Ventilation 27-Apr-17 Ventilation.

Ventilation 27-Apr-17 Ventilation

Pulmonary Ventilation
Tidal volume 500 ml Total ventilation 7500 ml/min Anatomical dead space 150 ml Frequency = 15 per min Alveolar gas 3000 ml Alveolar ventilation 5250 ml/min Pulmonary blood flow 5000 ml/min Pulmonary capillary blood 70 ml 27-Apr-17 Ventilation

Minute ventilation (VE) Volume of air inspired or expired per minute Depends on the frequency (f) Depth of breathing (tidal volume, VT) VE = ( VT * f) 27-Apr-17 Ventilation

At rest VT = 500 ml , f = 12 to 15 breath per minute VE = (500 * 12) = 6000 ml/min VE = (500 * 15) = 7500 ml/min 27-Apr-17 Ventilation

Anatomical Dead Space Trachea Conducting zone 1 The first 16 generation plus trachea and upper respiratory tract form Conducting zone of the airways Transport gas from & to exterior Bronchi 2 3 4 17 Respiratory bronchiole 18 Trachea, bronchi, bronchioles Warm & humidify air Distribute air to the depth of the lungs Part of the body’s defense system Removal of dust, bacteria, noxious gases from lungs 19 Respiratory zone 20 Alveolar duct 21 22 23 27-Apr-17 Ventilation

Anatomical Dead Space Made up of Upper respiratory tract Trachea
Conducting zone 1 Made up of Upper respiratory tract Trachea Bronchi, bronchioles, terminal bronchioles Constitute the anatomical dead space Bronchi 2 3 4 17 Respiratory bronchiole 18 Trachea, bronchi, bronchioles Warm & humidify air Distribute air to the depth of the lungs Part of the body’s defense system Removal of dust, bacteria, noxious gases from lungs 19 Respiratory zone 20 Alveolar duct 21 22 23 27-Apr-17 Ventilation

Dead Space Ventilation (VD)
This is a portion of the minute ventilation That fails to reach areas of lungs involved in gas exchange Portion of tidal volume air that remain in dead space (150 ml) Tidal vol = 500 ml Portion of tidal air that gets into alveoli (350 ml) Alveolar air 27-Apr-17 Ventilation

Dead Space Ventilation (VD)
Anatomical dead space (VD) Volume of gas occupying the conducting zone of airways Is equal to 150 ml Dead space ventilation Is equal to VD * f 150 * 15 = 2.25 l/min Portion of tidal volume air that remain in dead space (150 ml) Tidal vol = 500 ml Portion of tidal air that gets into alveoli (350 ml) Alveolar air 27-Apr-17 Ventilation

Function of Anatomical Dead Space
Conditioning of inspired air Warming the air to body temp Adding moisture Saturate with water vapour Addition of water vapour dilutes oxygen and nitrogen concentration of inspired air 27-Apr-17 Ventilation

Removal of foreign material Foreign particles Filtered by nose Impacted in lower airways Dissolved on moist surface of airways Small particles (soot, pollen) Impact on the surface of the airways 27-Apr-17 Ventilation

Impaction Stick to mucus lining Carried in the mucus towards the mouth Expectorated Swallowed Mucus is propelled upwards towards the mouth Cilia of the respiratory epithelium 27-Apr-17 Ventilation

Foreign materials in inspired gas (cigarette smoke, smog) Stimulate irritant receptors in the airways Cause coughing Increase secretion of mucus Hypertrophy of mucus glands 27-Apr-17 Ventilation

Prolonged breathing air containing foreign material Cause chronic bronchitis Increase airway resistance, difficult in breathing 27-Apr-17 Ventilation

