Anatomy of the Respiratory System

Organization of the Respiratory System

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

Upper Respiratory Tract

• The upper respiratory tract consists of the nasal cavities, 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 space where air enters the nasal cavity just inside the nostrils is referred to as the vestibule.
• The nasal cavities are subdivided into three air passageways, the meatuses, which are formed by three shelf-like projections called the superior, middle, and inferior nasal conchae.

Nasal Cavity

• The nasal cavity contains olfactory receptors, responsible for the sense of smell, around the superior conchae and upper 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.
• The mucus-covered dust particles are then propelled by cilia towards the pharynx, where they can be swallowed or expectorated.

External Nose

• The external nose extends the nasal cavities onto the front of the face and positions the naris 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

• The skeletal framework of the nasal cavities includes:
  • Unpaired bones: ethmoid, sphenoid, frontal bone, and vomer
  • Paired bones: nasal, maxillary, palatine, lacrimal bones, and inferior conchae

Floor

• The floor of each nasal 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

Kesselbach's Plexus/Little's Area

• Located in the anterior ethmoid, superior labial, and sphenopalatine arteries
• Greater palatine artery

Woodruff's Plexus

• Located in the pharyngeal and posterior nasal arteries of the sphenopalatine artery

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.
• NP (Nasopharynx): from the post border of the nasal septum to the tip of the uvula.
• OP (Oropharynx): from the tip of the uvula to the tip of the epiglottis.
• LP (Laryngopharynx): from the tip of the epiglottis to the bifurcation of the larynx and esophagus.

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.
• Above, it opens into the laryngopharynx and forms its anterior wall, while below it continues 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
  • 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
  • Lead to alveolar ducts
  • Lead to alveolar sacs
• Alveoli:
  • Consist of type I cells and basal laminae
  • Site of gas exchange between air and blood

Lungs

• The lungs are divided into distinct regions called lobes.
• There are three lobes in the right lung and two in the left.
• The heart, along with its major blood vessels, sits in a space between the two lungs called the cardiac notch.
• Each lung is surrounded by two thin protective membranes called the parietal and visceral pleura.
• The parietal pleura lines the wall of the thorax, whereas the visceral pleura lines the lungs themselves.
• The space between the two pleura, the pleural space, is minute and contains a thin film of lubricating fluid, which reduces friction between the two pleura, allowing the two layers to slide over one another during breathing.

Pleural Cavity

• Potential space between visceral and parietal pleura
• Normally contains only a thin film of serous fluid
• Pleural recess: a potential space where two adjacent parietal pleura are in contact (during expiration) and fill with lungs during deep inspiration.