Gas exchange: properties of gases + gas diffusion in lung

Question 1

How do we calculate an individual gas partial pressure?

Answer 1

We multiply the percent of the gas expressed in decimal form x the total pressure. If the total pressure were 100 kPa, and gas P1 made up 21% of the mixture, then gas P1’s partial pressure = .21 (21 ÷100) x 100 kPa = 21 kPa.

Question 2

○ Gas pressure is caused by collisions of gas particles with walls of container -pressure is the force generated by collisions. There is a TOTAL gas pressure in box.

What about when we have a MIXTURE of gases? How much pressure does each individual gas exert?

Answer 2

• In mixture of gases, (where constituent gases do not combine chemically), each component gas exerts a ‘partial pressure’ in proportion to its volume percentage in the mixture.
  The sum of the partial pressures of all the gases equals the total pressure.

Question 3

in body, gases diffuse down their ____ ____ gradient

Answer 3

in body, gases diffuse down their partial pressure gradient

Question 4

○ pCO2 – partial pressure CO2 – generic
○ pO2 – partial pressure O2 – generic

○ PACO2 – partial pressure CO2 in —-
○ PAO2 – partial pressure O2 in —-

○ PaCO2 – partial pressure CO2 in ——
○ PaO2 – partial pressure O2 in ——

○ PvCO2 – partial pressure CO2 in ——
PvCO2 – partial pressure CO2 in —

Answer 4

○ pCO2 – partial pressure CO2 – generic
○ pO2 – partial pressure O2 – generic

○ PACO2 – partial pressure CO2 in alveoli (big A=alveoli)
○ PAO2 – partial pressure O2 in alveoli (big A=alveoli)

○ PaCO2 – partial pressure CO2 in arterial blood (little a= arterial)
○ PaO2 – partial pressure O2 in arterial blood (little a= arterial)

○ PvCO2 – partial pressure CO2 in venous blood
PvCO2 – partial pressure CO2 in venous blood

Question 5

Typical total air volume in lung at rest is _____

Answer 5

Typical total air volume in lung at rest is ~ 3 litres – 3000 m

Question 6

why is o2 in venous blood low

Answer 6

venous blood o2 low, cuz tissues use up the o2 + give co2 (respiration)

Question 7

Explain how the oxygen bound to Hb will be released and diffuse into tissues

Answer 7

• O2 enters plasma & dissolves in it (how much dissolves depends on partial pressure and solubility)
• O2 enters RBC and binds to Hb
• Process continues till Hb fully saturated (each Hb molecules binds 4 O2 molecules)
• After Hb is fully saturated, O2 continues to move down its partial pressure gradient until equilibrium reached
• At equilibrium, pO2 of plasma = pO2 of alveolar air
• Oxygen on Hb does NOT contribute to partial pressure oxygen in blood – not “free” – but Hb by removing free oxygen allows more oxygen to be delivered to blood from alveoli
• Blood contains both dissolved and Hb bound oxygen
• pO2 reflects amount dissolved O2 in plasma.
• Dissolved O2 is available to diffuse into tissues down its partial pressure gradient
  As dissolved O2 leaves blood for tissues, it will be replaced by O2 that was bound to Hb unbinding from Hb and dissolving into plasma - and process continues

n.b. Haemoglobin is fully saturated when it binds 4 O2

Question 8

what is the equation for
Amount of gas dissolved in a liquid

Answer 8

Amount of gas dissolved = solubility coefficient × partial pressure of gas

Question 9

how do we calculate total content of gas in blood (plasma + RBCs)
n.b. o2 bound to haemoglobin doesn’t contribute to partial pressure of o2 in blood, only the o2 freely dissolved in blood does

Answer 9

Total content of gas in blood (plasma + RBCs) = dissolved gas + gas bound to or reacted with a component (Hb)

In context of oxygen, O2 dissolved plasma + O2 bound to Hb= total content of oxygen in blood

Question 10

if somebody has interstitial lung disease they have problems with o2 or co2

Answer 10

o2 as it has lower diffusion rate, so their alveoli are thickened so harder for them to get o2 in

Interstitial lung disease is characterized by inflammation and scarring+ thickening of alveoli that make it hard for the lungs to get enough oxygen. The scarring/thickening is called pulmonary fibrosis

Question 11

equation for diffusion coefficient calculation

Answer 11

diffusion coefficient= solubility/ √molecular weight

Question 12

co2 diffuses 20 times faster than o2, why?

Answer 12

even though o2 has a smaller molecular weight than co2. co2 is more soluble than o2. So co2 solubility is greater and therefore diffuses faster

Question 13

3 factors affecting rate of gas diffusion (+ disease examples)

Answer 13

• Thickness of membrane
  
  • Increased as a result of oedema fluid in the interstitial space and in alveoli – congestive heart failure
  • ILD/Lung fibrosis - increased thickness of alveolar capillary membrane
• Surface area of membrane
  
  • Decreased by removal of an entire lung
  • Decreased in emphysema - decreased surface area
  • Respiratory distress syndrome newborn- Fewer open alveoli – less surface area
• Diffusion coefficient of the gas:
  
  • CO2 always diffuses much faster than O2
  • In diseased lung with lower alveolar O2 partial pressure, and therefore less partial pressure gradient, O2 gas exchange more impaired than CO2 because of O2 slower diffusion rate – dyspnea will initially present during exertion – blood goes by alveoli more quickly – Interstitial Lung Disease/Pulmonary fibrosis
  • Diffusion of CO2 not affected à pCO2 normal until late disease
    Lung Function Tests: abnormal DLCO{is a measurement to assess the lungs’ ability to transfer gas from inspired air to the bloodstream} (Carbon monoxide diffusing capacity)

Question 14

our lungs work best when ventilation + perfusion are matched, what is this called.
define ventilation + perfusion

Answer 14

ventilation perfusion ratio (VQ ratio)

• Ventilation is the amount of air being delivered to alveoli
• Perfusion is the amount of blood being delivered to alveoli

Question 15

Why do we have V/Q mismatch in normal lungs? i.e. it’s not 1

Answer 15

As we need a reserve, e.g. if we are exercising we can increase ratio

Question 16

main scientific cause of hypoxaemia

Answer 16

is ventilation perfusion mismatch

if Ventilation is less than perfusion = low VQ ratio
If ventilation is more than perfusion = high VQ ratio

healthy ppl VQ ratio is about 0.9

Question 17

name some diseases with VQ mismatch

Answer 17

Diseases that have V/Q mismatch
* Asthma – airway narrowing but not uniformly dispersed in lung
* Early stages COPD – airway narrowing but not uniformly dispersed in lung
* Pneumonia – acute inflammatory exudate in affected alveoli
* Respiratory Distress syndrome in the Newborn – some alveoli open, others not
* Pulmonary oedema – fluid in alveoli
Pulmonary embolism - alveoli ventilated but not perfused

Question 18

In pulmonary embolism there are alveoli
that receive _____ , though they are
still ____
This is known as Dead Space

aka perfusion has gone down

how does this lead to death?

Answer 18

In pulmonary embolism there are alveoli
that receive NO BLOOD, though they are still VENTILATED
This is known as Dead Space

aka perfusion has gone down
This means that blood is diverted to other alveoli, this is a problem because if u over perfuse the other alveoli then VQ mismatch= impaired gas exchange. u get hypoxaemia and die

Question 19

define a SHUNT

Answer 19

no ventilation but still perfused

Shunting is defined as blood passing from right to left sides of the heart without ever seeing alveolar gas