Energy production and consumption

Question 1

What is the oxygen cascade?

Answer 1

A model where every ‘step’ is a potential barrier to gas exchange where partial pressure of oxygen decreases (so there is a gradient between air and mitochondria). An interference at any step would resut in significant injury downstream (hypoxia).

Question 2

What are the six steps of the oxgen cascade?

Answer 2

1. Humidification (‘dilution’ by water)
2. Alveolar gas (‘dilution’ by CO2)
3. Alveolar-capillary diffusion
4. Ventilation-perfusion mismatch and shunt
5. Tissue diffusion
6. Diffusion within cell

Question 3

What does the Krogh model show?

Answer 3

Tissue diffusion of oxygen-a cylinder representing tissue with a tube running through representing the capillary. Oxygenation is greatest in the tissue near the capillary and also neares the arterial end. The ‘lethal corner’ is the tissue furthest away from the capillary and nearest the venous end- most at risk of poor oxygenation.

Question 4

What is oxygen delivery?

Answer 4

Amount of oxygen leaving the heart in one minute (DO2) (approx 1000 ml/min)

Amount of O2 in blood x amount of blood leaving heart

Question 5

What is oxygen consumption?

Answer 5

Amount of oxygen used by the body in one minute (VO2)
(approx 250 ml/min)

Measured by difference in arteroal and venous CO2 or by the difference in inspired and expired O2 volume

Question 6

What three factors affect VO2

Answer 6

1. Age (peak 0-2 years then falls for rest of life)
2. Temperature (metabolic rate doubles with every 10C)
3. Exercise

Question 7

What is the effect of anaemia on cardiac output? Why?

Answer 7

Increases it because 2,3-DPG increased, O2-Hb dissociation curve to right (affinity reduced), increased O2 release into tissues, higher cardiac output to compensate and deliver more oxygen.

Question 8

What is the respiratory exchange ratio? What is its normal value?

Answer 8

The ratio of VCO2 to VO2. Normal value = 1

Question 9

What three factors affect RER?

Answer 9

• acid base balance
• hyperventilation
• metabolic fuel (carbohydrates = 1:1, fatty acids 0.7, protein 0.8)

Question 10

What can RER be used for?

Answer 10

Cardio-pulmonary exercise testing- RER is the slope on a graph of VO2 and VCO2. When more O2 taken in than CO2 produced (steepening of the line) this is the anaerobic threshold.

Question 11

What is oxygen required for?

Answer 11

To generate energy via a three stage process:

1. glycolysis (glucose –> pyruvate)
2. tricarboxylic acid cycle (pyruvate –> CO2)
3. Oxidative phosphorylation (H+ and O2 –> ATP + H2O)

Question 12

What are the three fundamental causes of cellular hypoxia?

Answer 12

Respiratory failure, lack of blood supply and anaemia (lack of Hb).
Could be more than one!