Lecture 32: Central and Peripheral Limitations to O2 Uptake

Question 1

Is respiration a limiting factor on O2 uptake?

Answer 1

No

Question 2

How is it known that respiration is not a limiting factor on O2 uptake?

Answer 2

Because ventilation increases at VO2 max when anaerobic exercise continues

Question 3

Can oxygen dissociation be a limiting factor in O2 uptake?

Answer 3

Yes

Question 4

Name two major factors in a decreased O2-haemoglobin affinity and name one minor factor.

Answer 4

Major: Temperature and pH
Minor: 2-3 DPG (from RBC glycolysis and increases when exposed to high altitudes)

Question 5

Is RBC concentration of the blood a limiting factor on O2 uptake?

Answer 5

Yes

Question 6

How can RBC concentration’s effect be tested in athletes? What is the result of the injection a certain chemical?

Answer 6

Injection of erthyropoietin

Increased time to exhaustion by 54%

Question 7

Is the pumping capacity of the heart a limiting factor in O2 uptake?

Answer 7

No, it is in fact the amount of muscle O2 uptake that is limiting

Question 8

Why could the pumping capacity of the heart be a limiting factor in O2 uptake?

Answer 8

Increasing pumping capacity of the heart results in increased stroke volume; heart rate; cardiac output

Question 9

What part of the heart has seen to be a major limiter in its pumping capacity?

Answer 9

Pericardium

Question 10

What did Davis & Sargaent’s research on muscle mass effect on O2 uptake show?

Answer 10

The fact that the one leg value was 70% of the two leg value implied that central circulation was the limiting factor as there was not enough blood to be supplied to both legs

Increased O2 uptake by muscles in 2 leg exercise when under hyperoxic conditions implies that it is central circulation as the limiting factor

The fact that O2 uptake remains the same in hyperoxic conditions in one leg, implies that skeletal muscle is the limiting factor in O2 uptake

Conclusion: amount of muscle mass active and amount of skeletal muscle blood flow are limiting factors in O2 uptake

Question 11

When exercising do we use all of our available blood flow?

Answer 11

No

Question 12

What is the commonly known blood flow range in a maximum intensity exercise?

Answer 12

50-80 ml.100g.min

Question 13

What is the effect of an increase in active muscle on blood flow per unit of volume?

Answer 13

The blood flow per unit volume decreases as more muscle is active (kg)

Question 14

What kind of muscle is required to identify skeletal muscle blood flow?

Answer 14

A muscle that is small enough not to overwhelm the pumping capacity of the heart

Question 15

What were the results of a knee extensor exercise measuring maximum blood flow and its effect on O2 uptake?

Answer 15

Blood flow increased to 240 ml.100g.min
O2 uptake increased
MAP increased at higher intensities
AVO2 difference plateau’d very early on

Question 16

Why is there less O2 extraction as intensity of exercise increases?

Answer 16

Because there is less transit time for RBC’s

Question 17

How long does blood remain in the capillaries at rest and then under intense exercise?

Answer 17

Rest: 0.75 seconds
Exercise: 0.3-0.4 seconds

Question 18

Why do elite endurance athletes have increased transit time?

Answer 18

Because they have a greater capillary density

Question 19

What is the main symptom of decreased transit time in the muscles?

Answer 19

Muscle hypoxia (not enough O2 extraction therefore decreased saturation)

Question 20

What causes the vasoconstriction of muscle blood flow to already working muscles, when new muscles become activated during exercise?

Answer 20

Catacholamine spillover

Question 21

What must be achieved to ensure muscle blood flow is maintained during strenuous exercise?

Answer 21

Vasoconstriction must prevail over vasodilatory tone

Question 22

What are the limiters of O2 uptake during exercise?

Answer 22

Cardiac circulation 
Muscle mass activated 
Heamoglobin affinity with oxygen 
RBC concentration 
Capillary density (transit time) 
Blood flow 
Vascular conductance (catacholamine spillover)

Question 23

What are the physiological responses to decreased temperature?

Answer 23

Decreased heart rate 
Decreased lipid mobilisation 
Decreased skin blood flow  
Increased central blood volume 
Increased submaximal VO2