Alveolar diffusion and airway resistance

Question 1

What is the partial pressure and the partial pressure gradients of inspired O2 and CO2 in air, alveoli and capillaries ?

Answer 1

• O2 in air- (PIO2)=20Kpa
• O2 in alveoli-(PAO2)=13Kpa
• O2 in capillaries-(PVO2)=12Kpa
• CO2 in air-(PICO2)=0Kpa
• CO2 in alveoli-(PACO2)=5Kpa
• CO2 in capillaries-(PVCO2)=5Kpa (ALMOST ALL CO2 from capillaries goes into alveoli)

Question 2

What factors effect O2 and CO2 partial pressures in the alveoli?

Answer 2

• Metabolic rate (more O2 needed and more CO2 produced)
○ Increase in metabolic rate will cause an increace in ventilation
○ The rate of ventilation will match metabolic demand therefore gases in blood are maintained at normal level
• Hyperventilation
○ More O2 intake will increase O2 concentration in alveoli so (PAO2 increase)
○ More CO2 out will reduce CO2 in alveoli (so PACO2 decreases)
○ (CO2 is being diluted so PACO2 decreases and PAO2 increases)
○ Ventilation rate increases without equivalent increase in metabolic rate
○ More O2 drawn in and absorbed then metabolic rate requires
○ Same amount of CO2 being produced and is in blood
○ More exhalation= more removal of CO2
○ So diffusion gradient remains steeper for longer resulting in more CO2 diffusing out of capillaries into alveoli
○ Therefore CO2 in capillaries decreases ( which will create H2CO3 in blood for buffering)
○ Results in decrease in both PACO2 and PaCO2
○ Hyperpnoea= over breathing
• Hypoventilation
○ Decreased ventilation rate without an equivalent decrease in metabolism
○ Decreased intake of O2 and decrease removal of CO2
○ less diffusion of CO2 out of capillaries into alveoli o CO2 is less dilute
○ Increase in PACO2 and PaCO2
○ Causes bronchoconstriction

Question 3

What factors affect the O2 and CO2 at the interface between alveolus and bronchi?

Answer 3

• Surface area
○ Larger surface area and thinner walls/shorter diffusion gradient will increase rate of diffusion ( alveoli have this)
• Size of partial pressure gradient
○ The PAO2=13Kpa and PaO2=5Kpa (large gradient)
○ The PACO2=5Kpa and PaCO2=5Kpa(small gradient)
§ Partial pressure gradient of CO2 is very shallow so CO2 must be more soluble as steep gradient not needed for diffusion
• Diffusion constant
○ Defined by molecular wight and solubility
○ CO2 has a bigger Mr (and is a bigger molecule) but is more soluble than O2)
○ Up to 20x easier diffusion for CO2 than O2
• See Fick’s law

	○ 

• The large difference in O2 partial pressure drives diffusion
• The blood flows slowly in capillaries (lots of time for diffusion) and after the first 25% equilibrium will be reached/no net movement (gives time just in case diffusion tales longer)
•

Question 4

What factors effect airflow in the airways?

Answer 4

• Density- higher density decreases rate of flow (harder to move through a crowd of people than past 1 person)
• He has a lower density than O2 (21% O2 and 79% He used to treat children- air has 21% O2)
• Viscosity- higher viscosity= lower rate
• Diving pressure (difference between the pressure in 2 areas) air flows from a high to low pressure therefore the larger the difference in pressure/diving force is the faster the air will more. Air is drawn into lungs.
• Type of air flow
○ Turbulent ( high velocity and large radius) air moves back on itself-so higher driving pressure needed
○ Laminar (small radius and low velocity) air goes forwards- less driving force needed
○ Transitional- drown separate branches
• Resistance
○ increased by- small radius
○ Mouth has a bigger radius than the radius of the nostrils (more resistance for nose breathing therefore more driving pressure needed) same idea for airway diameters
○ 40-50% resistant in conducting zone and 50-60% at exchange zone

Question 5

What are the sources of resistance in the airways?

Answer 5

• Resistance=(length of airways X viscosity of gas)/airway calibre to power of 4
○ Formula shows the larger the radius the smaller the resistance
○ More branches run in parallel increases surface are ( if add radi of all the branches the radius is much bigger so resistance decreases)

Question 6

Explain the relationship between airway radius and resistance.

Answer 6

• Airways and alveoli are connected
○ Alveoli pull outwards and force dilation of airwards so increase radius of airways and decrease resistance (increase air flow)
• Resistance goes down when radius increases

Question 7

What factors regulate airway calibre?

Answer 7

• Vagal nerve (parasympathetic) releases from propagandic fibres and activates M3 receptors causes:
○ More ACH = smaller radius and more mucus (decrease radius and increase Friction)
○ Bronchoconstriction-radius decreases so resistance increases and airflow decreases
§ Airways lined with smooth muscle to increase or decrease lumen size
○ Increased mucus secretion- increases resistance and decreases radius
• Adrenalin activates B2 (beta 2) receptor causing:
○ Bronchodilation
○ Agonist= activated reception
B2 agonists increase reception to increase radius/ relax muscles

Question 8

Explain how airway resistance increases during disease?

Answer 8

• Tongue/tongslis or foreign objects in the way
• Allergens can cause bronchoconstriction
○ 1stv sensitisation
○ 2nd = graduate to release histamine, prostaglandins and leukotrienes
○ These constrict airways

Question 9

How can drugs reduce airway resistance?

Answer 9

• Prophylaxis with steroids or anti-inflammatory
• Treated with drugs that stop conviction by shutting the process or immune system down.)