Respiratory Physiology: Gas ExchangeRespiratory Physiology: Gas Exchange

Dr Shihab Khogali

Ninewells Hospital & Medical School, University of Dundee

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 LectureAbout?

See blackboard for detailed learning objectives

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

Understand the followings in relation to the four factors which influence gas transfer across membranes:

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 airfrom inspiration

Airway dead-spacevolume (150 ml)

After inspiration,before expiration

Alveolar air

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.

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 airfrom inspiration

Airway dead-spacevolume (150 ml)

After inspiration,before expiration

Alveolar air

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

Flow

Lung PositionBottom Top

Blood Flow

Ventilation

V/Q Ratio

1

2

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.

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

Alveolar Dead Space

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

Area in which blood flow (perfusion)is greater than airflow (ventilation)

Helpsbalance

HelpsbalanceLarge blood flow

Small airflow

CO2 in area

Relaxation of local-airwaysmooth muscle

Dilation of local airways

Airway resistance

Airflow

O2 in area

Contraction of local pulmon-ary arteriolar smooth muscle

Constriction of local blood vessels

Vascular resistance

Blood flow

Area in which airflow (ventilation)is greater than blood flow (perfusion)

Helpsbalance

HelpsbalanceLarge airflow

Small blood flow

CO2 in area

Contraction of local-airwaysmooth muscle

Constriction of local airways

Airway resistance

Airflow

O2 in area

Relaxation of local pulmonary arteriolar smooth muscle

Dilation of local blood vessels

Vascular resistance

Blood flow

Note the Different Effects of O2

1. Partial Pressure Gradient of O2 and CO2

2. Diffusion Coefficient for O2 and CO2

3. Surface Area of Alveolar Membrane

4. Thickness of Alveolar Membrane

Four Factors Influence The Rate of Gas Exchange Across

Alveolar Membrane

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

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

What is Partial Pressure of Gas?

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.

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 gradientfrom alveoli toblood =

60 mm Hg (8 kP)100 – 40 mmHg i.e. (13.3- 5.3 kP)

CO2 partial pressure gradient from blood toalveoli =

6 mm Hg (0.8 kP)46 – 40 mmHg i.e. (6.1 – 5.3 kP)

Across Systemic Capillaries:O2 partial pressure gradientfrom blood totissue cell =

> 60 mm Hg (8 kP)100 – < 40 mmHg i.e. (13.3- < 5.3 kP)

CO2 partial pressure gradient from tissue cell toblood =

> 6 mm Hg (0.8 kP)> 46 – 40 mm Hg i.e. (> 6.1 – 5.3 kP)

Inspiration Expiration

Pulmonarycirculation

Systemiccirculation

Alveoli

Net diffusion gradientsfor O2 and CO2 betweenthe lungs and tissues

Tissues

Atmospheric air

But the partial pressure gradient for CO2 is much smaller than the partial pressure gradient for 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

What offset the difference in partial pressure gradient for CO2 and O2?

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

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

Respiratory Membranes

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