2. Basic Structure of The Airway

Question 1

Provide a brief overview of the basic structure of the respiratory system

Answer 1

It starts in the nose and nasal passages, then down to pharynx, larynx and trachea, which branches into the primary bronchi which supply the lungs

The lungs sit in the thorax, and are surrounded by pleura, the diaphragm, intercostal muscles and abdominal muscles

All of these components are essential for respiration

Question 2

What are the airways?

Answer 2

Air-filled spaces/tubes which take air from the outside to the alveoli

Question 3

What are the alveoli?

Answer 3

Microscopic spaces lined by very thin simple squamous epithelium through which oxygen and carbon dioxide exchange takes place

Gas exchange takes place within the blood in a network of alveolar capillaries surrounding the alveoli

Question 4

What are the alveolar capillaries?

Answer 4

These are present on the pulmonary circuit, bringing deoxygenated blood from the right ventricle of the heart via the pulmonary trunk and pulmonary arteries

Question 5

What is meant by the upper airways?

Answer 5

These comprise the nasal cavities, the nasopharynx (above roof of mouth), laryngopharynx (shared by the airway and foodway), and the larynx (voicebox/Adam’s apple; a valve that allows air into the lower airways but excludes liquids and solids)

Question 6

Outline the basic structure of the nasal cavities

Answer 6

Nearly a triangular cross-section, with fairly smooth medial and inferior walls but an elaborate lateral wall lined with hairy muscoa on three scroll-like plates of bone called the conchae

It has a complex and important vascular and nerves supply

The warm, moist plates heat up/humidify inspired air on the way down the respirator tract, protecting parts of the lower respiratory tract from cold shock and death

Nasal mucus and hair help to exclude a range of airborne particles; the complex, narrow passages of the nasal cavity provide a large resistance to airflow; because of this, open-mouth breathing is required in strenuous exercise

It has a secondary role in olfaction (smelling); the olfactory tract has a specialised epithelium with a specialised nerve supply

Question 7

What are the paranasal air sinuses and what are their functions?

Answer 7

They are 4 sets of blind-ended out-pocketings (i.e. holes) of the lateral walls of the nasal cavities

The air turnover in these is fairly slow and plays little role in heat and water transfer; ideas on their functions include reducing the weight of the facial bones, providing a “crumple zone” in facial trauma, acting as resonators for the voice, and insulating sensitive structures such as dental roots and eyes from the rapid temperature fluctuations in the nasal cavities

Infection of the maxillary sinus is common as the opening is high up; all sinuses anterior to the brain have a possible insulating effect on the brain

Question 8

What is meant by the lower airways?

Answer 8

These comprise comprise the trachea, the bronchi and the bronchioles (initially surrounded by smooth muscle but ending as respiratory bronchioles from which alveoli are direct or indirect buds)

The walls of the larynx, trachea and bronchi are held open by plates or crescents of cartilage (a non-mineralised connective tissue, supporting but flexible); the nasal cavities and pharynx are held open by attachments to nearby bones

The microscopic air spaces (alveoli and bronchioles) contain a surfactant phospholipid that prevents collapse caused by surface tension forces

Question 9

Outline the pharynx

Answer 9

After conditioning of the air, air passes down the back of the nasal cavity to the pharynx, which is the final part of the airway proximal to its separation from the oesophagus

The pharynx consists of 3 parts:

o Nasopharynx – posterior to the nasal cavity, and is the Eustachian tube opening

o Oropharynx – posterior to the tongue, consists of lymphoid tissue

o Laryngopharynx – after the epiglottis

Food is channelled posteriorly along the oropharynx to the oesophagus

Question 10

What is the Eustachian tube?

Answer 10

Also known as the auditory tube, this links the nasopharynx to the middle ear

Question 11

What is the epiglottis?

