resp2 -gas exchange
DESCRIPTION
This was discussed by Dra. Elena CabarlesTRANSCRIPT
3 important gas laws govern ambient air and alveolar ventilation:
1. Boyle’s law2. Dalton’s law3. Henry’s law
Dalton’s Law of Partial Pressures
states that the total pressure exerted by a mixture of gases is the sum of the pressures exerted independently by each gas in the mixture.
Further, the pressure exerted by each gas—its partial pressure—is directly proportional to the percentage of that gas in the gas mixture.
Barometric pressure (PB) – 760 mmHg
O2 – 21%
Po2 = 160 mm Hg
N2 – 79%
PN2 = 600 mm Hg
Dalton’s Law of Partial Pressures
HENRY’S LAW when a mixture of gases is in
contact with a liquid, each gas will dissolve in the liquid in proportion to its partial pressure
Thus the greater the concentration of a particular gas in the gas phase, the more and the faster that gas will go into solution in the liquid.
The amount of gas that will dissolve in a liquid also depends upon its solubility
Various gases in air have different solubilities: Carbon dioxide is the most soluble Oxygen is 1/20th as soluble as carbon
dioxide Nitrogen is practically insoluble in
plasma
Henry’s Law
FICK’S LAW OF DIFFUSION
The net diffusion rate of a gas across a fluid membrane is proportional to the difference in partial pressure proportional to diffusion coefficient ( molecular
weight and solubility of gas) proportional to the area of the membrane and inversely proportional to the thickness of the
membrane
Diffusion rate
Surface area x conc. gradient x membr. permeability
Membrane thickness
Transfer of gases occurs by simple diffusion
GAS EXCHANGE
• refers to DIFFUSION of O2 and CO2 in the lungs and in the peripheral tissues
The driving force for gas exchange is the partial pressure difference of the gas (ΔP) across the
membrane, NOT the concentration
difference.
Partial Pressure Gradients:
O2 flows “downhill”
Alveoli Blood Tissues
CO2 flows “downhill”
Alveoli Blood Tissues
RESPIRATORY MEMBRANE
Respiratory Membrane
only 0.6 m thick, allowing for efficient gas exchange
Have a total surface area (in males) of about 70 m2 (40 times that of one’s skin)
RESPIRATORY MEMBRANE
Diffusion of gases through the respiratory membrane depends upon:
1. Membrane thickness in fibrosis or pulmonary edema, diffusion
↓s
2. Diffusion coefficient of gas affected by solubility and molecular weight
CO2 more soluble in water than O2 Oxygen more soluble than nitrogen Equal amounts of carbon dioxide and
oxygen are exchanged
Diffusion of gases through therespiratory membrane depends upon:
3. Surface area Diseases like emphysema , lung
cancer, TB reduce available surface area
During exercise, additional capillaries are perfused with blood, which increases the surface area for exchange
Diffusion of gases through the respiratory membrane depends upon:
3. Surface area
When the total surface area is decreased to 1/3 -1/4 normal, it becomes significant
Highpressure
Lowpressure
Diffusion of gases through the respiratory membrane depends upon:
4. Partial pressure differences Gas moves from area of higher partial
pressure to area of lower partial pressure. Normally, partial pressure of oxygen is higher in alveoli than in blood. Opposite is usually true for carbon dioxide
Differences in partial pressure Small diffusion distance Lipid-soluble gases Large surface area of all alveoli
Gas exchange across respiratory membrane is efficient due to:
LUNG DIFFUSING CAPACITY (DL)
Combines :the diffusion coefficient of the gasthe surface area of the membrane
the thickness of the membrane
LUNG DIFFUSING CAPACITY (DL)
In emphysema, DL decreases because of destruction of alveoli resulting to decreased surface area
In fibrosis or pulmonary edema, DL decreases because membrane thickness increases
In anemia, DL decreases During exercise, DL increases
GAS EXCHANGE ACROSS PULMONARY CAPILLARIES
• Both oxygen and carbon dioxide diffuse down their concentration (partial pressure) gradients
LUNG PO2 = 100mmHgPCO2 = 40mmHg
PO2 = 40mmHgPCO2 = 46mmHg
PULMONARY CAPILLARIESPO2 = 100mmHgPCO2 = 40mmHg
GAS EXCHANGE ACROSS SYSTEMIC CAPILLARIES
• Both oxygen and carbon dioxide diffuse down their concentration (partial pressure) gradients
TISSUE PO2 < 40mmHgPCO2 > 46mmHg
PO2 = 40mmHgPCO2 = 46mmHg
SYSTEMIC CAPILLARIESPO2 = 100mmHgPCO2 = 40mmHg
VENTILATION, PERFUSION AND V/Q RELATIONSHIPS
VENTILATION and PERFUSION are important components of gas exchange in the lung
However, the major determinant of gas exchange and thus the level of PO2 and PCO2 in the blood is the relationship between ventilation and perfusion
VENTILATION
Process by which air moves in and out of the lung MINUTE VENTILATION ALVEOLAR VENTILATION
Minute and Alveolar Ventilation
