Energy Production & Oxygen Consumption Flashcards
What drives oxygen to different places?
the partial pressure of oxygen
What drives oxygen towards the mitochondria and why?
The pressure gradient drives oxygen towards the mitochondria
pO2 is low in the mitochondria as oxygen is being consumed
What is the oxygen cascade?
the oxygen cascade describes the different stages of the partial pressure of oxygen from when it is breathed in to when it reaches the mitochondria
What are the stages of the oxygen cascade, from when it is breathed in?
- dry air
- humid air
- alveolar gas
- pulmonary capillary
- arterial blood
- venous blood
- cytoplasm
- mitochondria
pO2 decreases as it moves down the cascade
What is the pO2 of dry inspired air?
21 kPa
What is the first thing that happens to dry inspired air?
The air is humidified by adding a fixed amount of water at 37oC
What is the result of humidification of dry inspired air?
What is the pO2 of inspired humid air?
Humidification means oxygen is diluted slightly
pO2 = 19.9 kPa
What happens to air as it enters the lung?
It meets CO2 that is leaving the lung
What is pCO2 in the alveolus and how does this influence air entering the lung?
pCO2 = 5-6 kPa
Oxygen entering the lung becomes diluted by the same amount as the amount of CO2 present
Every time pCO2 increases by 1 kPa, pO2 will decrease by 1 kPa
What is the pO2 of alveolar gas?
14.9 kPa
What is the pO2 in the pulmonary capillaries?
14.9 kPa
Why is pO2 in the pulmonary capillaries very similar to the pO2 in alveolar gas?
Diffusion from the air space in the alveoli to the haemoglobin is very efficient
All the structures are very thin and there is a large surface area of lung
What is meant by an area of the lung having a ‘low ventilation to perfusion ratio’?
Some areas of the lung do not get enough ventilation for the amount of blood which flows through them
Which areas of the lung tend to have a low ventilation to perfusion ratio?
Why?
The less well ventilated areas are at the base of the lung
This is due to the effect of gravity
Why may there be a lower pO2 in arterial blood than in pulmonary capillaries?
If there is not enough O2 in the alveolus, the blood is not fully oxygenated
This blood mixes with oxygenated blood to provide a lower pO2 in arterial blood
What is the pO2 of arterial blood?
13.6 kPa
How is arterial pO2 measured?
A needle is inserted into the radial artery
What happens once the blood has entered the capillaries?
Oxygen diffuses out of the capillary and into cellular mitochondria
This is a long distance
What is mean tissue pO2?
5 kPa
What pO2 is required for the mitochondria to work efficiently?
0.15 kPa
There is an excess supply of oxygen to the mitochondria
What happens if pO2 is less than 0.15 kPa at the mitochondria?
The mitochondria develop anaerobic respiration
What does the Krogh model describe?
How oxygen gets from the capillary into the cells of a tissue
According to the Krogh model, how are tissues and their capillary supply arranged?
All tissues are arranged in a cylinder with one capillary supplying each cylinder
The capillary is in the centre of the cylinder, with tissue surrounding it
What happens to pO2 along the capillary, according to the Krogh model?
pO2 falls in an exponential manner
This occurs as oxygen diffuses away from the tissues exponentially
According to the Krogh model, how does pO2 differ at the arterial and venous ends of the capillary?
pO2 of cells at the venous end of the capillary will always be lower than pO2 of the cells at the arterial end of the capillary
What is the ‘lethal corner’ in the Krogh model?
How is it clinically significant?
This is where the cells of a particular tissue cylinder have the lowest pO2
If there is a problem with oxygen delivery, these are the first cells to turn hypoxic
What is the definition of ‘oxygen delivery’?
The amount of oxygen leaving the heart in one minute
DO2
What is the equation used to calculate oxygen delivery?
DO2 = cardiac output x oxygen content of blood
What makes up the oxygen content of the blood?
Amount of dissolved oxygen and the amount of oxygen being carried by haemoglobin
What is the equation used to work out oxygen content of the blood?
What is normal oxygen content?
CO2 = (SO2 x [Hb] x 1.39) + 0.3
normal oxygen content is 19 ml/dl
In the oxygen content of the blood equation, what is normal SO2 and [Hb]?
SO2 = 0.97
This means the blood is 97% saturated with oxygen
[Hb] = 14 g/dl
When blood returns to the heart, what % of it is still oxygenated?
75%
Why is cardiac output important in the DO2 equation?
Cardiac output determines the speed of blood flow and the amount of oxygen delivered to tissues
In the oxygen delivery (DO2) equation, what are typical values for DO2, CO2 and CO?
DO2 = CO2 x CO
DO2 = 1000 ml/min
(oxygen delivery)
CO2 = 19 ml/dl
(oxygen content of blood)
CO = 5.25 L/min
(cardiac output)
What is the definition of oxygen consumption?
The amount of oxygen used by the body in one minute (VO2)
What is the basal metabolic rate and how is this related to oxygen consumption?
It is the minimum energy expenditure found in the body
This is oxygen consumption (VO2) at rest
What is normal VO2 (oxygen consumption) at rest?
200 ml/min
What are the values for BMR and VO2 when taking into account body size?
BMR is relative to body size so…
BMR = 45 W/m^2
VO2 = 4.8 ml/min/kg
What is basal metabolic rate measured in and why?
