Gas exchange in the lungs Lecture 18

Learning objectives Differences in ventilation & perfusion in different parts of the lungs Dead space(DS), anatomical DS, Physiological DS (Total DS) Bohr’s equation to estimate total DS Meaning of shunt Alveolar gas equation Diffusion and flow limited exchange Diffusion capacity for oxygen, carbon dioxide & carbon monoxide

Learning objectives At the of the lecture students should be able to; Explain the differences in ventilation & perfusion in different parts (zones)of lung Explain the term dead space; and describe the distinction between anatomic & physiologic (total)dead space Determine total dead space by Bohr’s equation

Explain the meaning of shunt; alveolar gas equation, diffusion limited & flow limited exchange Explain the diffusing capacity of the lung for oxygen, carbon dioxide, carbon monoxide Describe the structure of respiratory membrane & factors affecting diffusion of gases Describe the diffusion of oxygen from lungs to pulmonary capillary

Pressure gradients for gas transfer in the body at rest

Tracheo-bronchial tree Conducting pathway for the passage of air. Anatomical dead space Functional unit for Gas exchange

Anatomic dead space = Volume of air in the conducting zone which is not available for exchange Total (Physiologic) dead space = (volume of gas not equilibrating with blood, ie, wasted ventilation) + anatomical dead space. In healthy individuals, the two dead spaces are identical In disease states, there may be no exchange between the gas in some of the alveoli and the blood, and some of the alveoli may be overventilated. The volume of gas in nonperfused alveoli and any volume of air in the alveoli in excess of that necessary to arterialize the blood in the alveolar capillaries is part of the dead space (nonequilibrating) gas volume.

Measurement of anatomic dead space: single-breath N2 curves. Measurement of Total dead space is by Bohr’s equation: Total dead space calculated by Pco2 of expired air, Pco2 of arterial blood and the tidal volume.

Bohr’s equation to estimate dead space Alveolar CO2% - Expired air CO2% Dead space air = Expired air x Alveolar CO2 % Expired air 500ml Expired air CO2= 4.0ml% Alveolar CO2 =5.5 ml% (5.5 -4.0) Dead space air=550x = 150 ml 5.5

Physiological shunt: Shunt is the volume of blood that passes from the venous to the arterial side, without passing through ventilated alveoli 1. Some bronchial capillaries & veins anastamose with Pulmonary capillaries & veins bypassing the right ventricle. So partly venous blood without oxygenation enters left atria. &. 2. Flow of blood from coronary arteries into the chambers of the left side of the heart. Because of these two shunts the systemic arterial blood PO2 is lower than the blood of alveoli (from 104 comes down to 95 mm Hg)

Ultra structure of the alveolar respiratory membrane, shown in cross section.

Respiratory membrane /Alveolo-capillary membrane Fluid surfactant layer Alveolar Epithelium Epithelial basement membrane Interstitial space Capillary basement membrane Capillary endothelium

Factors that influence diffusion of gases across the membrane Pressure gradient Cross-sectional area of the lung Distance through which the gas must diffuse (thickness of the membrane)-0.5 µm Molecular Weight of the gas Solubility of the gas in the fluid

Diffusing capacity of lungs For O2 = 25 ml/min/mm Hg at rest Reduced when membrane is thickened eg. Fibrosis when surface area is reduced eg. Emphysema, Pneumonectomy Diffusion capacity of CO2 is much higher than O2 i.e the reason CO2 retension is rarely a proble as compared to Hypoxia.

Factors influencing diffusion cont…………….. Whether they for chemical combination & rate of combination (O2,CO2,CO) CO2 diffuses 20 times faster thanO2 ( Diffusion coefficient of CO2 is much higher) Ii Whether they are transported entirely in Physical solution (inert gases N2, Helium, Anaesthetic agents N2O)

Whether or not substance passing from the alveoli to the capillary blood reach equilibrium in 0.75 sec that blood takes to traverse the pulm capillaries depends on the reaction of the substance with the blood.

Eg; N2O does not react with blood, so reaches equilibrium within 0 Eg; N2O does not react with blood, so reaches equilibrium within 0.1 sec. so it is not diffusion limited , flow limited. CO taken up by RBC at a high rate so it is diffusion limited. O2 and CO2 are intermediate between N2O & CO, as they are taken up by Hb, but much less avidly than CO and it recahes equilibrium with capillary blood in about 0.3 sec. consdiered as perfusion limited.

diffusion limited and flow limited exchange

Alveolar gas equation PAO2 can also be calculated from the alveolar gas equation: where FIO2 is the fraction of O2 molecules in the dry gas, PIO2 is the inspired PO2, and R is the respiratory exchange ratio , ie, the flow of CO2 molecules across the alveolar membrane per minute divided by the flow of O2 molecules across the membrane per minute.

1.PA>Pa 2.Pa>PA 3.Pa>Pv

Blood flow in different zones of an erect lung. At the apex alveolar pressure is greater than the arteriolar pressure. It is poorly perfused In the middle zone the blood flow is normal as the arteriolar pressure is greater than the alveolar pressure. In lower zone arterial & pulmonary capillary pressure always remains higher than alveolar pressure so always greater flow.

Both ventilation & blood flow are gravity dependent & decrease from bottom to Top of lung Gradient of blood flow is steeper than that of ventilation so V/Q ratio increases up lung VA/Q Blood flow Ventilation or blood flow/unit of lung vol ventilation 2.0 VA/Q ratio 0.8 Bottom Top

V/Q ratio Apex 3.0 Mid zone 1.0 Base 0.6 In regions of high V/Q PO2 is high(130 mmHg) PCO2 low28 mmHg At the base V/Q is low PO2 is low 89mmHg & PCO2 is high

Posture & gravity influence blood flow in different parts of lung Erect posture –Flow greatest in the base & progressively decreases in the upper parts lowest in the apex ii. Supine position-BF to posterior portion is greater than to anterior parts iii. Lateral lying position blood flow to lower parts of lung is higher than to the upper parts

Effect of Gravity In the upright position, the upper portions of the lungs are well above the level of the heart, and the bases are at or below it. Consequently, there is a relatively marked pressure gradient in the pulmonary arteries from the top to the bottom of the lungs, resulting in linear increase in pulmonary blood flow from the apices to the bases of the lungs.

The pressure in the capillaries at the top of the lungs is close to the atmospheric pressure in the alveoli. Pulmonary arterial pressure is normally just sufficient to maintain perfusion, but if it is reduced or if alveolar pressure is increased, some of the capillaries collapse. Under these circumstances, there is no gas exchange in the affected alveoli and they become part of the physiologic dead space.