Alveolar Dead Space In health individuals In people with lung diseases
Trachea Conducting zone 1 In health individuals Anatomical dead space represent the entire dead space volume In people with lung diseases Some alveoli do not get blood supply Such alveoli do not participate in gas exchange They constitute alveolar dead space Bronchi 2 3 4 17 Respiratory bronchiole 18 Trachea, bronchi, bronchioles Warm & humidify air Distribute air to the depth of the lungs Part of the body’s defense system Removal of dust, bacteria, noxious gases from lungs 19 Respiratory zone 20 Alveolar duct 21 22 23 27-Apr-17 Ventilation

Total Dead Space Total (physiologic) dead space include
Trachea Conducting zone 1 Total (physiologic) dead space include Anatomical dead space Alveolar dead space Bronchi 2 3 4 17 Respiratory bronchiole 18 Trachea, bronchi, bronchioles Warm & humidify air Distribute air to the depth of the lungs Part of the body’s defense system Removal of dust, bacteria, noxious gases from lungs 19 Respiratory zone 20 Alveolar duct 21 22 23 27-Apr-17 Ventilation

Alveolar Ventilation Volume of fresh gas that reaches the alveoli per minute Participate in exchange of O2 & CO2 It is equal to Amount of new air reaching the alveoli times the breathing frequency Portion of tidal volume air that remain in dead space (150 ml) Tidal vol = 500 ml Portion of tidal air that gets into alveoli (350 ml) Alveolar air 27-Apr-17 Ventilation

Alveolar Ventilation Alveolar ventilation (VA) VA = (VT – VD) * f
VA = 4200 ml/min Portion of tidal volume air that remain in dead space (150 ml) Tidal vol = 500 ml Portion of tidal air that gets into alveoli (350 ml) Alveolar air 27-Apr-17 Ventilation

Alveolar Ventilation Alveolar ventilation Alveolar CO2 tension (PACO2)
Major factor in determining the conc of O2 and CO2 in the alveoli Alveolar CO2 tension (PACO2) Regulated at value of 40 mm Hg Determined by the Rate of production Portion of tidal volume air that remain in dead space (150 ml) Tidal vol = 500 ml Portion of tidal air that gets into alveoli (350 ml) Alveolar air 27-Apr-17 Ventilation

Alveolar Ventilation Alveolar O2 tension (PA O2)
O2 is continually removed from the alveoli by diffusion Inspiration brings Fresh air into the alveoli Maintain the alveolar O2 tension (PA o2)at about 100 mm Hg Portion of tidal volume air that remain in dead space (150 ml) Tidal vol = 500 ml Portion of tidal air that gets into alveoli (350 ml) Alveolar air 27-Apr-17 Ventilation

Alveolar – Capillary Gas Exchange
alveoli Pulmonary blood flow 5000 ml/min Pulmonary capillary blood 70 ml 27-Apr-17 Ventilation

Alveolar/capillary Exchange
CO2 Composition of alveolar gas mixture Contain respiratory gases Oxygen, carbon dioxide Together with Nitrogen, water vapour O2 Alveolar space CO2 O2 CO2 O2 27-Apr-17 Ventilation

CO2 The volume of alveolar space Functional residual capacity (FRC) 2.4 to 3 liters To this vol fresh air is added O2 is removed CO2 is added O2 Alveolar space CO2 O2 CO2 O2 27-Apr-17 Ventilation

CO2 The conc of O2 in the alveoli (FAO2) depends on Rate of diffusion of oxygen in blood (VO2) Oxygen uptake Rate of entry of O2 into the lung (FIo2) * (VA) O2 Alveolar space CO2 O2 CO2 O2 27-Apr-17 Ventilation

CO2 Where (FIO2) is the conc of O2 in inspired air (VA) is alveolar ventilation O2 Alveolar space CO2 O2 CO2 O2 27-Apr-17 Ventilation

CO2 The alveolar CO2 conc (FACO2) depends on Rate of excretion of CO2 from blood into alveolar Rate of CO2 removal from the alveoli (FACO2) * (VA) O2 Alveolar space CO2 O2 CO2 O2 27-Apr-17 Ventilation

