Ventilation And Gas Exchange Flashcards
What are the lungs ventilated by?
Air
What are the lungs perfused by?
Blood
What does gas exchange do?
- moves air into and out of the body
- for gas exchange to occur, the two elements must come into contact
- there is a small diffusion distance
Where does the air in our lungs go?
- doesn’t distribute evenly in the lungs
- gravity = upper parts of lungs are stretched, heavy bases sit on the diaphragm
Where does most of the inhaled breath go and why?
- lower zones of the lungs, as they are more compliant and distensible
- than the upper zones
What do pulmonary arteries do?
- supply all of the blood from the heart to the lungs
- accepts a huge volume at low pulmonary pressure
- to minimise the work of the right heart
How and why does perfusion of the lung work?
- the walls of the pulmonary arteries are very thin
- they branch, getting smaller to feed the lungs up to the level of the terminal bronchioles
- they then split into the capillary bed
What does distension of the capillaries do in lung perfusion?
- enhances gas exchange and reservoir action
- once the red blood cells become oxygenated, the capillary bed is drained into venules
- which join to form the pulmonary veins
- as the capillaries can distend allows the pressure in the pulmonary system to stay low
- despite the very high blood flow
What is capillary distension?
Involves the widening of the capillaries to accommodate increased blood flow
What is reservoir action?
Filling up of the lungs
Where does the blood then go?
- upright posture, not enough pressure in the pulmonary circulation to fill the upper parts of the lungs compared to the heart
- the bases are overperfused
At the bases….
Perfusion is greater than ventilation
- Q > V
At the top / apexes….
Ventilation is greater than perfusion
- V > Q
What does the V/Q ratio do from the top to the base of the lungs?
- ventilation and perfusion both increase at the base due to gravity
- increase in perfusion is greater
- v/q ratio decreases from the top to the bottom
Top = V/Q > 3
Middle = V/Q = 1.0
Bottom = V/Q = < 0.6
What is the v/q matching system?
A local mechanism to adjust for poor air flow or poor blood flow
What do low oxygen levels in the hypoxia of the lung do?
- direct vasoconstrictor effect on the pulmonary arteriole that supplies it
- the capillaries collapse
What is the beneficial effect of low oxygen levels in the hypoxia?
- diverts blood away from a poorly ventilated area to a better ventilated one
- maximises contact between air and blood
- very important for optimising gas exchange
Perfusion without ventilation?
V/Q = 0
Shunt
Ventilation without perfusion?
V/Q = infinity
Dead space
What is incompatible with life?
- pure shunt
- pure dead space
Causes of a shunt, leading to hypoxemia: (low blood pressure)
- pulmonary oedema
- pneumonia
- ards
- chronic bronchitis
- asthma
Causes of dead space:
- pulmonary embolism
- tumour
- shock
What is an embolism?
Blood clot
What is the mechanism of gas exchange?
- when there is a difference in partial pressure at the semi-permeable membrane of the alveoli
What is Dalton’s Law of Partial Pressures?
- the total pressure exerted by a mixture of gases results from the combined effect of each of the pressures, of the individual gases in the mixture
- partial pressure of a gas reflects the proportion of that particular gas in the whole mixture
What is partial pressure expressed as?
PX
X = formula for the gas
What is diffusion of gases?
- when the higher concentration of a gas moves to the lower concentration, until equilibrium is achieved
What is Fick’s Law of Diffusion?
- states that gas transfer across a membrane is directly proportional to the concentration gradient
Factors that increase diffusion:
- large surface area
- thin membranes
- high solubility of the molecule
Mechanism of gas exchange:
- air moves in and out of the alveolus
- blood low in oxygen, high in carbon dioxide is in the blood in red blood cells
- carbon dioxide diffuses from the blood to alveolus to be exhaled
- oxygen diffuses into the blood from the alveolus
- oxygen is transported around the body by red blood cells
- blood low in carbon dioxide and high in oxygen is transported around the body
Partial pressures of gases:
- oxygen from the alveolae diffuses across the semi-permeable membrane
- to replace lower concentration of oxygen in the deoxygenated blood
- the higher concentration of carbon dioxide will diffuse back
- this is because it is the respiratory function to maintain constancy of partial pressures of oxygen and carbon dioxide in arterial blood
What is an oxygen cascade?