Minute ventilation: total amount of air entering
or leaving the lungs each minute
Alveolar ventilation:volume of air that enters the alveoli each minute
MINUTE VENTILATION respiratory minute volume; pulmonary ventilation
VE =500 ml/breath X 12 breaths/min
Normally about 6 liters
VE = VT • f
ALVEOLAR VENTILATION
VA = (500 ml/breath -150 ml) X 12 breaths/min
Normally about 4.2 liters
VA = (VT-VD) • f
Alveolar Ventilation
Slow, deep breathing increases AVR
rapid, shallow breathing decreases AVR
VA = (VT-VD) • f
AMBIENT AIR
Gas mixture composed of N2 and O2 with minute quantities of CO2. argon and inert gases
As we inspire the air, it is warmed to body temperature and humidified
INSPIRED AIR
becomes saturated with water vapor, which exerts a partial pressure and dilutes the total pressure of the other gases
Water vapor pressure at body temperature is 47mmHg
GAS Atmospheric air(mmHg)
Humidified air
(mmHg)
N2 597 563
O2 159 149
CO2 0.3 0.3
H2O 3.7 47
ALVEOLAR GAS: composition
contain more CO2 and water vapor and much less O2
These differences reflect the effects of:
1. gas exchanges occurring in the lungs (O2 diffuses from the alveoli into the pulmonary blood and CO2 diffuses in the opposite direction)2. humidification of air by conducting passages3. the mixing of alveolar gas that occurs with each breath
GAS Atmospheric air
(mmHg)
Humidified air
(mmHg)
Alveolar air
(mmHg)
N2 597 563 569
O2 159 149 104
CO2 0.3 0.3 40
H2O 3.7 47 47
alveolar air is only partially replaced by atmospheric air with each breath
Because only 500 ml of air is inspired with each tidal inspiration, gas in the alveoli is actually a mixture of newly inspired gases and gases remaining in the respiratory passageways between breaths.
Rate at which alveolar air is renewed by atmospheric air:
FRC measures 2300ml BUT only 350ml of new air is brought into the alveoli with each normal respiration
Amount of alveolar air replaced by new atmospheric air with each breath is only 1/7
At normal alveolar ventilation , approximately half of the gas is exchanged in 17 seconds
What controls alveolar ventilation?
PaCO2 in arterial blood that regulates ventilation
↑ PaCO2 ventilation rises
↓ PaCO2 ventilation falls
DEAD SPACE VENTILATION
Ventilation to airways that do not participate in gas exchange
2 types of dead space:1. anatomic dead space2. physiologic dead space
ANATOMIC DEAD SPACE
composed of the volume of gas that fills the conducting airways
In a healthy individual, 30% of the inspired air is wasted
PHYSIOLOGICAL DEAD SPACE VENTILATION
The total volume of gas in each breath that does not participate in gas exchange
Includes the anatomic dead space and the dead space secondary to the ventilated but not perfused alveoli
nearly equal in normal
PERFUSION
Process by which deoxygenated blood passes through the lung and becomes oxygenated
PULMONARY CIRCULATION
Begins with the right atrium Total blood volume: 500ml Low pressure, low resistance
system
Regulation of Pulmonary blood flow
Hypoxic vasoconstriction Mechanism: direct action of alveolar
PO2 on the vascular smooth muscle of pulmonary arterioles
HYPOXIC VASOCONSTRICTION
Hypoxia causes depolarization of vascular smooth muscle cells → depolarization opens voltage-gated calcium channels → calcium influx → contraction
VENTILATION-PERFUSION RELATIONSHIPS
Ratio of ventilation to blood flow Ventilation and perfusion must be
tightly regulated for efficient gas exchange
VENTILATION-PERFUSION COUPLING (V/Q ratio)
Ratio of pulmonary ventilation to pulmonary blood flow
Ratio of CO2 excreted to the O2 taken up by the lungs
= CO2 consumption
O2 consumption
Respiratory exchange ratio Normally, 0.8
DIFFERENCES IN VENTILATION & PERFUSION IN DIFFERENT PARTS OF
THE LUNG
UPRIGHT POSITION Ventilation is greater at
the base Perfusion is greater at
the base
LATERAL POSITION The dependent lung is
best ventilated
Ventilation/Perfusion Ratios
In an upright subject, ventilation increases more slowly than blood flow from the apex of the lung to the base
Insert fig. 16.24
The difference in V/Q ratios is associated with a difference in alveolar O2 and CO2 content between apex and the base
APEX : high alveolar O2, low alveolar CO2
BASE : low alveolar O2, high alveolar CO2
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© 2005 Elsevier
Alveolar-arterial PO2 difference
Difference between alveolar O2 (PAO2) and arterial PO2 ( PaO2)
Normally, alveolar O2 (PAO2) is slightly greater than arterial PO2
( PaO2)
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© 2005 Elsevier
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Downloaded from: StudentConsult (on 2 April 2008 01:42 PM)
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If PAO2 normally averages 100 mmHg,
why is average PaO2 =95 mmHg??
