factors affecting gaseous exchange Flashcards
gas exchange definition
occurs via diffusion co2 and o2 diffuse down their concentration gradients
diffusion is dependent on the tissue area
what is ficks law and what does it mean
the amount of gas transferred through a sheet of tissue is proportional to the tissue area, diffusion constant D and their difference in partial pressures (p1-p2) and indirectly proportional to the tissue thickness T
what is ficks law equation
v= A x D X (p1-p2)/ T
V-volume of gas transferred through a sheet of tissue
A- tissue area
D- diffusion constant
T- tissue thickness
what are some respiratory factors affecting gaseous exchange
diffusion limitation
decreased po2- lower diffusion gradient
hypoventilation- less o2 enters the lungs and less co2 removed from the lungs
right to left shunt or a shunt like effect- areas of lung that are perfused but not ventilated. this can cause blood to bypass the lung
ventilation and perfusion inequality- mismatch between the two results in impaired gas exchange.
measuring gas exchange
what can we use to measure gas exchange
TCO total lung carbon monoxide absorbance TLCO
what factors affect TCLO total lung co absorbance
-surface area available for gas exchange (stretched out area of alveoli is 50-100m^2)
lung units
partial pressures of gases difference of partial pressures
in the alveolus
in the capillaries
diffusion constant of gas
proportional to solubility
inverse to molecular weight
thickness of alveolar barrier
blood flow and gas carrying capacity of haemoglobin
what form of oxygen is in alveoli
gaseous oxygen
what units are partial pressures expressed in
mmhg
what is mixed venous blood
refers to blood in pulmonary artery
is the partial pressure gradient greater for oxygen than it is for carbon dioxide
yes the partial pressure in mixed venous blood for oxygen is 40 whereas in alveoli is 100 the difference between the partial pressures of o2 in blood and alveoli is 60mmhg.
the venous blood partial pressure for co2 is 45 whereas the partial pressure for co2 in alveoli is 40 so the difference in partial pressures for co2 is 5. oxygen therefore has a greater partial pressure gradient than co2.
what is the difference between alveoli and blood for oxygen partially maintained by
binding to haemoglobin
diffusion constant is the solubility of the gas
what is proportional and inversely proportional to
diffusion constant is proportional to solubility of gas
inversely proportional to the square root of molecular weight
soluble, light molecules have a higher diffusion constant
compare diffusion constants of co2 and o2
co2 is more soluble than oxygen (20 times more soluble) and since they have roughly similar molecular weight, co2 diffuses across tissue barriers more efficiently
carbon monoxide diffusion constant
carbon monoxide diffuses very efficiently. used in a dclo test test for diffusion. because carbon monoxide is so soluble, it is quickly taken away from the lung by haemoglobin in erythrocytes
DCLO test diffusion capacity of the lung for carbon monoxide tests for what
the extent to which oxygen passes from air sacs of the lungs into the blood
nitrogen water solubility and diffusion constant
nitrogen has a low water solubility so diffuses very poorly (only occurs under high pressure breathing when alveolar nitrogen partial pressure increases e,g during diving
why does oxygen have a higher partial pressure gradient than co2 but lower diffusion constant and how does this keep gas exchange at a balance
the higher partial pressure gradient for oxygen compensates its low diffusion constant
the high solubility and high diffusion constant for co2 ensures its rapid removal from blood
balance is critical for efficient gas exchange
both uptake of o2 and removal of co2 take place at nearly equal rates to maintain respiratory haemostasis
alveolar structure
many small air sacs called alveoli that give an overall large surface area
each alveolus is surrounded by a capillary bed ( a network of pulmonary capillaries)
blood is at low pressure in the capillaries
the capillaries are highly distensible extensible which means that during exercise they can stretch to increase their surface area
what does it mean when we say that capillaries are highly distensible
during exercise they can stretch to increase their surface area
what are reducing transfer factors
reduced ventilation- pulmonary oedema
reduced perfusion- reduced transfer of gases pulmonary embolism- big clot in pulmonary arteries prevents blood from reaching pulmonary capillaries
reduced lung area- pneumonectomy
reduced haemoglobin- needs adjustment in individuals suffering from anaemia. reduces carrying capacity of haemoglobin.- anaemia
what are increasing transfer factors
cardiac output increased- more haemoglobin travelling through the pulmonary circulation
increased exercise
haemoglobin concentration
alveolar haemorrhage- bleeding within the alveoli, this is an increase factor because leaked blood is still absorbing the co used in the test
polycothaemia- high haemoglobin concentration in blood. could be because they are chronic smokers or lived at a high altitude for a long time.
