Lecture 21: Gas exchange and transport Flashcards
What is the ultimate purpose of breathing?
Continual supply of oxygen (O2)
Continual removal of carbon dioxide (CO2)
What determines the direction in which gas moves?
Partial pressure gradients
Alveolar capillary interface
See figure
Does all the oxygen from the alveoli enter the blood?
No
Only the amount of oxygen needed to oxygenate the blood will be transferred
This is why you can still give someone CPR with your “used” air
Composition of atmospheric air
Nitrogen ~79%
Oxygen~21%
Other (CO2, H2O, vapour, pollutants)
What is total atmospheric pressure?
Pressure exerted by all component gases
What is the pressure exerted by a gas proportional to?
% of the gas in the total mixture
What does gas exchange involve in pulmonary capillaries and tissue capillaries?
Simple diffusion of O2 and CO2 down the partial pressure gradients of these gases
Formula for partial pressure of a gas
Partial pressure of a gas = total pressure x fractional composition of the gas in the mixture
See figure
Contribution of water vapour to partial pressure
Air is humidified by the upper respiratory track
Water vapor is a component of partial pressure of gases (approximately
47 mm Hg at 37oC)
Decreases the available space in alveolar gas equation for PO2
What is the alveolar gas equation? Normal PAO2? A/a gradient? PaO2?
See figure
What is the alveolar gas equation if you breathe in 100% oxygen?
See figure
Altitude, barometric pressure and PiO2
Increasing altitude causes decrease in atmospheric pressure and decreased in inspired O2
See figure and table
Defence of alveolar PO2 by PCO2
Inspired O2 decreases as elevation increases
Hyperventilation kicks in to decrease alveolar CO2
This protects the levels of alveolar PO2
See figure
Alveolar gas equation after altitude adaptation
See figure
What determines gas exchange
Partial pressure gradient of gases: steeper gradient, more rapid gas transfer
Gas exchange surface area: exchange increases with surface area, decreases in disease
Thickness of the alveolar capillary interface: worsens in disease – water, pus, fibrosis
Diffusion coefficient of gas: CO2 is 20x more diffusible than O2
Process of gas exchange
Equilibration of gases at the alveolar capillary interface (but gases will not be equal on both sides)
Capillary blood has a high partial pressure of O2 (~ 100) compared to tissue cells (~40). Tissue cells are extracting O2.
Partial pressure for CO2 in capillaries is low (~40) compared to tissue cells (~ 46), which generate CO2 through their metabolism.
O2 diffuses from capillaries into tissue down its partial pressure gradient (100 to 40, higher to lower). CO2 diffuses in the opposite direction.
Equilibration with tissue cells: dumping off O2, picking up CO2
Blood leaving systemic capillaries is low in O2 and high in CO2
Blue blood returns to the lungs, to acquire O2 and release CO2 at the pulmonary alveolar / capillary interface
Tissue oxygen cascade
Amount of oxygen decreases as you move through the body
At higher altitude, you begin with less oxygen, so you deliver less oxygen
See figure
Diffusion of O2 and CO2 in alveoli and tissues
See figure
Diffusion gradient for CO2 is less steep than O2, but CO2 is much more diffusible
How is O2 transported in the blood?
O2 bound to hemoglobin
What is Hb?
Iron containing protein within red blood cells
Deoxyhemoglobin = no oxygen
Oxyhemoglobin = oxygen bound
See figure
Chemistry of different colours of blood
See figure
Red - hemoglobin, iron
Blue - hemocyanin, copper instead of iron
Green - chlorocruorin, similar to Hb
Purple - Haemorythrin
What form of Hb is favoured in the lungs?
Oxyhemoglobin
Hb combines with O2 as O2 diffuses form the alveoli into the pulmonary capillaries (oxygen is not very soluble in solution)
What happens to Hb at the tissue cells?
The dissociation of oxyhemoglobin into hemoglobin and free molecules of oxygen occurs
Reaction favored in this direction as oxygen leaves the systemic capillaries and enters tissue cells.
Location of O2 and CO2 in the circulatory system
O2: Dissolved 1.5%, Bound to Hb 98.5%
CO2: Dissolved 10%, Bound to Hb 30%, Bicarbonate 60 %
Most of CO2 is dissolved (as CO2 or HCO3-, acts as buffer)
Where is % Hb saturation high?
Where partial pressure of O2 is high (lungs)
Where is % Hb saturation low?
Where partial pressure of O2 is low (tissue)
O2 tends to dissociate from Hb at these sites
Oxygen-hemoglobin dissociation curve
If more oxygen present, more oxygen will be bound to Hb
S shape due to Hb’s properties (O2 binding strength changes with saturation)
Plateau: Partial pressure of oxygen is high (lungs). Ensures that Hb will be almost fully saturated even with a substantial drop in pO2.
Steep portion: O2 tension in systemic capillaries, where hemoglobin unloads oxygen to the tissue cells. Ensures that O2 is delivered where it is needed.
See figure
What causes a leftward shift in the Hb dissociation curve?
Lower PaCO2
Lower temperature
Higher pH
Picks up oxygen easier
See figure
What causes a rightward shift in the Hb dissociation curve?
Higher PaCO2 (picking up CO2 from the tissue cells decreases affinity of Hb for O2, promotes oxygen dissociation)
Higher temperature (exercise)
Lower pH
Drops off oxygen easier
How many molecules of oxygen can each Hb take up?
