral processes. The openings to the nose are the nares.

The nares are lined with coarse hairs to aid in the filtration of particulate matter.The visible nose is actually the entryway into the nasal cavity, where the major functions of the nose occur.  The area immediately inside the nares contains a large number of sebaceous glands, sweat glands, and hair follicles.

 The olfactory epithelium found in the nasal cavity is used to detect odors .The nasal cavity is divided into the right and left sides by the nasal septum.The roof of the nasal cavity consists of parts of the ethmoid and sphenoid bones.The of the nasal cavity floor is called the palate which forms the roof of the mouth. The palate  is separated into the hard and soft palate.

The anterior hard palate is formed from part of the palatine bone. The posterior soft palate  called the uvula does not contain bone. The uvula moves up during swallowing to close off the nasal cavity, thus preventing material from entering the mouth.

Within the bones surrounding the nasal cavity are hollow spaces called  paranasal sinuses , which function to make the skull lighter as well as moisten and warm incoming air. 

The nasal cavity opens into the nasopharynx.

Pharynx (Throat)

The pharynx connects the nasal passage to the larynx in the throat. The pharynx  has three distinct areas namely, the nasopharynx, the oropharynx, and the laryngopharynx. 

The pharyngeal tonsils are located at the top of nasopharynx. The pharyngeal tonsils contains rich supply of lympocytes and ciliated epithelium which traps and destroys the invading pathogens .

The pharyngeal tonsils are large in children, but tend to regress as they age and eventually may even disappear.

Auditory or the Eustachian tubes open into the nasopharynx. This explains  why colds often lead to ear infections.

Larynx (Voice Box)

After air leaves the pharynx, it enters a complex cartilaginous structure called the larynx. It extends from the laryngopharynx and the hyoid bone to the trachea.

The larynx serves to regulate the volume of air that enters and leaves the lungs.

The larynx is formed by several pieces of cartilage of which the thyroid cartilage is the largest piece of cartilage that makes up the larynx.

The thyroid cartilage consists of the laryngeal prominence, or “Adam’s apple,” which is more prominently seen in males.

The larynx directs air and food to the appropriate tubes.  During swallowing the airway is blocked by closing off the opening of the trachea with a structure called  epiglottis.

The larynx also contains the  vocal cords, which are used in making sounds and speech.

Trachea (Windpipe)

 The trachea or the wind pipe  is a cylindrical structure about 25 to 35 cm long. It  begins just below the larynx, and ends with the terminal bronchioles.The trachea funnels the inhaled air to the lungs and the exhaled air back out of the body.It is made of C-shaped rings of cartilage and smooth muscle. The cartilage provides strength and support to the trachea to keep the passage open. The last cartilage in the trachea . The trachea is lined with cells that have cilia and secrete mucus. The mucus catches particles in the inhaled air, and the cilia moves the particles toward the pharynx.

Bronchi and Bronchioles

The end of the trachea divides into two bronchi that enter the right and left lungs.

Air enters the lungs through the primary bronchi. Each primary bronchus divides further creating smaller and smaller diameter bronchi.

The bronchi further divide and are called broncioles when the passages are under 1 mm (.03 in) in diameter.. Like the trachea, the bronchus and bronchioles are also made of cartilage and smooth muscle.

Bronchi are supplied with nerves of both the parasympathetic and sympathetic nervous systems.

These nerves control muscle contraction (parasympathetic) or relaxation (sympathetic) in the bronchi and bronchioles,

The final bronchioles are the respiratory bronchioles.

Lungs

 Lungs are a pair of organs on each side of the heart, inside the chest cavity.

They are the main organs of the respiratory system.

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

The left lung is slightly smaller than the right lung,

During breathing in, the air enters the airways and travels down into the air sacs, or alveoli, in the lungs. This is where gas exchange takes place. 

Alveoli

The end of each respiratory bronchiole are attached to the Alveolar ducts .

 At the end of each duct are alveolar sacs, each containing 20 to 30 alveoli (air sacs). Gas exchange occurs only in the alveoli.

The alveoli are thin-walled tiny bubbles within the alveolar sacs and are in direct contact with the capillaries of the circulatory system.

The oxygen diffuses from the alveoli into the blood and carbon dioxide  diffuses from the blood into the alveoli which is then exhaled