Week 9

• Introduction to Biomedical Physiology and Kinesiology: Blood Flow and Gas Transport
• Course learning outcomes include cardiovascular system function during rest and exercise and maximal aerobic power in relation to health
• Learn how to measure and predict maximal aerobic power (VO2 max) related to physiological responses

Cardiac Output and Oxygen Transport

• Stroke volume is the amount of blood pumped by either the left or right ventricle per beat, measured in ml
• Cardiac output ("Q") is the amount of blood pumped by either the left or right ventricle of the heart per minute, measured in L/min
• Both left and right ventricles have the same cardiac output
• Blood flow through the pulmonary and systemic circuits is maintained equally
• Cardiac output = heart rate X stroke volume
• Blood volume / minute = beats / minute X blood volume / beat
• Average Resting levels= 5 L / minute = 62 beats / minute X 80.6 ml / beat X 1 L / 1000 ml

Fick Equation

• Oxygen transported by blood relates to increases in cardiac output during exercise
• VO₂ = HR x SV x (a-vO2) diff is the Fick equation
• VO2 = oxygen uptake or utilization by the tissues in the body
• (a-vO2) diff = arterial-mixed venous oxygen difference, showing amount of oxygen extracted at tissue capillary beds
• Increasing oxygen uptake requires increasing cardiac output and/or extracting more oxygen from the arterial blood
• Higher maximal stroke volume leads to higher maximal cardiac output and higher maximum oxygen uptake (VO2max.)
• Cardiac output rises with work rate (VO2); at rest it is 5 L/min, on average max it is 20 L/min, and 30L/min for an athlete
• Cardiac output required for a given workload is similar for trained and untrained subjects
• Heart rate increases linearly with work rate and O₂ consumption
• A person's max HR = 220 - their age (one standard deviation is +/- 12bpm)
• Trained subjects will have a lower exercise heart rate for any given workload (VO2)
• Trained subjects have a higher stroke volume than untrained subjects (Cardiac output = HR X SV)
• The heart rate at a given oxygen uptake is higher when the exercise is performed with the arms than with the legs
• Smaller muscle mass, increased intra-thoracic pressure, and less effective muscle pump can contribute to this
• Ejection fraction Increases with exercise in trained individuals

Stroke Volume

• Stroke volume = end-diastolic volume minus end-systolic volume
• Diastole is the resting phase of the cardiac cycle, between heart beats
• Systole is the contraction phase of the cardiac cycle, when the ventricles pump out their stroke volumes
• End-diastolic volume (EDV) is the volume of blood in each ventricle at the end of diastole, it is 120 ml in an untrained person at rest
• End-systolic volume (ESV) is the volume of blood that remains in each ventricle after the ventricles have finished contracting; it is 50 ml in an untrained person at rest
• Stroke volume = 120ml – 50 ml = 70 ml
• Stroke volume rises with exercise, then plateaus at ~40% VO2max
• At rest it is 60-100ml; during exercise it is 100-120 ml (elite much higher)
• The mechanism of increase in stroke volume during exercise happens through greater systolic emptying = greater ejection fraction
• The heart has a functional residual volume, with only 50 - 60% of the blood in the ventricle pumped out during contraction
• 50 to 80 ml of blood remains in the ventricle at rest, in an upright position
• During graded exercise, the heart progressively increases stroke volume by means of a more complete emptying during systole
• This increase is due to effect of sympathetic hormones

Distribution of Blood Flow During Exercise

• At rest 15-20% of the systemic blood flow goes to the skeletal muscles.
• During maximal exercise 85% of the cardiac output can be diverted to the working skeletal muscles.
• This increased blood flow to the working muscles is caused by increased blood pressure
• Dilation of arterioles happen in working muscles due to relaxation of the smooth muscle in the walls of the arterioles + release of local factors as a result of muscle contraction
• Constriction of arterioles occur in the gut area (liver, intestines, stomach, kidneys) and non-working muscles due to Sympathetic Nervous System stimulation

Poiseuille's Law of Distribution of Blood Flow

• Resistance to flow = Fluid viscosity X Tube length / Radius of tube 4
• Decreasing tube radius by a factor of 2 will increase resistance to flow by a factor of 16, decreasing flow by a factor of 16
• A 33 % decrease in the radius of the arterioles will produce a 400 % increase in resistance to flow.
• A small change in blood vessel radius dramatically alters blood flow

Aerobic Power

• VO2 max provides an integrated measurement of the capacity of physiological systems related to O₂ transport and O₂ utilization
• Systems include the cardiovascular, respiratory, neural, and muscular systems
• Physiological Determinants of VO2max: depends on the ability to maintain body homeostasis
• Maximal aerobic power (VO2 max) describes the maximal capacity to utilize oxygen per minute
• Liters of oxygen * minute-1 and ml oxygen Kg body weight -1 minute -1 are ways to measure
• The most important factors that determine VO2 max. in each person: the ability to ventilate the lungs and oxygenate the blood passing through the lungs, the ability of the heart to pump blood, the oxygen carrying capacity of the blood and the ability of the working muscles to accept a large blood supply, the ability of muscle fibers to extract oxygen from the capillary blood and use it to produce energy
• During As the duration of events requiring heavy continuous energy expenditure becomes progressively greater than one minute, aerobic capacity becomes increasingly important as a determining factor for success.