4
How much oxygen does Hb take up at alveolar capillary interface?
Hemoglobin takes up oxygen continuously until hemoglobin is as saturated as possible (saturation is 97.5% at 100 mm of Hg).
Extra oxygen sits in alveoli
What is Hb’s maximum saturation?
Hemoglobin can’t get more than 100% saturated
Only way to get more oxygen is to increase Hb (doping)
How is unloading of oxygen from Hb promoted?
By lower oxygen tension
What occurs with Hb at the tissue cells?
At the tissue cells hemoglobin rapidly dumps oxygen into the
blood plasma; from there, oxygen diffuses into the tissue cells.
What is the Bohr effect?
This shift of the hemoglobin curve to the right (more oxygen
dissociation)
Hb affinity for CO
Hemoglobin has greater affinity for carbon monoxide compared to oxygen
Trying to get oxygen to person who has CO poisoning is hard
Check your CO detectors!
CO2 transport as HCO3-
Most CO2 (about 60%) is transported as the bicarbonate ion
CO2 + water = carbonic acid
Carbonic acid dissociates into hydrogen ions and bicarbonate ion
Carbonic anhydrase facilitates this in red blood cells
The reverse of this process occurs in the lungs (bicarbonate ion combines with hydrogen to form free molecules of CO2)
Partial pressure of gas in a liquid - O2 and CO2
Gas dissolved in liquid also has ‘partial pressure’
Depends on how soluble the gas is in liquid
The amount of O2 and CO2 dissolved in pulmonary capillary blood is directly proportional to the alveolar PO2 and PCO2
Generally much smaller than the amount of oxygen transported by binding to hemoglobin
When does decompression sickness occur?
Most common in divers where the high pressure environment forces inert gases (Nitrogen) to dissolve in their blood from the lungs.
Resurfacing too quickly, moving from high to low pressure environment causes the gas to leave solution and form bubbles in the circulation.
Symptoms of decompression sickness, treatment
Joint pain, neurological symptoms, visual disturbances, swelling, patchy discoloration on skin.
Remedied by ensuring a controlled ascent where the dissolved nitrogen can escape the body through the lungs.
What is Hypoxia?
Abnormality in arterial PO2
Condition of having insufficient O2 at the cell level
Types of hypoxia
Hypoxic: Insufficient oxygen supply or uptake, Inadequate hemoglobin saturation
Anemic: Reduced O2 carrying capacity of blood
Circulatory: Blood (and thus O2) not reaching tissues
Histotoxic: Cells cannot use delivered O2
What is hyperoxia?
Abnormality in arterial PO2
Condition of having an above-normal arterial PO2
Can only occur when breathing supplemental O2
Can also be dangerous
What is Hypercapnia?
Abnormality in arterial PCO2
Condition of having excess CO2 in arterial blood
Caused by hypoventilation
What is hypocapnia?
Abnormality in arterial PCO2
Below-normal arterial PCO2 levels
Brought about by hyperventilation: Anxiety states, Fever, Aspirin poisoning
What is apnea?
Respiratory state
Stop breathing (temporary)
What is dyspnea?
Respiratory state
Difficult breathing
What is eupnea?l
Respiratory state
Normal
What is hyperpnea?
Respiratory state
“Big” breaths (e.g. exercise)
What is asphyxia?
Respiratory state
No O2 in tissues – Carbon Monoxide
What is suffocation?
Respiratory state
O2 deprivation – Airflow cutoff
What is cyanosis?
Respiratory state
Color due to asphyxia/lack of O2
What is respiratory arrest?
Respiratory state
Stop breathing - permanent
What is Hypercapnea?
Respiratory state
Too much CO2
What is hyperventilation?
Respiratory state
Too little CO2 – due to increased alveolar ventilation
What is Hypocapnea?
Respiratory state
Low CO2
What is hypoventilation?
Respiratory state
Too much CO2 – due to under-ventilation
What is oxygen pressure?
PaO2
PaO2 is the partial pressure of oxygen in arterial blood
The number of O2 molecules dissolved in plasma determines how
many molecules will bind to hemoglobin
What is oxygen saturation?
SaO2
The percentage of available heme sites saturated with oxygen in arterial blood is the hemoglobin oxygen saturation (SaO2)
What is oxygen content?
CaO2
Neither PaO2 nor SaO2 tell you how many molecules of oxygen are in the blood.
How much O2 per unit volume of blood is the oxygen content (ml O2 per dl Hgb), which incorporates the blood hemoglobin content.
What are indicators of severity of V/Q mismatch?
Alveolar-arterial difference
Oxygenation index
What is alveolar-arterial difference?
Indicates severity of alveolar diffusion gradient
AaDO2 = PAO2 - PaO2
What is oxygenation index?
Indicates how difficult it is to maintain an aerated lung for adequate ventilation
ie. how much pressure and oxygen is required to achieve the measured arterial oxygen partial pressure
OI = FiO2 x MAP/ PaO2
MAP - mean airway pressure
Normal arterial blood gas
Oxygenation indicators:
PO2: 90-100
Ventilation indicators:
pH: 7.35-7.45
pCO2: 35-45
HCO3 (bicarbonate): 22-26
What happens in hypoxic respiratory failure?
PO2 drops
What happens in hypercapnic respiratory failure?
PCO2 rises
What happens in metabolic acidosis?
HCO3 drops
Case studies
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