1. Dr. Zainab H.H Dept. of Physiology Lec.3,4
RESPIRATION
Dr. Zainab H.H
Dept. of Physiology
Lec.3,4

2. objectives Define physical laws of gases
List types of dead spaces and their effect on the alveolar ventilation
Describe physical properties of the lung
List the functions of the surfactant
Describe the work of breathing

3. Physical Laws of Gases
Dalton’s Law: Total pressure exerted by a mixture of gases = sum of the pressures exerted by the individual gases.
Partial pressure:
The pressure that an particular gas exerts independently.
Partial pressures = % of that gas x total pressure.

4. Partial Pressure of Gases
PATM = PN2 + PO2 + PCO2 + PH2O = 760 mm Hg.
In atmosphere:
N2 = 79%, 760 mmHg x 0.79 = 600mmHg PN2
O2 = 21%, 760 mmHg x 0.21 = 159 mmHg PO2
CO2 = 0.04%, 760 mmHg x = 0.3 mm Hg PCO2

5. Partial Pressure of Gases
PH2O contributes to partial pressure (47 mmHg).
Water vapor also has a Partial pressure in humidified air as in the respiratory passages.
Atmospheric PO2 decreases on a mountain and increases as one dives into the ocean.
The partial pressure of CO2 is negligible at 0.03 mm Hg

6. Effect of Water Vapor
As fresh air enters the nose and mouth it is immediately mixed with water vapor
Since the total pressure remains constant, the water vapor lowers the partial pressure of all other gases
For this reason, the PO2 is lowered to about 149 mmHg

7. Dead Space
the volume of inspired air that is not involved in gas exchange.
Types of dead space volume are:
Anatomic dead space (VANA ): formed by the gas conduction parts of the airway that are not involved in gas exchange, such as the mouth, nasal cavity, pharynx, trachea and upper bronchial airways.
Usually estimated ~ 2.2 ml per kg body weight
~154 ml in 70 kg man)

8. Dead Space
Alveolar dead space: composed of those alveoli that are being ventilated but not perfused.
They are therefore, in effect, not contributing to gas exchange
Physiologic dead space: this is the sum of the two volumes above.
further lowers the PO2 to about 100 mmHg

9. Factors affecting VANA
Size of subject: VANA ↑ with ↑ in body size
Age: from early adulthood, VANA ↑ ~ 1 ml/year
VANA ↑ slightly in inspiration: airway diameter larger
Tracheostomy →  VANA
Breathing through snorkel: additional tubes →↑ dead space

10. Alveolar Air Inspired air is humidified (water vapor)
Fresh inspired air mixes with the large volume of old air and the dead space
At the end of each inspiration, less than 15% of the air in the alveoli is fresh air.

11. Alveolar Air
Atmospheric CO2 mixes with high CO2 levels from residual volume in the alveoli increasing PCO2 to 40 mmHg
Atmospheric O2 mixes with “old” air already in alveolus to arrive at PO2 of 105 mmHg

12. Partial Pressures of Gases in Inspired Air & Alveolar Air

13. Alveolar Ventilation VA
Volume of fresh air entering alveoli per minute
VA = (TV – VANA) X f, where
TV = tidal volume
VANA = anatomical dead space
f = respiratory rate

14. Alveolar Ventilation VA
TV = 500ml
body weight = 70 kg
Respiratory rate = 10/min
= ( ) X 10 = 3.46 L/min VA
Normal VA ~ 4L/min (adult male)
Normal pulmonary ventilation ~ 5-6L/min

15. Alveolar Ventilation VA
Affected by:
Total Flow in and out
Anatomic Dead Space
Functional Dead Space
Gas Mixing

16. Atmospheric Air ≠ Alveolar Air. Why?
Alveolar air is only partially replaced by atmospheric air.
O2 is constantly being absorbed into the pulmonary blood from the alveolar air
CO2 is constantly diffusing from the pulmonary blood into the alveoli
Dry atmospheric air that enters the respiratory passages is humiliated before it reaches the alveoli

17. Disorders Caused by High Partial Pressures of Gases
Nitrogen narcosis:
At sea level nitrogen is physiologically inert.
Under hyperbaric conditions:
Nitrogen dissolves slowly.
Can have deleterious effects.
Resembles alcohol intoxication.
Decompression sickness:
Amount of nitrogen dissolved in blood as a diver ascends decreases due to a decrease in PN2.
If occurs rapidly, bubbles of nitrogen gas can form in tissues and enter the blood.
Block small blood vessels producing the “bends.”

18. Physical Properties of the Lungs
Ventilation occurs as a result of pressure differences induced by changes in lung volume.
Physical properties that affect lung function:
Compliance.
Elasticity.
Surface tension.

19. Compliance (Stretchability)
Ease with which the lungs can expand.
DV/DP = 200ml/cmH2O
Under normal physiological situations
Variations in the respiratory cycle: compliance is greater after expiration.
Position: Compliance is less in lying down due to less FRC(functional residual capacity)
The lung 100 x more distensible than a balloon.
Compliance is reduced by factors that produce resistance to distension.

