Exercise 6: Respiratory System

The respiratory system is responsible for breathing and the exchange of gases that allow respiration to occur at the cellular level. Breathing (ventilation) is the mechanical movement of air into and out of the lungs. The gases, oxygen and carbon dioxide, are exchanged between the lungs and the blood. The blood transports the oxygen to the tissues where the cells use the oxygen for respiration. Respiration is the chemical process of using this oxygen to produce energy. A waste product of the respiratory process is the gas, carbon dioxide, which is taken up from the tissues into the blood and transported back to the lungs to be exhaled back into the environment.

Below is the list of the principal organs that contribute to the respiratory system, we will delve deeper into the structures and functions associated with each organ. They are listed in the order where air enters the body, up to where gas exchange happens. These are the basic structures that students should know and understand:

Nose ⇒ Pharynx ⇒ Larynx ⇒ Trachea ⇒ Bronchi ⇒ Lungs

The Respiratory system can be categorized into 2 types of divisions, which are discussed further in this reading:

1. Anatomical Division, which mainly encompasses structures that are found in the respiratory tracts.

2. Functional Division, which mainly encompasses structures that are further divided based on their function of either being a passageway or for gas-exchange.

Anatomical Divisions of the Respiratory System

Upper Respiratory Tract

• Organs found in head and neck

  • Such as the nasal cavity, pharynx and larynx

Lower Respiratory Tract

• Organs of thorax

  • Such as the trachea through lungs

An upper respiratory tract infection (or URTI, or URI) refers to a general and acute infection (often viral) of one or a number of parts of the upper respiratory tract, including nose, sinuses, pharynx, or larynx. The infection would cause inflammation of these regions and possibly an over-production of mucus. The common cold is classified as a URTI and a sore throat often associated with this condition.

Functional Divisions of the Respiratory System

Conducting Division (passages for airflow)

• All cavities and structures (nostrils to bronchioles) that allow for air into and from the alveoli

Respiratory Division

• Gas-exchange surfaces at the level of the alveoli

The Upper Respiratory Tract

Nose

Functions:

• To warm, cleanse, and humidify inhaled air

• To detect odors (through olfactory receptors located on the cribriform plate)

• As a resonating chamber that amplifies the voice

Structures

Bony and cartilaginous support include:

• Superior half: nasal bones

• Inferior half: lateral and alar cartilages

• Ala nasi: flared portion shaped by dense connective tissue that forms the lateral wall of each nostril

Nasal Cavity

It is the internal chamber of the nose and is divided into right and left halves called nasal fossae. The dividing midline wall is a vertical plate, the nasal septum, composed of 2 cranial bones (vomer and perpendicular plate of the ethmoid bone) and a hyaline cartilage, called the septal cartilage. It is bordered by the palate (hard and soft) from the oral cavity.

The conducting division begins in the nasal cavity and consists of:

Nasal Conchae 

• These are 3 folds of tissue, namely superior, middle, and inferior on the lateral wall of nasal fossa:

  • That help increase the surface area for the incoming air. 

  • They also are lined with mucous membranes to help sense odors and trap air particulates/pathogens. There are 2 types of mucosa lining these folds:

    • Olfactory mucosa: lines the roof of the nasal fossa. The mucus produced here helps to dissolve gaseous odors for binding onto the chemosensory olfactory neurons. 

      • They are lined with stratified squamous epithelium which contains mucous glands that supplement the mucus produced by the goblet cells. 

    • Respiratory mucosa: lines the rest of the nasal cavity with ciliated pseudostratified columnar epithelium. It has goblet cells that secretes mucus and its cilia propel the mucus posteriorly toward the pharynx.

    • Note: mucus is a noun and mucous is an adjective. The actual fluid that comes out of the nose is mucus and the linings in your body that secrete mucus are mucous membranes.

Nasal Meatuses

• These are narrow air passages, namely superior, middle, and inferior, located beneath each concha which help to ensure that air comes into contact with mucous membranes

Pharynx

As the air moves from the nasal cavity into the pharynx or the “throat”, there are 3 general areas that are recognized as the air moves toward the lungs:

Nasopharynx: the transition between the nasal cavity and the pharynx that comes into contact with air only. The uvula (the soft “punching bag” in the back of the throat) is found here, which helps to prevent food from entering the nasopharynx when swallowing. 

Oropharynx: the transitional region between the oral cavity and the pharynx that comes into contact with air, food and liquids. It is lined with stratified squamous epithelium. This space is between the soft palate and root of tongue, contains palatine and lingual tonsils, and extends inferiorly as far as the hyoid bone.

Laryngopharynx: the transition between the pharynx and the area of bifurcation (split) between the larynx and the esophagus. It is the region between the hyoid bone to the level of cricoid cartilage. It is lined with stratified squamous epithelium.

Larynx

From the pharynx, the air comes into the larynx or the "voicebox", which is outlined by the hyoid bone and 9 cartilages. The glottis, made up of the vocal cords and opening between, is inferior to the epiglottis, which is a flap of tissue that guards the glottis and directs food and drink to the esophagus (not into the trachea).

