53.2 Exercise Physiology

Question 2

What is exercise?

Answer 2

The voluntary exertion of muscles.

Question 3

In exercise, what do we use as a typical indicator of metabolic rate?

Answer 3

Oxygen consumption

Question 4

What is the basal oxygen consumption?

[IMPORTANT]

Answer 4

250ml/min (at STP)

Question 5

For a normal mixed diet, how much energy is each ml of oxygen consumed associated with?

Answer 5

20J

Question 6

Estimate the power dissipation of the body under basal conditions.

Answer 6

Question 7

How does power dissipated by the body change from the basal state to doing mild exercise (stepping up on a box repeteadly)?

Answer 7

Oxygen consumption rises hugely, so the power dissipated does also.

Question 8

What are the ATP yields per molecule of:

• Glucose (aerobic)
• Glucose (anaerobic)
• Fatty acids

[IMPORTANT]

Answer 8

• Glucose (aerobic) -> 36-39 ATP/molecule
• Glucose (anaerobic) -> 2-3 ATP/molecule
• Fatty acids -> 16 ATP/CH₂CH₂ unit

Question 9

Draw a graph to show how the usage of different fuel sources changes over time in exercise.

Answer 9

Question 10

What is the respiratory quotient?

[IMPORTANT]

Answer 10

• The ratio of CO₂ produced to O₂ consumed
• RQ = CO₂ eliminated / O₂ consumed

Question 11

Compare the respiratory quotient for pure carbohydrate metabolism and pure lipid metabolism.

[IMPORTANT]

Answer 11

• Carbohydrate -> RQ = 1
• Lipid -> RQ = 0.7

Question 12

How do the ratio of carbohydrate to lipid usage and the respiratory quotient change during prolonged exercise?

[IMPORTANT]

Answer 12

• At first, carbohydrates provide most of the energy, meaning the RQ is closer to 1
• As time goes on, lipids provide progressively more of the energy, meaning the RQ is closer to 0.7

Question 13

How much blood flow is there to muscles during exercise?

Answer 13

It is a rise of 20-fold (compared to just a 4-fold increase in cardiac output).

Question 14

Describe how much oxygen consumption of muscle changes during exercise and how this is achieved.

Answer 14

VO₂​ increases about 40-fold due to:

• Blood flow increases 20-fold due to:
  
  • 4-fold increase in blood velocity
  • 5-fold increase in number of capillaries open
• Oxygen extraction from blood doubles

4 x 5 x 2 = 20

Question 15

State the relationship between cardiac output and blood pressure.

Answer 15

BP = CO x SVR

(Note that blood pressure here ignores venous pressure because it is so small, but this induces some error. So could consider it as the difference in blood pressure, really.)

Question 16

How are vagal efferents to the heart shut down during exercise?

Answer 16

Inhibition by nerves from the insular cortex which projects neurons to the nucleus ambiguus inside the medulla.
This region controls vagal outflow to the heart and is activated upon the anticipation of exercise.
The cell bodies of the vagus nerves contain many GABAergic binding sites, so when stimulated by efferents from NA vagal efferent activity is inhibited

Question 17

How do local metabolites affect the cardiovascular system? Give examples

Answer 17

Cause local vasodilation = adenosine, CO2 and K+ act either on VSMC or endothelial cells

Question 18

How does adenosine cause vasodilation?

Answer 18

• binds to adenosine 2 (A2) coupled to Gs protein
• increases [cAMP]
• inhibits MLCK activity.

Question 19

How does metabolic rate change during exercise? (Give numbers)

Answer 19

1 ml O2 = 20J
Exercise oxygen consumption can reach 2500ml/min
2500 X 20 = 50000 J/min
50000 / 60 = 833 J/s (watts)
At rest = 83 watts
Increases by 10 times

Question 20

How does potassium cause vasodilation?

Answer 20

K+ hyperpolarises VSMC which causes vasodilation, hyperpolarisation spreads through gap junctions to relax further smooth muscles

Question 21

How does the type of fuel being used affect the CO2 production? Why is this important in calculations?

Answer 21

• Carbohydrates being utilised = 16kJ/g and 1 molecules of CO2 being produced per O2 consumed (R = 1)
• Lipids being utilised = 37kJ/g and 0.7 molecules of CO2 being produced per O2 consumed (R = 0.7)

R values (respiratory quotient) tells you how much energy per litre of O2 is being used so combining fuel energy values with oxygen consumption gives you metabolic rate

Question 22

How is the arterial baroreflex overcome?

Answer 22

Central command resets sensitivity of arterial baroreceptors to a higher threshold so that the heart only slows when the MAP increases more dramatically
(Sends group III fibres to a high pressure pressure threshold)

Question 23

How much does cardiac output increase by from rest to during exercise?

Answer 23

Increases 4 to 5 times that at rest

Question 24

How much energy is released per gram of fatty acids, amino acids and glucose?

