Pulmonary Gas Exchange and Gas Transport

Dr. Meg-angela Christi Amores

Physiologic Anatomy

One of the most important problems in all the respiratory passages is to keep them open to allow easy passage of air to and from the alveoli

• Trachea – with cartilage rings 5/6 of the way around• Bronchi – walls have less extensive cartilage plates• Bronchioles – no plates. Diameter <1.5mm, all smooth

muscles– Kept expanded by same transpulmonary pressures that expand

the alveoli

Physiologic Anatomy

All areas of the trachea and bronchi not occupied by cartilage plates, walls are composed of smooth muscles

Resistance to flow is greatest NOT in the minute air passages of terminal bronchioles but in some of the larger bronchi near to the trachea.

Smaller airways are easily occluded ; smooth muscles = contract easily

Pulmonary Circulatory System

• Pulmonary Vessels – Pulmonary artery (5 cm, thin, 2x VC, 1/3 aorta)• Right and Left main pulmonary branches – lungs• Large compliance (7 mL/mmHg)• Allows pulmonary arteries to accommodate 2/3 of

stroke volume output of Right Ventricle– Bronchial Vessels – arterial supply to the lungs• 1/3 of cardiac output• Supplies supporting tissues (CT, septa, bronchi)• Drains to pulmonary veins

Pulmonary vs. Alveolar Ventilation

• Pulmonary Ventilation – Inflow and outflow of air

between the atmosphere and the lung alveoli

• Alveolar Ventilation– Rate at which new air reaches the areas in the

lung where it is in proximity to the pulmonary blood or gas exchange areas (alveolar sacs, ducts, respiratory bronchioles)

Diffusion of Gases• Diffusion – Random molecular motion of

molecules with energy provided by kinetic motion of the molecules

– All molecules are continually undergoing motion except in absolute zero temperature

– Net diffusion • Product of diffusion from high to low

concentration

Gas Pressures

• Partial Pressure– Pressure is directly proportional to the

concentration of gas molecules; caused by impact of moving molecules against a surface

– In respiration, there’s mixture of gases: O2, N2, CO2

– Rate of diffusion of each gas is directly proportional to the pressure caused by each gas alone

• AIR = total Pressure 760 mmHg• 79% N, 21% O2 = PP N = 600mmHg , PP O2 =160mmHg

Gas Pressure in Fluid

• Determined by its concentration and by solubility coefficient

• If gas is repelled, pressure increases• HENRY’s LAW : Pressure = concentration

solubility coefficient

Solubility of Gases in body temp.

• O2 = 0.024

• CO2 = 0.57 - 20x more soluble than O2

• CO = 0.018• N2 = 0.012• He = 0.008

Factors that affect Rate of Gas Diffusion thru Respiratory Membrane • Respiratory Unit: – Respiratory bronchiole– Alveolar ducts– Atria– Alveoli (300 Million in

both lungs) (0.2mm)*their membranes make up the

respiratory membrane

Respiratory Membrane• Layers:

1. Layer of fluid lining alveolus (surfactant)

2. Alveolar epithelium3. Epithelial basement

membrane4. Interstitial Space5. Capillary basement

membrane6. Capillary endothelial

membraneOverall thickness: 0.2um (ave: 0.6 um)Total surface area: 70 m2

Factors that affect Rate of Gas Diffusion thru Respiratory Membrane

1. Thickness of membrane• Inc. in edema and fibrosis

2. Surface area of membrane• Dec. in removal of lung and emphysema

3. Diffusion coefficient of Gas in substance of membrane• Gas’ solubility

4. Pressure difference• Difference between partial pressure of gas in alveoli

and pressure of gas in pulmonary capillary blood

Ventilation-Perfusion Ratio

• A concept developed to help us understand respiratory exchange where there is imbalance between alveolar ventilation and alveolar blood flow

• Areas in lung with well ventilation but no bloodflow or excellent blood flow but no ventilation

• Va – alveolar ventilation• Q – blood flow

Ventilation-Perfusion Ratio

Va/Q = normal• If Va is 0 (zero), but with perfusion: Va/Q = 0• If Va is present, but no perfusion Va/Q =

infinity• In both: there is no gas exchange

Ventilation-Perfusion Ratio

• Normal person :– Upright: Va and Q are less in Upper part but Q is more– At top of lung: Va/Q 2.5x > as ideal = physiologic dead

space (ventilation but less blood flow)– At bottom: Va is less than Q

• Va/Q is 0.6 < as ideal = physiologic shunt

• COPD patient:– Smoker, emphysema, alveolar walls destroyed– Wasted blood flow = severe shunting

Transport of O2 and CO2

• Pressure differences causes gas to diffuse

Alveolus Capillaries Tissues (fluid) Tissues (cells)

pO2 104 mmHg 95 mmHg 40 mmHg 5-40 (ave 23) mmHg

pCO2 40 mmHg 45 mmHg 45 mmHg 46 mmHg

Transport of O2 and CO2

• CO2 can diffuse about 20 times as rapidly as O2

Transport of O2 in blood:

• 97% of O2 from lungs to tissues are carried in combination with hemoglobin

• O2 combines loosely and reversibly with heme

pO2 – O2 combines with heme (pulm capi)

pO2 – O2 is released (tissue capillaries)

• For the next meeting, read on Regulation of Respiration

• Guyton Textbook of Medical Physiology, 10th edition Chapter 41