VO2 max Values

• Untrained Canadian male (20-29 years): 40-50 ml·kg-¹·min-1
• Untrained Canadian female (20-29 years): 30-40 ml·kg-¹·min-1
• World class endurance athlete (M): 80-90 ml·kg-¹·min-1
• World class endurance athlete (F): 65-75 ml·kg-¹·min-1
• Soccer, ice hockey, basketball (M): 54-60 ml·kg-¹·min-1
• Baseball, football, thrower, sprinter : 40-50 ml·kg-¹·min-1
• The VO2max of Frank Shorter, US Olympic Marathon winner, 71.3
• Lance Armstrong had a score of 83, and Bjørn Dæhlie, cross-country skier, had a score of 96
• VO2max Test Protocols: The test protocol should exceed 6 minutes but be less than 15 min, and should incorporate a warm-up period
• The test protocol should be arranged in stages, with each stage progressively increasing in intensity until the termination criteria is reached.

Criteria for Attainment of VO2max

• The oxygen consumption ceases to increase linearly with increasing work rate and approaches a plateau, the last two values agreeing within + 2 ml/kg/min.
• Heart rate should be close to the age-predicted maximum (220 - age), test and protocol dependent
• An age-predicted maximum heart rate of a 20 year old = 220 - 20 = 200 ± 11 bpm
• Blood lactate levels should be 8 millimoles/liter or greater, 3-5 minutes post exercise
• Indicates significant contribution from anaerobic metabolism
• Respiratory exchange ratio (VCO₂ divided by VO₂) should be greater than 1.15
• Indicates anaerobic metabolism and metabolic acidosis
• Subjective observations include the subject looking exhausted at the end of the test
• In most subjects, the highest VO2max values can be obtained during uphill treadmill running, 5-7% higher than on a bicycle ergometer
• Higher values are due to activation of a larger muscle mass on the treadmill
• Competitive athletes are able to achieve VO2max values equal to, or higher than, their treadmill scores while doing their own sport.
• Athletes should ideally be tested in the mode of exercise used in their sport, due to local muscle capillarization and aerobic enzyme levels being important determinants of VO2 max
• There are 2 main types of bicycle ergometers: mechanical and electrically braked
• The advantages of bicycle ergometers as compared to treadmills for exercise testing: they cost less, are portable, don't require electricity, the patient is more stable and body weight is supported allowing with collecting physiological data, and work rate is easier to quantify
• The disadvantages of bicycle ergometers are that you cannot obtain as high a VO2max and cycling is not a common mode movement like walking

Factors Affecting VO2Max

• Genetics determine 40-50% of VO2max, research includes studies of identical and fraternal twins
• Aerobic capacity with training normally improves between 6 and 20%
• VO2 max.(liters/min) increases with age and reaches its peak between 18 and 25 years of age
• VO2 max. declines approximately one percent per year so that by age 55 it is on average 25-30% below values reported for a 20 year old
• Before puberty, there is no significant difference in VO2 max. between boys and girls
• After puberty, the average male has a VO2 max. (ml·kg-¹·min-1) that is 20-25% higher than the average female.
• The sex difference is due to differences in body composition, where muscle is more metabolically active and fat takes up space, also hemoglobin concentrations are 10-14% higher in men
• In the normal population, there are many females who have VO2 max. scores higher than less-fit males.
• Reasons for the decrease in VO2max with age: decreased max heart rate, stroke volume and cardiac output
• Changes occur because of negative changes in other components of the oxygen uptake and transport systems
• Active individuals maintain a considerably higher VO₂ max as they age as compared sedentary individuals.
• An endurance trained 60 year old can have a higher VO₂ max than a sedentary 20 year old

Tests to Predict VO2max

• Predictive tests are less expensive as well as require less specialized equipment
• Motivation is less required and safety risks are lower due to the tests being often submaximal + can be administered to large groups
• Prediction is done through submaximum exercise testing + measuring heart rate
• Tests are based on a linear relation between heart rate and oxygen uptake, true over a wide range of exercise intensities, however this linear nature breaks down in some subjects at heavy work rates
• Assumes a similar maximum heart rate for all subjects
• Estimates are based on 220-age
• A mechanical efficiency is assumed when predicting VO2, which could vary by 6% on a bicycle ergometer, for example, and introduce error
• Heart rate varies from day to day, about ± 5 beats/min. with day to day testing at the same work rate.
• The variation in environmental temperature, time of day, diet, drugs, preliminary rest, or clothing also contribute variabilty
• The VO2 max. predicted from submaximal heart rate is generally within 10 to 20% of the person's actual value for normal subjects.
• The type of subject for whom these tests are poor predictors tends to be in the very low or very high VO2max. categories.
• The error in a well performed direct measurement of VO2max. is ± 4 - 5%.

Efficiency of Muscular Work

• Muscular work is the percentage of chemical energy converted to mechanical energy, with the remainder lost as heat.
• Computation of mechanical efficiency: %EFF = Work performed (kcal) / Energy expended (kcal)*100
• The efficiency of large muscle activities, such as walking, running, and cycling is usually 20 to 25 percent.