Blood Gas Transport

Question 1

Where do gases dissolve into first before transportation in blood

Answer 1

Gases carried in the blood first dissolve in plasma before mostly being transported in other forms.

Question 2

Outline what happens to oxygen brief

Answer 2

Oxygen for example diffuses into the plasma and then binds to haemoglobin where it is then
pumped to tissues. At tissue, it moves back into the plasma and then diffuses from the plasma into
tissues. Around 98% of oxygen is bound to haemoglobin at any 1 time with only around 2% of it
dissolved in plasma.

Question 3

How is carbon dioxide transported in the blood

Answer 3

In the case of carbon dioxide, it can be transported as carbonic acid (HCO3
-) or
can bind to haemoglobin. It is produced by tissues and dissolved into plasma before it moves on to
form one of its two forms.

Question 4

When does co2 transportation change

Answer 4

The process is reversed at the lungs where it then diffuses out into the 
alveoli during gas exchange. Around 70% of carbon dioxide is transported as carbonic acid, 23% binds
to haemoglobin (at a different site to oxygen) and 7% is dissolved in plasma.

Question 5

Why is there more co2 in plasma than oxygen

Answer 5

There is more carbon
dioxide in plasma because it has a higher water solubility than oxygen.
Haemoglobin is very critical to oxygen transport. This is because of oxygen’s low solubility in plasma
(0.223ml/L/kPa). It would be impossible to reach the high alveolar partial pressure required to
oxygenate blood.

The oxygen binding protein haemoglobin greatly increases the oxygen carrying
capacity of blood that is especially important at gas exchanging surfaces and respiring tissue.

Question 6

How can the oxygen concentration in the blood be defined

Answer 6

The
vast majority of oxygen in blood is carried by haemoglobin (over 98%). The oxygen content of the
blood can be defined by a number of different measurements.
1. Oxygen partial pressure (PaO2) is a measure of the amount of oxygen dissolved in plasma in response to a certain partial pressure of oxygen in the gas phase.
2. A second measure of oxygen content of the blood is to calculate the total oxygen content (CaO2) that is expressed as mL of oxygen per litre of blood (ml/L). This measure of oxygen content includes both the oxygen carried in the plasma and the oxygen bound to haemoglobin.
3. The final measure of oxygen content in blood is through oxygen saturation (SaO2 in
arterial blood and SpO2 when using pulse oximetry). This calculates the available haemoglobin
binding sites that are occupied by oxygen as a percentage of the total available haemoglobin binding
sites.

Question 7

How can the relationship between all three ways if measuring oxygen concentration be measured

Answer 7

The relationship between these three measurements can be shown using an oxygen-
haemoglobin dissociation curve (ODC).

Question 8

What does an oxygen dissociation curve describe

Answer 8

This describes the affinity between oxygen and haemoglobin
at different oxygen partial pressures (see right). The curve has a sigmoid shape due to the
relationship of oxygen with haemoglobin.

Question 9

What is shown at the beginning of an oxygen dissociation curve

Answer 9

At the beginning of the curve, there is an acceleration due
to the cooperative binding of oxygen to haemoglobin. This is where once the first oxygen binds to
haemoglobin, subsequent oxygens will find it easier to bind to haemoglobin (hence the acceleration).

Question 10

Describe what happens when the oxygen dissociation curve plateaus

Answer 10

The saturation of oxygen binding sites is what then causes the deceleration and plateau of the curve.
There are a number of reasons why haemoglobin is so effective at transporting oxygen in the body.
The first reason is that the structure of haemoglobin has a high affinity to oxygen at its oxygen
binding sites. This results in a high level of Hb-O2 binding at a relatively low oxygen partial pressure
(i.e. get below 90% saturation for example, a very low partial pressure of around 7 kPa is required to
achieve this). There are also a very large number of haemoglobin molecules in RBCs that gives blood
a very high carrying capacity. There are 4 haem groups on haemoglobin and around 270 million
haemoglobin molecules per RBC and 5 billion RBCs/ml of blood.

Question 11

What is a key feature of heamoghlobin that makes it a good transporter of oxygen

Answer 11

Haemoglobin is also extremely good
at its role of transporting oxygen because haemoglobin’s affinity for oxygen changes based on its
surrounding environment. This enables it to offload oxygen at tissues that demand it.