CO2 Where (FACO2) is the alveolarCO2 conc (VA) is alveolar ventilation O2 Alveolar space CO2 O2 CO2 O2 27-Apr-17 Ventilation

Alveolar Partial Pressures
In a mixture of gases Each gas exerts its own partial pressure (tension) According to Dalton’s law Partial pressure equal Fraction of gas present (concentration) times the total pressure Partial pressure of gas in a mixture is a measure of the concentration of the gas in the mixture 27-Apr-17 Ventilation

Partial Pressure % Composition of dry air at sea level contain
Total pressure * % conc For O2 Po2 = 760 * = 159 mm hg 27-Apr-17 Ventilation

Partial Pressure For CO2 For N2 PCO2 = 760 * 0.0003 = 0.2 mm Hg
PN2 = 760 * = 600 mm Hg 27-Apr-17 Ventilation

Partial pressures & conc of O2, CO2 in alveoli
Oxygen Conc of O2 in alveoli (FAO2) & PAO2 Depend on Rate of diffusion into blood (VO2) Rate of entry of O2 in lungs (FIO2) * (VA) Alveoli FAO2 PACO2 PAO2 CO2 O2 CO2 O2 Pulmonary capillary 27-Apr-17 Ventilation

Hence If you increase O2 consumption (VO2) You need to increase alveolar ventilation (VA) To maintain PAO2 at 100 mmHg Alveoli FAO2 PACO2 PAO2 CO2 O2 CO2 O2 Pulmonary capillary 27-Apr-17 Ventilation

Partial Pressures & conc of O2, CO2 in Alveoli
When the oxygen uptake (VO2) is 250 ml/min You require alveolar vent of about 5 liters /min to maintain PAO2 = 100mm Hg VO2 = 250 ml/min 150 100 PAO2 = 100 mm Hg PAO2 & PACO2 mm Hg VO2 = 1000 ml/min 40 PACO2 = 40 mm Hg 20 30 5 10 15 Alveolar ventilation (L/min) 27-Apr-17 Ventilation

When the oxygen uptake (VO2) is 1000 ml/min You require alveolar vent of about 20 liters /min to maintain PAO2 = 100mm Hg VO2 = 250 ml/min 150 100 PAO2 = 100 mm Hg PAO2 & PACO2 mm Hg VO2 = 1000 ml/min 40 PACO2 = 40 mm Hg 20 30 5 10 15 Alveolar ventilation (L/min) 27-Apr-17 Ventilation

For CO2 The alveolar CO2 conc (FACO2) and the PACO2 depend on rate of Excretion of CO2 from blood into the alveoli CO2 removal from alveoli (VA * FACO2) Alveoli FAO2 PACO2 PAO2 CO2 O2 CO2 O2 Pulmonary capillary 27-Apr-17 Ventilation

Partial Pressure of Respiratory Gases (mm Hg)
Atmospheric air Alveolar gas Expired air O2 159.0 (20.84%) 104.0 (13.6%) 120.0 (15.7%) CO2 0.3 (0.04%) 40.0 (5.3%) 26.0 (3.6%) N2 597.0 (78.62%) 569.0 (74.9%) 566.0 (74.5) H2O 3.7 (0.5%) 47.0 (6.2%) Total 760 (100%) From Guyton 27-Apr-17 Ventilation

Diffusion 27-Apr-17 Ventilation

Diffusion of Gases Through the Respiratory Membrane
Fick’s law The rate of transfer of gas through a sheet of tissue is proportional to Tissue area Diffusing gas partial pressures Is inversely proportional to Tissue thickness Vgas  (A/T)D(P1-P2) D  Sol/ √ MW P1 P2 Vgas = gas transferred A =area T = thickness D = diffusion const A T 27-Apr-17 Ventilation

With respect to the lungs The area of blood gas barrier is large Thickness is very small The dimensions are ideal for diffusion Vgas  (A/T)D(P1-P2) D  Sol/ MW P1 P2 Vgas = gas transferred A =area T = thickness D = diffusion const A T 27-Apr-17 Ventilation