Answer 11

The epiglottis is a flap made of elastic cartilage covered with a mucous membrane, attached to the entrance of the larynx

It closes during swallowing to prevent aspiration, forcing swallowed liquids/foods down the oesophagus instead

There are also tastebuds on the epiglottis

Question 12

Outline the larynx

Answer 12

The larynx is a cartilaginous structure supported from the roof of the mouth by the hyoid bone

Is associated with the lateral carotids

Superior and posterior to the thyroid gland, superior to the trachea

Develops differently in men and women
Entire structure is lined by a membrane, which forms a complete sheath on the inside of the trachea

Without the larynx, voice would be monotonous and of low pitch - i.e. role of larynx is modulation of sound

Also, closure of vocal folds increases the pressure in the thorax and abdomen, which can lead to an expulsive force e.g. during sneezing, childbirth and vomiting

Question 13

Explain the importance of the arytenoid cartilage in the larynx

Answer 13

The arytenoid cartilage is attached to vocal ligaments which open and close entry to the larynx

This is crucial; it acts as a sphincter, preventing entry into the lower airways

They are open during inspiration and closed during phonation (production of sounds)

When the vocal folds are partially open, and air is passed through and sound is made - this is the mechanism of vocalisation in the mouth

Question 14

Outline the trachea

Answer 14

The trachea has a regular cartilage arrangement of ~20 horseshoe shaped cartilage rings which keep the trachea open

The anterior surface is lined with epithelium

The posterior surface consists of the trachealis muscle, which is anterior to oesophageal muscle and is needed for swallowing

The posterior surface is where the cartilage ring is not continuous

Question 15

Outline the tracheobronchial tree

Answer 15

The sternal angle at T4 marks where the trachea branches

There is a dimorphism between the primary bronchi; the right side is larger and more vertical therefore more things are inhaled into the right lung

The secondary bronchi supple each lung lobe; within each lobe, tertiary bronchi then supply each pulmonary segment

With branching of the bronchi, the number of
cartilage rings decrease and the amount of smooth muscle increases

Question 16

Outline the position of the lungs within the thorax

Answer 16

The lungs lie within two pleural cavities separated by a central partition of tissue called the mediastinum (the mediastinum contains the trachea, the oesophagus (gullet), the heart and great arteries and veins and various important nerves and lymph vessels)

Question 17

Outline the position and orientation of the lungs relative to the pleura and the different surfaces of the lung

Answer 17

Each lung and the inside of the pleural cavities are covered by a thin, shiny, moist layer of tissue called the pleura, which allows each lung to slide smoothly within its pleural cavity during breathing

Each lung has a convex surface facing the ribs (the costal surface), a surface moulded to the mediastinum (the mediastinal surface) and an inferior (lower or diaphragmatic) surface, which is concave and moulded to the diaphragm, a sheet-like muscle that separates the thoracic and abdominal cavities

The highest part of each lung, the apex, projects 2-3 cm above the clavicle in an adult and really lies in the root of the neck

Question 18

Briefly outline alveolar gas exchange

Answer 18

Gas exchange occurs in the alveoli and alveolar capillaries within the bronchopulmonary segments

Alveoli are air sacs in close proximity to alveolar capillaries, forming a blood-air barrier

The pressure gradient of oxygen drives oxygen across this barrier

Question 19

Contrast partial pressure of oxygen in the air to that in the blood

Answer 19

PO2 air = 100mmHg (13.3kPa)

PO2 blood = 40 mmHg (5.3kPa)

Question 20

Describe the position of the diaphragm

Answer 20

A margin is attached to the costal margin

The centre of dome bulges up because of pressure difference between the pleural and abdominal cavities; this bulge (and hence the pressure difference) is highest during expiration

Question 21

Describe the mechanism of breathing

Answer 21

Breathing is produced by two main sets of muscles

Contraction of the diaphragm (which is attached to the costal margin, the lower border of the rib cage) pulls the domed centre of the diaphragm down and so increases the height of the pleural cavities

Contraction of the intercostal muscles, which almost fill the spaces between adjoining ribs, pulls the ribs upwards towards the relatively fixed first rib; the ribs slope down towards their anterior (front) ends so the lifting movement of the intercostal muscles increases the depth and width of the pleural cavities

Expansion of the pleural cavities produces a drop in the pleural pressure, so air flows through the airways into the lungs, which expand with the increase in pleural cavity volume

The lower part of each lung expands downwards to occupy much of the costo-diaphragmatic recess (the lowest region of each pleural cavity, which in expiration contains no lung because the margin of the diaphragm is pressed closely against the lower part of the rib cage)

Question 22

Which nerve supplies motor innervation to the diaphragm and where does this originate from?

Answer 22

The phrenic nerve

Originates at C3 - C5

Question 23

What is the function of the Eustachian tube?

Answer 23

The primary function of the Eustachian tube is to ventilate the middle ear space, ensuring that its pressure remains at near normal environmental air pressure

The secondary function of the Eustachian tube is to drain any accumulated secretions, infection, or debris from the middle ear space