Question 12

What are the conditions that affect the affinity of oxygen

Answer 12

The particular
conditions that affect the affinity of oxygen to haemoglobin include temperature, pH, 2,3-DPG levels
(produced by anaerobic respiration) and carbon dioxide concentration. Lower affinity for oxygen is
stimulated by higher carbon dioxide levels (produced by respiring tissues), lower pH levels (as a
result of lactic acid produced from anaerobic respiration and carbonic acid produced from carbon
dioxide), higher 2,3-DPG levels (produced by anaerobic respiration) and higher temperature
(produced by working tissues).

Question 13

What happens to the graph when a lower affinity of oxygen is present

Answer 13

The lower affinity for oxygen produces a right shift on the oxygen-
haemoglobin dissociation curve where oxygen is now released at higher oxygen partial pressures.

Question 14

What are the conditions for a left shift if the oxygen dissociation curve

Answer 14

A
leftward shift in the curve indicates a higher Hb-O2 affinity. This is stimulated by lower CO2 levels,
higher pH, lower numbers of 2,3-DPG and lower temperature.

Question 15

What effect is the shift if the curve

Answer 15

The overall shifts in the curve are
known as the Bohr effect. This effect is crucial to supplying working tissues that require the most
amount of oxygen with the most amount of oxygen.

Question 16

Explain how the lungs have a high oxygen saturation

Answer 16

In the lungs for example, there are high levels of
oxygen partial pressure, low levels of carbon dioxide partial pressure and high pH so oxygen
saturation is high.

Question 17

How is oxygen able to move into resting tissue

Answer 17

In resting tissues, there are lower levels of oxygen partial pressure (compared to
the lungs) and this causes oxygen saturation to decrease. Some oxygen therefore moves from
haemoglobin to tissue.

Question 18

How is oxygen able to move into working tissue

Answer 18

In working tissues, oxygen partial pressures are very low and anaerobic
respiration produces lactic acid (increases acidity), carbon dioxide and 2,3-DPG. Oxygen saturation
will now decrease further that causes a large amount of oxygen to move from haemoglobin to
tissues.

Question 19

Compare myoglobin to heamoghlobin

Answer 19

Myoglobin another type of oxygen binding protein has a much
higher affinity to oxygen than haemoglobin does. It acts as a
reservoir of oxygen in respiring muscles that is only released at very
low oxygen partial pressures (see right).

Question 20

How does myoglobin effect muscle contraction

Answer 20

Myoglobin can ensure

muscle contraction occurs for a slightly longer period of time.

Question 21

Compare foetal heamoghlobin to heamoghlobin

Answer 21

Foetalhaemoglobin also has a higher oxygen affinity than adult
haemoglobin. This allows it to effectively steel oxygen from maternal
haemoglobin when they both come into contact in the placenta.

Question 22

Compare the two main colours of heamoghlobin ( clinical )

Answer 22

There are some clinical aspects of haemoglobin and oxygen transport. The colouring of the blood can
determine its oxygenation to a certain degree. Haemoglobin has a different pigmentation based on
whether it is oxygenated or not. Oxyhaemoglobin has a bright red appearance whereas
deoxyhaemoglobin appears blue. The proportion of each type of haemoglobin will affect the
appearance of blood. This is why arterial blood is bright red whilst venous
blood is a mixture of red and blue that creates a crimson/purple colour.

Question 23

What can happen to patients that have Hugh amounts of deoxygenated blood

Answer 23

Patients that have high amounts of deoxygenated blood can develop
cyanosis. This is purple discolouration of the skin and tissue that occurs
when deoxyhaemoglobin is excessive (see right peripheral cyanosis).
Cyanosis can in fact be central of peripheral.

Question 24

What is central cyanosis

Answer 24

Central cyanosis is the bluish
discolouration of core mucous membranes and extremities. It is caused by inadequate oxygenation
of blood due to hypoventilation, V/Q mismatch and other causes.

Question 25

What is peripheral cyanosis

Answer 25

Peripheral cyanosis is the bluish colouration confined to the extremities (e.g. fingers) that is caused by inadequate oxygen supply to the extremities. The cause of this type of cyanosis is small vessel circulation issues.