The rate of transfer is proportional to a diffusion constant which depends on Properties of the tissue Particular gas The diffusion constant is Proportional to solubility of the gas Inversely proportional to MW of the gas Vgas  (A/T)D(P1-P2) D  Sol/ MW P1 P2 Vgas = gas transferred A =area T = thickness D = diffusion const A T 27-Apr-17 Ventilation

Hence CO2 diffuses about 20 times more fast than O2 because Has much higher solubility But not very different MW Vgas  (A/T)D(P1-P2) D  Sol/ MW P1 P2 Vgas = gas transferred A =area T = thickness D = diffusion const A T 27-Apr-17 Ventilation

The partial pressure of the respiratory gases in the alveoli PAO2 = 100 mmHg PACO2 = 40 mm hg In the capillary at arterial end Pvo2 = 40 mmHg Pvco2 = 46 mm hg Alveoli PACO2 = 40 PAO2 = 100 CO2 O2 CO2 O2 PvCO2 = 46 mm Hg PaCO2 = 40 mm Hg PvO2 = 40 mm Hg PaO2 = 100 mm Hg 27-Apr-17 Ventilation

In the capillary at venous end PaO2 = 100 mmHg PaCO2 = 40 mm Hg Thus there is Partial pressure difference which form the driving force for diffusion of O2 and CO2 Alveoli PACO2 = 40 PAO2 = 100 CO2 O2 CO2 O2 PvCO2 = 46 mm Hg PaCO2 = 40 mm Hg PvO2 = 40 mm Hg PaO2 = 100 mm Hg 27-Apr-17 Ventilation

Diffusion Path in the Lungs
Basement membrane Capillary endothelium Alveolar capillary membrane Made up of Capillary endothelium Single layer endothelial cells Basement membrane Elastic collageneous tissue Alveolar epithelium Single layer epithelial cells Alveolar epithelium Surface lining Diameter of RBC = 7.5 m Alveolar diameter = 300 m Alveolar capillary membrane (0.2 m) 27-Apr-17 Ventilation

Alveolar Capillary Membrane
Basement membrane Capillary endothelium Also to be included RBC membrane Alveolar epithelium Surface lining Diameter of RBC = 7.5 m Alveolar diameter = 300 m Alveolar capillary membrane (0.2 m) 27-Apr-17 Ventilation

Diffusion Capacity of the Lung
Ability of respiratory membrane (RM ) To exchange gas between alveoli & pulmonary blood Diffusion capacity Volume of gas that will diffuse through the RM/min/mm Hg Alveoli CO2 O2 Pulmonary capillary 27-Apr-17 Ventilation

Factors affecting diffusing capacity of the lung include Membrane component Blood component Pulmonary diseases may affect diffusion process by  The SA (destruction of alveoli)  Diffusion distance (oedema) 27-Apr-17 Ventilation

Reducing the partial pressure gradient for the diffusion of gases Ventilation/perfusion abnormalities 27-Apr-17 Ventilation

Blood component Chemical combination of gases with Hb require finite time  In Hb conc enhances the transfer of gases Anaemic individuals would have impaired diffusion capacity Increase in cardiac output (C.O) enhance diffusion capacity 27-Apr-17 Ventilation

Diffusion Capacity for O2
The extent to which diffusion can occur in the whole human lung Can be obtained from Fick’s law of diffusion Vgas  (A/T)D(P1 – P2) PO2 = 60 PO2 = 0 Alveoli PO2 = 100 PO2 = 100 PO2 = 40 PO2 = 100 Pulmonary capillary 27-Apr-17 Ventilation

Vgas = K(A/T)P VO2 = K(A/T)PO2 The amount that diffuses must be identical to the oxygen uptake (VO2) K, A, & T can not be measured in the human lung PO2 = 60 PO2 = 0 Alveoli PO2 = 100 PO2 = 100 PO2 = 40 PO2 = 100 Pulmonary capillary 27-Apr-17 Ventilation

