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USA Track and Field Level II Coaching Education: Physiology

Part 2 – Cardiovascular physiology

The opening screen. The ability of the cells to exchange

materials with their immediate environment is an absolute

requirement of life. Oxygen is particularly important – cells rely

in a continuous supply of oxygen for their energy supply. They

do have other means for covering their energy needs in the

absence of oxygen, but as you now know these are rather

inefficient and in the long run they all eventually require support

by mitochondrial ATP production – and this energy production

method demands oxygen. Mitochondria are fundamental

constituents of all living cells beyond bacteria and are specially

designed for the ATP energy production task.

Cells of the body are located far away from the source of oxygen

supply –which is the atmosphere. The process by which a flow of

oxygen to the tissues can be maintained involves a method of first extracting the oxygen from the atmosphere – and this

is done through the lungs, and a way of moving that oxygen to the cells needing it – and this is done by the blood

vessels, the blood and a pumping mechanism – the heart, Fuel supplies are also needed – and, like oxygen has to be

moved from outside the body to the cells located inside the body. Together, the lungs, heart, blood and blood vessels

permit amazing physical feats such as running a marathon. Elite athletes are blessed with lungs, heart, blood vessels and

blood that has an extremely efficient capacity to deliver oxygen and nutrients to working cells. Some of this capacity is

inherited, and some of it is acquired through training.

Probably the most amazing organ we have is the heart. It is a muscular organ whose function it is to generate the force

that propels blood through the blood vessels. It can adjust its pumping capacity depending on the oxygen demands of the

mitochondria located in the working muscle. The relevant structural parameter of the pump is the size of the heart, which

determines the amount of blood pumped with each contraction. On average, the heart is approximately .58% of body

mass and this size holds true over a wide range of mammals from mice to cows. The thickness of the heart muscle is

proportional to the work required. It is interesting to note that maximal beating capacity of the heart is no different

between trained and untrained individuals. However, the heart size can be up to twice as large in elite athletes. When

compared to the non-athlete. It is still unclear though, if these elite athletes have larger hearts to begin with or whether

trainability of their hearts is better.

Click the Start Button: Remember that five physiological

systems work together. So, while we are now going to discuss

the cardiovascular and respiratory systems as if they work

independently, keep in mind that these systems are intricately

linked to the energy needs of the working muscle cells.

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Go to the next screen: This screen overviews the interaction

between the cardiovascular and respiratory systems. Even the

very mild exercise like walking demands substantial ATP energy

and it is the task of the cardiovascular and respiratory systems to

provide the cells with the oxygen and fuel supplies so that they

can produce their ATP supplies. Let’s just talk about the pathway

for oxygen at the moment. Oxygen enters the lungs from the

atmosphere (A) where it diffuses from the small air sacs in the

lungs called alveoli into the bloodstream (this is depicted as the

white gas). Carbon dioxide diffuses from the bloodstream into the

alveoli – depicted as the purple gas.

The next component of the system is the heart that is actually two

pumps sitting side by side (B). The right pump sends the blood off to the lungs to be oxygenated and for the elimination

of carbon dioxide. The left pump sends the oxygenated blood off to the rest of the body.

Oxygen poor blood returns to the heart after circulating through the body. The right side of the heart is composed of the

right atrium and right ventricle and collects the blood and pumps it to the lungs through the pulmonary arteries. The

lungs refresh the blood with a new supply of oxygen. Oxygen rich blood then enters the left side of the heart that’s

composed of the left atrium and ventricle and is pumped through the aorta to supply the body tissues with oxygen.

Once the circulating blood reaches working tissues oxygen diffuses from the capillaries into the working cells and

carbon dioxide diffuses out of the cell and into the bloodstream where it then travels to the right heart and then to the

lungs. (C). As oxygen enters the cell it goes directly to the mitochondria (D) to the inner membrane where the electron

transport chain is located. Here, the electron transport chain uses the oxygen to resynthesize ATP from ADP and free

phosphate. The ATP is then used by the sarcomere of the muscle cell so that muscle contraction and relaxation can occur

(E). We will come back and talk about how the muscle cell uses ATP in Part 3.

The diagram in the middle also shows the fuel supplies. Fuel enters through the gut as food, is processed into the type of

fuel the cells can use, and whatever is not used right away is stored in the liver and muscle as glycogen and in the fat

cells as triglycerides for future use.

Go to the next screen: This is the module index screen. You

can navigate anywhere in the module by clicking the buttons of

this screen.

Click the Goals Button: These are the goals for this

module. These topics are all very important background

knowledge for a coach.