Question 26

Why can cyanosis be less obvious in some patients

Answer 26

Cyanosis is often less obvious in patients with less RBC density due to the concentration of haemoglobin overall that causes the colour change in the first place.

Question 27

What can insufficient heamoghlobin conc cause

Answer 27

Insufficient haemoglobin can cause tissue hypoxia despite adequate ventilation and perfusion. This is because the blood is simply not able to carry enough oxygen to meet tissue demands.

Question 28

What is the main characteristic of anaemia + symptoms

Answer 28

A lack of haemoglobin is characteristic of a patient suffering from anaemia. Anaemia has many causes including iron deficiency that is required as a prosthetic group of each haem subunit, and haemorrhage where blood is simply lost and therefore RBCs are lost. Anaemia has a number of symptoms including pale skin and fatigue.

Question 29

Describe the transports of carbon dioxide compare with oxygen

Answer 29

Carbon dioxide transport in the blood differs from oxygen transport. Carbon dioxide has a higher water solubility than oxygen which means a greater percentage of CO2 is transported dissolved in plasma (7% of CO2 compared to 2% of O2). Carbon dioxide also binds to haemoglobin at different binding sites to oxygen (R-NH2 residues at the end of peptide chains forming carbamino-Hb) and at a decreased affinity. This means a lower percentage of carbon dioxide is transported in this manner compared to oxygen (23%). Carbon dioxide also reacts with water to form carbonic acid (see reaction right). This in fact accounts for the majority of carbon dioxide transported in blood (around 70%).

Question 30

What is the haldane effect - explain

Answer 30

Venous blood carries more carbon dioxide than arterial blood in what is known as the Haldane effect (at equivalent partial pressures). The fundamental reason behind this is that deoxyhaemoglobin has a higher affinity for carbon dioxide and hydrogen ions (biproduct of carbonic acid) than oxyhaemoglobin does. This is because when the oxygen binds to haemoglobin, it changes the affinity of haemoglobin to carbon dioxide and hydrogen ions. This means that the more haemoglobin is 6 saturated with oxygen, the more carbon dioxide is removed from haemoglobin and dissolved in RBCs and the more of this dissolved carbon dioxide moves into the plasma. This makes more carbon dioxide available for removal in the lungs as only a limited amount of carbon dioxide can be stored in RBCs or plasma.

Question 31

What happens when there is excess co2 in plasma at tissue

Answer 31

Excess carbon dioxide in plasma at tissues however (if the carbon dioxide cannot be removed by ventilation) accumulates resulting in acidosis.

Question 32

How is more co2 able to dissolve into plasma

Answer 32

Carbon dioxide is produced by respiring cells and dissolves in the plasma. The dissolved carbon dioxide enters RBCs where it is converted to carbonic acid in a reaction facilitated by carbonic anhydrase. The production of carbonic acid enables further carbon dioxide to dissolve into RBCs. The hydrogen ions produced by this reaction cannot leave the RBC as the RBC membrane is impermeable to hydrogen ions.

Question 33

What prevents the accumulation of h plus ions

Answer 33

The accumulation of these hydrogen ions is prevented by deoxyhaemoglobin binding to them and acting as a buffer. The movement of oxygen from haemoglobin at tissues increases the number of deoxyhaemoglobins and therefore increases the RBC carbon dioxide carrying capacity.

Question 34

What happens when there is an increased hco3 conc

Answer 34

The increased HCO3 - ions creates a diffusion gradient for these h ions to leave RBCs and be exchanged with chloride ions to maintain electrical neutrality. The carbon dioxide diffused in blood is carried to the lungs where the low alveolar partial pressure creates a diffusion gradient pushing carbon dioxide out of the blood into the lungs. The increased oxygenation of haemoglobin frees up a number of hydrogen ions that induces the increased formation of carbonic acid. This would eventually lead to increased CO2 formation that contributes to plasma saturation by CO2. The reduced carbonic acid reduces the number of carbonate ions that results in carbonate ions external of the cell entering back into RBCs in exchange for chloride ions. The most important part of carbon dioxide blood transport is that deoxygenated blood carries more carbon dioxide than oxygenated blood an oxygenation of blood causes CO2 to leave.