Challenges to Normal Respiration Flashcards
How much oxygen is required per minute at rest?
What is this equivalent to?
250 ml of oxygen is required per minute at rest
This is equivalent to 1 MET
What are METs?
Metabolic equivalents
They are a multiple of the value for oxygen consumption at rest (250 ml/min)
Why is there not a single MET value for activities such as walking and swimming?
The METs consumed depends on the intensity of the exercise
How is minute ventilation increased?
What is the result of this?
The volume of air that is moved in and out of the lungs per minute is increased
This increases the quantity of oxygen that the body receives
How can oxygen delivery to the tissues be increased?
- through increased cardiac output
2. increased arterial O2 content
Why can arterial O2 concentration not really be increased in healthy individuals?
The haemoglobin is already around 98% saturated
Although arterial O2 concentration cannot be increased, what other factor can be changed when oxygen demand increases?
Oxygen extraction is increased
This is the amount of oxygen being taken out of the Hb by the tissues as it passes through
At 3 METs, by how many times has cardiac output increased?
4 times
How does minute ventilation compare to cardiac output at rest?
Minute ventilation is approximately the same as cardiac output - 5 L/min
At 2 METs, by how many times has minute ventilation increased?
10 times
When oxygen demand increases, is it easiest for the body to increase cardiac output?
No - the capacity to increase the amount of oxygen moved in and out of the body (MV) is much greater than the capacity to increase cardiac output
In what ways is the body’s ability to increase oxygen delivery limited?
The oxygen content of the blood cannot really be increased
Cardiac output can only be increased by around 5 times
What are the major responses of the body to increased oxygen demand?
- increased minute ventilation
2. increased oxygen extraction from Hb
How is mixed venous oxygen content measured?
A catheter is placed in one of the great veins of the neck or the femoral vein
It is then passed into the vena cava
Why is mixed venous oxygen content measured?
How does it vary?
It allows measurement of the oxygen saturation of the blood
70% saturated at rest
20% saturated during exercise
What is the difference between oxygen-requiring and non-oxygen-requiring processes?
Oxygen requiring processes are aerobic
Non-oxygen requiring processes are anaerobic
Why is aerobic respiration more efficient than anaerobic respiration?
Aerobic respiration, in the presence of O2, generates 36 ATP from 1 glucose
Anaerobic respiration, with no O2, generates 2 molecules of ATP per glucose
At the start of light exercise, what type of respiration is used and why?
Combination of both aerobic and anaerobic respiration
Because there are many different types of muscle in the body
Why do some muscles require more oxygen than others?
Some muscles depend almost entirely on oxygen as they have many mitochondria and generate ATP very efficiently
Some muscles have few mitochondria and little capacity for aerobic respiration
What do VO2 and VCO2 stand for?
VO2 - oxygen consumption
VCO2 - carbon dioxide production
During a period of light exercise, how does work done vary?
The work being done is constant
How do VO2 and VCO2 vary during a period of light exercise?
VO2 builds up after a few minutes and is closely followed by VCO2
The 2 variables plateau with VO2 being higher than VCO2
What do the values of VO2, VCO2 and lactate show about respiration during light exercise?
The majority of work is being done aerobically
Lactate in the blood increases very slightly showing some anaerobic respiration
During heavy exercise, how to the values of VO2, VCO2 and lactate vary compared to light exercise?
All 3 variables still reach a plateau
VCO2, VO2 and lactate concentrations are all higher, with a gap opening up between VCO2 and VO2
Why does the component of anaerobic respiration increase in heavy exercise?
The work being done increases so more anaerobic respiration occurs to supplement the aerobic respiration
During severe exercise, how do the values of VCO2, VO2 and lactate vary compared to heavy exercise?
None of the variables reach a plateau
Lactate rises slowly
VCO2 and VO2 rise quickly at first and then continue to rise more slowly
What is the rate limiting factor in severe exercise?
Why do people stop exercising at this point?
Lactate accumulation
Cramp is caused by the build-up of lactate in the muscles
Why does lactate concentration act as a rate limiting factor in exercise?
When lactate reaches a concentration of 10 mmol/L, the body cannot tolerate the metabolic acidosis associated with increased lactate concentration
What pattern is shown by the increase in minute volume of ventilation during exercise?
Increase in ventilatory capacity during exercise is reasonably linear
It follows a linear pathway until it reaches Owles point
What is Owles point?
It describes the level of oxygen consumption at which the relationship with minute ventilation veers upwards
What do the limits of tolerance represent on the graph of oxygen consumption against minute volume of ventilation?
They represent the VO2 max in individuals of varying levels of physical fitness
How can improving physical fitness affect the limit of tolerance?
Improving physical fitness increases the degree of oxygen consumption before the linear relationship changes
There is a limited capacity after the linear relationship ends, before the limit of tolerance is reached
What is the VO2 max?
The amount of oxygen consumption which can be maximally generated
At rest, what mechanism predominantly controls minute ventilation?
Contraction and recoil of the diaphragm
As the demand for oxygen increases, how is the control of minute ventilation changed?
Different muscle groups are utilised to increase minute ventilation:
- accessory muscles of ventilation
- external intercostals on inspiration
- internal intercostals on expiration
What is minute volume and the equation?
It is the product of the tidal volume and the respiratory rate
volume of breath x number of breaths per minute
How do tidal volume and respiratory rate change during exercise?
Tidal volume and respiratory rate increase
This also increases minute volume
At maximum exercise, by how much is tidal volume increased?