K(A/T) = DL DL new constant Equals the diffusion capacity of the lung Oxygen uptake VO2 = DLO2 * (meanPO2) PO2 = 60 PO2 = 0 Alveoli PO2 = 100 PO2 = 100 PO2 = 40 PO2 = 100 Pulmonary capillary 27-Apr-17 Ventilation

DLO2 is the diffusion capacity of the lung for O2 MeanPO2 is the mean oxygen partial pressure difference between the alveolar space and the blood in the lung It is about 10 mm Hg PO2 = 60 PO2 = 0 Alveoli PO2 = 100 PO2 = 100 PO2 = 40 PO2 = 100 Pulmonary capillary 27-Apr-17 Ventilation

In the human lung VO2 = 250 ml/min MeanPO2 = 10 mm Hg Thus DLO2 = (VO2)/ MeanPO2 = 250/10 = 25 ml of O2 / min/ mm Hg PO2 = 60 PO2 = 0 Alveoli PO2 = 100 PO2 = 100 PO2 = 40 PO2 = 100 Pulmonary capillary 27-Apr-17 Ventilation

Changes in O2 diffusion capacity During exercise there is increase Pulmonary blood flow Alveolar ventilation Diffusion capacity for O2 increase Maximum of about 3 times resting value PO2 = 60 PO2 = 0 Alveoli PO2 = 100 PO2 = 100 PO2 = 40 PO2 = 100 Pulmonary capillary 27-Apr-17 Ventilation

The increase is due to Opening up of dormant capillaries Extra dilatation of already open capillaries All these lead to Increase in blood flow Increase in SA PO2 = 60 PO2 = 0 Alveoli PO2 = 100 PO2 = 100 PO2 = 40 PO2 = 100 Pulmonary capillary 27-Apr-17 Ventilation

There is also better matching between Ventilation of alveoli Perfusion of capillaries PO2 = 60 PO2 = 0 Alveoli PO2 = 100 PO2 = 100 PO2 = 40 PO2 = 100 Pulmonary capillary 27-Apr-17 Ventilation

Diffusion Capacity for CO2
Diffusion capacity of the lung for CO2 Has been estimated to be equal to 400 to 450 ml of CO2 /min/mm Hg PO2 = 60 PO2 = 0 Alveoli PCO2 = 40 PCO2 = 40 PCO2 = 46 PCO2 = 40 Pulmonary capillary 27-Apr-17 Ventilation

Equilibration for O2 Diffusion of O2 occurs from alveolar gas to pulmonary capillary blood Normal Alveolar O2 tension (PAO2) = 100 mm Hg Oxygen tension of blood entering the capillary (PvO2) = 40 mm Hg PO2 = 60 PO2 = 0 Alveoli PAO2 = 100 PAO2 = 100 PvO2 = 40 PaO2 = 100 Pulmonary capillary 27-Apr-17 Ventilation

Equilibration for O2 Diffusion of O2 occurs from alveolar gas to pulmonary capillary blood Normal Alveolar O2 tension (PAO2) = 100 mm Hg Oxygen tension of blood entering the capillary (PvO2) = 40 mm Hg PO2 = 60 PO2 = 0 Alveoli PAO2 = 100 PAO2 = 100 O2 PvO2 = 40 O2 PaO2 = 100 Hb Hb O2 Hb Pulmonary capillary 27-Apr-17 Ventilation

Equilibration for O2 After crossing the alveolar/capillary membrane
O2 diffuse in plasma Raising plasma O2 tension Cause O2 to diffuse into RBC PO2 = 60 PO2 = 0 Alveoli PAO2 = 100 PAO2 = 100 O2 PvO2 = 40 O2 PaO2 = 100 Hb Hb O2 Hb Pulmonary capillary 27-Apr-17 Ventilation