- describes the sequential reduction in partial pressure of oxygen from the atmosphere to the cellular mitochondria
What is a summary of the key steps in oxygen transport?
- uptake in the lungs
- alveolar-arterial gradient
- oxygen carrying capacity of the blood is maximised
- global delivery from lungs to tissue
- regional distribution within the tissue
- diffusion from the capillaries to the cells
- cellular use by mitochondria
Oxygen transport:
- small amount of oxygen is dissolved in the plasma
- most oxygen is attached to haemoglobin
- forms oxyhaemoglobin
What is oxygen attached to in the haemoglobin?
- iron containing haem groups
How many oxygen molecules can each haemoglobin molecule bind to?
- 4
- saturated
Oxyhaemoglobin dissociation curve summary:
- shows the relationship between oxygen saturation of haemoglobin
- and the partial pressures of oxygen in the blood
- determined by haemoglobin affinity for oxygen
Above 100mmhg = fully saturated
Carbon dioxide transport in the body:
- 7% dissolves in the plasma
- 23% carried on the amino acid chains of haemoglobin, forms carbaminohemoglobin (HBCO2)
- 70% dissolves into water to form carbonic acid
- that dissociates into H+ and HCO3- (bicarbonate ions)
The bicarbonate buffer system:
- important in regulating acid-base homeostasis
- involves the balance of carbonic acid, bicarbonate ions and carbon dioxide in order to maintain ph in the blood and tissues
- to support proper metabolic function
What is the equation for the bicarbonate buffer system?
Carbonic anhydrase rapid dissociation
CO2 + H2O ———> H2CO3 —————-> H+ + HCO3-
How is the pH balanced?
- by the presence of both a weak acid (H2CO3)
- and its conjugate base (HCO3-)
- any excess acid or base added into the system is neutralised
What happens if the bicarbonate buffer system fails?
- if it fails to function properly, acid-base imbalance occurs
- acidosis
- alkalosis
What is acidosis?
Ph < 7.35
What is alkalosis?
PH > 7.45
Summary of respiratory acidosis:
- caused by hypoventilation
- decreases the rate at which carbon dioxide is lost by the respiratory process
- increase in partial pressure carbon dioxide and an increase in H+ ions
- too much acid present
- blood ph decreases
Summary of respiratory alkalosis:
- caused by hyperventilation
- increases the rate at which carbon dioxide is lost by the respiratory process
- decrease in partial pressure carbon dioxide, decrease in H+ ions
- too little acid
- increased blood pH
What is the Bohr effect?
- the effect that carbon dioxide has on influencing the release of oxygen to the tissues
What does higher pH do according to the Bohr effect?
- increases the affinity of haemoglobin for oxygen
What does lower pH do according to the Bohr effect?
- triggers haemoglobin to release the oxygen bonded to it
What happens in the lungs based on the Bohr theory?
- low carbon dioxide levels
- the pH is higher
- oxygen is picked up by haemoglobin as it has a higher affinity for it
Where does it then go based on the Bohr theory?
- the tissues
- high carbon dioxide levels
- pH is lower
- oxygen is released by the haemoglobin molecules
What way does exercise shift the oxyhaemoglobin dissociation curve?
- shifts it right
- pH decreases
- as carbon dioxide levels are increasing
- temperature increases
- 2,3-biphosphoglycerate increases
What is 2,3-biphosphoglycerate do?
- present in red blood cells
- binds to haemoglobin that has no oxygen
- to promote the release of the remaining oxygen molecules it has
- near the tissues that need it the most
What causes a shift to the left on the oxyhaemoglobin dissociation curve?
- carbon monoxide poisoning
- very low blood carbon dioxide
- too low
What is arterial blood gas and what does it measure?