It is increased by 5-6 times the value of resting tidal volume
This is 50% of the vital capacity
When exercising, at what point does minute ventilation begin to increase?
Instantaneously as exercise begins or even slightly before
Breathing instantly becomes harder and faster
What are the 3 phases of minute volume change during exercise?
Phase I - rapid increase in minute ventilation at time 0
Phase II - Gradual increase in minute ventilation as exercise proceeds
Phase III - a plateau is reached and maintained
What happens during the recovery phase after exercise?
There is an oxygen deficit which is compensated for
This is the oxygen debt
What is the partial pressure of oxygen at sea level and why?
Atmospheric pressure is 100 kPa
pO2 is 21% of the atmospheric pressure
pO2 is 21 kPa at sea level
What is the saturated vapour pressure of water at body temperature?
6.3 kPa
What happens to oxygen as it is inhaled through the nose?
it takes up water vapour and becomes humidified
As altitude increases, how does atmospheric pressure change?
How does this affect pO2 and saturated vapour pressure of water?
Atmospheric pressure decreases
pO2 in air decreases
saturated vapour pressure of water does not change
What is PiO2 and why is it greater than alveolar pressure?
PiO2 - pO2 of air entering the trachea
Alveolar pressure is less as it is occupied with CO2 coming out of the pulmonary veins
As altitude increases, what mechanism can be used to increase oxygen delivery?
Hyperventilation
As altitude increases, what is the tolerance limit?
Tolerance limit is at 4 kPa
If pressure falls below 4 kPa, it is unsurvivable as the tissues are not receiving enough oxygen
Doubling the minute ventilation will not have an effect
As altitude increases, how does this affect alveolar pressure, PiO2 and atmospheric pressure?
They all decrease as altitude increases
What chemical mechanism triggers hyperventilation?
Peripheral chemoreceptors detect the decreased oxygen level of the blood
Where are peripheral chemoreceptors found?
In the carotid body and the aortic arch
What limits excessive hyperventilation?
Alkalosis
Respiratory alkalosis occurs due to a reduced level of CO2 in the blood
There is also a fall in [H+], especially in the CSF
How does the body compensate for respiratory alkalosis?
What does this allow for?
Excretion of bicarbonate from the CSF
This allows for hyperventilation to continue
What are the consequences of secreting bicarbonate by the kidneys?
Diuresis - more frequent urination
Hyponatraemia - sodium levels in the blood fall below normal
What is acute mountain sickness predominantly caused by?
The hypoxic vasoconstriction response in the pulmonary arteries
In acute mountain sickness, what happens when oxygen tension in the inspired area is low?
The pulmonary arteries constrict to try and move blood to regions of the lung which are better oxygenated
In acute mountain sickness, what happens if all of the pulmonary arteries in the lung constrict?
Increased pressure on the right side of the circulation
This leads to increased fluid being shifted out of the pulmonary circulation and into the lungs
This causes pulmonary oedema
What is a dangerous consequence of acute mountain sickness?
Fluid may come out into the brain and cause cerebral oedema
What is erythropoiesis?
What is the mechanism involved?
Increase in the amount of RBCs, and consequently Hb
This involves an increase in erythropoietin (hormone) which stimulates production of RBCs
By how much are Hb levels increased through erythropoiesis?
There is a 30 - 40% increase in haemoglobin by 5 g/dL
Other than erythropoiesis, what are a further 2 examples of acclimatisation to high altitude?
- sustained hyperventilation and increase in cardiac output
2. less 2,3-DPG is produced which enables more oxygen to be released to tissues
What happens to the pressure on the body as depth increases?
The pressure of the water above acts on the body, increasing the pressure on the body
What is the “breaking-point” of breath-holding and how is it reached?
It determines how long someone can hold their breath
When someone holds their breath, pO2 falls and pCO2 rises until the breaking point is reached
How can the time someone can hold their breath for be prolonged?
Inspiring air with a high oxygen concentration beforehand
Hyperventilation also gives a longer breath-holding period
How much does pressure increase by when someone descends 10 m underwater?
For every 10m that someone descends, there is an increase in pressure of 1 atmosphere
This is 100 kPa
How does the pO2 change as depth increases?
How does this affect the amount of oxygen needed?
As total pressure increases, pO2 also increases
The amount of oxygen that needs to be inspired is reduced
How does nitrogen affect breathing?
Nitrogen limits the depth at which air can be breathed
At what pressure is/does:
i - nitrogen detectable
ii - there serious impairment
iii - anaesthesia occurs
i - nitrogen is detectable at > 4 atm
ii - there is serious impairment > 10 atm
iii - anaesthesia occurs > 30 atm
How does density affect breathing?
Density increases with high pressures which leads to an increased work of breathing
What happens to nitrogen on ascent?
How is this overcome?
Nitrogen comes out of solution on ascent
This is overcome through using helium/oxygen mixtures for diving
On descent, by how much is lung volume compressed and what does this lead to?
Lung volume of 6L is compressed to 600 ml at 10 atm
This leads to a loss of buoyancy
On descent, how does alveolar CO2 compare to mixed venous CO2?
Alveolar CO2 > mixed venous CO2
On ascent, how does alveolar pCO2 change?
Alveolar pCO2 decreases
How is barotrauma brought about on ascent?
Expansion of air filled spaces
What is involved in decompression sickness on ascent?
Bubbles form in the tissues and arterial gas embolism