Click the Home Menu Button

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Click the Oxygen Pathway Button: The major

problem the body faces is how to move oxygen from the

atmosphere to the sites of ATP utilization. Two processes

are used to accomplish this task. One method is by a

mechanism called diffusion and the other is by a process

referred to as convection or mass transport. Diffusion occurs

at two locations – the first is at the lungs. This is how

oxygen moves through the lungs and into the bloodstream.

The second place diffusion occurs is at the working tissue

level. This is how oxygen moves from the bloodstream into

the cells and mitochondria, Moving oxygen quickly from the

lungs to the cells however, requires a much faster method –

diffusion is far too slow for such long distances. So, a

process called convection or mass transport is used. The

oxygen hitches a ride on transport carriers in the red blood

cell called hemoglobin that is pumped by the heart through the blood vessels at a rate that depends upon the oxygen

needs of the cells. We are now going to talk about diffusion and convection in a bit more detail.

Go to the next screen and Click the Button

labeled #1: Diffusion. Diffusion relies on the uneven

distribution of a gas or fluid between two different

compartments. In the alveoli of the lung – which is one

compartment - there is a high concentration of oxygen and

in the blood vessel – which is another compartment - there

is a low concentration of oxygen. The driving force for

oxygen from the alveoli to the capillary is the size of the

concentration gradient for oxygen. This is referred to as the

difference between the partial pressures for oxygen. Just

think of partial pressure as an indication of the concentration

of a gas – it actually could be oxygen or carbon dioxide.

Note that carbon dioxide is moving from the blood capillary

into the alveoli. This is because the carbon dioxide

concentration in the blood when it returns to the lungs is

much higher that the concentration of carbon dioxide in the alveoli so carbon dioxide will diffuse down its concentration

gradient into the alveoli of the lung. The principle of diffusion explains why you smell the perfume when the lid of the

bottle is removed. The molecules of perfume are in very high concentration inside the bottle. If you remove the lid the

perfume molecules will move down their concentration gradient into the air around you. The molecules will continue to

move down their concentration gradient away from the bottle. When blood is moving through the tissues, as you seen in

the movie clip at the bottom of the screen note how carbon dioxide is moving down its concentration gradient from the

cell into the blood vessel. Oxygen is moving from the blood vessel into the cell.

Go to the next screen: Diffusion requires a gas

exchanger in which air and blood come into very intimate

contact over a sufficient large area. The lungs appear to be

literally “bathed” in a sheet of blood but there is a very thin

barrier between the alveoli from the capillary. The speed of

diffusion actually depends on three things – a very thin

barrier, a large surface area, and a high concentration

gradient. To ensure an oxygen flow rate of up to 3 liters per

minute required by the muscles of an athlete, the air-blood

contact surface is about the size of a tennis court and the

barrier separating air and blood is around 50 times thinner

than a sheet of air-mail paper. The total amount of blood that

is pumped by the heart goes through the lungs. In the human

lung alveoli measure about ! mm in diameter and there are

about 300 million of them in the adult lung. The capillary

network contains approximately 200 ml of blood that is spread out in a film so thin that it each red blood cell is exposed

to air on all sides.

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Go to the next screen. Here’s an electron micrograph

of the alveolar wall and a thin capillary. To prevent damage

to these very thin cellular barriers the blood pressure is very

low – in fact the pulmonary vascular bed has 1 fifth the

pressure of the systemic circuit but the blood flows at the

same rate of speed. Each capillary is about the long as a

single alveolus – in other words they are very short. The

blood flows through the capillaries in the lungs at a high

rate of speed. It is estimated that at rest the red blood cell

spends less than 1 second in the alveolar capillary. The

hemoglobin of that red blood cell will be completely

saturated with oxygen by the time it is one third along the

capillary – in a minute we’ll talk about what we mean that

the hemoglobin is saturated with oxygen. During exercise

the entire capillary may be needed for hemoglobin

saturation because the velocity of blood flow is much faster – in this situation the red blood cell will only spend about

0.3 seconds in the capillary. At rest the lung has a considerable diffusion reserve, but this reserve can be completely

exploited when the gas exchanger is stressed to the limit in strenuous exercise. As well, at high altitude we need most of

the capillary length for saturating the blood with oxygen.

Go to the next screen: Here’s a summary of the factors

affecting gas exchange. Remember that diffusion is a

passive process that depends on the surface area, the

thinness of the barrier and the concentration gradient.