Equilibration for O2 Equilibration time
Enough O2 diffuse across the alveolar/ capillary membrane Blood O2 tension and alveolar O2 tension Equalize in about 0.25 seconds PO2 = 60 PO2 = 0 Alveoli PAO2 = 100 PAO2 = 100 O2 PvO2 = 40 O2 PaO2 = 100 Hb Hb O2 Hb Pulmonary capillary 27-Apr-17 Ventilation

Equilibration for CO2 Diffusion of CO2 occurs from pulmonary capillary blood to alveolar gas Normal Alveolar CO2 tension (PACO2) = 40 mm Hg CO2 tension of blood entering the capillary (PvCO2) = 46 mm Hg PCO2 = 6 PCO2 = 0 Alveoli PACO2 = 40 PACO2 = 40 PvCO2 = 46 PaCO2 = 40 Pulmonary capillary 27-Apr-17 Ventilation

Equilibration for CO2 CO2 diffuse
From capillary blood into alveoli It is estimated that the time required for The blood CO2 tension and the alveolar CO2 tension to equalize Is approximately 0.25 sec PCO2 = 6 PCO2 = 0 Alveoli PACO2 = 40 PACO2 = 40 CO2 PvCO2 = 46 CO2 PaCO2 = 40 Hb Hb Hb CO2 Pulmonary capillary 27-Apr-17 Ventilation

Equilibration Alveolus Blood transit time during its passage through the capillaries At rest transit time is 0.75 sec By 0.25 sec blood and alveolar air have equalized for O2 and CO2 tensions During exercise blood transit time Reduced to 0.34 sec CO2, O2 RBC 100 mm Hg Oxygen 46 Carbon dioxide 40 seconds 0.25 0.50 0.75 Transit time 27-Apr-17 Ventilation

Factors Affecting Gas Exchange
Amount of gas exchanged across the respiratory membrane may be dependent on Perfusion or Diffusion properties Alveoli Pulmonary capillary 27-Apr-17 Ventilation

Perfusion Limited Gas Exchange
Alveolus As soon as the O2 equilibrates Net transfer of O2 ceases No additional uptake of O2 occurs until Capillary blood is replaced by new blood CO2, O2 RBC 100 mm Hg Oxygen 46 Carbon dioxide 40 seconds 0.25 0.50 0.75 Transit time 27-Apr-17 Ventilation

Alveolus Increase in gas exchange can only Be achieved by increase in blood flow Average RBC Spends 0.75 sec in pulmonary capillary O2 equilibration occurs in 0.25 sec CO2, O2 RBC 100 mm Hg Oxygen 46 Carbon dioxide 40 seconds 0.25 0.50 0.75 Transit time 27-Apr-17 Ventilation

Alveolus There is normally no increase in the O2 content for the last 0.5 sec This provides for a safety factor CO2, O2 RBC 100 mm Hg Oxygen 46 Carbon dioxide 40 seconds 0.25 0.50 0.75 Transit time 27-Apr-17 Ventilation

Diffusion Limited Gas Exchange
Alveolus Occurs whenever Equilibration does not occur Many pulmonary diseases Reduce the rate of O2 transfer By altering with RM Reduce alveolar O2 tension Reduces diffusion rate CO2, O2 RBC 100 mm Hg Oxygen 46 Carbon dioxide 40 seconds 0.25 0.50 0.75 Transit time 27-Apr-17 Ventilation

Diffusion Limited Gas Exchange
Alveolus  PAO2 The diffusion rate can be increased by Raising the alveolar O2 tension (PAO2) CO2, O2 RBC 100 mm Hg Oxygen 46 Carbon dioxide 40 seconds 0.25 0.50 0.75 Transit time 27-Apr-17 Ventilation

Blood Flow Q 27-Apr-17 Ventilation

Pulmonary Blood Flow The entire blood flow from the right ventricle
Distributed to the pulmonary vessels Pulmonary blood flow is essentially equal to cardiac output (5 l/min) Alveoli Q Pulmonary capillary 27-Apr-17 Ventilation

