11 - GAS EXCHANGE 1: EXTERNAL RESPIRATION

Question 1

External respiration

Answer 1

Oxygen diffuses from alveoli into pulmonary capillaries.

Carbon dioxide moves in the opposite direction

Question 2

Internal respiration

Answer 2

Oxygen diffuses from the systemic capillaries into the tissues, and carbon dioxide in the opposite direction

Question 3

What is gas exchange?

Answer 3

• Gases moves from the alveolar air and blood by passive diffusion.
• Normally, blood is in contact with the alveoli for 0.75 seconds, and PO2 reaches equilibrium in about 0.25 seconds; therefore, PO2 is not diffusion limited.
• If exercising, blood flow increases so time for diffusion decreases

Question 4

DALTON’S LAW AND PARTIAL PRESSURES

Answer 4

• The total pressure of a gas mixture is equal to the sum of the pressures of each gas in it.
• Partial pressure: the pressure of an individual gas; can be measured by multiplying the % of that gas by the total pressure O2 makes up 21% of the atmosphere, so :partial pressure of O2 = 760 X 21% = 159 mmHg.
In practical terms: Each gas exerts a partial pressure according to the proportion of air it occupies

Question 5

PARTIAL PRESSURE: AIR COMPOSITION

Answer 5

Composition of air:
• 78.09% nitrogen PP=593.5mmHg
• 20.95% oxygen PP=159.2mmHg
• 0.93% argon (& other inert gases)
• 0.03% carbon dioxide PP= 0.23mmHg
Air pressure:
• at sea level = 760 mm Hg = 1 atmosphere (Water contains only 5-10 ml oxygen/litre)

Question 6

HOW DOES ALTITUDE AFFECT PARTIAL PRESSURE?

Answer 6

At sea level, the limiting factor is co2.
As atmospheic pressure drops, teh amount of oxygen drops, amount of oxygen in alveoli and arterial blood decreases. the availabiity of oxygen is the limiting factor at altitude. chest is wder and deeper to maximise intake of air, emphysema and COPD causes barrel chest to try and increase oxygen intake

Question 7

HOW DOES ALTITUDE AFFECT PARTIAL PRESSURE?

Answer 7

At sea level, the limiting factor is co2.
As atmospheric pressure drops, the amount of oxygen drops and the amount of oxygen in alveoli and arterial blood decreases.
The availability of oxygen is the limiting factor at altitude. Chest is wider and deeper to maximise intake of air, emphysema and COPD causes barrel chest to try and increase oxygen intake

Question 8

What is external respiration?

Answer 8

External respiration is the gas exchange between the lungs and blood
• Occurs across the respiratory 
membrane:
• Alveolar walls
• Blood vessel walls

Question 9

Alveolus

Answer 9

• Alveolar wall has elastic fibres for movement and stretch
• Macrophages (dust cells) for filtration
• 2 types of alveolar cell (pulmonary epithelial cells)

Question 10

Alveoli and surfactant

Answer 10

Alveoli are lined by type 1 and type 2 alveolar epithelial cells
• Type 2 cells release lipid-rich surfactant
• Lowers the surface tension of the fluid lining the alveoli
• An increase in surface area on lung inflation would ordinarily increase surface tension and cause lung collapse
• Surfactant prevents this (Laplace’s Law) by decreasing the distending Pressure.

Question 11

RESPIRATORY DISTRESS SYNDROME

Answer 11

• Surfactant produced from 26 weeks prenatally so premature infants are vulnerable to collapsed lung (respiratory distress syndrome-RDS)
• Cortisol treatment for mother in labour can help stimulate surfactant production
• Infants treated with O2 to resolve RDS.