Oxygen and carbon dioxide molecules easily slip through

the cellular barrier separating the air in the alveolus from the

red blood cell. For the most part the lung volume is limited

by the space available in the chest cavity. The gas exchanger

has an appreciable excess capacity that is not used by non-

athletes except when they are at altitude. Athletes, however,

can reach the limit of the gas exchange mechanism during

strenuous exercise. Some very elite athletes have shown

some hypoxemia, which simple means that the hemoglobin

does not become completely saturate with oxygen after

passing past the alveoli. This is some evidence that elite

athletes tend to have larger lungs than non-athletes that gives them a diffusion surface advantage. As well, chronic

exercise can enable the athlete to use more of their available lung volume due to the training effect on the respiratory

muscles making them stronger allowing them to expand the chest. This permits more alveoli in the lung to fill with air.

There is also an endurance training effect on the respiratory muscles that permits them to work for longer periods of time

before showing the effects of fatigue. There is no evidence that training increases the actual size of the lung, or the

number of capillaries after adulthood. The jury is still out about the training effect during on the size of the lung during

childhood.

Click the Return Button

Click the Button labeled #2: Transport of

oxygen. Oxygen does not dissolve very well in water. One

liter of blood can only hold about 3 mls of oxygen and the

heart would have to pump over 1000 liters of blood per

minute to deliver the 3 liters of oxygen that the muscles of

an athlete may need to cover the demands of their muscles

during a strenuous run. The strategy used to improve the

situation is to add an oxygen carrier to the blood that

appreciably increases its oxygen carrying capacity that

reduces the blood flow requirements. Let’s take a brief look

at how the oxygen carrier works.

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Click the continue button: The oxygen carriers used

in all vertebrates and many other animals is hemoglobin.

The structure of hemoglobin includes four heme sites for

oxygen to attach. This improves the oxygen carrying

capacity of blood by about 30 times higher than a solution of

water. This makes the oxygen capacity of blood similar to

that of air. Oxygen that diffuses into the red blood cells

combines chemically with hemoglobin to form

oxyhemoglobin. So, while there is a little bit of oxygen

dissolved in the plasma most of the oxygen is transported by

the red blood cells. In fact, the red blood cells owe their

intense red color to their high hemoglobin content.

Hemoglobin fills about a quarter of the red blood cell’s

internal space. Most other structures you see in other cells

are eliminated in the mature red blood cell. There is no

nucleus and none of the structures that are needed to repair the cell and there are no mitochondria. The red blood cell

makes all its ATP through glycolysis. The life span of the red blood cell is about 140 days on average.

Go to the next screen: What you see here is the

oxyhemoglobin curve. During rest when the red blood cells

leave the lungs the hemoglobin is 98% saturated with

oxygen and at this level of hemoglobin saturation the partial

pressure of oxygen or PO2 is 100 mmHg. Remember that

the partial pressure of a gas simply gives you a measure of

the concentration of the gas dissolved in the plasma. After

passing through the tissues the red blood cells return to the

lungs with hemoglobin that is 75% saturated with oxygen.

The PO2 of blood in the veins is around 40 mm Hg so, you

can see that the amount of oxygen that is dissolved in the

plasma is much lower in venous blood. The PO2 is related

to the amount of oxygen left on the hemoglobin. The

difference between the hemoglobin oxygen saturation

before entering the tissues and the hemoglobin oxygen saturation after leaving the tissues is called the arteriovenous

oxygen difference or a-v O2 difference) and represents how much oxygen is consumed by the tissues. Every time the red

blood cells passes through the lungs it is replenished with oxygen and the hemoglobin becomes 98 percent saturated with

oxygen again. The conditions in the tissues during exercise stimulate the red blood cells to release more oxygen and the

PO2 of blood can drop to around 20 mmHg depending on how much oxygen the cells extract from the blood – note that

the hemoglobin when the PO2 is 20 mmHg is still around 35% saturated with oxygen. Elite athlete can extract a little

more oxygen at the working tissue level– perhaps leaving the hemoglobin about 30% saturated with oxygen - but the

driving force for oxygen into the cell at these low PO2 levels becomes quite weak due to the diminished concentration

gradient for oxygen into the mitochondria. The key point is that the a-vO2 difference represents how much oxygen is

extracted or consumed by the tissues.

Click the Return Button

Click the Button labeled #3: Transport of

carbon dioxide. Carbon dioxide or CO2 is the gaseous

waste produced by the Krebs cycle. As fast as the CO2 is

produced the gas diffuses out of the cell, and into the

capillary. As the blood is passing through the tissues it is

dumping off the oxygen the cells need and at the same time

it will pick up the carbon dioxide. The PCO2 or partial

pressure of carbon dioxide in the cells is higher than it is in

the blood that is entering the tissue bed. At rest, the PCO2 of

cells is around 46 mmHg but it can get a lot higher than this

during exercise. The PCO2 entering the tissue capillaries is

around 40mmHg. As a result, the CO2 diffuses down its

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concentration gradient out of the cell and into the capillary. Now we will take brief look at how CO2 is transported back

to the lungs where it is expelled into the atmosphere.

