Respiratory Physiology: Gas Exchange Dr Shihab Khogali Ninewells Hospital & Medical School, University of Dundee

See blackboard for detailed learning objectives Understand the difference between pulmonary ventilation and alveolar ventilation, and the significance of anatomical dead space Understand the basic principles of ventilation perfusion matching Understand the significance of alveolar dead space Know that the physiological dead space = anatomical + alveolar dead space Understand the four factors which influence the gas transfer across the alveolar membrane Know the non-respiratory functions of the respiratory system What is This Lecture About? See blackboard for detailed learning objectives

Understand the followings in relation to the four factors which influence gas transfer across membranes: The Dalton’s Law of partial pressures. Know that gases move across membranes by partial pressure gradient The role of diffusion coefficient on gas transfer across membranes The effect of membrane surface area and membrane thickness on gas transfer, with the Fick’s Law of diffusion

Figure 13.2: Anatomy of the respiratory system. (a) The respiratory airways. (b) Enlargement of the alveoli (air sacs) at the terminal end of the airways. Most alveoli are clustered in grapelike arrangements at the end of the terminal bronchioles. (Source: Adapted from Cecie Starr and Ralph Taggart, Biology: The Unity and Diversity of Life, 8th ed., Fig. 41.10a, p. 696. Copyright © 1998 Wadsworth.)

Pulmonary Ventilation & Alveolar Ventilation Some inspired air remains in the airways (anatomical dead space) where it is not available for gas exchange Pulmonary Ventilation = tidal volume (ml/ breath) x Respiratory Rate (breath/min) = 0.5 L X 12 breath/min = 6 L/min under resting conditions Alveolar Ventilation is less than pulmonary ventilation because of the presence of anatomical dead space. Alveolar Ventilation = (tidal volume – dead space volume) x Respiratory Rate = (0.5 – 0.15) x 12 = 4.2 L/min under resting conditions. Pulmonary Ventilation & Alveolar Ventilation Fresh air from inspiration Airway dead-space volume (150 ml) Alveolar air After inspiration, before expiration

Figure 13.22: Effect of dead space volume on exchange of tidal volume between the atmosphere and the alveoli. Even though 500 ml of air move in and out between the atmosphere and the respiratory system and 500 ml move in and out of the alveoli with each breath, only 350 ml are actually exchanged between the atmosphere and the alveoli because of the anatomic dead space (the volume of air in the respiratory airways). Fig. 13-22, p. 472

Pulmonary Ventilation: Is the volume of air breathed in and out per minute Alveolar Ventilation: Is the volume of air exchanged between the atmosphere and alveoli per minute This is more important as it represent new air available for gas exchange with blood.

Pulmonary Ventilation & Alveolar Ventilation To increase pulmonary ventilation (e.g. during exercise) both the depth (tidal volume) and rate of breathing (RR) increase. because of dead space: It is more advantageous to increase the depth of breathing Pulmonary Ventilation & Alveolar Ventilation Fresh air from inspiration Airway dead-space volume (150 ml) Alveolar air After inspiration, before expiration

It is more advantageous to increase the Depth of Breathing

Ventilation Perfusion The transfer of gases between the body and atmosphere depends upon: Ventilation: the rate at which gas is passing through the lungs. Perfusion: the rate at which blood is passing through the lungs

Ventilation Perfusion Both blood flow and ventilation vary from bottom to top of the lung The result is that the average arterial and alveolar partial pressures of O2 are not exactly the same. Normally this effect is not significant but it can be in disease. V/Q Ratio 1 2 Flow Lung Position Bottom Top Blood Flow Ventilation

Alveolar Dead Space The match between air in the alveoli and the blood in the pulmonary capillaries is not always perfect Ventilated alveoli which are not adequately perfused with blood are considered as alveolar dead space In healthy people, the alveolar dead space is very small and of little importance (note: the physiological dead space = the anatomical dead space + the alveolar dead space) The alveolar dead space could increase significantly in disease

Ventilation Perfusion Match in the Lungs Local controls act on the smooth muscles of airways and arterioles to match airflow to blood flow Accumulation of CO2 in alveoli as a result of increased perfusion decreases airway resistance leading to increased airflow Increase in alveolar O2 concentration as a result of increased ventilation causes pulmonary vasodilation which increases blood flow to match larger airflow

Dilation of local airways Area in which blood flow (perfusion) is greater than airflow (ventilation) Helps balance Helps balance Large blood flow Small airflow CO2 in area O2 in area Relaxation of local-airway smooth muscle Contraction of local pulmon- ary arteriolar smooth muscle Dilation of local airways Constriction of local blood vessels Airway resistance Vascular resistance Airflow Blood flow