Question 12

RESPIRATORY MEMBRANE STRUCTURE

Answer 12

• Type 1 alveolar cells
• Alveolar basement membrane
• Interstitial space: elastic fibres
• Capillary basement membrane
• Capillary endothelium

Question 13

FACTORS AFFECTING GAS EXCHANGE

Answer 13

1. Surface area
2. Diffusion distance (i.e. thickness of membrane)
3. Diffusion gradient
   - Fick’s law

Question 14

Surface area

Answer 14

• Very large in healthy lungs (spherical structures have very large surface area (80-100 square metres)
• Inflation increases the available surface area for exchange
• Multiple small alveoli increases surface area
• Larger spaces/cavities have smaller surface areas
- Emphysema patients have degraded alveolar walls. This results in a smaller surface area for gas exchange.

Question 15

DIFFUSION DISTANCE

Answer 15

• Normally very short in healthy lungs (0.4 to 2.0 µm)
• Increased if there is fluid in the lungs (e.g. in pneumonia) or mucus in the lungs (e.g. cystic fibrosis)
- If alveolar PO2 is low or the diffusion resistance is high, capillary PO2 may not reach equilibrium with alveolar PO2. ie There is not enough difference between the 2 to allow diffusion
- Rate of diffusion is explained by Fick’s law and means that diffusion of gas is slow if the diffusion thickness increases.

Question 16

DIFFUSION : FICK’S LAW

Answer 16

R = D x A x delta p/t
R = rate of diffusion
D = diffusion constant for gas ie O2
A = surface area (70 m2 in human lungs)
p = differences in partial pressures across the membrane
t = thickness of respiratory membrane (0.5 - 1.5 mm in human lung)
- Diffusion is proportional to surface area and concentration difference.
- It is inversely proportional to diffusion distance.
- It requires there to be a difference in pp across the membrane

Question 17

DIFFUSION GRADIENT

Answer 17

• Gases diffuse from high to low PP
• Increased by repeated replenishment of air with high PO2 and low PCO2 (breathing)
• Decreased if ventilation or blood flow are reduced e.g. lung blood flow reduced due to pulmonary embolism

Question 18

DIFFUSION GRADIENT

Answer 18

• Gases diffuse from high to low PP
• Increased by repeated replenishment of air with high PO2 and low PCO2 (breathing)
• Decreased if ventilation or blood flow are reduced e.g. lung blood flow reduced due to pulmonary embolism

Question 19

VENTILATION AND PERFUSION

Answer 19

• The rate of oxygen uptake depends on the rate at which it is supplied (ventilation), and the rate at which it is removed (perfusion).
• V/P ratio determines blood O2 and CO2concentration
• Mismatch leads to respiratory failure
V/P scan: using scintigraphy and radioisotopes. Good for detecting embolism

Question 20

Ventilation

Answer 20

amount of air reaching the alveoli/minute

Question 21

Perfusion

Answer 21

amount of blood reaching the alveoli/minute

Question 22

VENTILATION AND PERFUSION INEQUALITY

Answer 22

Due to gravity, blood flow is greater at the base than at the apex of the lungs
1. Apex of the lungs has a higher V/Q ratio
There is more ventilation here , and V/Q = α when there is ventilation and no perfusion.
2. Base has lower V/Q as gravity means there is more blood at the base (higher perfusion)
V/Q can be 0 if there is perfusion but no ventilation.

Question 23

LUNG PERFUSION

Answer 23

V/Q = Ratio : Average 0.8 for the whole lung
• Higher at Apex (>0.8)
• Due to gravity and height above or below heart
• Means more perfusion than ventilation (<0.8 for base)
• Disease alters V/Q ratio

Question 24

Factors that affect perfusion

Answer 24

• cardiac output

* Pulmonary vascular resistance

Question 24

Factors that affect perfusion

Answer 24

• cardiac output

* Pulmonary vascular resistance

Question 25

DECREASED V/Q

Answer 25

• Decreased ventilation in lung
• No effect on blood flow
• Low arterial PO2
• Associated increase in PCO2.
• Chronic bronchitis, asthma, acute oedema

Question 26

INCREASED V/Q

Answer 26

• Increases PO2 and dead space in lungs (high ventilation)
• Decrease in arterial O2 saturation
• Seen in emphysema where there is lots of ventilation but small area for blood exchange
Tachypnea