Blood gas transport

Question 1

What are the main ways in which oxygen is transported throughout the body?
What % of all oxygen in the blood is transported by each mode of transport?

Answer 1

HbO2 (O2 bound to haemoglobin) = 98%

Dissolved O2 = 2%

Question 2

What are the main ways in which carbon dioxide is transported throughout the body?
What % of all carbon dioxide in the blood is transported by each mode of transport?

Answer 2

HCO3- = 70%
HbCO2 (CO2 bound to haemoglobin) = 23%
Dissolved CO2 = 7%

Question 3

Does the partial pressure of a gas in arterial blood refer to the overall content of that gas within the blood?

Answer 3

No it only refers to the amount of that gas dissolved in the plasma not for example, how much is bound to Hb

Question 4

Why is a greater proportion of CO2 transported as dissolved in the blood than oxygen?

Answer 4

Because CO2 is more soluble in aqueous solution than O2

Question 5

Why is haemoglobin critical to oxygen transport?

Answer 5

Oxygen has a very low solubility in blood plasma. This means in order to dissolve the amount of O2 needed to supply tissues, an impossibly high alveolar PO2 would be required.
Presence of haemoglobin overcomes this problem – it enables O2 to be concentrated within blood

Question 6

What are the 3 main ways in which the oxygen content of the blood is measured?

Answer 6

O2 partial pressure (PaO2) - expressed as kPa
Total O2 content (CaO2) - expressed as mL of O2 per L of blood (ml/L)
O2 saturation (SaO2) - expressed as %

Question 7

What is it that the O2 partial pressure is actually measuring?

Answer 7

How much does the pressure of oxygen contribute to the overall pressure within a gas phase

Question 8

What is it that the O2 saturation is actually measuring?

Answer 8

What % of total available haemoglobin binding sites are occupied by oxygen

Question 9

What is it that the O2 saturation is actually measuring?

Answer 9

What % of total available haemoglobin binding sites are occupied by oxygen

Question 10

How is O2 saturation measured?

Answer 10

Measured using pulse oximetry

Question 11

What is the oxygen-haemoglobin dissociation curve?

Answer 11

A curve that illustrates the relationship between O2 concentration, partial pressure of oxygen and the O2 saturation.

Question 12

The oxygen-haemoglobin dissociation curve has a sigmoidal shape. Why is this the case?

Answer 12

As partial pressure of O2 begins to increase you get an accelerated rate of increase in the O2 saturation due to the cooperative binding of O2 to Hb - once first O2 molecule binds to one of the haem groups of the Hb it becomes easier for subsequent O2 molecules to bind to the
others.

Curve begins to plateu because at some point the oxygen saturates the Hb meaning no more oxygen can bind to it.

Question 13

Why is haemoglobin so effective at transporting oxygen?

Answer 13

Structure of Hb produces high O2 affinity, therefore a high level of Hb-O2 binding (and saturation) is achieved at relatively low PO2.

High concentration of heme groups & Hb contained in RBCs enables high oxygen carrying capacity.

Haemoglobin’s affinity for oxygen changes depending on the environment of the blood - e.g. pH of blood; PACO2; temperature and 2,3-diphosphoglycerate (2,3-DPG).

Question 14

Changes in haemoglobin’s affinity for oxygen cause the oxygen-haemoglobin to shift slightly, what is the name of this shift?

Answer 14

Bohr shift/Bohr effect

Question 15

What changes in the variables that affect haemoglobin’s affinity for oxygen cause the oxygen-haemoglobin curve to shift to the left?

Answer 15

Decreased CO2
Increased pH
Decreased 2,3 DPG
Decreased temperature

Question 16

How does the oxygen-haemoglobin curve shifting to the left affect haemoglobin’s affinity for oxygen?

Answer 16

Shift to left means there’s a higher Hb-O2 affinity which means that Hb binds more O2 at a given PO2

Question 17

What changes in the variables that affect haemoglobin’s affinity for oxygen cause the oxygen-haemoglobin curve to shift to the right?

Answer 17

Increased CO2
Decreased pH
Increased 2,3 DPG
Increased temperature

Question 18

How does the oxygen-haemoglobin curve shifting to the left affect haemoglobin’s affinity for oxygen?

Answer 18

Shift to right means there’s a lower Hb-O2 affinity which means that Hb binds less O2 at a given PO2

Question 19

Why is the Bohr effect important for oxygen delivery to tissues?

Answer 19

It allows oxygen delivery to be coupled to oxygen demand - allows tissues that need most oxygen to be given the most oxygen

Question 20

Explain, using examples, how the bohr shift and the environment of the blood within certain parts of the body help to ensure that the tissues with highest demand for oxygen get the highest supply

Answer 20

Lungs: Blood here has a high PO2, low PCO2 and a high pH. Higher PO2 naturally results in a high O2 saturation but the high pH and low PCO2 increase Hb’s affinity for oxygen which shifts O2-Hb dissociation curve to the left which further increases O2 saturation - all this means oxygen moves from the lungs to the Hb.

Resting tissue: Blood here has lower PO2 than in lungs so O2 saturation lower in resting tissue than lungs. Also, conditions mean O2-Hb dissociation curve shifts back to normal which further decreases O2 saturation - all this means oxygen moves from Hb to the resting tissue.

Working tissue: Due to anaerobic respiration/hypoxia these tissues produce lots of lactic acid, causes blood pH to decrease, they also produce CO2 and 2,3-DPG. This causes the O2-Hb curve to shift to the right which decreases O2 saturation. Also PO2 in these tissues is lower than in resting tissues so O2 saturation decreases even further - all this means even more oxygen moves from Hb to working tissue than in resting tissue.

