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Module 10 - Respiratory System Introduction and Anatomy I

Breathing and Respiration

There are three processes required for respiration to actually occur:

Ventilation (breathing) is the movement of air into and out of the lungs.
Respiration can be divided into internal and external respiration:
- External respiration is the gas exchange between the air in the lungs and the blood that moves through those lungs. It is the transportation of oxygen and carbon dioxide into the blood from the external environment. air in lungs → blood
- Internal respiration is gas exchange between the blood and the tissues. We take oxygen and carbon dioxide from the blood and remove it or add it to body tissues. blood → tissues

Other Respiratory System Functions (PPROV)

Production of chemical mediators. For example, the lungs produce angiotensin-converting enzyme (ACE), which helps control blood pressure.
Protection. A variety of structures, such as nostril hairs and mucous membranes that line the respiratory system, help remove and protect against any micro-organisms entering or attempting to enter the body.
Regulation of blood pH. The bicarbonate system helps buffer any pH changes when there are alterations in blood pH. The respiratory system can alter blood pH by changing blood carbon dioxide levels.
Olfaction. The olfactory region is in the nasal cavity, which is part of the respiratory system and causes the sensation of smell.
Voice production. The movement of air past the vocal folds can produce various sounds along with movements of the mouth and tongue. The nasal passages and sinuses also act as resonating chambers to help create the variety of sounds that make up speech.

General Respiratory System Anatomy

The respiratory system can be divided into two regions:

The upper respiratory system, which contains the external nose, nasal cavity, pharynx (throat), and larynx (voice box). Larynx = Language
The lower respiratory system, which contains the trachea (windpipe), bronchi (the branches off the trachea that lead into the lungs), and the lungs.

The respiratory system can also be classified into two zones:

The conducting zone, which is where we have movement of air but no gas exchange. Everything in the respiratory system except the alveoli is part of the conducting zone (all the structures that bring air in from the external environment towards the alveoli).
The respiratory zone, which is where gas exchange is occurring. The alveoli is the only place where gas exchange is occurring, so it is the only respiratory zone.

Nose and Nasal Cavity

The nose is composed of bones and cartilage. The cartilage helps create openings known as external nares/naris, which are our nostrils. The external nares are where air enters the nasal cavity.

Just inside the external nares/nostrils is the nasal vestibule, which contains stratified squamous epithelial cells aligned with hairs. The nasal vestibule acts as our first line of defence in the respiratory system by enabling us to trap large particles floating in the air. The nasal vestibule is lined with stratified squamous epithelium because it is closest to the external environment and this type of epithelial cell, with its many layers, provides the most protection.

The base of the nasal cavity contains the hard palate, which is composed of the maxilla and palatine bones. The hard palate separates the superior nasal cavity from the inferior oral cavity.

Ridges line the lateral sides of the nasal cavity on both the right and left sides. These ridges are called nasal conchae. We contain superior, middle, and inferior nasal conchae. The nasal conchae help create turbulent flow as the air moves into the nasal passageway. They also create a large surface area for mucous membranes.

All structures found within the nasal cavity are lined with mucous membranes that produce mucous to help trap any smaller particles as they move through the nasal passageways. They are also highly vascular, so they help heat up air as it moves through the passageways. They also add some moisture to the air.

Between the nasal conchae are canals called nasal meatuses. The superior nasal meatus is between the superior nasal concha and the middle nasal concha. There are also middle and inferior nasal meatuses. These are the passageways where air will actually travel from the nasal vestibule towards the back of the nasal cavity. The ridges create surface area so the air travelling through has lots of contact with mucous membranes as it travels to the back of the throat.

On the superior region of the nasal cavity is the olfactory epithelium.

In the nasal cavity bones there are also sinuses, small cavity structures within the bones. The frontal bone contains a small sinus called the frontal sinus and the sphenoid bone contains the sphenoidal sinus. Together, these two sinuses make up the paranasal sinuses. The paranasal sinuses are lined with mucous membranes and they can secrete substances into the nasal cavity (specifically the superior and middle meatuses) to provide layers of protection for air as it moves through.

These skull openings also make the skull lighter and, in the case of the paranasal sinuses, help with speech production. In addition to the nasal cavity itself, the sinuses help to resonate sounds. This is why the voice sounds funny when you have a sinus infection—the sinuses are blocked so we cannot get the same resonating sounds typically heard when speaking.