- it measures the acidity and the levels of oxygen and carbon dioxide in the blood, coming from an artery
- it is used to check how well your lungs are able to move oxygen into the blood and move carbon dioxide out of the blood
Where is arterial blood obtained from?
- obtained from the radial (wrist) or brachial (artery)
- measures pH = H+ ions of the blood
What should the pH of the blood be?
7.35-7.45
What does partial pressure of carbon dioxide measure?
The pressure of carbon dioxide dissolved in the blood and how well the carbon dioxide can move out of the body
What does partial pressure of oxygen measure?
The pressure of oxygen dissolved in the blood and how well oxygen can move from the airspace of the lungs into the blood
What does bicarbonate do?
- acts as a buffer to keep the pH of the blood becoming too acidic or basic
What does oxygen saturation measure?
- measures the amount of oxygen in the blood, how much haemoglobin in red blood cells is currently bound to oxygen
Normal arterial blood gas values:
PH = 7.35-7.45
Partial pressure carbon dioxide = 34-45mmhg
Partial pressure oxygen = 90-105mmhg
Bicarbonate = 22-26mmol/L
Oxygen saturation = 96-99%
What is the arterial blood gas test used for:
- check for severe breathing problems and lung diseases in:
- asthma
- cystic fibrosis
- copd
- see how well the treatment for lung disease is working
- find out if extra oxygen or mechanical ventilation is needed
- find out if the patient is receiving the right amount of oxygen
Who would you want to measure the acid-base level of the blood, in what kind of people?
- heart failure
- kidney failure
- uncontrolled diabetes
- sleep disorders
- severe infections
- drug overdose
What is the alternative to arterial blood gas test?
- pulse oximiter
- as arterial blood gas in not ideal each time as:
- blood has to be drawn from an artery and analysed each time
- provides non-continuous monitoring which is not ideal when you need to closely monitor an unstable patient
How does the pulse oximiter work?
- probe clips onto patients finger
- light sensors continuously monitor wavelengths of light coming from oxygenated and deoxygenated haemoglobin
- displays a % of haemoglobin that is saturated with oxygen
- 96-99%
Does the pulse oximiter give you the same information as the partial pressure of oxygen from arterial blood gas?
- no
- pulse oximiter measures the haemoglobin oxygen saturation
- arterial blood gas measures the pressure of oxygen gas dissolved in the blood, NOT bound to haemoglobin
Relationship of partial pressure of oxygen and carbon dioxide:
- fall in oxygen, no change in carbon dioxide
- disturbance in v/q matching
- type 1 respiratory failure
- low oxygen, normal / low carbon dioxide
- increase in carbon dioxide, proportionate fall in oxygen
- under ventilation
- type 2 respiratory failure
- low oxygen, high carbon dioxide
What does the alveolar gas equation do?
- calculates the alveolar partial pressure of oxygen
- which allows the alveolar-arterial gradient to be calculated
Respiratory quotient equation:
RQ = carbon dioxide produced / oxygen absorbed
- normally about 0.8
Oxygen absorbed equation:
Oxygen absorbed = partial pressure of carbon dioxide / 0.8
Alveolar gas equation:
Alveolar gas equation = inspired oxygen - oxygen absorbed
= inspired oxygen - (PACO2/0.8)
Alveolar arterial gradient equation:
= alveolar partial pressure of oxygen - alveolar arterial pressure of oxygen
= P(BIG A) - P(small a)
What is the alveolar arterial gradient for young healthy adults?
5-10mmHg
For every decade a person lives, their a-a gradient decreases by….
1mmHg
NORMAL A-A gradient equation:
(Age in years + 10) / 4
Summary of the main functions of the respiratory system:
- ventilate the lungs
- extract oxygen from the air and transfer it to the bloodstream
- excrete carbon dioxide
- maintain the acid base of the blood
- pH and partial pressure carbon dioxide are primary stimuli for breathing to occur
- hypoxia only acts as a stimulant, when partial pressure of oxygen is less than 60mmHg
