Lecture 21: Gas exchange and transport Flashcards Preview

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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?


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%


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?


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?



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


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



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 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?


The percentage of available heme sites saturated with oxygen in arterial blood is the hemoglobin oxygen saturation (SaO2)


What is oxygen content?


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