what is the oxygen content of the blood dependent on
the pao2 and haemoglobin concentration
what is the cardiac output dependent on
stroke volume and heart rate
what do blood vessels do
allow for the transfer of oxygen to the tissues and removal of waste
most of oxygen is bound to haemoglobin, describe how hamoglobin binds to and carries oxygen
each hb molecule can carry up to 4 o2 molecules
about 250 million haemoglobin molecules per erythrocyte
oxygen is picked up by haemoglobin in the lungs, carried in the blood and released in tissues
haemoglobin oxygen saturation measured using pulse oximteres
99% of total oxygen content
oxygen transport by blood
dissolved oxygen pao2 is measured using arterial blood gas. this tells us the po2 which correlates to the amount of oxygen dissolved in the blood
only about 1% of oxygen is carried in the blood.
haemoglobin has a high affinity for oxygen. its affinity is reduced if (these conditions are found in tissue capillaries to aid the release of oxygen)
-low pao2
high paco2
low ph
higher temperature
higher concentration of 2,3 diphosphoglycerate dpg is a by prodyct of erythrocyte metabolism
oxygen should be kept above 90% saturation
explain the oxygen dissociation curve
concentration of oxygen in the x axis
haemoglobin saturation on the y axis
the curve shifts to the left when affinity of haemoglobin for oxygen has increased and haemoglobin saturation increases (ph increased, pco2 lower lower temperature and lower2,3 dpg
curve shifts to the right (lower affinty for oxygen by haemoglobin if ph decreased pco2 increased temperature increased 2,3 dpg increased
if the curve shifts to the right increasing o2 release
factors increase and co2 is increased reducing ph
if the curve shifts to the left less o2 is released
factors reduce and less co2 is produced so ph increases
ventilation is what
V is gas flow
what is perfusion
q blood flow- distribution of blood to tissues
what is a perfect v to q ratio and what does it mean
p= v/q is 1 gas flow is equal to blood flow
what is the normal v/q ratio
0.8
what is the conducting zone
all parts of anatomy that conduct gas down to the terminal bronchioles. this is known as deadspace because respiratory exchange does not take place here (no perfusion) average value of conducting zone is 150ml
what is respiratory zone
part of the anatomy where gas exchange occurs alveoli
what is deadspace
part of the tidal volume that does not come into contact with perfused areas where gas exchange can take place. this leads to normal gas flow V but reduced q blood flow so the v/q ratio is infinite
shunt- a percentage of the cardiac output bypasses the ventilated alveoli. this is normally 1-2% . leads to a normal q blood flow but reduced gas flow so the v/q ratio is low. areas of lungs are perfused but not
ventilated
shunting- normal perfusion but no ventilation
deadspace- explain what this is
no respiratory exchange, normal ventilation but no perfusion
the v gas flow is normal but the q blood flow is reduced
so vq ratio is infinite
distribution of alveolar ventilation
alveolar ventilation does not occur in lungs
alveoli in the lower regions recieve more ventilation. this is due to the effects of gravity which lead to intrapeural and transpulmonary pressures varying from apex to base.
these presure changes mean that alveoli are smaller at the base.
these smaller alveoli are more compliant so alveolar ventilation increases
alveoli in the upper parts of the lungs are less compliant
increased expansion at the base of the chest where the lung increases in volume the most, thus more ventilation occurs (more compliant alveoli in the base of the lung)
distribution of blood flow
there is a greater blood flow per alveoli in the lower regions of the lung than in the upper regions
this is caused by gravity
the intravascular pressure is greater in lower regions of the lung therefore resistance is also lower due to recruitment and distension.
