Gross structure of respiratory system and ventilation process.
Human respiratory system opens to exterior environment through nostril. Nostrils opens to the nasal cavity and it contain two chambers, which separated by Longitudinal septum. Nasal cavity combines with pharynx. Pharynx contain three parts namely Nasopharynx – behind the nose, Oropharynx – behind the mouth, Larynopharynx – behind larynx. Pharynx lead to Larynx. Larynx has cartilages and vocal cords and it leads to trachea, which contain C shaped cartilages. Trachea divides in to two at the distal end forming two bronchi. Bronchi divide into bronchioles after they enter to the lungs. Bronchioles divide again into terminal bronchioles, respiratory bronchioles, alveolar duct and finally into alveoli. Alveoli are bubble shaped. There are two pairs of lungs which are cone shaped. Right lung divides into three lobes and left lung divide into two lobes.
Ventilation consists of two phases called Inspiration and Expiration. Inspiration is an active process. Following things occur in inspiration.
· External intercostal muscles contracts.
· Ribs moves upward and forward.
· Sternum moves forward.
· Thoracic cavity enlarges sideways and anterior posteriorly.
· Diaphragm contracts or flattens.
This causes the pressure in the plural cavity, between plural membranes to decrease. So as a result lung volume increases. Pressure within the lung decreases than the atmospheric pressure and air flow into the lungs until the pressure within the lungs equals to that of atmospheric pressure.
Expiration is a passive process. Following things occur in expiration.
· External intercostal muscles relax and Internal intercostals muscles contracts.
· Rib cage moves downward and backward.
· Sternum moves backward.
· Volume of thoracic cavity reduces anterior posteriorly and also in side ways.
· Diaphragm gains its original position.
The pressure between plural membranes to increase due to the decrease in volume in the thoracic cavity. It reduces the lung volume and pressure within the lungs increases than the atmospheric pressure. Air flow out of lungs till the pressure inside the lungs equals to that of atmospheric pressure.