10.4 Oxygen, Heat and Fluid Flashcards
Describe what type of fuel is consumed in exercise (Aerobic vs. Anerobic)
Anaerobic glycolysis is initially used but after around 2 minutes (when muscle glycogen stores are used up) aerobic metabolism is utilised
Describe the amount of oxygen consumed with time
There is a stepwise increase early on (when there is an oxygen deficit where creatine phosphate and other substrate level processes are important in supplementing energy supply). This increase slope plateaus eventually
How is oxygen consumption changed with intensity of workout?
It is dependent on intensity. The larger the intensity, the more energy consumption/uptake is required.
Increase power output increase energy uptake (beyond a level there is no change and start using other sources) = VO2 max
Describe the mechanical efficiency of oxygen uptake
Mechanical efficiency is constant (doesn’t change that much with exercise level for cycling) - different types of exercise, weight bearing then O2 consumption depends on body mass and technique.
What are the cardiovascular responses to exercise? [4]
- Increase O2 supply to skeletal and cardiac muscle
- VO2=CO x (CaO2–CvO2) - Difference between arterial O2 content and venous content at a whole body level
- Facilitation of removal of heat and CO2 (peripheral vasodilation and sweating)
- Maintenance of arterial Mean Arterial Pressure (the baroreceptor is active but reset)
Describe what happens to the Cardiac Output in exercise
The increased demand of muscles of the body requires an increased blood flow and this is addressed by an increase in cardiac output. It is able to rise from a resting CO = 5-6 L/min to up to 25L/min
Explain the distribution of Cardiac Output during exercise compared to at rest
Differential central and peripheral vasoconstriction and vasodilation means that blood is increased to the working muscles and skin. Blood to the gut and kidney is decreased.
Blood flow to the brain and heart is generally preserved.
In exercise the vasodilation of the periphery redistributes up to 90% of the blood volume. What does this mean for cardiac output and TPR?
In order to maintain MAP, you have to increase CO to compensate. TPR is also significantly decreases and to minimise this fall there is vasoconstriction of less important vascular beds like gut, kidney and reduced GFR and more concentrated urine)
Vasodilation is an important mechanisms to dissipate heat production in the muscles out of the body. However, this leads to problems with preservation of blood volume and blood pressure (the fall in TPR impacts CO).
What happens when the CV systems start to be stressed?
When it comes to decision then cardiovascular regulation takes over and selectively chooses to vasoconstrict skin to preserve function. This leads to problems with hyperthermia.
(The blood flow to muscles eventually become constricted to preserve blood flow to the brain)
Describe the trend between rising O2 uptake and rising CO
Resting O2 uptake is 250mL/min. For each L of O2 uptake increases there is about 5-6L of increase of CO.
What is a consequence of a sustained vasoconstriction to the gut?
Blood flow can be severely compromised leading to hypoxic states. Chronic (long term blood flow diversion) can lead to infarction and necrosis of the gut.
Decreased flow also leads to an increase in permeability of the gut (as the tissue dies) leading leakage of contents to and from the gut (acidity, sepsis, etc)
Exercise induced ischemia also leads to a decrease in gastric emptying and a speed of intestinal transit leading to impaired absorption.
What is meant by exercise hyperaemia?
An excess of blood in the vessels supplying the muscles and skin during exercise.
What are the important features of exercise hyperaemia?
- Metabolic vasodilators from contracting skeletal muscle, endothelium and red blood cells
- The muscle pump action of contracting muscles
- “Conducted Vasodilation” = Gap junctions in smooth muscle causing upstream vasodilation of resistance arterioles. Increased CO shear stress in arterioles contributes to vasodilation downstream
- Funtional Sympatholysis: Sympathetic vasoconstrcition is desensitised in the presence of metabolic dilators (NO etc)
What is meant by functional sympatholysis
Sympathetic vasoconstriction is markedly blunted in exercising muscles to optimise blood flow to the metabolically active muscle fibres
This is done by the action of local factors like Nitric oxide
What is the major explanation for why some athletes faint after a major exercise bout?
