The Respiratory System

Respiratory System

Provides oxygen to body tissues.
Removes carbon dioxide.
Non-vital functions:
- Sensing odor.
- Speech production.
- Straining (coughing/sneezing).

Conducting zone: provides a route for incoming and outgoing air.
- Removes debris and pathogens from incoming air.
- Warms/humidifies incoming air.
- Structures: nose, mouth, pharynx, larynx, trachea, and primary bronchi (outside of the lungs).
Exchange (Respiratory) zone

The mouth and nose are major entrances for the respiratory system.
- The mouth is shared with the digestive system and will be discussed in that chapter.
External nose: mostly cartilage and soft tissue.
- Two openings: Nares/nostrils.
Two bones-nasal, at the top (bridge) of the nose (where glasses rest).
Internal nose: Nares/nostrils open into an expanded cavity (vestibule).
Nasal septum: Separates right and left halves.
Conchae: Further divide up the nasal cavity (help make air flow less of a straight shot vs the flow into the mouth).
Mucus lines the cavity to trap larger particles (preventing them from moving further into the respiratory tract).
Hair, mucus, conchae all help to filter, warm, and moisten air as it makes its way farther into the respiratory system (breathing through the mouth doesn’t do much of any of these).
When breathing dusty or cold air the preference is to breathe through the nose to somewhat clean up the air.

Air breathed in through the nose hits the back of the nasal cavity (where pharyngeal tonsils are located) before making a sharp downward turn into the pharynx (throat).
Air flows from the pharynx into the larynx, where the vocal cords (true and false) are located.
Both vocal cords are another means of preventing breathing in larger particles.
- The lower (true) vocal cords produce sound.
The lower portion of the pharynx and most of the rest of the conducting division are lined with ciliated epithelium.

Nasopharynx
- Airway only.
- Pharyngeal tonsils.
Oropharynx
- Passageway for air and food.
- Palatine & lingual tonsils.
Laryngopharynx
- Opens into larynx anteriorly.
- Opens into esophagus posteriorly.
- Lined with pseudostratified columnar epithelium.

Cartilaginous structure.
Connects pharynx to trachea.
Stays open always (pharynx and esophagus collapse/close).
Glottis:
- Opening into larynx.
- Closed by the epiglottis (flap above) when swallowing.
True vocal cords: below the vestibular folds (false vocal cords).

From the larynx, air flows into the trachea, a tube that extends from the neck into the chest (thoracic cavity).
16-20 C-shaped pieces of hyaline cartilage make up the trachea, with soft tissue “closing” the C in the back/posterior (allows for easier swallowing).
Trachea branches into bronchi (primary) which enter the lungs.
Primary bronchi branch into smaller secondary bronchi, which branch again into even smaller tertiary bronchi, and so on about 16 times (until microscopic).
The branching creates what is called the bronchial tree.
Bronchioles are small bronchi.

Bronchioles lead to alveoli, which are sac-like structures bounded by a single layer of epithelium.
The alveoli are surrounded by capillaries (which are also bounded by a single layer of squamous epithelium).
These two thin epithelial layers make up what is called the respiratory membrane (oxygen and carbon dioxide move across it).
Alveoli make up what is called the respiratory (exchange) division/zone.

Alveolar ducts (terminal and respiratory bronchioles also) are lined with smooth muscle that keeps airways (at rest) somewhat closed (less “junk” entering them).
Type I cells: the main alveolar cells, simple squamous epithelium (contain ACE receptors, now of Covid-19 infamy).
Type II cells: secrete surfactant (to reduce sticking together of wet alveoli).
Alveolar macrophages: phagocytic (immune) cells that remove debris and pathogens.

Bronchi and alveoli make up what we call the lungs.
Right lung: divided into 3 lobes (in humans).
Left lung: divided into 2 lobes.
Like the heart, lungs are surrounded (protected from friction) by a fluid-filled sac.
The fluid inside is watery, but it is much more vital to the function of breathing (lungs will collapse/can’t breathe if pleural fluid or pressure are inadequate).
Similar to the pericardium, there is a visceral (inner- touches the lungs), a parietal (outer) pleura, and a pleural cavity.
Intrapleural pressure changes with breathing (in vs out) but it is less than what it is in the lungs (to keep them open, prevents collapsing of lungs).
The intrapleural pressure is less than the intra-alveolar pressure, but do NOT memorize the values.

Respiration is divided into 4 events (ventilation, external, internal, and cellular respiration).
Ventilation (what we think of as breathing) moves air in and out of the lungs (alveoli).
Contractions of different muscles allow for ventilation.
Diaphragm: major muscle.
Intercostals (between the ribs-external and internal).
Chest muscles (sternocleidomastoid, scalenes, etc.).

Two steps:
- Inspiration – air enters the lungs.
- Expiration – air leaves the lungs.
Quiet breathing-at rest.
- Contracts external intercostals and diaphragm for inspiration, relaxes them for expiration (gravity pulls down chest).
- NO energy used for expiration.
Forced breathing
- The same two muscles contract, but other chest muscles also.
- Contracts muscles on expiration as well (to get the air out faster) thus also requiring energy on expiration.