Anatomy 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.

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.

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.

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.

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

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

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

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.

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.

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.

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

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

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.

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.