Answer 24

Fatty acids- 38kJ per gram (stored as triglycerides)
Glucose- 17kJ per gram (complete oxidation)
Amino acids - 17kJ per gram

Question 25

What are the key physiological changes that occur during exercise?

Answer 25

Increased cardiac output (70% diverted to skeletal muscle)
Increased heart rate
Vasoconstriction of GI (splanchnic) tract and renal circulations
Local vasodilation of respiring muscle
Increased ventilatory rate
Increased venous return to the heart

Question 26

What are the two ways in which muscle contraction directly affects the cardiovascular system?

Answer 26

mechanical = increased venous return
chemical = lcoal vasodilation and exercise pressor reflex

Question 27

What are the ventilatory phases of exercise?

Answer 27

Phase 1 - steep increase in ventilation and heart rate due to anticipation top down control
Phase 2- Steady increase in ventilation
Phase 3- Reach Vmax

Question 28

What is the effect of the increased sympathetic drive on the body during exercise? How do the three main receptors (a-1, b-1, b-2) coordinate each response?

Answer 28

Noradrenaline and adrenaline:

Alpha 1 receptors: (Gq linked)
-Vasoconstriction of GI and renal smooth muscle
-Peripheral vascular smooth muscle contraction to increase blood pressure

Beta 1 receptors: (Gs linked)
-Increased chronotropy and ionotropy of the heart to increase stroke volume and cardiac output

Beta 2 receptors: (Gs linked)
-VSMC on skeletal muscle
-Relaxation of smooth muscles on bronchioles = bronchodilation to increase mass flow through anatomical dead zone
-Liver and muscle increases glycogenolysis

Question 29

What is the exercise pressor reflex?

Answer 29

As skeletal muscle contracts, stretch receptors are activated (golgi tendon organs) and chemoreceptors detect a build up of metabolites such as K+ and H2PO4-
Sensory afferents (C fibres) relay this to the nucelus tractus solitatrius within the medulla and PAG in the pons to increase the activity of the sympathetic nervous system

Question 30

What is the main driver for exercise?

Answer 30

Increased metabolic demand

Question 31

What is the main driver for increased ventilation during exercise?

Answer 31

NOT BLOOD GASES
K+ or lactate may stimulate chemoreceptors
Cortical areas (SMA and PMA) active during the imagination of exercise and incereases in ventialtion by double were seen (Thornton et el, 2001)

Question 32

What is the most important driver of cardiac output in individuals with heart transplants?

Answer 32

local mechanical pumps are more important in driving a cardiovascular response than the nervous system.
These individuals lack the nervous control of the heart, so they rely predominately on stroke volume to drive an increase in cardiac output.
As skeletal muscles contract, they compress on veins which facilitates venous return to the heart.
According to starlings’ law an increase in preload leads to greater contraction of the cardiac muscle and thus a greater cardiac output.

Question 33

What receptors detect increased blood pressure?

Answer 33

Baroreceptors in the carotid sinus and aorta normally send afferents (glossopharyngeal and vagus) to the NTS which signals to the nucleus ambiguous to send parasympathetic afferents to lower blood pressure

Question 34

When is the respiratory quotient 1?

Answer 34

The body is utilising carbohydrates as its main source of emery

Question 35

When is the respiratory quotient less than one?

Answer 35

Body is using either fat or protein as its energy source (gives less CO2)

Question 36

Which region in the brain coordinates the response to exercise? What does it connect with?

Answer 36

Central command in the cerebral cortex includes medial prefrontal cortex (planning exercise) and cortical parts of the limbic system (insula and anterior cingulate gyrus)
Projects to lateral hypothalamus, rostral and ventrolateral medulla and NTS which signal to the autonomic nervous system

Question 37

Why are blood gases not a driver for ventilation in exercise?

Answer 37

Arterial blood gas and pH composition does not fluctuate during exercise

Question 38

Why does heart rate increase prior to exercise?

Answer 38

Lateral column in periaqueductal grey activated in anticipation of exercise which leads to symapathetic stimulation and increases heart rate

Question 39

Why is it postulated that K+ may be a driver for ventilation in exercise

Answer 39

K is able to stimulate chemoreceptors (when removed there is a less steep increase in ventilation) but not central chemoreceptors as it cannot cross BBB
K follows profile of ventilatory rate

Question 40

Why is the ability of the body to move inefficient?

Answer 40

Only around 20% efficient (20% metabolic fuel being used for movement itself)
Rest of the fuel is lost as heat from homeothermic organism and ether systems not linked to muscle contraction

Question 41

What is the comparison between shivering and exercise in terms of energy production?

Answer 41

• Shivering can increase heat production by approx 200%, but this is small compared to exercise-induced increases which can be >10 fold
• This leads to shivering and exercise-seeking behaviours in cold conditions