TBR: Respiration During Exercise

[justadream]

TBR says that initially during exercise, you do anaerobic respiration since your body doesn't initially have enough Oxygen.

Okay, I can understand that.

Then, TBR says after 90 seconds, your body switches to aerobic respiration (since you are breathing in more Oxygen).

My question is: Does the body switch to anerobic respiration at some point if the exercise is super vigorous? Is that when lactic acid is produced?

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[sillyjoe]

TBR says that initially during exercise, you do anaerobic respiration since your body doesn't initially have enough Oxygen.

Okay, I can understand that.

Then, TBR says after 90 seconds, your body switches to aerobic respiration (since you are breathing in more Oxygen).

My question is: Does the body switch to anerobic respiration at some point if the exercise is super vigorous? Is that when lactic acid is produced?

Yes, you are correct. Let me explain the physiology of this so you get a bigger picture.

When you start exercising your body withdraws parasympathetic stimulation of the body and it increases the sympathetic outflow. What this does is cause a general tonic constriction in most of your blood vessels. Seemingly, this is the antithesis of what you desire, since we need tons of blood flow to the active muscles to bring oxygen. At this moment, in the start of the vigorous exercise your muscles are working, but they do not have the oxygen to run oxidative respiration so it mainly supplies it's ATP via glycolysis. What this does is produce vast amounts of metabolites, such as lactic acid, CO2, etc as end products of glycolysis. These LOCAL (produced in the area by the muscle) metabolites cause a vasodilation that is far greater than the sympathetic stimulation that causes tonic vasoconstriction throughout the body. This allows for an increase in the cardiac output that reaches the active muscles, while keeping inactive areas (without the local metabolites) relatively lower in receiving the percentage of cardiac output. The end result is that oxygen is transported to where it needs to be, which is the active muscles.

If you continue vigorous exercise, no amount of vasodilation will bring enough blood to supply enough oxygen. At that point, oxygen is a limiting reagent, you switch to glycolysis and get lactic acid build-up.

This is probably more physiology than you need to know for the MCAT, but it can't hurt to know.

 

[justadream]

@sillyjoe

I appreciate the explanation!

A few questions:
1) I thought sympathetic stimulation only constricted blood vessels in the digestive system (and related things) whereas it would dilate blood vessels in skeletal muscle in your arms/legs (isn't this the point of flight or fight?)

2) You say "you switch to glycolysis" if you continue vigorous exercise. Do you mean that the level of glycolysis increases when you continue vigorous exercise?

Because even when you have oxygen, you need glycolysis to provide the pyruvate (acetyl CoA).

An easy way to answer my question would be to answer: If you were to graph the level of glycolysis over time during intense exercise, what would it look like?

 

[sillyjoe]

@sillyjoe

I appreciate the explanation!

A few questions:
1) I thought sympathetic stimulation only constricted blood vessels in the digestive system (and related things) whereas it would dilate blood vessels in skeletal muscle in your arms/legs (isn't this the point of flight or fight?)

2) You say "you switch to glycolysis" if you continue vigorous exercise. Do you mean that the level of glycolysis increases when you continue vigorous exercise?

Because even when you have oxygen, you need glycolysis to provide the pyruvate (acetyl CoA).

An easy way to answer my question would be to answer: If you were to graph the level of glycolysis over time during intense exercise, what would it look like?

Sympathetic stimulation constricts most blood vessels. This is part of fight or flight via the previous mechanisms I described. Let me give you a metaphor:

If there was a statewide emergency, the government would shut down the highways. This allows for only emergency vehicles to travel to the incident without impeded flow.

During exercise the cardiac output to muscles vastly increases. This is set up and can only happen due to the sympathetic outflow that constricts vessels. It is shunting blood away from inactive areas to active areas that need oxygen. Remember, the local metabolites are a stronger stimulus than the sympathetic outflow, so you will get vasodilation in active areas, and thus increased blood flow.

Know that most events in the body are not simply on and off switches. There is usually a basal level of activity. For example, kinases and phosphatases are balanced by the ratio of phosphorylation to dephosphorylation. The body regulates the shifting of that ratio. The same is true for glycolysis and exercise.

In the beginning of vigorous exercise your body withdraws parasympathetic outflow, increases sympathetic outflow, which results in a generalized vasoconstriction. Your muscles do not have enough O2 to run oxidative phosphorylation so the ratio of anaerobic to aerobic respiration increases. As glycolysis continues, the by-products cause a local vasodilation allowing for the vast majority of blood to flow to the working skeletal muscle. Now there is enough oxygen to supply oxidative phosphorylation and the ratio of anaerobic to aerobic respiration decreases. As the vigorous exercise continues, the body cannot supply enough O2 even with all of the compensatory mechanisms. This now causes an increase in the ratio of anaerobic to aerobic respiration.

