Blood gas transport

Question 1

How does oxygen get from the blood into cells?

Answer 1

• O2 transported in blood, predominantly bound to haemoglobin.
• O2 diffuses into cells/tissues for use in aerobic respiration.

Question 2

Describe how gases are carried in the blood first dissolves in the plasma before mostly being transported in other forms

Answer 2

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Question 3

Why is haemoglobin critical to O2 transport?

Answer 3

• Oxygen has low solubility in plasma (0.225mL/L/kPa). In order to dissolve the amount of O2 needed to supply tissues, an impossibly high alveolar PO2 would be required.
• The presence of haemoglobin overcomes this problem – it enables O2 to be concentrated within blood (↑ carrying capacity) at gas exchange surfaces and then released at respiring tissues.
• The vast majority of O2 transported by the blood is bound to haemoglobin (>98%).

Question 4

Describe the adaptation of haemoglobin and what effect this has on the oxygen-haemoglobin dissociation curve

Answer 4

Adaptations within the structure of the haemoglobin molecule mean that oxygen affinity is itself affected by the number of oxygen molecules bound (cooperative binding). This means that the oxygen-haemoglobin dissociation curve (a graph showing the effect of PaO2 on Hb-oxygen saturation) shows a non-linear (sigmoidal) relationship, where the gradient of the line initially accelerates, before reaching a plateau, which reflects the saturation and decreased availability of free O2 binding sites on the haemoglobin molecule:

The effect of this relationship and the high overall oxygen affinity of haemoglobin is that a relatively low PO2 is required for high saturation of Hb binding sites. Also, O2 release from Hb increases as PO2 falls (i.e. Hb will release oxygen if the surrounding oxygen pressure decreases, such as in respiring tissues). The means that in normal circumstances, Hb readily take large quantities of oxygen at respiratory surfaces and give it up to respiring tissues.

Question 5

Why is haemoglobin so effective at transporting O2 within the body?

Answer 5

1. The structure of Hb produces high O2 affinity, therefore a high level of Hb-O2 binding (and saturation) is achieved at relatively low PO2.
2. The concentration of heme groups & Hb contained in RBCs enables high carrying capacity. O2 carrying capacity = 3 ml/L (plasma) + 197 ml/L (Hb) = 200 ml/L (total)
3. The oxygen-haemoglobin binding curve shifts to offload oxygen to demanding tissues
   a. Leftward shift = higher Hb-O2 affinity = Hb binds more O2 at a given PO2
   b. Rightward shift = lower Hb-O2 affinity = Hb binds less O2 at a given PO2
4. Hb O2 affinity changes depending on the local environment, enabling O2 delivery to be coupled to demand

Question 6

What is hypoxia?

Answer 6

Hypoxia (deficient supply of oxygen to tissues) can occur despite adequate ventilation and perfusion of the lung if the blood is not able to carry sufficient oxygen to meet tissue demands.

Question 7

What is anaemia?

Answer 7

This is a decrease in the number of red blood cells per unit volume of blood

Question 8

What causes anaemia?

Answer 8

• Decrease in RBC production

* Rapid excess loss of blood cells

Question 9

What are the effects of anaemia?

Answer 9

A decrease in red blood cell density will result in a reduction in the concentration of haemoglobin, total oxygen binding sites, and oxygen-carrying capacity. However, the affinity of haemoglobin is unchanged. Therefore Hb-O2 saturation and O2 partial pressure within the plasma will be normal, whereas overall total O2 content of the blood will decrease, as will the overall concentration of both oxyhaemoglobin and deoxyhaemoglobin, as reflected in the symptoms of pale skin and conjunctiva). Anaemia has the following effect on the Hb-O2 dissociation curve compared to normal:

Question 10

What is carbon monoxide poisoning caused by?

Answer 10

Caused by exposure to excessive levels of CO, a gas produced by incomplete combustion of fossil fuels when oxygen levels are deficient.

Question 11

What are the effects of carbon monoxide poisoning?

Answer 11

• It involves a decrease in total oxygen carrying capacity due to a decreased number of Hb binding sites being available.
• In CO poisoning the overall concentration of Hb in the blood remains constant. CO displaces O2 at Hb binding sites, as it binds with much greater affinity.
• As binding sites are occupied by CO, less O2 can bind and so less is transported. Therefore the total O2 content of the blood will decrease, as will the concentration of oxyhaemoglobin.
• Oxygen-Hb saturation readings may decrease depending on how they are measured: arterial blood gas measurements (SaO2) will fall as they compare the concentration of oxyhaemoglobin to total haemoglobin. However pulse oximetry readings (SpO2) may remain normal as the technique cannot reliably differentiate between O2-Hb and CO-Hb.

Question 12

What causes the bohr curve to be shifted to the left during CO poisoning?

