[017] factors affecting gas exchange in lungs and in blood

Question 1

diffusion depends on ficks law

what is the fick’s law equation

Answer 1

Question 2

apart from the components of Fick’s Law,

what other factor (s) affect gas exchange

Answer 2

blood flow and gas carrying capacity of haemoglobin

fick’s law components:
surface area, partial pressure of gases in alveolus and capillaries, diffusion constant of gas, thickness of alveolar barrier

Question 3

what factors are proportional and inversely proportional to

diffusion constant

Answer 3

• proportional to gas solubility

- inversely proportional to square root of molecular weight

Question 4

carbon dioxide vs oxygen

diffusion constnat

Answer 4

carbon dioxide has higher diffusion constant, diffuses across tissue barriers more efficiently than oxygen

Question 5

nitrogen diffusion constant

Answer 5

low,
has low water solubility so diffuse very poorly
only occurs under high pressure breathing when alveolar nitrogen partial pressure increased eg diving

Question 6

mechanisms/ forms of oxygen transport by blood

Answer 6

1. dissolved oxygen (PaO2)
   - measured in blood gas
   - only about 1% of O2 carried in blood
2. bound to haemoglobin (Hb) in red blood cells
   - each Hb molecule can carry up to 4 O2 molecules
   - Hb oxygen saturation measured using pulse oximeters
   - 99% if total oxygen content

Question 7

what reduces Hb affinity for oxygen

Answer 7

low PaO2
high PaCO2
low pH
higher temperature
higher concentrations of 2,3-diphosphoglycerate (DPG)

–> right shift

Question 8

V/Q mismatch occurs during?

Answer 8

V: gas flow
Q: perfusion, blood flow

deadspace
shunt

Question 9

deadspace

Answer 9

well ventilated, but not perfused, eg deadspace (mouth and trachea, no capillaries to exchange gases)

Question 10

shunts

Answer 10

V low/ Normal Q
occur when deoxygenated venous blood passes unventilated alveoli,

eg atelectais (collapsed alveoli), consolidation of lungs, pulmonary oedema

Question 11

is distribution of alveolar ventilation even?

Answer 11

No
alveolar ventilation is not even
alveoli in lower regions receive more ventilation
- intrapleural pressure is less negative in lower regions, caused by gravity and mechanical interactions between lung and chest
(aka intrapleural pressure increases down the chest)

Question 12

is distribution of blood flow even in lungs?

Answer 12

no
there is greater blood flow per alveoli in lower regions of lung than in upper regions
caused by gravity, intravascular pressure is greater in lower regions of lung, therefore resistance is also lower due to recruitment and distension

Question 13

pulmonary and systemic circulation

Answer 13

Question 14

pulmonary vs systemic circulation

vessel differences

Answer 14

pulmonary artery vs aorta
- pulmonary artery shorter and much thinner
pulmonary arteries vs systemic counterparts
- contain less vascular smooth muscle ( thus less capacity to contract), have less elastin

pulmonary veins
- thinner than systemic and have little vascular smooth muscle

Question 15

Q perfusion vs V ventilation

changes down the lung

Answer 15

Q increases more than V down lung
blood flow shows about a 5 fold difference between top and bottom of lung, while ventilation shows about a 2-fold difference

Question 16

Q perfusion vs V ventilation

changes down the lung

Answer 16

Q increases more than V down lung
blood flow shows about a 5 fold difference between top and bottom of lung, while ventilation shows about a 2-fold difference

Question 17

responses to ventilation-perfusion inequalities

Answer 17

1. hypoxic (low PO2) VASOconstriction of pulmonary arterioles
   - constrict blood away from shunt or poorly ventilated alveoli (hypoventilated) towards better ventilated albeoli
2. Hypocapnic (low PCO2) BRONCHOconstriction of bronchioles
   diverts ventilation away from deadspace or poorly perfused (hyperventilated) alveoli towards alveoli that are better perfused

Question 18

what does extensive ventilation perfusion mismatch lead to?

Answer 18

hypoxia

Question 19

does nerves control pulmonary blood flow?

Answer 19

while nerves innervate lung tissues, effects perhaps too little to be more than marginally important

sympathetic nerves vasoconstrict (noradrenaline)
- alpha 1 receptors on smooth muscle of arteries and arterioles

parasympathetic nerves vasodilate (acetylcholine)
- muscarinic (M3) receptors on endothelium leads to release of NO

Question 20

control of pulmonary blood flow

Answer 20

passive – differences in pressure between extra- and intra- alveolar vessels

Question 21

what is pulmonary hypertension

Answer 21

results from constriction, or stiffening of pulmonary arteries that supply blood to lungs
consequently, it becomes more difficult for heart to pump blood forward through lungs
stress on heart leads to enlargement of right heart, eventually fluid can build up in liver and other tissues eg in leg

Question 22

hypoxia vs hypoxemia

Answer 22

hypoxia: deficiency in amount of oxygen reaching tissues
hypoxemia: oxygen concentration within arterial blood is abnormally low

it is possible to experience hypoxia (eg due to anaemia) but maintain high oxygen partial pressure (pO2)

hypoxaemia is a cause of hypoxia

Question 23

types of hypoxia

Answer 23

hypoxemic hypoxia (wrt insufficient oxygen reaching blood)

anaemic hypoxia ( wrt oxygen-carrying capacity)

stagnant hypoxia ( wrt blood flow)

histotoxic hypoxia (impaired use of oxygen by tissues)

Question 24

apex vs base of lung

V/Q ratio , hypo perfused or hyperperfused

Answer 24

apex of lung

• hypoperfused in relation to its alveolar ventilation
• high V/1 ratio

base of lung

• hyperperfused in relation to its alveolar ventilation
• low V/Q ratio