The larynx of an infant lies higher in throat than in the adult and it forms a continuous airway from nasal cavity allowing for breathing while swallowing. By 2 years of age the child develops a more muscular tongue, which ultimately forces the larynx down to its typical position.

9 Cartilages of the Larynx: forms the framework of the larynx.

Unpaired and Large Cartilages

• Epiglottic cartilage (1): the most superior and spoon-shaped. It is the cartilage that makes up the epiglottis.

• Thyroid cartilage (1): the largest and have a shield-like shape. The laryngeal prominence forms here, which is more prominent in men than women. The presence of this prominence results in a deeper voice.

• Cricoid cartilage (1): inferior to the thyroid cartilage is this ring-like cartilage. It connects larynx to trachea and constitutes as the "box" of the voicebox.

Paired and Smaller Cartilages

• Note: they are all found posterior to the thyroid cartilage.

• Arytenoid cartilages (2): lie posterior to thyroid cartilage, resembles a ladle. Functions in speech.

• Corniculate cartilages (2): attach to the upper end of arytenoid cartilages, like a pair of little horns. Functions in speech.

• Cuneiform cartilages (2): wedge-like that supports soft tissue between arytenoids and epiglottis.

Ligaments: fibrous ligaments that binds the cartilages of the larynx together and forms a suspension system for the upper airway.

• Thyrohyoid ligament: broad sheet that suspends the larynx from the hyoid bone above. Found between the thyroid cartilage and hyoid bone.

• Cricothyroid ligament: connects the cricoid cartilage to the thyroid cartilage. Site for cricothyrotomy, which temporarily treats airway obstruction. It is done via an incision on this ligament.

• Cricotracheal ligament: suspends the trachea from the cricoid cartilage.

Vocal Cords: the walls of the larynx have 2 folds on each side, from the thyroid to arytenoid cartilages:

• Vestibular folds (false vocal cords): lie as a superior pair. It functions to close the glottis during swallowing and it doesn't play any role in speech.

• Vocal cords (true vocal cord): produce sound. As mentioned before, these and the opening between them, forms the glottis

The Lower Respiratory Tract

Trachea

The air passes from the larynx into the trachea. 

• This structure is a rigid tube extending ~4.5 inches long and ~2.5 inches in diameter. 

• It lies anterior to the esophagus. 

• It is supported by 16–20 C-shaped cartilaginous rings called tracheal cartilages. 

  • Wherein, they reinforce the trachea and prevent it from collapsing during inhalation.

  • The openings of these C-shaped rings face posterior toward the esophagus. 

  • The trachealis muscle spans where the opening occurs in these C-shaped rings:

    • Allowing for adjustments in airflow by expanding or contracting.

    • It also allows room for the esophagus to expand when swallowing food. 

• Site for tracheotomy, an airway surgical management procedure to help with long-term breathing issues. It is done via an incision on the anterior side of the trachea.

• The trachea is lined with ciliated pseudostratified columnar epithelium, which contains goblet cells, for mucus production and cilia for propelling mucus to the pharynx:

  • Which functions as a mucociliary escalator to bring the mucus (and the particulates/pathogens it traps) back to the oropharynx, where it can be expelled or swallowed to the digestive system.

Carina: The final ring with an internal median ridge. It is positioned where the primary/main bronchi form. From here it directs the airflow to the right and left lungs.

Bronchial Tree

• The left and right primary (or main) bronchi (singular: bronchus) begin the branching of the bronchial tree. 

• These primary bronchi divide into smaller secondary (or lobar) bronchi 

  • There are 2 secondary bronchi on the left lung (left superior and left inferior lobar bronchi) and 3 secondary bronchi (right superior, middle, and right inferior) on the right lung, which serve their respective lobes of each lung. 

• The secondary bronchi then branch into the smaller tertiary (or segmental) bronchi, which serve the respective segments of each lobe they are found in.

• Then the tertiary bronchi branch further into quaternary, etc. - until the air passages are 1 millimeter in diameter, at which point they are referred to as bronchioles, which lack cartilage. 

Bronchioles

• The passages become smaller until they are 0.5 mm in diameter, at which point they are referred to as terminal bronchioles. 

  • These terminal bronchioles represent the end of the conducting division of the respiratory system.

• The respiratory bronchioles begin the respiratory division of the respiratory system. Each of these respiratory bronchioles, divides into 2-10 elongated thin walled passages called the alveolar ducts the ducts then end in the “grapelike” sacs or alveolar sacs, which are clusters of alveoli.

Alveoli

• The alveoli (singular: alveolus) are pouches surrounded by capillary networks that place the red blood cells in close proximity to the air within them. An alveolus is made up 2 types of cells: 

  • The thin and broad squamous (type I) alveolar cells, which makes up 95% of the alveolar surface area. Its thinness allows for rapid gas exchange.