Pressure in Pulmonary System
Pressure in the pulmonary system Pressure in the RV = 25/0 mm hg In the PA = 25/8 mm hg Mean pressure of 15 mm hg Capillary = 7 mm hg LA & PV = 2 mm hg Varies between 1 – 5 mm hg 27-Apr-17 Ventilation

Blood Volume Blood volume of the lungs Is about 450 ml
9% of total blood volume About 70 ml of this is in the capillaries The remaining is divided equally between arteries and veins 27-Apr-17 Ventilation

Distribution of Blood Flow
Effect of gravity Gravity has marked effect on pulmonary circulation In upright position Upper portion of the lung are well above the level of the heart The bases are well below the level of the heart Zone PA >Pa >Pv Zone Pa >PA >Pv Level of RA Zone Pa >Pv >PA 27-Apr-17 Ventilation

There are marked pressure gradients In the pulmonary arteries from top to bottom of the lung Zone PA >Pa >Pv Zone Pa >PA >Pv Level of RA Zone Pa >Pv >PA 27-Apr-17 Ventilation

Pressure in capillaries at apex (zone 1) Close to atmospheric in the alveoli Pulmonary arterial pressure is normally sufficient to maintain perfusion Zone PA >Pa >Pv Zone Pa >PA >Pv Level of RA Zone Pa >Pv >PA 27-Apr-17 Ventilation

If it is reduced or if alveolar pressure increases Some capillaries collapse Thus there will be No gas exchange Cause alveolar dead space Zone PA >Pa >Pv Zone Pa >PA >Pv Level of RA Zone Pa >Pv >PA 27-Apr-17 Ventilation

In the middle of the lung (zone 2) Pulmonary arterial pressure exceed alveolar pressure Venous pressure is still low Blood flow is determined by difference between arterial & alveolar pressure Zone PA >Pa >Pv Zone Pa >PA >Pv Level of RA Zone Pa >Pv >PA 27-Apr-17 Ventilation

In the lower portion of the lung (zone 3) The alveolar pressure is Lower than pressures in all parts of the pulmonary circulation Blood flow is determined by Arterial – venous pressure difference Zone PA >Pa >Pv Zone Pa >PA >Pv Level of RA Zone Pa >Pv >PA 27-Apr-17 Ventilation

Control of Distribution of Blood Flow
When conc of O2 in the alveolus decease Less than 70% normal ; or <73 mm Hg Adjacent blood vessel constrict within 3 to 10 sec This increases resistance Well ventilated alveolus PAO2 = 104, PACO2 = 40 vasoconstriction under ventilated alveolus PAO2, PACO2 27-Apr-17 Ventilation

This restrict blood flow through the affected alveoli Diverts blood to well oxygenated alveoli An important mechanism for Balancing blood flow and ventilation Well ventilated alveolus PAO2 = 104, PACO2 = 40 vasoconstriction under ventilated alveolus PAO2, PACO2 27-Apr-17 Ventilation

Generalized hypoxia as in Exposure to high altitude (>5000 – 7000 feet) Hypoventilation Hypoxic vasocosntriction can cause Increase in total pulmonary resistance Pulmonary hypertension Well ventilated alveolus PAO2 = 104, PACO2 = 40 vasoconstriction under ventilated alveolus PAO2, PACO2 27-Apr-17 Ventilation

Ventilation – Perfusion Ratio
The alveolar O2 (PAO2) tension and CO2(PACO2) tension Determined by the rate of Alveolar ventilation (VA) and Transfer of O2 & CO2 through the respiratory membrane Alveoli VA Q Pulmonary capillary 27-Apr-17 Ventilation

In the lung with normal ventilation & blood flow some areas are well Ventilated but poorly perfused Perfused but poorly ventilated In either of these situation Gas exchange at the respiratory membrane would be impaired Alveoli VA Q Pulmonary capillary 27-Apr-17 Ventilation