comparison of pulmonary and systemic circulation
pulmonary artery
shorter than the aorta and much thinner. also thinner than systemic counterparts contains less vascular smooth muscle thus less capacity to contract and have less elastin (less damping)
comparison of pulmonary vein and systemic counterparts
pulmonary vein thinner than systemic and have little vascular smooth muscle
comparison of pulmonary capillaries and systemic counterparts
pulmonary capillaries sandwiched between the alveoli so blood flows through them like a sheet. this provides a large but thin surface area for exchange.
the lung is characterised by low pressure, low resistance and high flow
low pressure- all the cardiac output goes through the lungs
low resistance- given lung blood flow is the same as cardiac ouput and low arterial pressure is 1/5 that of the systemic, the pulmonary resistance must be much less than systemic
high flow- all of the cardiac output goes through the lungs
what happens to cardiac output with exercise
cardiac ouput may rise 2-3 times but inflow pressure to the lungs rises only slightly
what happens to capillaries when blood flow increases
capillaries that were previously closed collapsed are recruited (opened)
capillaries that were previously opened, distend enlarge or balloon
what are the two types of control of pulmonary blood flow
neural control
passive control
stimulation of vagal fibres to the lungs causes a decrease in pulmonary vascular resistance and stimulation of the sympathetic cause a slight increase in resistance
neural control- sympathetic nerves vasconstrict (noreadrenaline) alpha adrenoreceptors on smooth muscle of arteries and arterioles
parasympathetic nerves vasodilate (acetylcholine) muscarinic m3 receptors on endothelium leads to release of no
passive control- differences in pressure between extra and intra alveolar vessels
when do you have the lowest total vascular resistance
at the end of expiration there is a balance between the pressure inside the alveoli and that exerted from the surrounding vasculature
when is the total vascular resistance low but still increases
during inspiration as the alveoli expand, the pulmonary capillaries are compressed and elongated increasing intra alveolar resistance. however since the chest wall is expanded, there is little pressure on extra alveolar vessels so extra alveolar resistance decreases. total vascular resistance is low but still increases
when does total vascular resistance increase
in forced expiration, vascular resistance increases. this is because the large positive intra peural pressures needed to force the air out causes compression of the extra alveolar vessels by the chest wall, increasing vascular resistance. intra alveolar pressure falls through because the alveoli are not expanded.
what happens in inspiration in terms of functional residual capacity and total lung capacity
you move from functional residual capacity towards total lung capacity
what happens in passive expiration in terms of total lung capacity and functional residual capacity
you move from total lung capacity to functional residual capacity
what happens in forced expiration in terms of total lung capacity and residual volume
you move from total lung capacity to residual volume
effect on posture on v/q in the lungs
both v and q increase down the lungs
q increases more than v down the lung due to increase in intravascular pressure this reduces resistance due to distension and recruitment
v/q ratio changes down the lungs
if the patient is supine v/q changes front to back
ventilation and blood flow are dependent on what factor
gravity dependent
airflow and blood flow increase down the lung
q increases more than v down the lung what is the difference between blood flow and ventilation between top and bottom of lung
blood flow shows a five fold difference between top and bottom of the lung
ventilation shows a 2 fold difference between the top and bottom of the lung
what does zones of west mean
describe what happen to perfusion at different zones of the lung
when standing upright is perfusion uniform
no
lung can be split into 3 zones to describe the different perfusion pressures from top to bottom
what does PA pa and pv stand for
PA- alveolar pressure
pa- arteriolar pressure
pv-venous pressure
arteriolar pressure is always greater than venous pressure
zone 1 apex of the lung
PA>pa>pv alveolar pressure is greater than arteriolar pressure which is greater than venous pressure
blood flow is low
region at the top of the lings in which arteriolar pressure falls below alveolar pressure. this does not happen under normal conditons but can result from hypotension which decreases arterial pressure or positive pressure ventilation (mechanical respiratory which will increase alveolar pressure)
zone 2 mid