  • The round to cuboidal great (type II) alveolar cells. which makes up the other 5% . It has 2 functions:

    • Aids in repairing damaged alveolar epithelium.

    • Secretes pulmonary surfactant, which coats the alveoli and smallest bronchioles to prevent them from collapsing during exhalation. 

The Lungs

These are conical organ with a broad, concave lung base resting on the diaphragm and a blunt peak called the lung apex, projecting slightly above the clavicle. The macrostructure of the lungs consists of 5 lobes in total.

• The right lung has 3 lobes, which are named:

  • Right superior lobe (paired)

    • In between these 2 lobes is an unpaired deep groove called the horizontal fissure.

  • Middle lobe (unpaired)

    • In between these 2 lobes is a paired deep groove called the right oblique fissure.

  •  Right inferior lobe (paired)

• The left lung has 2 lobes, which are named:

  • Left superior lobe (paired)

    • In between these 2 lobes is a paired deep groove called the left oblique fissure. 

  • Left inferior lobe (paired)

• The lungs are surrounded by two layers of serous membranes, known as pleural membranes:

  • Parietal pleura: this outer layer lines the interior of the thoracic cavity and superior surface of the diaphragm

  • Visceral pleura: the inner layer that comes into direct contact with the lungs.

  • The pleural space between these two layers is filled with pleural fluid to:

    • Reduce friction

    • Create a pressure gradient (lower pressures assist in lung inflation)

    • Compartmentalization (to prevent the spread of infections)

• The medial side of the lungs contains an indentation called the lung hilum. 

  • It is the location where the primary bronchi, pulmonary artery and vein, afferent and efferent nerves, hilar lymph nodes and lymphatic vessels enter and exit the lungs.

• Note: the right lung is shorter than the left lung due to the placement of the liver, while the left lung is narrower due to the left tilting position of the heart.

• The left lung contains a concave indentation on its medial surface called the cardiac impression, where the heart presses against it. 

• Also on the left lung, we will find a crescent-shaped indentation called the cardiac notch,that is only visible anteriorly on the margin of the lung. 

• The lungs have 3 surfaces surrounding it:

  • Costal surface: a broad surface pressed against the rib cage.

  • Diaphragmatic surface: an inferior surface pressed against the superior side of the diaphragm.

  • Mediastinal surface: a smaller and concave surface facing medially, it contains the lung hilum.

Breathing Mechanics

The goal of breathing is to bring air inside the lungs (inhalation) and push air out of the lungs (exhalation). To achieve these actions requires a change in pressure within the thoracic cavity, which is accomplished by the coordinated contraction and relaxation of muscles including the diaphragm and external intercostal muscles.

• Inhalation: Contracting the diaphragm (which has a flattening effect) and external intercostal muscles increases the area of the thoracic cavity (e.g., volume) around the lungs. The alveolar pressure drops from 760 mmHg to 758 mmHg and air flows into the lungs (from high to low pressure). The flow of air continues until the alveolar pressure is restored to 760 mmHg.

• Exhalation: The muscles involved in inhalation relax, subsequently decreasing the area of the thoracic cavity. The elastic nature of the lungs and associated structures helps in the recoiling effect (think rubber band) needed for exhalation. Together, there’s a net increase in alveolar pressure to 762 mmHg within the thoracic cavity and air flows along its gradient until the alveolar pressure reaches 760 mmHg. Note: the internal intercostal muscles are involved during forced exhalation.

Oxygen Hemoglobin Dissociation Curve

The Oxygen Hemoglobin Dissociation Curve represents the percent of total blood hemoglobin that is saturated with oxygen at a given partial pressure of oxygen (PO₂). 

• The partial pressure of oxygen will change throughout the body. 

• The higher the PO₂, the more oxygen (O₂) combines with hemoglobin.

• This relationship helps us to understand the relative PO₂ and the percent saturation found in different regions of the body. 

  • For example, as expected, the highest percent saturation of hemoglobin would be found within the blood within the lungs, which is where the PO₂ (~100 mmHg) is also located.

• At rest (e.g., couch-potato mode), our bodies do not require a lot of oxygen for aerobic energy production when compared to active/exercising muscles, so the saturation of hemoglobin will be different at these two levels of physiological demand for oxygen.

• Similarly, the blood’s biochemistry can influence the binding characteristics of oxygen to hemoglobin, and therefore the position of this sigmoid-shaped line making up the Oxygen Hemoglobin Dissociation Curve. 

  • For example, blood biochemistry can be influenced by heavy carbon dioxide production from aerobic respiration, body temperature, respiration rate, or exposure to high altitude.

• Shifting to the left means increased affinity/attraction to oxygen and reduced oxygen delivery to the tissues. This can be caused by:

  • low temperature

  • more alkaline/basic (high pH of 7.6)

  • lower PCO2

• Shifting to the right means reduced affinity/attraction to oxygen and increased in delivering of oxygen to the tissues. This can be caused by:

  • higher temperature

  • more acidic (low pH of 7.2)

  • higher PCO2