STRUCTURE :

• Nasal Cavity and Nose : Nose and its cavity are the first part of the Respiratory System and form its main external opening. Nose is a structural organ present in the centre of the Face above Mouth. It contains hollow cavity called Nasal Cavity. Nasal Cavity contains inner walls made up Hair and Mucous Membrane. It is present inside Nose and Skull. It warms, moistures and filters the air entering the Respiratory System.

• Mouth/Oral Cavity : Mouth forms the secondary external opening to the Respiratory System. It lacks hair cells unlike the Nose and is also shorter. Therefore it doesn’t filter, warm or moist the air entering the Respiratory System like Nose. However it provides alternate route for breathing when a person is unable to breath through Nose (for example in flu, nasal bleeding etc.). It also provides shorter route and more volume to be transferred quickly.

• Pharynx : It is a cavity present inside the Neck. It is a funnel shaped cavity that descends from Oral and Nasal Cavity into the Larynx. It is further divided into Oropharynx, Nasopharynx and Laryngopharynx. Oropharynx is the part that starts from the Oral Cavity. Nasopharynx starts from the Nasal Cavity. Both Oropharynx and Nasopharynx converge to form Laryngopharynx. Laryngopharynx ends at the opening of the Larynx which is guarded by the Epiglottis. Epiglottis is a flap like structure which prevents the food from going into the Larynx and allows air to pass through. As the food approaches the Epiglottis it comes down and cover the opening of the Larynx and diverts the food into the Esophagus.

• Larynx : It is present in the anterior part of the Neck. It is small section of the Airways situated between the Laryngopharynx and Trachea and connects the two. On the superior aspect is Epiglottis and anteriorly lies Thyroid Cartilage. Vocal Cords are situated in it which are folds of the skin which produce sound via vibration. They are protected by the Thyroid Cartilage. Larynx is also known as Voice Box.

• Trachea : It is a long tube of about 5 inch length. It connects the upper airways with the Bronchi inside Thorax. It is made up of C shaped cartilages along its length which keep it patent. Open end of C is posteriorly, allowing Esophagus to expand as food passes by. Inner wall is lined with Pseudostratified Ciliated Columnar Epithelium. This epithelium has cilia on its surface and produces mucus. Movement of the Cilia pushes mucus upwards.

• Bronchi and Bronchioles : Respiratory System after Trachea divides into 2 Primary Bronchi, one on each side. Each of these Bronchi runs into the respective lung and branches further into Secondary Bronchi. Secondary Bronchi are related to the lobes of the Lungs hence there are 2 on left and 3 on right side. They then branch and split again into many Tertiary Bronchi. Tertiary Bronchi end into numerous small branches in side lungs with very small diameter which supply the whole lung. These are called Bronchioles. Bronchioles at their termination form small branches with less than a millimetre thick diameter called Terminal Bronchioles. Terminal Bronchioles form baloon like dilations at their endings known as Alveoli. Alveoli perform main function of the respiration.

Bronchi and Bronchioles both have smooth muscle cells in their walls which control their diameter hence flow of air through them. However Only Bronchi have C shaped Cartilage in their walls which keep them patent.

• Lungs : Every normal Human has a pair of Lungs in his Thoracic Cavity. Each Lung is a spongy organ capable of expanding. Each lung is of different shape and size because of the Heart. Heart is present to the left of Thoracic Cavity hence left Lung is smaller with 2 lobes and right Lung is larger with 3 lobes. Lungs are covered from outside by Pleural Membrane which lubricates them and allows them to expand.

Each lung inside is having a large number of capillaries in connection with the small sac like air spaces known as Alveoli. Both lungs have almost 30 million of these small sac like spaces. Blood is in contact with air here through their very thin wall (1 cell thick).

• Muscles of Respiration : These are the muscles around the Lungs which help them to inhale and exhale the air. These are Diaphragm, Internal Intercostals and External Intercostals. Diaphragm is a dome like muscle in the inferior part of the Thoracic Cavity separating it from the Abdominal Cavity. It is a powerful muscle whose contraction creates negative pressure inside the Thoracic Cavity and makes the Lungs expand. Thus inhalation takes place. It relaxation allows passive slow exhaling of the air out of the Lungs.

Internal Intercostals are a group of muscles present between the Ribs anteriorly. They lie internal to External Intercostal Muscles and their contraction causes Thoracic Cavity to decrease in size. Hence air is exhaled out of the Lungs by their active contraction.

External Intercostal lie superficial to the Internal Intercostals. They expand the Rib Cage hence Thoracic Cavity when contract. Hence they also help in expansion  of Lungs and Inhalation of the Air like Diaphragm.

FUNCTION :

• Main function of the Respiratory System is to bring about the process of respiration. Airways filter, moist and warm the air as it is inhaled by the help of Muscles described earlier into the Lungs. They also adjust the amount of air flow by adjusting sizes of their lumens. The inhaled air is then brought in contact with the Blood through alveoli. Oxygen enters the Blood via capillaries and Carbon Dioxide exits from blood into the Alveoli. Then this air is exhaled out.
• Respiratory System also buffers and balances the pH of the Blood by maintaining the level of Carbon Dioxide in the Blood.
• Respiratory System is also involved in the Voice production through Larynx.
• Respiratory System is also involved in the Immunity against diseases by the help of Cough and Sneeze reflexes and by the White Blood Cells and Immunoglobulins present in it.