The Respiratory System

Respiratory System

Respiratory Functions

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

Divisions of the Respiratory System

• Two ways to divide the respiratory system:
  • Anatomically
  • Physiologically

Physiologic Divisions

• 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

Nose

• 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.

Pharynx

• 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.

Sections of the Pharynx

• 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.

Larynx

• 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).

Anatomical Divisions

• Anatomical divisions include the upper and lower respiratory tracts.
• The boundary between them is the larynx, more specifically the vocal cords.
• Upper respiratory tract: All respiratory structures above the vocal cords.
• Lower respiratory tract: All respiratory structures below the vocal cords.

Trachea & Bronchi

• 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.

Respiratory Zone

• 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.

Alveoli

• 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.

Gross Anatomy

• 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.

Muscles Around the Lungs

• 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.).

Pulmonary Ventilation

• 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.