The end of the nasal cavity contains an opening called the choana/choani. We have a right choana and a left choana because we have a right and left nasal cavity separated by a septum. The choani are also known as the internal nares because, like the nostrils, these are openings. The external nares are openings into the nasal cavity and the internal nares are openings into the pharynx/throat.

The job of a lot of the nasal cavity structures is to increase mucous membrane surface area so air is more likely to come in contact with a mucous membrane before it enters the rest of the respiratory system. In addition to the paranasal sinuses draining mucous into the nasal cavity, the lacrimal duct also drains mucous into the inferior meatus. The lacrimal duct also carries tears into the nasal cavity, which helps provide moisture to the air as it moves through the nasal passage.

Functions of the Nasal Structures

The olfactory epithelium, found on the roof of the nasal cavity, is used for our sense of smell. The nasal cavity, unlike the nasal vestibule (which contains stratified squamous epithelium), is composed of pseudostratified ciliated columnar epithelial cells that contains goblet cells. This epithelium has several jobs:

The mucous membranes made by the epithelium in the nasal cavity help to warm the air due to high vascularity.
The mucous produced by the goblet cells helps to moisten the air and trap dust.
The cilia moves the mucous towards the pharynx in order for us to either swallow it or spit it out.

Pharynx

The pharynx/throat is a muscular tube about 13cm in length. It is composed of skeletal muscle and mucous membranes. The pharynx has several functions:

It acts as a passageway for food and air.
It can act as a resonating chamber for speech production.
It contains several tonsils, masses of lymphatic tissue involved in immune responses.

The pharynx can be divided into three main regions: the nasopharynx, the oropharynx, and the laryngopharynx.

The pharynx starts at the choana and travels down to the opening of the esophagus. The esophagus and trachea start at approximately the same level, serving as openings into the digestive and respiratory systems, respectively.

The nasopharynx is the superior part of the pharynx, stretching from the choana to the soft palate.

The soft palate, a combination of muscle and mucous membrane, acts as a continuation of the hard palate, blocking the nasal cavity during swallowing to prevent food and fluids from entering the nasal cavity.
The uvula, located at the end of the soft palate, assists in sealing the passage to the nasal cavity. It separates the nasopharynx from the oropharynx when swallowing and is attached posteriorly to the soft palate.
The auditory (eustachian) tube opens in the nasopharynx, equalizing air pressure on both sides of the tympanic membrane, connecting to the middle ear.
Pharyngeal tonsils (adenoids) are located in the superior nasopharynx, potentially inflaming during illness.

Extending from the soft palate to the epiglottis, the oropharynx serves as the passage for food and fluids from the oral cavity to the pharynx.

The fauces is the opening from the oral cavity into the oropharynx.
The oropharynx also contains superior palatine tonsils and inferior lingual tonsils, which are both commonly removed in tonsillectomies.

The laryngopharynx runs from the epiglottis to the start of the esophagus, transporting both food and fluids towards the esophagus and air towards the trachea.

The laryngopharynx, alongside the oropharynx, support the passage of both air and ingested substances.

While the nasopharynx primarily carries air, the oropharynx and laryngopharynx facilitate the movement of both air and food/fluids. A malfunctioning uvula can cause fluids to move in the reverse direction.

Pharynx Epithelium

The nasopharynx is an extension of the nasal cavity, positioned posterior to the choana and superior to the soft palate, serving exclusively as an air passageway. It is lined with pseudostratified ciliated columnar epithelial cells, similar to the nasal cavity, which aids in air filtration and humidity control.

Both the oropharynx and laryngopharynx function as passageways for air, food, and drink. The oropharynx extends from the soft palate to the epiglottis, while the laryngopharynx runs from the epiglottis to the esophagus. These regions are lined with stratified squamous epithelial cells to provide protection against the mechanical stress induced by the passage of food and fluids.

The distinction in tissue lining between the nasopharynx and the oropharynx/laryngopharynx reflects their functional differences. The nasopharynx, similar to the nasal cavity, requires a lining that supports air movement and filtration, hence the presence of pseudostratified ciliated columnar epithelium with goblet cells. In contrast, the oropharynx and laryngopharynx, which accommodate both air and ingested materials, are lined with stratified squamous epithelium to withstand wear and tear.