Question 21

What is Myoglobin?

Answer 21

An iron containing oxygen binding protein found in muscle tissues.

Question 22

How differently would the shape of a oxygen-myoglobin dissociation curve be compared to an oxygen-haemoglobin dissociation curve?
Why is this the case?

Answer 22

The oxygen-myoglobin curve would be shifted more to the left and would be less sigmoidal and more linear.
The reason for this is that myoglobin has a much higher affinity for oxygen than haemoglobin does.
Also, myoglobin only releases O2 when PO2 is extremely low so myoglobin is saturated at much lower PO2 than haemoglobin.

Question 23

What is the difference in oxygen affinity between foetal and adult haemoglobin?

Answer 23

Foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin.

Question 24

What does the difference in oxygen affinity between foetal Hb and adult Hb mean when they mix within the placenta?

Answer 24

Foetal Hb’s higher oxygen affinity means that when adult and foetal Hb come into contact oxygen from the adult Hb will be transferred to the foetal Hb.

Question 25

What colour does oxyhaemoglobin appear?

Answer 25

Appears red

Question 26

What colour does deoxyhaemoglobin appear?

Answer 26

Appears blue

Question 27

What is it that determines the colour of blood?

Answer 27

Relative concentrations of oxyhaemoglobin and deoxyhaemoglobin present

Question 28

What is Cyanosis?

Answer 28

Purple discoloration of the skin and tissue that occurs when the deoxyhaemoglobin becomes excessive.

Question 29

What are the 2 types of cyanosis?

Answer 29

Core cyanosis: discolouration of core, mucous membranes and extremities

Peripheral cyanosis: discolouration of the extremities, e.g. fingers.

Question 30

Why is cyanosis often less obvious in patients with lo)w red blood cell density?

Answer 30

Patients with low RBC density will have lower amounts of both oxyhaemoglobin and deoxyhaemoglobin. Cyanosis occurs due to presence of large amounts of deoxyhaemoglobin so having lower amounts of it means it’s less likely to build up to levels seen in cyanosis.

Question 31

How can hypoxia still occur if there’s adequate ventilation and perfusion of the tissues?

Answer 31

Blood is not able to carry sufficient oxygen to meet tissue demands due to the blood lacking RBCs or haemoglobin (anaemia).

Question 32

What are some causes of anaemia?

Answer 32

Iron deficiency - decreases Hb deficiency

Haemorrhage - causes major RBC loss

Question 33

Explain how transport of CO2 differs from transport of O2?

For each way the trasnport differs explain why this is the case

Answer 33

Greater % of CO2 is transported simply dissolved in plasma - this is because CO2 has a higher H2O solubility than O2 does

Less % of CO2 is transported bound to haemoglobin - this is because CO2 binds to Hb at different sites than O2 and binds with decreased affinity.

CO2 reacts with water to form carbonic acid, which accounts for the majority of CO2 transported.

Question 34

Why does venous blood carry more CO2 compared to arterial blood?

Answer 34

Deoxyhaemoglobin has a higher affinity for CO2 and H+ than oxyhaemoglobin does and because venous blood has a higher concentration of deoxyhaemoglobin (less oxygenated) compared with arterial blood it means it is able to carry more CO2.

Question 35

Briefly describe the processes that result in CO2 from tissues being turned into the various forms in which it is transported within the blood?)

Answer 35

CO2 from tissues dissolves in plasma and becomes PaCO2.
Some stays as PaCO2 but some CO2 diffuses/dissolves in RBC’s
Some CO2 within RBC’s binds to Hb while some of it gets converted to carbonic acid (H2CO3)
Some of this H2CO3 gets oxidised to form H+ and HCO3-
Finally, some of the H+ produced also binds to Hb

Question 36

What is the Haldane effect?

Answer 36

Property of haemoglobin that allows it to carry more CO2 in its deoxygenated state compared to its oxygenated state

Question 37

Explain the Haldane effect?

Answer 37

When blood becomes oxygenated, the O2 binding to Hb causes a change in structure of Hb.
This causes Hb’s affinity for CO2 and H+ to decrease leading to some CO2 and H+ unbinding from Hb.
The CO2 previously bound to Hb will go back to the RBCs and then will go back to the plasma.
The H+ previously bound to Hb reforms H2CO3 which reforms CO2 in the RBC. This CO2 will also end up in the plasma.
All this means oxygenated blood is able to carry less CO2 because oxygenation causes less CO2 to be transported in all its other forms within the blood. Also, Plasma isn’t able to take up extra CO2 formed from oxygenation because it has a certain amount of CO2 it can transport.

Question 38

Why is the Haldane effect a good thing at the lung?

Answer 38

Because at the lung, Haldane effect means that oxygenation of blood enables greater CO2 release into alveoli

Question 39

Why is the Haldane effect a bad thing for respiring tissues?

Answer 39

Haldane effect means that plasma in oxygenated blood carries less CO2.
This results in less CO2 produced by the tissues being able to dissolve in the plasma which can lead to CO2 build up in the tissues which could lead to acidosis.

Question 40

Why is rapid oxygen therapy in hypercapnic patients with COPD dangerous?

Answer 40

Hypercapnic COPD patient is also hypoxaemic - this means that their blood is able to carry more CO2 due to low levels of O2.
Sudden oxygenation of their blood means CO2 is displaced from the blood, and the blood is able to transport less CO2 bound to Hb due to the Haldane effect.
This leads to sudden very high levels of CO2 within the body which can potentially leads to dangerous acidaemia (very low blood pH).
Patient isn’t able to get rid of CO2 from their bodies by increasing ventilation as lungs don’t function properly due to COPD.