20. Factors that Determine Compliance:
1. Elastic forces of the lung tissue itself (elastin and collagen fibers intermingle among the lung parenchyma 2. Elastic forces caused by surface tension of the fluid that lines the inside walls of the alveoli and other lung spaces Surface Tension accounts for 2/3 of total elastic forces in normal lung

21. Elasticity This is 1/compliance.
Thus, it is a measure of the elastic recoil of the lung (tendency to return to initial size after distension).
What generates this force?
The elastic properties of the elastin and collagen network of the lung
Surface tension at the alveolus
Elastic tension increases during inspiration and is reduced by recoil during expiration.

22. Surface Tension
Force exerted by fluid in alveoli to resist distension.
Lungs secrete and absorb fluid, leaving a very thin film of fluid.
This film of fluid causes surface tension.
Fluid absorption is driven (osmosis) by Na+ active transport.
Fluid secretion is driven by the active transport of Cl- out of the alveolar epithelial cells.
H2O molecules at the surface are attracted to other H2O molecules by attractive forces.
Force is directed inward, raising pressure in alveoli.

23. Surface Tension (continued)
Law of Laplace:
Pressure is directly proportional to surface tension; and inversely proportional to radius.
Pressure in smaller alveolus would be greater than in larger alveolus, if surface tension were the same in both.

24. Surface Tension (continued)
In a model with two bubbles of different radii in communication with each other
absence of surfactant:
the pressure p of the small bubble is higher than p of the large bubble
airflow is generated from the smaller into the larger bubble.
small alveoli collapsing and larger alveoli expanding.

25. Surface Tension (continued)
It is thus important that the surface tension (T) changes in proportion with the radius of curvature (r) of the surface on which tension exerts.
The surface-active material (pulmonary surfactant), lining the alveoli, helps to stabilize alveolar surface forces.
The surfactant lowers T of the less inflated alveolus, such that its recoil pressure is not higher than that of the bigger one.

27. Surfactant
Surface active agent secreted by type II alveolar epithelial cells
Complex mixture of phospholipids monolayer lining the alveoli
Dipalmitoylphosphatidylcholine 62%
Phosphatidylglycerol 5%
Other phospholipids 10%
Neutral lipids 13%
Proteins 8%
Carbohydrate 2%
Begin to be secreted between the 6th and 7th month of gestation

29. Surfactant
Spreads over the surface of a fluid
The polar heads point at the alveolar wall, the lipophilic side chains point at the lumen
Reduces attractive forces of hydrogen bonding by becoming interspersed between H20 molecules).
Surface tension in alveoli is reduced.
As alveoli radius decreases, surfactant’s ability to lower surface tension increases.

30. Importance of Surfactant
Lowers surface tension
Stabilizes the size of the alveoli
Prevents the accumulation of fluid
Keep airways and alveoli open during end expiration
Cause even distribution of air during late inspiration.

32. An infant born prematurely in gestational week 25 has neonatal respiratory distress syndrome.
Which of the following would be expected in this infant?
(A) Arterial PO2 of 100 mm Hg
(B) Collapse of the small alveoli
(C) Increased lung compliance
(D) Normal breathing rate
(E) Lecithin:sphingomyelin ratio of greater than 2:1 in amniotic fluid

33. Surface Tension of Different watery fluids
72 dynes/cm – pure water
50 dynes/cm – normal fluids lining the alveoli but without surfactant
5 to 30 dynes/cm – fluids lining the alveoli with surfactant included.

34. Airway Resistance lung volume determine the airway resistance:
The greater the lung volume, the less the overall airway resistance due to the effects of radial traction.
Radial traction is due to the elastin and collagen in the airways.
The larger the lung volume, the greater the elastic recoil forces across the airways  pulls the others around it open  increasing their caliber  reducing their resistance by the Poiseuille equation.

35. Airway Resistance
The physiologic factors that influence the diameter of the airways, and hence airways resistance are:
Lung volumes
Respiratory secretions
Activity of airway smooth muscle cells determined by the ANS and chemical mediators

36. Airway Resistance
At lower lung volumes, the effect of radial traction is diminished  reducing the caliber of the airways.
Notes:
only airways that are unsupported by mural cartilage are subject to the effects of radial traction.
radial traction is lost in emphysema. This leads to air trapping and increased lung volumes.

37. Work of Breathing
1. Compliance work or Elastic work – that required to expand the lungs against the lung and chest elastic forces 2. Tissue resistance work – required to overcome the viscosity of the lung and chest wall structures 3. Airway resistance work – required to overcome airway resistance to movement of air into the lungs

38. Work of Breathing
Compliance and Tissue resistance work – increased by diseases that cause fibrosis of the lungs as in tuberculosis
Airway resistance work – increased by diseases that obstruct the airways as in asthma

39. Work of Breathing
Airway resistance requires actual work to be done to overcome it.
Airway resistance to flow is present during both inspiration and expiration.
The energy required to overcome it, which represents the actual work of breathing, is dissipated as heat.

40. Work of Breathing
The work of breathing is best displayed on a pressure-volume curve of one respiratory cycle.
Shows the different pathways for inspiration and expiration, known as hysteresis

42. Energy Expenditure is Increased When:
Pulmonary compliance is decreased
Airway resistance is increased
Elastic recoil is decreased
There is need for increased ventilation