Structure of the Larynx

The larynx/voice box facilitates air passage from the pharynx to the trachea. It is located anterior to the esophagus and plays a crucial role in breathing, voice production, and protecting the airway during swallowing.

The larynx extends from the pharynx to the trachea level. The thyroid gland, which encircles the trachea, is often depicted with the larynx in anatomical illustrations to highlight their proximity.

Composed of nine pieces of cartilage, the larynx includes three unpaired (single) and six paired (three pairs of smaller) cartilages. These cartilages are interconnected by ligaments and muscles, with many attaching to the hyoid bone—a unique "floating" bone behind the mandible.

Unpaired Cartilages (EpiCriT):

Epiglottis: A flap of elastic cartilage, distinct for its flexibility compared to the hyaline cartilage of other laryngeal structures. It covers the tracheal opening during swallowing, preventing food and fluids from entering the airway.
Thyroid Cartilage: Also known as the Adam's apple, more prominent in males due to changes during adolescence. It helps maintain the laryngeal opening for air passage.
Cricoid Cartilage: A ring-shaped structure at the larynx base, supporting the trachea's top rings and ensuring an open airway.

The larynx moves upward during swallowing, with the epiglottis effectively covering the trachea to direct food and fluids away from the airway. The thyroid and cricoid cartilages contribute to keeping the airway open for respiration.

Within the larynx, pairs of ligaments covered by mucous membranes form the vestibular folds (false vocal cords) and vocal folds (true vocal cords). These are lined with stratified squamous epithelium and adjust shape for sound wave creation, playing a pivotal role in voice production.

Unpaired Cartilages of the Larynx

The thyroid cartilage forms the Adam's apple.

The epiglottis is a leaf-shaped piece of elastic cartilage. During swallowing, the entire larynx will move upwards and the epiglottis will cover over the tracheal opening. The opening to the throat where we have the structures that make up the vocal cords is known as the glottis.

The other unpaired cartilage is the cricoid cartilage, which is a ring of cartilage that is attached to the top of the trachea. There are also three additional pairs of cartilages.

Larynx Structure Continued

The larynx sits atop the trachea, which is characterized by C-shaped cartilage rings. These rings are open on the posterior side, providing structural support while allowing flexibility and expansion during respiration.

Key Structures of the Larynx:

Epiglottis: Visible in both anterior and posterior views, showcasing its leaf shape. Attached to the thyroid cartilage via ligaments, it acts as a movable flap that covers the airway during swallowing, directing food and liquids into the esophagus.
Thyroid Cartilage (Adam's Apple): Prominently located on the anterior side of the larynx, it is connected to the hyoid bone by the thyrohyoid membrane. The structure is notably more significant on the anterior side, with minimal presence posteriorly.
Cricoid Cartilage: Forms a complete ring at the larynx's base and is particularly prominent on the posterior side. It provides substantial support to the larynx and is positioned directly above the trachea.

Paired Cartilages (CuCoA):

Cuneiform Cartilages: Embedded in the mucous membrane anterior to the corniculate cartilages, they support the lateral aspects of the epiglottis and vocal cords, reinforcing the structure and integrity of the larynx.
Corniculate Cartilages: Small cartilages located at the arytenoid cartilages' tips, playing a supporting role in the larynx's structure and function.
Arytenoid Cartilages: Articulate with the cricoid cartilage at the posterior superior part. These cartilages are crucial for their role in attaching to both the vestibular and vocal folds, implicating them in voice production mechanisms.

The larynx's structural complexity, highlighted by its cartilaginous framework, ensures its role in breathing, voice production, and protecting the respiratory tract during swallowing. The arytenoid cartilages, in particular, are very important in speech production, facilitating the movement and tension of the vocal folds necessary for sound generation.

Vocal Folds

The larynx's vocal folds play an important role in breathing, preventing material entry into the trachea, and speaking.

The thyroid cartilage forms the anterior portion, followed by the cricoid cartilage, which completes a ring structure beneath the thyroid cartilage.

The arytenoid cartilages, located at the posterior part of the cricoid cartilage, are crucial for vocal fold attachment and movement. These cartilages, along with the corniculate and cuneiform cartilages, are enveloped by mucous membranes, contributing to the larynx's functional anatomy.

