Oxygen Transport & pH homeostasis

Question 1

What percentage of oxygen is physically dissolved in blood plasma?

Answer 1

~1.5%

Question 2

Each polypeptide has a haem group capable of binding to oxygen, how many polypeptides make up a haemoglobin protein?

Answer 2

4

Question 3

What is the most common type of haemoglobin found in an adult’s blood?

Answer 3

HbA (2 alpha and 2 beta chains)

Question 4

What features would you see from PaO2 and SpO2 levels from someone with methemoglobinemia?

Answer 4

low SpO2 and normal PaO2

Question 5

What causes the plateau of the oxygen dissociation curve after 75% saturation?

Answer 5

Haemoglobin molecule is already bound to 3 O2 molecules, the rate of reaction is slower to bind to the fourth.

Question 6

What shape is the oxygen dissociation curve?

Answer 6

Sigmoid curve

Question 7

When is the rate of O2-Hb dissociation the fastest and why?

Answer 7

Between 25-50% saturation, due to co-operative binding

Question 8

What does a right-hand shift in the oxygen dissociation curve mean for affinity?

Answer 8

Hb has reduced affinity for O2. More O2 is unloaded to the tissues.

Question 9

Acidosis, hypercapnia and hyperthermia are all reasons for what shift in the oxygen dissociation curve?

Answer 9

Right-hand shift

Question 10

Alkaline pH, hypocapnia and hypothermia are all reasons for what shift in the oxygen dissociation curve?

Answer 10

Left-hand shift

Question 11

What does a left-hand shift in the oxygen dissociation curve mean for affinity?

Answer 11

Hb has a higher affinity for O2. Less O2 is unloaded to the tissues.

Question 12

Carbon-monoxide poisoning moves the oxygen dissociation curve which way?

Answer 12

Left-hand shift

Question 13

Where is myoglobin found and why?

Answer 13

Cardiac muscle, it has a high affinity for oxygen so does not unload it easily, allows storage even at low partial pressures.

Question 14

What percentage of CO2 is transported in the plasma?

Answer 14

9%

Question 15

How is the majority of CO2 transported in the blood?

Answer 15

Undergoes a reaction with water to produce carbonic acid, which readily dissociates into H+ and HCO3-.

Question 16

What is the chloride shift and why is it important?

Answer 16

HCO3- diffuses out of systemic circulation, Cl- diffuses into systemic circulation to maintain electrical balance

Question 17

What maintains the pH of systemic circulation despite the protons produced by the dissociation of carbonic acid?

Answer 17

Deoxyhaemoglobin acts as a buffer, binding to H+.

Question 18

What are the three main buffer systems?

Answer 18

Protein buffer system, phosphate buffer system and carbonic acid-bicarbonate buffer system

Question 19

What is the most preferable buffer system and why?

Answer 19

Bicarbonate buffer system because of it’s abundance and easy regulation.

Question 20

What is the main limitation of the bicarbonate buffer system?

Answer 20

Cannot protect against pH changes due to respiratory issues because it’s production is tightly regulated by CO2 abundance

Question 21

Describe the relationship between pH, bicarbonate ion and carbon dioxide.

Answer 21

pH inversely proportional to CO2: if CO2 is high, pH is low and vice versa.
pH proportional to HCO3-: if HCO3- is low, pH is low and vice versa.

Question 22

How does the metabolic system compensate for respiratory acidosis?

Answer 22

Increased excretion of protons, which increases the plasma level of bicarbonate ion (alkalotic substance)

Question 23

How might gastric vomiting cause metabolic alkalosis?

Answer 23

Removal of HCl in stomach, decrease in protons in circulation.

Question 24

What is a base excess?

Answer 24

Theoretical amount of acid needed to bring blood back to a normal pH.

Question 25

Type 2 respiratory failure occurs by what mechanism?

Answer 25

Alveolar hypoventilation because of increased resistance and compliance, meaning slower breaths are required to maintain ventilation equilibrium in the alveoli.

Question 26

What is the most common mechanism for hypoxaemia?

Answer 26

VQ mismatch. Where alveoli is ventilated but not perfused (deadspace ventilation) because of a blockage in blood flow. Or the alveoli is perfused but not ventilated because of a blockage in the airways.

Question 27

Explain what a diffusion deficit is and what type of respiratory failure is caused by it?

Answer 27

Disruption in the gas exchange membranes (from fluid, scarring or destruction of alveolar epithelium), which affects the diffusion of O2 across the membrane causing hypoxaemia. Type 1.