Ventilation – perfusion ration Expressed as VA/Q Where VA = alveolar ventilation for a given alveolus Q = capillary blood flow for the same alveolus Alveoli VA Q Pulmonary capillary 27-Apr-17 Ventilation

For the entire lung VA = 4.2 liters / min Q = 5 liters/ min Thus the VA/Q = 4.2/5 = 0.84 This is the normal ratio Alveoli VA (4.2) Q (5) Pulmonary capillary 27-Apr-17 Ventilation

Effect of Ventilation-perfusion Ratios
If an alveolus is well ventilated & well perfused The VA/Q = 0.84 In this case there will be normal gas exchange The alveolar gas equilibrates with the capillary blood partial pressures of O2 & CO2 PAO2 = 104 PACO2 = 40 VA PVO2 = 40 Q PVCO2 = 46 PaO2 = 98 PaCO2 = 40 VA / Q = 0.84 27-Apr-17 Ventilation

If an alveolus is not ventilated but is well perfused The VA/Q = 0 In this case there will be no gas exchange Pulmonary capillary blood not oxygenated Shunt PAO2 = 40 PACO2 = 46 VA PVO2 = 40 Q PVCO2 = 46 PaO2 = 40 PaO2 = 46 VA / Q = 0 27-Apr-17 Ventilation

The alveolar gas equilibrates with the venous blood partial pressures of O2 & CO2 If an alveolus is well ventilated but not perfused The VA/Q = ∞ PAO2 = 149 PACO2 = 0 VA PVO2 = 40 Q PVCO2 = 46 VA / Q = ∞ 27-Apr-17 Ventilation

In this case there will be no gas exchange Pulmonary capillary blood not oxygenated The alveolar gas equilibrates with the atmospheric air partial pressures of O2 & CO2 Dead space PAO2 = 149 PACO2 = 0 VA PVO2 = 40 Q PVCO2 = 46 VA / Q = ∞ 27-Apr-17 Ventilation

Physiologic Shunt In a poorly ventilated alveolus
VA is low while Q is normal The VA/Q < 0.8 VA PVO2 = 40 Q PVCO2 = 46 VA / Q < 0.8 27-Apr-17 Ventilation

Physiologic Shunt Certain portion of venous blood does not become oxygenated Poorly aerated blood leaves pulmonary capillary (shunted blood) VA PVO2 = 40 Q PVCO2 = 46 VA / Q < 0.8 27-Apr-17 Ventilation

Physiologic Shunt Physiologic shunt There is a fall in PaO2
Only slight elevation of PaCO2 CO2 is eliminated in ventilated alveoli VA PVO2 = 40 Q PVCO2 = 46 VA / Q < 0.8 27-Apr-17 Ventilation

Physiologic Dead Space
When VA is normal but Blood flow (Q) is decreased The VA/Q > 0.8 Some of the alveolar ventilation (VA) is wasted No blood flow to carry out gas exchange This is physiologic dead space VA PVO2 = 40 PVCO2 = 46 Q VA / Q > 0.8 27-Apr-17 Ventilation

Ventilation – Perfusion Ratios in Lung
In the lung of upright individual Upper part is less well ventilated than the lower part, but It is also poorly perfused VA/Q > 0.8 This amounts to Dead space Zone 1 VA/Q > 0.8 Zone 2 VA/Q = 0.8 Level of RA Zone 3 VA/Q < 0.8 27-Apr-17 Ventilation

Ventilation – Perfusion Ratios in Lung
In the lung of upright individual Lower part is well ventilated, but It is also very well perfused VA/Q < 0.8 This amounts to physiologic shunt Zone 1 VA/Q > 0.8 Zone 2 VA/Q = 0.8 Level of RA Zone 3 VA/Q < 0.8 27-Apr-17 Ventilation

Ventilation 27-Apr-17 Ventilation.

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