Vocal folds attach anteriorly to the thyroid cartilage and posteriorly to the arytenoid cartilages. The arytenoid cartilages' lateral and medial movements adjust the vocal folds' opening, which is crucial for differentiating between breathing and speaking modes.

Abduction of the vocal folds occurs when muscles contract to pull the arytenoid cartilages outward, enlarging the gap for breathing. Adduction involves contracting different muscles to bring the arytenoid cartilages together, closing the vocal folds for sound production or to protect the airway.

Sound is generated when air is forced through the closed vocal folds, creating varying air pressure zones that result in sound waves. The glottis, the space between the vocal folds, plays a central role in modulating airflow and sound characteristics.

Pitch alteration is achieved by adjusting the tension of the vocal folds through the anterior or posterior movement of the arytenoid cartilages. Moving them posteriorly increases tension and elevates pitch, while anterior movement reduces tension, lowering the pitch.

The larynx's design allows for precise control over vocal fold position and tension, enabling a wide range of vocal expressions and protective reflexes. The mucous membranes and associated cartilages not only support vocal fold movement but also ensure the larynx's structural integrity and functionality in voice production and airway protection.

Trachea Structure and Function

The trachea measures approximately 12cm in length, extending from the larynx down to the level of the fifth thoracic vertebra (T5).

Comprised of 16 to 20 C-shaped rings of hyaline cartilage, these structures are crucial for maintaining the trachea's open and rigid form. The cartilage rings provide structural support amidst a matrix of dense, regular connective tissue and smooth muscle.

The posterior side of the trachea, where the C-shaped rings are open, accommodates the esophagus. This design allows for the esophagus's expansion during the passage of food, ensuring that the tracheal structure does not interfere with esophageal function. The primary purpose of these C-shaped cartilages is to prevent the trachea from collapsing, ensuring a clear airway for respiration.

The gaps in the cartilage rings are bridged by elastic membranes and the trachealis muscle, contributing to the trachea's flexibility and ability to adjust its diameter.

Located at the trachea's base, the carina is a cartilaginous structure marking the trachea's division into the right and left bronchi, leading to each lung. The carina is especially sensitive to irritation, equipped with a membrane that triggers a cough reflex upon detecting particulate matter, thereby protecting the respiratory system from potential obstructions or infections.

A transverse section through the trachea reveals the C-shaped rings of hyaline cartilage, which structurally support the trachea. The lumen of the trachea, where air passes, is centrally located within these rings.

The open part of the C-shaped cartilage rings on the trachea's posterior side is designed to accommodate the esophagus. This structural adaptation ensures that the trachea and esophagus can coexist without compromising the function of one another.

The gap in the C-shaped cartilage is bridged by a fibromuscular membrane comprising smooth muscle, specifically the trachealis muscle, and elastic fibres. This arrangement provides flexibility and the capacity for minor adjustments in diameter.

The presence of the trachealis muscle and elastic membrane allows for dynamic changes in the trachea's size. This flexibility accommodates the esophagus during the swallowing of large food items, allowing it to expand into the trachea's space temporarily. It can also increase space within the trachea for enhanced air flow when necessary, to support both respiratory and digestive functions.

Trachea Epithelium

The trachea, like many other air passages in the respiratory system, is lined with pseudostratified ciliated columnar epithelium, which features goblet cells. This type of epithelium is specialized for trapping debris and particulate matter in the mucus produced by goblet cells.

Goblet cells within this epithelium secrete mucus, which serves to capture airborne particles and bacteria, preventing them from reaching the lungs. The cilia, hair-like structures on the surface of epithelial cells, then propel this mucus, along with the trapped particulate matter, towards the pharynx. This mechanism is essential for maintaining the cleanliness and health of the respiratory tract.

This protective epithelial lining is present not only in the trachea but also in the larynx, with an exception at the sites of the vestibular and vocal folds, where stratified squamous epithelium is found instead. The purpose of the differing epithelium in these regions is to provide protection against mechanical stress and potential damage from food particles during swallowing.

In both the trachea and larynx, the direction of mucus movement is consistently towards the pharynx, facilitating the removal of trapped particulate matter by swallowing. Similarly, in the nasal cavity and nasopharynx, cilia move the mucus downward towards the pharynx, ensuring that foreign particles are efficiently cleared from the respiratory system.