Respiratory System Physiology

• The lecture covers the respiratory system physiology.
• The end objectives are to describe the functional anatomy of the respiratory system, recognize its functions, discuss breathing centers, explain breathing genesis, and discuss breathing mechanics.

Respiratory Structures

• Key components comprise the nasal cavity, mouth, pharynx, larynx, trachea, bronchi, bronchioles, alveoli, lungs, ribcage, diaphragm.

Respiration

• This process supplies tissues with oxygen (O2) and eliminates carbon dioxide (CO2)
• O2 consumption at rest is about 250 ml/min.
• CO2 elimination at rest is approximately 200 ml/min.
• Respiration is classified into external, transport of gasses, and internal.

External Respiration (Pulmonary Respiration)

• Pulmonary ventilation involves continuous inflow and outflow of air, including inspiration, expiration, and a short expiratory pause.
• Normal respiratory rate (RR) in infants is 40/min. and 12-16/min in adults.
• Pulmonary gas exchange refers to O2 diffusion from the alveolar air into the alveolar capillary blood while CO2 diffuses in the opposite direction.

Transport of Gasses

• This involves blood transport O2 from the lungs to tissues and CO2 in the opposite direction.

Internal Respiration (Tissue Respiration)

• O2 is utilized with the production of CO2 and gaseous exchange occurs between the cells and their fluid medium.

Respiratory System Components

• It consists of the respiratory tract (airways) and lungs, the thoracic cage and respiratory muscles, and the breathing centers in brain stem and tracts.

The Airways

• Both upper and lower airways exist.
• The upper airways include the nasal cavity and pharynx.
• The lower airways consist of the larynx, trachea, bronchi, and bronchioles.
• The respiratory system can be divided into the upper and lower air passages and can also be classified as the conducting zone of lower air passages and the respiratory zone.
• The upper air passages and conducting zone are responsible for air conditioning
• The respiratory zone is responsible for gas exchange

Trachea and Bronchial Tree

• The structures involved are larynx, trachea, two main bronchi, and bronchioles
• Cilia are present until the beginning of the respiratory bronchioles, bent upwards, acting as a ciliary escalator.
• Kartagener's Syndrome occurs when this mechanism is defective, leading to chronic sinusitis and recurrent lung infections.
• Bronchi diameter is greater than 1.5 mm
• Bronchioles diameter is less than 1.5 mm
• There are 20-25 (average 23) generations in the bronchial tree.
• The first 11 generations include the bronchi and small bronchi
• Generations 12-16 consist of bronchioles, during which gas exchange does not occur.
• The 16th generation, which includes terminal bronchioles with diameters ≤ 1mm
• The 17th, 18th, and 19th generations consist of respiratory bronchioles, alveolar ducts, atria, alveolar sacs, and alveoli (last four generations).
• The total surface area of alveoli in contact with capillaries is 70 m2.

Functionally, the Bronchial Tree

• The tree is divided into conducting, transition, and respiratory zones.
• In the conducting zone (anatomic dead space) no gas exchange occurs, includes the trachea and terminal bronchioles, and is supplied by the bronchial artery.
• In the transition zones gas conduction occurs and some exchange(alveoli) is present. Includes the respiratory bronchioles.
• In the respiratory zone gas exchange occurs and the structures include alveolar ducts and sacs while being supplied by the pulmonary artery.

Respiratory Unit

• A full respiratory unit includes a respiratory bronchiole, alveolar ducts, atria, air sacs, and alveoli.
• The alveoli are the site of gas exchange and are lined by a thin film of fluid resulting in increased surface tension, alveolar distension resistance, and increased recoil forces in the lungs.

Epithelial Lining of the Alveoli

• Consists of the following two types of cells:
• Type I cells: flat cells that constitute the main lining cells.
• Type II cells: granular pneumocytes aiding surfactant production.

Functions of the Respiratory System

• Functionally divided into the following two categories:
• Respiratory
• Non-Respiratory

Respiratory Function

• The main respiratory function comprises gas exchange

Non-Respiratory Functions

• Breathing movements affect venous return and lymph flow rates.
• The venous blood is filtered.
• Acid-base balance is maintained.
• Excretion of acetone, methane, and alcohol occurs.
• The respiratory system has metabolic and endocrine functions and contributes to the smell sensation along with phonation or vocalization.
• The respiratory system will help with heat loss and adjustment of air flow resistance by controlling the bronchial tone.
• This provides protective (defensive) mechanisms.
• Conducting zone functions include the perception of smell (nose) and phonation or vocalization (air in the larynx).
• The water evaporation promotes heat loss and air flow resistance adjustment is completed by controlling the bronchial tone.

Respiratory Protective Mechanisms (Lung Defense Mechanisms)

• In the alveoli, micro-organisms ≤ 2 μ in diameter are attacked by PAMS.
• In the respiratory passages, air conditioning (humidification and temperature of inspired air) occurs before entry into the trachea, contains Ig= Abs.
• Particles > 10 μ are trapped by hairs of nostrils or impacted in tonsils.
• Particles 2-10 μ stick to the mm lining the trachea and bronchi, potentially causing small air passage or bronchial constriction and cough.

