Anatomy of Respiratory System

Respiratory System Organization

• The respiratory system is divided into the upper and lower respiratory tracts
• The upper respiratory tract and the uppermost section of the lower respiratory tract form the conduction region, in which air is conducted through a series of tubes and vessels
• The respiratory region is the functional part of the lungs, in which oxygen diffuses into the blood

Upper Respiratory Tract

• The upper respiratory tract includes the nasal cavity, pharynx, and larynx
• Air enters the body via the nasal and oral cavities
• The nasal cavity is divided into two equal sections by the nasal septum
• The vestibule is lined with coarse hairs that filter incoming air, ensuring that large dust particles do not enter the airways
• The conchae are lined with a mucous membrane that contains a network of capillaries and mucus-secreting cells
• The blood flowing through the capillaries warms the passing air while the mucus moistens it and traps any passing dust particles

External Nose

• The external nose extends the nasal cavities onto the front of the face and positions the nares so that they point downwards
• It is pyramidal in shape with its apex anterior in position
• The upper angle of the nose between the openings of the orbits is continuous with the forehead

Skeletal Framework

• Bones that contribute to the skeletal framework of the nasal cavities include:
  • Unpaired ethmoid, sphenoid, frontal bone, and vomer
  • Paired nasal, maxillary, palatine, and lacrimal bones, and inferior conchae

Floor of the Nasal Cavity

• The floor of each nasal cavity is smooth, concave, and much wider than the roof
• It consists of soft tissues of the external nose, the upper surface of the palatine process of the maxilla, and the horizontal plate of the palatine bone, which together form the hard palate

Pharynx

• The pharynx is a 12-14 cm long, musculomembranous tube shaped like an inverted cone
• It extends from the cranial base to the lower border of the cricoid cartilage, where it becomes continuous with the esophagus
• The pharyngeal cavity is a common pathway for air and food
• The pharynx is separated from the vertebral column behind by a thin retropharyngeal space containing loose connective tissue

Larynx

• The larynx is an air passage, a sphincter, and an organ of phonation
• It is a hollow musculoligamentous structure with a cartilaginous framework that caps the lower respiratory tract
• It extends from the tongue to the trachea
• It projects ventrally between the great vessels of the neck
• It opens into the laryngopharynx and forms its anterior wall while continuing into the trachea

Trachea

• The trachea is a tube, 10-11 cm long, formed of cartilage and fibromuscular membrane
• It descends from the larynx and extends from the level of the sixth cervical vertebra to the upper border of the fifth thoracic vertebra
• It lies approximately in the sagittal plane but its point of bifurcation is usually a little to the right
• The trachea is flexible and can rapidly alter in length
• It is flattened posteriorly, so that in transverse section it is shaped like a letter D

Bronchi

• The right main bronchus is wider and takes a more vertical course through the root and hilum than the left main bronchus
• Therefore, inhaled foreign bodies tend to lodge more frequently on the right side than on the left
• Secondary (lobar) bronchi: three on the right and two on the left
• Tertiary (segmental) bronchi: branch into each lung segment
• Bronchioles: little bronchi, less than 1 mm in diameter
• Terminal bronchioles: less than 0.5 mm in diameter

Respiratory Zone

• Consists of air-exchanging structures
• Respiratory bronchioles: branch from terminal bronchioles and lead to alveolar ducts
• Alveoli consist of type I cells and basal laminae
• Cuboidal epithelial cells (type II cells) secrete surfactant, a lipid-protein complex that reduces surface tension and minimizes breathing effort