Answer 12

Finally, a slight increase in O2-Hb affinity is observed in CO poisoning, as CO inhibits the production of 2,3-DPG, shifting the curve to the left

Question 13

Describe the clinical aspects of anaemia and carbon monoxide poisoning (Total arterial {O2}, PaO2, Hb SaO2 (% saturation), {HB} and {Hb-O2})

Answer 13

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Question 14

What is Cyanosis and what causes it?

Answer 14

Cyanosis is a blue-purplish discoloration of the skin and tissues that occurs when the concentration of deoxyhaemoglobin present within the blood becomes excessive. This is because oxyhaemoglobin appears red, whereas deoxyhaemoglobin appears blue. The relative concentration of these two proteins determines the colour of blood and innervated tissues.

Question 15

What is central cyanosis?

Answer 15

Central cyanosis (discoloration of the core, mucous membranes and extremities) reflects inadequate oxygenation of blood within the lungs (e.g. due to hypoventilation, gas exchange defects or V/Q mismatch).

Question 16

What is peripheral cyanosis?

Answer 16

Peripheral cyanosis (discoloration confined to the extremities) reflects inadequate oxygen supply to only these tissue (e.g. due to small vessel circulation problems)

Question 17

Describe the role of erythropoietin in RBC production and oxygen transport

Answer 17

• A hormone which induces production of red blood cells within the bone marrow, secreted from the kidney.
• A method of compensating for the presence of chronic hypoxia is to increase haemoglobin concentration; although Hb saturation will decrease due to reduced PaO2, this is somewhat compensated for by a greater overall number of haemoglobin.
• Cardiac output will increase via increased heart rate in response to hypoxia to increase overall oxygen transport
• Increased EPO secretion occurs in chronic hypoxic respiratory disease, as well as individuals exposed to high altitude (again, due to the chronic hypoxia involved). The resulting increase in the number of RBCs per unit of plasma is term polycythaemia.

Question 18

How and why does transport of CO2 differ to O2?

Answer 18

1. CO2 has a higher H2O solubility than O2 does – therefore a greater % of CO2 is transported simply dissolved in plasma (CO2 ≈ 7%, O2 ≈ 1%)
2. Concentration=Partial pressure ×Solubility

CO2 binds to Hb at different sites than O2 (R–NH2 residues at the end of peptide chains, forming carbamino-Hb, R-NHCOOH) and with decreased affinity. Thus, a lower % of CO2 is transported in this manner (≈ 23%).

3. CO2 reacts with water to form carbonic acid, which accounts for the majority (≈70%) of CO2 transported.
   CO2 + H2O  H2CO3  H+ + HCO3-

Question 19

Why does venous blood carry more CO2 than arterial blood?

Answer 19

Deoxy-Hb has a higher affinity for CO2 and H+ than oxy-Hb does.

Therefore: an increase in Oxy-Hb means a decrease in CO2 carried

Question 20

Describe how carbon dioxide is transported from respiring tissues to the lungs

Answer 20

At tissues:

1. CO2 is produced by respiring cells and dissolves in the plasma + enters RBCs.
2. Conversion of CO2 + H2O to H2CO3 within RBCs (catalysed by carbonic anhydrase)
3. The effective removal of CO2 by (2) enables further CO2 to diffuse into the RBC (& more can then enter the plasma).
4. H2CO3 ionises to HCO3- + H+. The RBC cell membrane is impermeable to H+, therefore H+ cannot leave
5. Accumulation of H+ within cell, and ∴ cessation of (2), is prevented by deoxy-Hb acting as a buffer and binding H+. Movement of O2 into tissues from RBCs ∴ ↑[deoxy-Hb] and enables more CO2 to be transported.
6. The increased [HCO3-] creates a diffusion gradient for HCO3- to leave the cell. It is exchanged for Cl- to maintain electrical neutrality.

At the Lungs:

1. Low PACO2, creates a diffusion gradient for CO2 to diffuse out of the blood into the airspace
2. Increased PAO2 leads to O2-Hb binding. O2-Hb binds less H+ than deoxy-Hb, increasing free [H+]
3. Increased free [H+] leads to increased H2CO3 and ultimately CO2 which contributes to CO2 plasma saturation.
4. The changing equilibrium of carbonic acid reaction, also leads to decreased [HCO3-], as it binds the free H+. This creates a diffusion gradient that allows HCO3- ions to entry the RBC in exchange for Cl-.

Question 21

What is the net effect of how carbon dioxide is transported on carbon dioxide and oxygen

Answer 21

• Deoxygenated blood carries more CO2

* Oxygenation of blood causes CO2 to leave (both points = “the Haldane effect”).

Question 22

How is the relationship between PCO2 and [H2CO3] mean CO2 transport is important in acid-base balance?

Answer 22

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Question 23

Describe how the kidney maintains blood pH by maintaining [HCO3]

Answer 23

• Renal regulation of HCO3-

E.g. regulating reabsorbtion/ excretion in glomerular filtrate (timeframe = hours to days)

Question 24

Describe how the lungs maintain the blood pH by maintaining PaCO2

Answer 24

• Respiratory regulation of PaCO2

E.g. regulating ventilation (timeframe = minutes)