Functions of the Nose

• These are warming, humidification, inspired air, particle trapping (> 10μ), sneezing initiation, and smell perception.

Functions of the Larynx

• These are air passage and vocalization (phonation), produced voluntary by the vagus X nerve.

Bronchial Tone

• Parasympathetic stimulation via muscarinic M3 receptors causes bronchoconstriction and increased bronchial secretion, and is maximal at 6 AM.
• Sympathetic stimulation via B1 and B2 receptors causes bronchodilation and decreased bronchial secretion, and is maximal at 6 PM.

Bronchial Asthma

• This is characterized by extreme difficulty in breathing due to airway obstruction.
• An allergic hypersensitivity to foreign substances in the air (e.g., plant pollens) is usually the cause.
• Treatments include β-adrenergic stimulants, muscarinic receptor antagonists, anti-histaminic drugs, and glucorticoids.

Control of Breathing

• Involuntary (subconscious) automatic mechanisms exist.
• It is a basic control mechanism, performed by respiratory centers.
• Voluntary (conscious) mechanisms also exist for short periods. Decreased activity is observed in talking and voluntary apnea, while increased activity is observed during muscle exercise and voluntary hyperpnea.
• Voluntary control is generated by signals discharged from the motor area of the cerebral cortex via the pyramidal tract.

Respiratory Centers

• These are located bilaterally in the RF of the brain stem, concerning automatic breathing control.
• These centers include those in the medulla and the pons.

Medullary Centers

• The prebotzinger complex (pacemaker) is located in the medulla .
• The dorsal Respiratory Group (DRG) contains inspiratory neurons responsible for the contraction of inspiratory muscles, especially the diaphragm.
• The Ventral Respiratory Group (VRG) in the medulla, containing expiratory neurons responsible for contraction of expiratory muscles.

Pontine Centers

• This consist of the apneustic center in the lower pons that can increases the length and depth of inspiration, as well as containing pneumotoxic center in the upper part of the pons.

Genesis of the Breathing Rhythm (Role of Respiratory Centers)

• The medullary center has an inherent rhythmic activity generated in the DRG, using the pre-Bötzinger complex network of neurons.
• These neurons are located near the upper end of the medullary respiratory center.
• They act as a pacemaker, rhythmically firing to the DRG, and their discharge rendered intermittent by two factors: the pneumotaxic center that discharges rhythmic inhibitory signals to the apneustic center and the DRG; and afferent fibers in the vagi from the inflated lungs during inspiration to the apneustic center.

Mechanism (Mechanics) of Inspiration

• Inspiration is an active process, and I neurons signals descend descending tracts.
• C3, 4, and 5 stimulate the phrenic nerve, leading to diaphragm contraction that moves downward, increasing the vertical diameter of the chest (1.5 cm in Eupnea, up to 7 cm in deep inspiration).
• All thoracic segments stimulate the intercostal nerves, leading to contraction of external intercostal muscles that move downward & forward, eversion of ribs, movement of sternum forwards & upwards, and elevation of ribs, leading to bucket handle movement and increased lateral (transverse) diameter.
• As a consequence, the parietal pleura moves outwards, decreasing intrapleural pressure (more negative), leading to expansion of the lungs and decreased intrapulmonary pressure (0 to -1 mmHg).
• This causes a rush of about 500 ml of atmospheric air into the lungs, resulting in inflation.
• During Eupnea (quiet breathing) diaphragm and external intercostals m are used along with the intercartilaginous part of internal intercostals m.
• During forced inspiration the above muscles are used and accessory inspiratory muscles such as sternomastoid and scalene, Quadratus lumborum & serratus posterior inferior, Levator costarum & serratus posterior superior are employed

Abdominal & Costal Breathing

• The diaphragm is involved in abdominal breathing during contraction
• The external intercostal muscles are involved in costal breathing during contraction
• During eupnea, 75% of pulmonary ventilation is due to diaphragm contraction; costal breathing can maintain life if its movement is compromised.
• During deep breathing, the ventilation involves 50% costal and 50% abdominal movement.
• During resting breathing, expiration is a passive process, and E-neurons are quiet but can discharge during sleep or rapid breathing.

Mechanics of Expiration

• During Eupnea, the process is passive, with no muscle activity.
• It occurs after relaxation of the inspiratory muscles by: elastic property of lungs, force of surface tension that lines the alveoli, elevation of the dome of the diaphragm, and the weight of thoracic cage.
• Chest volume decreases, which causes an increase in intrapleural pressure (less negative), helping the lung recoil.
• The intrapulmonary pressure increases to +1 mmHg, leading to a rush of 500 ml of air towards the external atmosphere, and the lungs are deflated slowly due to partial contraction of the inspiratory muscles in the early expiration, exerting a braking action on the lung recoil forces.

Forced Expiration

• This occurs during muscle exercise, asthma, and emphysema.
• Anterior abdominal wall muscle contracts, increasing intra-abdominal pressure and bulging the diaphragm into the chest cavity, reducing chest cavity size.
• Internal intercostal muscles contract downwards & posteriorly, causing depression of ribs and reduction of chest dimensions.