Cardiovascular system 2

Cardiovascular responses to

exercise

Blood supply during rest and exercise

• Blood pressure must be maintained at the correct level so that there is sufficient blood flow

around the body. Coordinated relaxation and constriction of some blood vessels maintains

pressure, but also redistributes blood flow to the active muscles during exercise.

• Cardiac output provides the most important indicator of the circulatory system’s

functional capacity to meet the demands for physical activity. As with any

pump, the rate of pumping (heart rate) and quantity of blood ejected with

every stroke (stroke volume)

• Cardiac output (L./min-1) = (heart rate x stroke volume) ÷ 1000• Cardiac output increases in proportion to the intensity of the exercise up to a maximum

attainable value. Increased cardiac output is a product of increased stroke volume (more

venous back flow to heart and increased contraction strength) and increased heart rate.

Stroke volume and heart rate increase during exercise

Cardiac output in different populations

• With the ventilatory system

providing sufficient gas

exchange in healthy individuals,

the cardiovascular system is

crucial to maintain function and

to maintain homeostasis during

exercise, or to meet the

demands for physical activity.

• Factors such as gender, age

and fitness status influence the

total cardiac output during rest

and exercise.

Resting CO (L/min)

HR(b.)

Stroke V. (ml.)

Male 5.6 70 72

Female 4.1 76 55

Adult 5.3 73 73

Child 4.8 85 56

Trained 5.0 42 120

Untrained

4.9 64 76

Cardiac Output (CO) during rest

Why would stroke volume increase when HR is low?

Cardiac output in different populations

Exercise CO (L./min)

HR (b.)

Stroke V. (ml)

Male 39.0 195 200

Female 31.2 195 160

Adult 35.1 195 180

Child 30.0 208 144

Trained 34.9 195 179

Untrained

22.0 195 113

Differences in stroke volume between men and women (with similar training) are mostly due to smaller body size. Stroke volume for women usually averages 25% below men’s values.

The difference in size and age influences the cardiac output of a child compared to an adult. A child has a 20% smaller stroke volume but a higher Maximum HR.

The difference in cardiac output between trained and untrained individuals relies solely on the stroke volume. Two factors probably interact as aerobic fitness improves- Increased vagal tone slows the

heart, allowing more time for ventricular filling ( Vagal tone)

- Enlarged ventricular volume and a more powerful myocardium eject a larger volume of blood with each systole

• Average adult cardiac

output: ± 5 L.

(5000ml)

• Average adult

Systolic and diastolic pressure during exercise

Systolic pressure the force exerted by blood on

arterial walls during ventricular contraction.

In resting healthy adult the pressure is 120mmHg,

this is the ideal balance for emptying and filling the

heart chambers

• During exercise the blood pressure changes and

is dependent upon intensity and type of exercise

• During high intensity isometric and anaerobic

exercise, both systolic and diastolic pressure rise

significantly due to increased resistance of the

blood vessels. This is a result of muscles

squeezing veins, increasing peripheral resistance

and an increase in intra-thoracic pressure due to

the contracting of the abdomen.

Diastolic pressure the force exerted by blood on

the arterial walls during ventricular relaxation.

• In resting adult the pressure is 80 mmHg. The

pressure lessens as the blood goes from arteries

to arterioles to capillaries. The pressure in the

venules and veins is low and consistent.

• During steady aerobic exercise involving large

muscle groups, the systolic pressure increases

as a result of an increased cardiac output, while

the diastolic pressure remains constant (or in

well trained athletes may even drop)

• High blood pressure can cause serious

complications to the heart, brain and kidneys,

whereas low pressure can result in insufficient

oxygen and other nutrients reaching the muscle

cells.

