Physiology: Respiratory System

Pressure - Atmospheric

Atmospheric pressure - 760mmHg or 1atmosphere

Transmural pressure: Inside - outside

Respiratory Pressures

• Pleural pressure is 4mmHg below atmospheric pressure

• Atmospheric pressure = alveoli pressure

• Transpulmonary pressure = alveoli pressure - pleural pressure.

Thoracic cavity expansion - Inspiration at rest

• Contraction of the diaphragm:

• increases vertical dimension of the thoracic cavity

• Elevation of ribs causes sternum to move out, increasing Front- to back dim

• Contraction of external intercostal muscles:

• Causes elevation of ribs and increase side-side dimension

Thoracic cavity compression (Passive or active)

Passive:

• Everything occurs in the opposite direction of inspiration

Active:

• Contraction of internal intercostal muscles flattens ribs and sternum, further reducing side-to-side and front-to-back dimensions

• Contraction of abdominal muscles forces diaphragm upward, further reducing vertical dimension.

Boyle’s Law

• the volume of a given mass of gas varies inversely with the pressure when the temperature is kept constant.

Definitions: Tidal volume, Functional residual capacity

• TV: Volume of air breathed in and out of the lung under resting conditions

• FRC: Volume of air in the lung after a normal tidal expiration.

Definitions: Total lung capacity, Residual volume

• TLC: Volume of air in the lung after maximal inspiration

• RV: Volume of air in the lung after a maximal forced expiration

Definitions: Inspiratory Capacity, Inspiratory reserve volume

• IC: Maximal volume of air that can be inhaled into the lung above a normal tidal inspiration

• IRV: Volume of air that can be inhaled into the lungs above a normal tidal inspiration.

Definitions: Expiratory reserve volume, Vital capacity

• ERV: Volume of air that can be exhaled out the lungs from the functional residual capacity (FRC)

• VC: Vital capacity: Volume of air that can exhaled out the lungs from TLC during a non-forced manoeuvre.

Helium dilution

• lung volume can be determined from the gas dilution

• C1 x V1 = C2 x V2

• C1 x V1 = C2 x (V1 + FRC)

• FRC = (C1 x V1/C2 ) - V1

Airway resistance

• Causes a loss of enegery due to the air molecules colliding itch the liminal surface of the airway.

• Lumen radius is therefore. Critical determinant of resistance

• Small change in adius procedure a large change in resistance

Airway resistance in the lungs

• Determines by the number and the size of airways

• Resistance is greatest in the more proximally located towards the trachea

• The distal airways (towards the periphery) have very little resistance due to large cumulative cross-sectional area and are often referred to as the ‘silent zone’

Airway resistance - smooth muscle contraction

• Bronchoconstriction increases airway resistance and makes breathing more difficult,

• From a 5 mm readout to a 1mm radius, there is a 600 fold increase in resistance.

Airway resistance - asthma

• contraction of the airway smooth muscle in diseases like asthma occurs due to release of histamine (allergic cascade) which binds to the histamine - 1 receptors. (Causes inflammation and mucus production)

• Parasympathetic nerve stimulation releases acetylcholine that binds to muscarinic - 2 receptors on the muscle cell and induces contraction

Bronchocontraiction

• produced by parasympathetic nerve stimulation

• Blocked by atropine

Lung compliance - Pressure-volume curve

• compliance determines how easy it is to inflate the lung

• Volume change per pressure (L/cmH2O)

• Lung is more compliant at lower lung volumes and become stiff towards total lung capacity

Ventilation - Minute versus alveolar

• Approximated at 0.5L per breath and 15 breaths/min

• Minute ventilation = tidal volume x breathing frequency

  • 0.5 × 15 = 7.5L/min

• Alveolar ventilation = (tidal volume - dead space volume) x breathing frequency.

Partial Pressure of O2

Pressure total = P_N + P_O2 + P_argon + P_CO2

• P_O2 in the atmosphere = barometric pressure x 0.21

• 760mmHg x 0.21 = 160mmHg

Partial pressure of 02 - atmosphere to the blood

1. Air that enters the lung is humidified - water vapour exerts. Partial pressure

   • Partial pressure of H2O at 37º is 47mmH

   • P_iO2 = (barometric pressure - 47mmHg) x 0.21

Partial Pressure of CO2 - Atmosphere to the blood

Alveolar gas equation

• P_AO2 = P_iO2 - P_ACO2/R

• R = the respiratory exchange ration which relates the amount of CO2 produced to amount of O2 utilised.

Oxygen-hemoglobin Dissociation curve

• % Hemoglobin saturation = sites bound with O₂ / Total binding sites

Automatic control of breathing - medulla

• Dorsal respiratory group (DRG) contains neurons that stimulate inspiration

• Ventral respiratory gourd (VRG) are neurons that are inactive during quite breathing, but active during forced breathing

Automatic control of breathing - Pons

• Pneumotaxic centre sends neural impulses to the DRG to halt inspiration

• Apneustic centre sends neural impulses to the Inspiratory from getting switched off.

Voluntary Control of Breathing - Motor cortex

• Directly modifies respiratory muscle contraction

  • breath hold

  • Deliberately hyperventilate

  • Blowing up balloon

Chemoreceptors - Central

• Detects changes in H+ in extracellular fluid within the brain

  • H+/HCO₃- can’t pass through the blood brain barrier but CO₂ can → decreasing pH

• Contributes 80% of the overall response to an increase in PaCO₂

Chemoreceptors - Peripheral

• Located in the aortic and carotid oldies

• Detects changes in O₂ , CO₂ and H+

  • Oxygen: Not activated until PaO₂ falls below 60mmHg

  • Carbon dioxide: weakly responsive

  • Arterial H+: Highly responsive

• Contributes 20% of overall response to an increase in PaCO₂

Hering - Breuer Reflex - Stretch receptor

• Respond to stretch

• Receptors located in the airway smooth muscle

• Inhibits Inspiratory drive (negative feedback) and prevents overinflation