blood gas transport

Question 1

What is plasma?

Answer 1

The aqueous portion of blood

Question 2

Describe the different mechanisms of gaseous transport within the body for oxygen and carbon dioxide

Answer 2

Oxygen:
98% transported via HbO2
2% transported dissolved in plasma 
Carbon dioxide:
70% transported as HCO3- 
23% transported as HbCO2 
7% transported dissolved in plasma

Question 3

Why is haemoglobin critical in oxygen transport?

Answer 3

Oxygen has low solubility in plasma so only 2% of oxygen can be transported this way
Haemoglobin transports the other 98% of oxygen by binding to oxygen and travelling as oxyhaemoglobin until it reaches respiring tissues; where it then dissociates and releases oxygen to respiring tissues.

Question 4

How is the oxygen content of the blood measured?

Answer 4

1. Oxygen partial pressure (PaO2): expressed as kPa
   This is the partial pressure of oxygen within a gas phase that would yield this much oxygen in the plasma at equilibrium
   
   2. Total oxygen content (CaO2): expressed as ml of oxygen per litre of blood (ml/L)
      What volume of oxygen is being carried in each litre of blood, including oxygen dissolved in the plasma AND oxygen bound to Hb
   3. Oxygen saturation (SaO2)
      • Can be measured directly in arterial blood (SaO2)
      OR
      • Can be estimated by pulse oximetry (SpO2)
      Quantifies the % of available Hb binding sites that are occupied by oxygen

Question 5

What does the oxygen-hemoglobin dissociation curve represent?
What is the sigmoidal shape of the curve produced by?

Answer 5

The oxygen-haemoglobin dissociation curve represents the affinity of haemoglobin for oxygen. It shows the relationship between O2 concentration, partial pressure and saturation.
The sigmoidal shape of the curve is produced by:
- cooperative binding (that the oxygen-haemoglobin affinity increases as more oxygen molecules bind, due to changes in the shape of the protein)
- saturation of oxygen sites

Question 6

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

Answer 6

1) The structure of haemoglobin produces a high O2 affinity, therefore, a high level of Hb-O2 binding (and saturation) requires relatively low O2.
2) The concentration of haem groups and haemoglobin contained in the RBCs enables a high carrying ability.
3) The oxygen-haemoglobin binding curve shifts to offload oxygen to demanding tissues.
4) Haemoglobin O2 affinity changes depending on the local environment, enabling O2 delivery to be coupled on demand.

Question 7

Describe the left shift in the oxygen dissociation curve

Answer 7

Occurs due to higher HbO2 affinity meaning Hb binds more oxygen at a given PO2

Question 8

When can a left shift occur?

Answer 8

1. Low CO2
   
   2. High pH
   3. Low 2,3-DPG
   4. Low temperature
      Remember: LEFT= LOW except for pH

Question 9

Describe the right shift of the oxygen dissociation curve

Answer 9

Occurs due to lower HbO2 affinity meaning Hb binds less oxygen at a given PO2
Hb gives up oxygen more readily for example in respiring tissues

Question 10

When can a right shift occur (Bohr effect)?

Answer 10

1. High CO2
   
   2. Low pH
   3. High 2,3-DPG
2. High temperature

Question 11

What color is oxyhaemoglobin in comparison to deoxyhaemoglobin?

Answer 11

Oxyhaemoglobin = RED 
Deoxyhaemoglobin = BLUE

Question 12

What is cyanosis?

Answer 12

It is the purple discoloration of the skin and tissues when the concentration of deoxyhaemoglobin becomes excessive.

Question 13

What is central cyanosis?

Answer 13

• Blueish discoloration of cure, mucous membranes and extremities

Inadequate oxygenation of blood

Question 14

Give 2 examples of where central cyanosis can occur

Answer 14

Hypoventilation

V/Q mismatch

Question 15

What is peripheral cyanosis?

Answer 15

• Blueish discoloration confined to extremities (eg-fingers)

Inadequate oxygen supply to extremities

Question 16

Give one example of where peripheral cyanosis can occur

Answer 16

Small vessel circulation issues

Question 17

What is hypoxia?

Answer 17

Low levels of oxygen
Hypoxia can occur despite adequate ventilation and perfusion, if the blood is not able to carry sufficient oxygen to meet tissue demands

Question 18

What are the 2 main causes of anemia?

Answer 18

Iron deficiency (less production)

Hemorrhage (more loss)

Question 19

How, and why, does the transport of CO2 differ to the transport of O2?

Answer 19

• CO2 has a higher H2O solubility than O2 does - therefore, a greater percentage of CO2 is transported simply dissolved in plasma.
• CO2 binds to haemoglobin at different sites than O2 does, and with a decreased affinity. Thus, a lower percentage of CO2 is transported in this manner.
• CO2 reacts with water to form carbonic acid, which accounts for the majority (about 70%) of the CO2 transported.

Question 20

What type of blood carries CO2 and how can it do this?

Answer 20

VENOUS BLOOD

Venous blood contain deoxyhaemoglobin, deoxyHb has a higher affinity for CO2 and H+ than oxyHb

Question 21

Describe, in detail, how carbon dioxide is transferred to the lung alveoli from the RBCs.

Answer 21

1) A low PACO2 creates a diffusion gradient for CO2 to diffuse out of the blood into the airspace.
2) An increased PAO2 leads to oxygen-haemoglobin binding. Oxyhaemoglobin binds less H+ than deoxyhaemoglobin, increasing the concentration of free H+.
3) Increasing the concentration of free H+ leads to increased H2CO3 and, ultimately, CO2, which contributes to CO2 plasma saturation.
4) The changing equilibrium of the carbonic acid reaction also leads to decreased HCO3- concentration, as it binds the free H+. This creates a diffusion gradient that allows HCO3- ions to enter the RBC in exchange for Cl-.