• The blood pressure is regulated by the

vasomotor control center. The redistribution is

controlled primarily by vasoconstriction and

vasodilatation

VO2Max, Fick equation

The most commonly

used marker of an

individual’s aerobic

fitness is the

relationship between

cardiac output, oxygen

uptake, and difference

between the oxygen

content of arterial and

mixed-venous blood (a-

vO2 difference). This

principle was discovered

by German physiologist

Adolf Fick in 1870

Cardiac output (mL./min-1) =

[VO2(mL/min-1) / a-vO2(ml/dl blood-1)] x

100

Stroke volume = 71 ml/b

HR = 70 b

Cardiac Output = 5000 ml

aVO2 rest = 20ml/dl

mixed venous vO2 rest = 12-15 ml/dl

mixed vO2 max = 2 – 4 ml/dl

a-vO2 diff. at rest = 5 ml/dl

=> 75% O2 is “unused” bound to HbIn reality, this method is rarely used due to the difficulty of collecting and analyzing the gas concentrations. However, by using an assumed value for oxygen consumption, cardiac output can be closely approximated without the cumbersome and time-consuming oxygen consumption measurement

VO2

Maximal aerobic and anaerobic capacities

• Oxygen consumption (uptake) is the amount

of oxygen a person consumes per unit of time

(usually a minute).

• The oxidation (burning) of fuel foods requires

a definite amount of oxygen per unit mass of

fuel. This amount can be measured indirectly

by collecting expired air and comparing it

with the composition of inspired air (how

much oxygen has been used and CO2

produced).

• At rest the oxygen uptake varies between 0.2

and 0.3 L./min-1

• (Based on that 1 L. of oxygen liberates 22kJ of

energy from glycogen, the BMR (basal

metabolic rate) converts to between 4.4 and

6.6 kJ. For a 60kg person the BMR would be

between 0.073 and 0.110 kJ per minute per

kg of body mass.)

• During exercise the total body oxygen uptake

increases proportionally to the intensity of the

exercise, until a maximal work rate is reached.

• Highest VO2 achieved is VO2max, this maximum

oxygen uptake is known also as aerobic power

• VO2max can therefore be quantitatively

represented as the maximum amount of oxygen

that a person can consume per minute during a

progressive exercise test to exhaustion. The

highest value represents the individual’s

maximal physiological capacity to transport and

use oxygen

• A mean value of VO2max for male students is

±3.5L/min and females ±2.7L./min

• Endurance athletes may reach between 4 and

6L./minVO2max depends on body mass as well as physical fitness, so often expressed in milliliters per kilogram of body mass per

minute (mL kg-1 min-1) so that comparisons can be made

Factors affecting maximum aerobic power• Chemical ability of the muscular tissues

to use oxygen in breaking down fuels

• Combined ability of the cardiovascular

and the pulmonary systems to

transport oxygen to the muscular

system

VO2max decreases by about 10% per

decade with ageing, starting in the late

teens for women and mid-20’s for men.

Oxygen consumption as an indirect way of

measuring energy costs: a hypothetical

example

Net oxygen cost (oxygen consumed during exercise above

which is needed during rest)

0.3 L/min at rest

2.275 L/min (65% of VO2max in males @

3.5L/min)

20 min Jog x net oxygen cost = total net

oxygen cost

20 x 1.975 = 39.5 L.

1 liter of oxygen produces 22kJ of heat

energy in combination with food fuel:

22 x 39.5 = 869 kJ

Exercising in a hot environmentCardiovascular drift

When exercising in a warm or neutral

environment for 15 min or more your

HR increases.

This increase in HR is to compensate

for the decrease in pulmonary

arterial pressure and reduced stroke

volume. To maintain cardiac output

at reduced pressure the heart rate

must be increased.

Under these circumstances, a person

usually must exercise at a lower

intensity than if cardiovascular drift

did not occur.

Cardiovascular drift• The drop in venous return is a

result of the muscles demand for

more blood, however, in warm

environments the blood is also

shunted to the skin for

themoregulation (cooling) causing a

shift in fluids to skin tissue.

• The redistribution of blood plasma

and water loss through sweating

causes a reduced pulmonary

arterial pressure and decreased

stroke volume

Prevention / minimization

• Consistently replace fluids and electrolyte imbalance

• Acclimatize to environment• Weight training to

supplement cardiovascular efforts