Because over the course of the exercise they have built up a large cardiac output and have peripherally vasodilated to redistribute the CO to the skin and muscles.
The muscle pump action of exercise was critical to matinaining venous return. Cessation of exercise causes blood to pool in the legs and in the periphery (they are still vasodilated) causing a decrease in venous return -> decreased CO -> decreased brain perfusion = Post exercise hypotension
What is Fick’s Law of Diffusion?
It is a measure of bulk flow of oxygen in the system
VO2 = CO x Oxygen Extraction
(where oxygen extraction = O2 concentration of arteries - veins)
VO2 max = CO x Oxygen extraction
VO2 max increases with exercise. Which of these 2 factors contributes the most to this increase?
The cardiovascular response to exercise is mostly due to increase max cardiac output with modest increase in O2 extraction
Since Cardiac output is the major contributer to the rise in VO2 max, which of the factors of CO (HR and SV) impact this?
In the trained state, there is a marked bradycardia at all VO2. Thus the max HR in athletes is slightly reduced - due to remodelling of the heart (LV hypertrophy = achieve a higher SV)
SV levels off: increased HR (systolic period is fixed due to finite time to electrically activate and systole), the period impacted is diastolic so diastole shortens (less filling)
What is the cardiovascular response to prolonged exercise?
CARDIOVASCULAR DRIFT
- Over time HR drifts up (due to increased temperature, loss of blood volume an decreased SV
- Hyperthermia
- Dehydration
- Increased plasma concentration of adrenaline
- Peripheral displacement of blood volume due to cutaneous vasodilation
Describe the neural control over the changes in circulation in exericise
Central command is a descending activation of CV system (linked to motor cortex activation) - cortical influence and anticipation causing HR. Powerful descending cortical drive
This is modified by feedback from the muscles (important for driving the response) chemical and mechanical receptors, feedback from baroreceptor reflex (central command is the resetter of the reflex).
Describe the autonomic control (both parasympathetic and sympathetic) on changes in cardiovascular responses during exercise (ie. the increase in HR)
Early part of increase in HR is due to vagal withdrawal and the later is driven by sympathetic system activation = reduction in certain blood flow areas. There is also increase in lactate.
What are the two major microvascular adaptations to exercise training?
Increased capillary density and increased capillary recruitment
ie. Trained muscles have increased pump (muscular hypertrophy) and increased plumbing to better distribute the cardiac output to wht working muscle
What happens to the minute ventilation as exercise (VO2 max) increases
Ventilation increases to match exercise VO2 max
It rises in 3 phases:
- Linear phase
- Then a steeper slope linear phase after a point where increased CO2 is produced and increased lactic acid is produced
- More steeper increase due to heat, potassium and adrenaline causing Hyperventilation
Describe what happens to pulmonary gas exchange during exercise in challenged situations such as decreased partial pressure of atmospheric oxygen
For trained athletes with large cardiac outputs the transit time of blood is further reduced.
Additive to this, decreases in atmospheric pO2 causes a limitation to blood flow
Describe the control of ventilation
The medullary centre for respiratory control
- Feedback comes from muscles, chemoreceptors and other drivers
- CO2 is the major driver of ventilation and not hypoxic drive which would be dangerous - No evidence to suggest hypoxia plays any role in driving respiration
What are the determinants of VO2 max?
- Respiration
- Central circulatory factors
- Peripheral circulation
- Metabolism
What are the main mechanisms of heat exchange during exercise?
What are the mechanisms of heat loss during different types of exercise?
What is the change in Cardiac ouput in a 25 degree heat enviroment compared to a 40 degree environment?
At rest in 40 degree conditions, the body’s cardiac output rises to the equivalent of exercising at 5-6 METs in normal conditions
What body compartments are impacted on in exercise induced dehydration?
Whole body dehydration affects all compartments of the body: intracellular and interstitial dehyration
How does dehydration impact performance?
It decreases it
Overcome by drinking only if it works (challenge is if exercise is intense and hot there is changes in gastrointestinal motility making it difficult to replace fluid losses)
Dangers in over drinking and get water intoxication (oedema)
