Respiratory tract is meant for conducting atmosphere air from the environment to the pulmonary alveoli where the gases are exchanged with blood. The organs of respiratory tract are nostrils, nasal cavities, pharynx, larynx, trachea, bronchi, bronchioles, atria and alveolar ducts (Fig 11b).
Nasal cavity: A pair of nostrils is the first structures through which air enters the body. In the nasal cavity the air gets saturated with water vapours (humidified) and its temperature is raised to body temperature. The mucus, secreted by the membrane of nasal cavity, traps the dust particles and microorganisms and allows only clean air to pass further. Pharynx is a common passage for food and air and located behind the mouth and nasal cavity. Sinuses are hollow air filled spaces in the bones of face around the nasal cavity. Total volume of sinuses equals almost that of nasal cavity (about 25-50 ml in adult).
Larynx: Warm, humidified and clear air through pharynx passes into larynx. Closure of epiglottis prevents the entry of food into larynx during swallowing. Larynx is pitcher shaped organ situated at the top of trachea. Inside the larynx special folds of mucous membrane, called vocal chords are situated which are involved in vocalization function (production of sound).
Trachea (Wind pipe): is long cartilaginous tube extending through ventral aspect of neck connecting head and thorax. Numerous cartilage rings does not allow collapse of trachea during movement of neck and maintains its patency. Inside the thorax, trachea divides into two bronchi (left & right) each of it enters the respective lung. Bronchus (singular) undergoes extensive branching (arborization) inside the lung giving rise to bronchioles, terminal bronchioles, respiratory bronchioles, atria and alveoli (alveolus singular). Alveolus is a blind sac filled with air and surrounded by numerous thin wall blood capillaries.
Respiratory tract consists of all above organs starting from nasal cavity up to respiratory bronchioles. The respiratory tract is filled with air which does not participate in exchange of gases with blood. Air occupied in aria and alveoli is only available for gas exchange.
Lungs: Lungs are a pair of multilobed, parenchymatous organs situated in thoracic cavity. Each lung is narrower on anterior side (called apical part) and broader on posterior aspect (called diaphragmatic part). The way each lung assumes an approximate triangular shape (Fig 33b). The lungs are individually covered with membranous covering known as pleura which is filled with watery pleural fluid. Pleural fluid provides a protective cushion and prevents injury to lungs during its distension (inflation) and compression (deflation) with each respiration. Parenchyma of lungs composed of bronchi, bronchioles, alveoli and other structures of respiratory tract enclosed by connective tissue and blood vessels. Due to presence of millions of air filled alveoli, the texture of lung appear spongy and due to this, the lungs floats in water (have lower specific gravity).
Diaphragm is a muscular partition between thoracic and abdominal cavity. It is a dome like in arrangement with its convexity facing thorax and concavity towards abdomen.