CO2 in area O2 in area Airway resistance Vascular resistance Airflow Area in which airflow (ventilation) is greater than blood flow (perfusion) Helps balance Helps balance Large airflow Small blood flow CO2 in area O2 in area Contraction of local-airway smooth muscle Relaxation of local pulmonary arteriolar smooth muscle Constriction of local airways Dilation of local blood vessels Airway resistance Vascular resistance Airflow Blood flow

Note the Different Effects of O2

Partial Pressure Gradient of O2 and CO2 Diffusion Coefficient for O2 and CO2 Surface Area of Alveolar Membrane Thickness of Alveolar Membrane Four Factors Influence The Rate of Gas Exchange Across Alveolar Membrane

Dalton’s Law of Partial Pressures Gases move across cell membranes etc by pressure gradient The partial pressure of a gas determines the pressure gradient for that gas The partial pressure of gas (1) in a mixture of gases that don’t react with each other is: The pressure that gas (1) would exert if it occupied the total volume for the mixture in the absence of other components Thus if the total pressure of the gas mixture is 100 kPa; and half of the mixture is gas (1): the partial pressure for gas (1) is 50 kPa What is Partial Pressure of Gas? Ptotal = P1 + P2 +…+ Pn Dalton’s Law of Partial Pressures The Total Pressure exerted by a gaseous mixture = The sum of the partial pressures of each individual component in the gas mixture

The partial pressure of gas is: The pressure that one gas in a mixture of gases would exert if it were the only gas present in the whole volume occupied by the mixture at a given temperature.

Figure 13.24: Concept of partial pressures. The partial pressure exerted by each gas in a mixture equals the total pressure times the fractional composition of the gas in the mixture.

Overview of Respiration Gases move from higher to lower partial pressures (partial pressure gradient) Note units in the diagram (mmHg). Here in UK you we use kPa (kilopascals) but Americans and American texts use mmHg. To convert divide mmHg by 7.5.

Across Pulmonary Capillaries: O2 partial pressure gradient Atmospheric air Across Pulmonary Capillaries: O2 partial pressure gradient from alveoli to blood = 60 mm Hg (8 kP) 100 – 40 mmHg i.e. (13.3- 5.3 kP) CO2 partial pressure gradient from blood to alveoli = 6 mm Hg (0.8 kP) 46 – 40 mmHg i.e. (6.1 – 5.3 kP) Inspiration Expiration Net diffusion gradients for O2 and CO2 between the lungs and tissues Alveoli Pulmonary circulation Across Systemic Capillaries: O2 partial pressure gradient from blood to tissue cell = > 60 mm Hg (8 kP) 100 – < 40 mmHg i.e. (13.3- < 5.3 kP) CO2 partial pressure gradient from tissue cell to blood = > 6 mm Hg (0.8 kP) > 46 – 40 mm Hg i.e. (> 6.1 – 5.3 kP) Systemic circulation Tissues

The diffusion coefficient for CO2 is 20 times that of O2 But the partial pressure gradient for CO2 is much smaller than the partial pressure gradient for O2??? What offset the difference in partial pressure gradient for CO2 and O2? CO2 is more soluble in membranes than O2. The solubility of gas in membranes is known as the Diffusion Coefficient for the gas. The diffusion coefficient for CO2 is 20 times that of O2

Effect of surface Area & Membrane Thickness on Gas Diffusion The lungs provide a very large surface area with thin membranes to facilitate effective gas exchange The airways divides repeatedly to increase the surface area for gas exchange The small airways form outpockets (the alveoli). This help increase the surface area for gas exchange in the lungs The lungs have a very extensive pulmonary capillary network Remember: the pulmonary circulation receives the entire cardiac output Effect of surface Area & Membrane Thickness on Gas Diffusion Fick’s Law of diffusion The amount of gas that moves across a sheet of tissue in unit time is proportional to the area of the sheet but inversely proportional to its thickness

The Respiratory Tree

Respiratory Membranes Alveoli: Thin-walled inflatable sacs Function in gas exchange Walls consist of a single layer of flattened Type I alveolar cells Pulmonary capillaries encircle each alveolus Narrow interstitial space

Four Factors Influence the Rate of Gas Transfer Across The Alveolar Membrane

Nonrespiratory Functions of Respiratory System Route for water loss and heat elimination Enhances venous return (Cardiovascular Physiology) Helps maintain normal acid-base balance (Respiratory and Renal Physiology) Enables speech, singing, and other vocalizations Defends against inhaled foreign matter Removes, modifies, activates, or inactivates various materials passing through the pulmonary circulation Nose serves as the organ of smell