1. Shunt
- small fraction of pulmonary blood flow bypasses the alveoli
2. V/Q differences from apex to base
SHUNT 2 sources:
1. some bronchial and mediastinal veins drain into the pulmonary veins
2. Small portion of coronary venous blood (thebesian vessels) that drains directly into the left ventricle rather than going to the lungs to be oxygenated
SHUNT (VENOUS ADMIXTURE)
Normally 1-2% of total cardiac output
Alveolar-arterial PO2 difference
An increased in AaDO2 is a hallmark of abnormal O2 exchange
In normal individuals breathing room air, AaDO2 is less than 15mmHg
The A-a Gradient
The A-a gradient = PAO2 – PaO2
Normal A-a gradient ≈ (age + 10) / 4 Can range between 5-20 mmHg
The A-a Gradient
useful in determining source of hypoxemia
INCREASES with age For every decade a person lived,
gradient is expected to increase by
1 mmHg
The A-a Gradient
If abnormally INCREASED, denotes poor gas exchange
Seen in : Defect in diffusion V/Q defect R to L shunt
V/Q DEFECTS
Ventilation/perfusion defects:
VA/Q = 0• •
PAO2 = 40PACO2 = 45
PvO2 = 40PvCO2 = 46
Pc'O2 = 40Pc'CO2 = 46
PO2 = 150PCO2 = 0
• Without alveolar ventilation, VA/Q = 0
pO2 in the alveolus falls
Alveolar PO2 will decrease to 40
mmHg pCO2 rises
Alveolar PCO2 will increase to 45
mmHg
•This is “shunt”
V/Q ratio=0
Shunts: when perfusion exceeds ventilation, a shunt exists. Blood bypasses the alveoli w/o gas exchange occurring.
Airway obstruction, mucous plugs, right to left cardiac shunts
V/Q ratio=0
Results in an increase in “physiologic shunt blood”- blood that is not oxygenated as it passes the lung
PAO2 = 150PACO2 = 0
PO2 = 150PCO2 = 0
VA/Q = ∞• •
Ventilation/perfusion defects: Dead space
• Without perfusion, VA/Q = ∞
• Ventilation exceeds perfusion
• Alveoli do not have adequate blood supply for gas exchange to occur
• •
PAO2 = 150PACO2 = 0
PO2 = 150PCO2 = 0
VA/Q = ∞• •
Ventilation/perfusion defects: ↑ V/Q ratio
• Alveolar gas tensions would equal those in inspired air
• This is “dead space” or wasted
Pulmonary emboli, pulmonary infarction, cardiogenic shock
• •
HIGH V/Q
High ventilation relative to perfusion usually because blood flow is decreased
Unlike dead space, which has no perfusion, high V/Q regions have some blood flow
Pulmonary capillary blood has a high PO2 and a low PCO2
LOW V/Q
Regions have low ventilation relative to perfusion
Pulmonary capillary blood has a low PO2 and a high PCO2
IN SUMMARY ……
the gas exchanges that occur between the blood and the alveoli and between the blood and the tissue cells take place by simple diffusion driven by the partial pressure gradients of O2 and CO2 that exist on the opposite sides of the exchange membranes.
Pulmonary blood flow and ventilation – unevenly distributed
Apex – blood flow is lowest as well as ventilation
V/Q highest at the apex, with an average value of 0.8
Where V/Q is highest, PaO2 is highest and PaCO2 is lowest
V/Q defects impair gas exchange If ventilation is decreased relative to
perfusion, then PAO2 and PACO2 will approach their values in mixed venous blood
If perfusion is decreased relative to ventilation, then PAO2 and PACO2
will approach their values in inspired air
QUESTIONS?
Thank you