BMR is measured per metre squared of body surface area
This is because larger people will have more tissues
What is VO2 measured in?
Why is this not entirely accurate?
ml/min per kg of bodyweight
not entirely accurate as fat has a different oxygen concentration
What are the 4 ways in which oxygen consumption may be measured?
- direct calorimetry
- indirect calorimetry
- arterio-venous CO2 (oxygen content of blood) difference
- inspired-expired O2 volume difference
How is arterio-venous CO2 difference measured?
A sample of arterial and venous blood is taken
the oxygen concentration in each is measured
This allows you to see how much oxygen has been consumed by the tissues
What is the drawback of measuring oxygen consumption through arterio-venous CO2 difference?
It is difficult to get a venous sample as different parts of the body are using different amounts of oxygen
A mixed blood sample from a pulmonary artery would be needed
How is inspired-expired O2 volume difference performed?
The amount of oxygen inspired through a closed-system mask is measured
The amount that is expired is then measured
How does calorimetry measure energy consumption?
By seeing how much heat is given off
All functions that require oxygen give off heat, so this shows baseline energy usage
Which 3 factors affect oxygen consumption?
- age
- temperature
- exercise
How does VO2 vary with age?
VO2 peaks at 2 years of age and then declines for the rest of life
Why are older people more prone to hypothermia?
They have a lower basal metabolic rate
Why do males and females have different metabolic rates?
There is a difference between the fat and muscle ratio in males and females
Fat has a low metabolic rate and muscle has a high metabolic rate
How does temperature affect metabolism?
metabolism doubles (or halves) for every 10 degrees
What is VO2 at rest?
How does this change with exercise and how is it measured?
VO2 at rest is 0.25 L/min
Oxygen consumption increases with intensity of exercise
It is measured in METs
What are METs?
METs show how many multiples of resting oxygen consumption are being used
If someone was using 10 METs whilst exercising, what would this mean?
They are using 10 times as much oxygen than they would be using at rest
Why does anaemia of chronic diseases occur?
The bone marrow is not working as well as it should, so is not producing enough red blood cells
How does the O2-Hb curve shift in anaemia and why?
It shifts to the right due to an increase in 2,3-DPG
This means Hb will give up its oxygen more easily to the tissues
What physiological mechanism means that anaemia patients often look pale?
The autonomic nervous system reduces blood flow to non-essential organs
There is reduced blood flow to the skin
How does the saturation of Hb in venous blood vary in anaemia patients and why?
There is increased oxygen extraction from the blood
Hb in venous blood falls to 50% saturation
How does the heart modify its behaviour in response to anaemia?
Increased cardiac output to maintain oxygen delivery
Why must cardiac output be doubled in anaemia patients?
Oxygen carrying capacity of the blood falls from 19ml to 8ml
doubling CO means the same amount of oxygen is reaching the tissues as a a healthy person
What is the potential negative outcome of increasing cardiac output in anaemia patients?
Doubling CO puts a lot of extra strain on the heart
This can lead to ischaemic heart disease and angina
How does partial pressure of oxygen vary with altitude?
pO2 decreases with increasing altitude
How does oxygen saturation of Hb change with increasing altitude?
What is the critical value?
It decreases
It is incompatible with life if oxygen saturation falls below 60%
How does [Hb] change with altitude?
What does this allow for?
At increasing altitudes, Hb concentration increases
This allows arterial oxygen content to remain reasonably normal
What is the respiratory exchange ratio (RER)?
What is the name for the RER at rest?
It is the ratio of carbon dioxide production (VCO2) to oxygen consumption (VO2)
The RER at rest is the respiratory quotient
What is the normal value for RER?
What 3 factors may affect this?
Normal value = 1
- acid-base balance
- hyperventilation
- metabolic fuel
What is cardio-pulmonary exercise testing used for?
It is used in clinical medicine to get an objective measure of how often a patient exercises
What graph is plotted from the results of cardio-pulmonary exercise testing?
VCO2 is plotted against VO2
What does the gradient of the line in the cardio-pulmonary exercise test graph show?
A value for RER
How would you identify the anaerobic threshold from the cardio-pulmonary exercise test graph?
It appears where the graph starts to curve
What is the anaerobic threshold?
The pO2 at which the patient begins to respire anaerobically
Oxygen consumption does not change, but more CO2 begins to be produced
When is cardio-pulmonary exercise testing used in practice?
To see how well a patient will respond to major surgery
It is a way of quantifying risk so that a patient can make an informed decision
What are the 3 fundamental causes of cellular hypoxia?
- anoxic
- stagnant
- anaemic
all 3 causes can overlap in a Venn diagram type way
What does anoxic mean in terms of hypoxia?
There is a lack of O2 in the blood, meaning insufficient oxygen is delivered to the cell
What type of condition usually leads to anoxic conditions?
A lung problem such as respiratory failure
What is meant by stagnant in terms of hypoxia?
There is a lack of blood flow to the tissue
What conditions often cause stagnant hypoxia?
cardiac problems
e.g angina, heart failure, arrhythmia
What is meant by anaemic in terms of hypoxia?
There is not enough haemoglobin in the blood, which reduces oxygen delivery
Why do anaemic patients often experience their angina becoming worse?
The causes of hypoxia overlap - stagnant & anaemic