pa>pA>pv arteriolar pressure is greater than alveolar pressure which is greater than venous pressure
arterial pressure is greater than alveolar pressure but venous pressure remains below alveolar pressure. perfusion is dependent on the gradient from arterial pressure to alveolar pressure as vessels collapse at the point where venous pressure has fallen below alveolar pressure limiting flow
venous pressure has no influence on flow but perfusion increases from top to bottom of zone 2 as arterial pressure rises further above alveolar pressure
zone 3 base of lungs
pa>pv>PA
both arterial and venous pressure exceeds alveolar pressure. airway pressure no longer influences perfusion as flow is dependnet on the arterial venous gradient
flow increases in moving down zone 3 because the mean volume of the vessels does as both arterial and venous pressure rise, distending capillary bed. the pressure gradient remains the same
zone 3 comprises the majority of lungs in health
when a person is standing upright the vq ratio is lowest in what zone and highest in what zone
lowest-zone 1
highest- zone 3
what are 2 responses to ventilation perfusion inequalities called
hypoxic vasconstriction
hypocapnic bronchoconstriction
explain what hypoxic vasoconstriction is
low po2 pulmonary arterioles constrict blood away from a shunt or poorly ventilated (hypoventilated ) alveoli towards better ventilated alveoli to increase gas exchange
explain what hypocapnic bronchoconstriction is
low pco2 bronchioles divert ventilation away from deadspace or poorly perfused (hyperventilated) alveoli towards alveoli which are better perfused. for instance oxygen would be diverted from a faulty bronchus
extensive ventilation perfusion mismatch leads to hypoxia
what is pulmonary hypertension right ventricle pumping against high resistance (normally low resistance to lungs)
results from constriction or stiffening of the pulmonary arteries that supply blood to the lungs. consequently it becomes more difficult for the heart to pump blood forward through the lungs. this stress on the heart leads to enlargement of right heart and eventually fluid can build up in the liver and other tissues such as in the legs.
pylmonary arterial hypertension- pah is defined as sustained elevation of mean pulmonary arterial pressure to more than 25mmhg at rest or to more than 30mmhg whilst exercising
right heart catherisation studies can be used to accurately determine the pressure within the right ventricle
what studies can be used to accurately determine the pressure within the right ventricle
right heart catherisation studies
what is pulmonary arterial hypertension
pah is defined as sustained elevation of mean pulmonary arterial pressure to more than 25 mmhg at rest or to more than 30mmhg whilst exercising.
what does extensive ventilation perfusion mismatch lead to
hypoxia
hypoxia meaning
deficiency in the amount of oxygen reaching the tissues
hypoxemia meaning
the oxygen concentration within arterial blood is abnormally low
how is it possible to experience hypoxia due to anaemia but maintain a high oxygen partial pressure po2
enough oxygen in the blood but not enough going to right parts of body
hypoxaemic hypoxia
low concentration o2 in the blood
low inspired po2 high altitude
hypoventilation opiates
impairment of diffusion fibrosis
right to left shunts typiacally collapsed alveoli but also due to gravity causing the apex to be under perfused
ventilation perfusion inequalities typically pulmonary embolism
anaemic hypoxia
reduction in oxygen carrying capacity of blood owing to decreased total haemoglobin or altered haemoglobin constituents
decreased concentration of functional haemoglobin po2 normal but low o2 content
reduced number of erythrocytes haemorrhage
stagnant hypoxia
failure to transport sufficient oxygen because of inadequate blood flow . in people with severe pulmonary embolism or heart failure
reduced blood flow
histotoxic hypoxia
impaired use of oxygen by cells
normal oxygen delivery to cells but poisoning at the tissue level by poisons cyanide which disable oxidative phosphorylation
cyanide binds to cytochrome c oxidase fourth complex in electron transport chain. it binds tightly so that it cannot transport any electrons to oxygen therefore halting production of atp.
what is transpulmonary pressure
alveolar pressure- intrapleural pressure
what is alveolar pressure
pressure of air inside the alveoli of the lungs
what is intrapleural pressure
the pressure within the pleural cavity space between parietal pleura and visceral pleura
it is typically negative
how does pneumothorax occur
if the pleural cavity is breached the intrapeural pressure becomes equal or higher than atmosphere pressure
this eliminates pressure gradient causing lung collapse
what is the purpose of the pleural cavity
reduce friction during breathing and helps lung adhere to the chest wall