I don't think you need a graph of glycolysis vs exercise time. Besides, glycolysis occurs during aerobic respiration as well. If anything you want to see anaerobic respiration vs time. IMHO, you just need to understand that it is a balancing of the ratio.

 

[justadream]

@sillyjoe

So lactic acid is produced at all times, correct? (since there is always at least some basal level of anaerobic respiration).

It's just that during exercise, the amount of lactic acid produced (from the high anaerobic:aerobic ratio) outweighs the body's ability to transport it to the liver and process it (into pyruvate ==> glucose)?

 

[sillyjoe]

So lactic acid is produced at all times, correct? (since there is always at least some basal level of anaerobic respiration).

Essentially. There will always be a basal amount of most metabolites.

This concept can be even more broader applied to the idea of vasoconstriction discussed above. The blood vessels will be dilated if the ratio of local metabolites : sympathetic stimulation is high.

It's just that during exercise, the amount of lactic acid produced (from the high anaerobic:aerobic ratio) outweighs the body's ability to transport it to the liver and process it (into pyruvate ==> glucose)?

Not quite. While this may be a factor, the availability of glucose isn't the major issue causing the "switch" to anaerobic respiration. It is really the lack of oxygen that is causing the buildup of lactic acid. You see the distinction?

 

[The Brown Knight]

general order of energy usage in muscle:
creatine phosphate (quickly provides energy; runs out quickly) --> anaerobic respiration (medium; medium) --> aerobic respiration (slowly provides energy; lasts much longer)
powerlifting = you rely on creatine and some anaerobic respiration; doesn't necessarily make you lose fat
swimming/sprinting = anaerobic; hence fat-burning exercises
marathons = aerobic; make you skinny
and this all depends on the type of muscle fibers recruited

 

[drechie]

thank you

Yes, you are correct. Let me explain the physiology of this so you get a bigger picture.

When you start exercising your body withdraws parasympathetic stimulation of the body and it increases the sympathetic outflow. What this does is cause a general tonic constriction in most of your blood vessels. Seemingly, this is the antithesis of what you desire, since we need tons of blood flow to the active muscles to bring oxygen. At this moment, in the start of the vigorous exercise your muscles are working, but they do not have the oxygen to run oxidative respiration so it mainly supplies it's ATP via glycolysis. What this does is produce vast amounts of metabolites, such as lactic acid, CO2, etc as end products of glycolysis. These LOCAL (produced in the area by the muscle) metabolites cause a vasodilation that is far greater than the sympathetic stimulation that causes tonic vasoconstriction throughout the body. This allows for an increase in the cardiac output that reaches the active muscles, while keeping inactive areas (without the local metabolites) relatively lower in receiving the percentage of cardiac output. The end result is that oxygen is transported to where it needs to be, which is the active muscles.

If you continue vigorous exercise, no amount of vasodilation will bring enough blood to supply enough oxygen. At that point, oxygen is a limiting reagent, you switch to glycolysis and get lactic acid build-up.

This is probably more physiology than you need to know for the MCAT, but it can't hurt to know.

 

[kingtal0n]

In my opinion, when you first start exercising, you are as aerobic as you were the second before you started. In other words, the transition between anaerobic and aerobic occurs once the blood oxygen level begins to deplete, and if that is correct, then it does not matter what you do for the first 5 seconds of exercise, since blood oxygen levels are still high (normal) and respiration is normal regardless of what you are doing.

Once oxygen is depleting, the body calls for more (increase respiration rate). As you pass a certain threshold, and it may differ from cell type to cell type (some cells preferentially use glucose regardless of oxygen blood content) the body is forced to perform the anaerobic pathways.

If you were to graph this on paper, the graph will be a plot of blood oxygen concentration vs time.
If we did one graph for a person who starts walking on a treadmill, or riding a bike slowly and gently, you will see the blood oxygen levels remain fairly consistent, and high, that is, the body sticks to aerobic respiration. This is the pathway for burning fat, since it is primarily resting muscle fiber's mitochondria which burn long chain fatty acids during this oxygen rich time. The same pathway is used for watching TV on a couch, because just like walking slowly, blood oxygen is high.

If we did a graph for someone who starts doing heavy squats, or sprinting, or clean and jerks, you will see that blood oxygen level depletes rapidly, and the body is forced to use the anaerobic pathway, which means that muscle fiber is now relying on stores of glucose (as glycogen) to produce ATP. Continually tapping into this pathway tells the body to increase its glycogen stores (makes muscle appear larger) and store more myoglobin (a protein that holds oxygen at the ready for these anaerobic bursts) and many other influences bodywide (such as increased concentration of red blood cells). This is the Olympic lifter athletes pathway.