Go to the next screen: Carbon dioxide is transported

in the blood in three forms. A small amount –less than 10%

dissolves in the plasma and will be transported that way.

Carbon dioxide is about 20 times more soluble in water than

oxygen so there is much more CO2 than O2 in simple

solution. The remaining CO2 enters the red blood cell. Once

in the red blood cell a small amount will bind to hemoglobin

forming carbaminohemoglobin. Up to around 20% can be

transported this way. The bulk of the CO2 however will be

converted to bicarbonate ions. The red blood cell contains a

very high level of carbonic anhydrase that is an enzyme that

accelerates the conversion of CO2 into bicarbonate. As the

CO2 enters the red blood cell carbonic anhydrase combines

it with water to make carbonic acid or in H2CO3. Carbonic

acid rapidly breaks up into a bicarbonate ion and a hydrogen

ion. The bicarbonate moves into the plasma while the hydrogen ion is buffered by the hemoglobin.

Go to the next screen: Now the blood has reached the

lung. As it passes by the alveolus the carbon dioxide will

once again move down its concentration barrier out of the

blood vessel and into the alveolus. Note the partial pressure

differences for carbon dioxide between the blood vessel and

the alveolus. The first carbon dioxide to diffuse out in the

alveolus is the CO2 that is dissolved in the plasma. Now let’s

take a look at what happens to the bicarbonate ion and how

it is converted back to carbon dioxide.

Go to the next screen: When the CO2 that is dissolved

in the plasma this creates a concentration gradient for the

CO2 that is attached to the hemoglobin and it will be

released from the hemoglobin, diffuse out of the red blood

cell and then into the alveolus. Once this happens there is

now a low concentration of CO2 in the red blood cell and

bicarbonate ions will move back into the red blood cell

where it will spontaneously combine with the hydrogen that

are released from the hemoglobin to from carbonic acid.

The enzyme carbonic anhydrase converts the carbonic acid

into CO2 and water. The CO2 then moves out of the red

blood cell into the plasma and then down its concentration

gradient into the alveolus.

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Go to the next screen: You now understand that

oxygen and carbon dioxide diffuse down their concentration

barriers. At the lung oxygen will diffuse into the blood

stream and carbon dioxide will diffuse from the bloodstream

into the lung. Once in the capillaries passing through the

lung oxygen is then moved to the left side of the heart where

it will be pumped to the working tissues.

When the blood moves into the working tissue capillary bed

there are a number of factors that change hemoglobin’s

affinity for oxygen and carbon dioxide. At the working

muscle level the temperature of the blood increases. As

well, the acidity or pH causes a change due to the CO2 that

is being produced. Another organic compound called 2.3

DPG or diphosphoglycerate that is a side product of

glycolysis of the red blood cells also increases. All these will decrease the affinity hemoglobin has for oxygen. The O2 is

more easily dumped from the hemoglobin. Just as a side comment 2,3 DPG also increases during an ascent to high

altitude and this is one of the arguments for altitude training. However, the 2,3 DPG effect seems to be fairly small

compared with the temperature and acidity effect. As the hemoglobin loses it affinity for oxygen it increases its affinity

for CO2 and as the blood moves along the capillary in the tissue bed it picks up the CO2 from the cells dropping the pH

from around 7.4 to 7.2 – or lower when the athlete is exercising very strenuously.

When the blood passes through the lung capillaries the reverse occurs as CO2 is discharged into the air. The pH rises

again and the temperature of the blood cools as it moves towards the lungs. This causes the hemoglobin to weaken its

affinity for CO2 and increases its affinity for oxygen.

Click the Return Button

Click the Important Terminology Button: The

normal heart is a muscle about the size of a fist. When the

athlete is at rest every beat it drives about 3 ounces of blood

through its chambers. Inside it is divided into two parts.

Blood enters the heart on the right side which contracts

sending it from the upper right chamber – the right atrium –

into the lower chamber – the right ventricle. Another beat

pushes the blood again into the lungs where it picks up

oxygen before moving into the left atrium which pumps it

into the left ventricle. The left ventricle has to pump blood

to the brain and to the most distant cells in the toes. When an

athlete starts running the working muscle cells need more

oxygen. To meet this demand two things happen – 1) the

heart speeds up its delivery of blood and 2) there is a

redistribution of blood flow from inactive organs to the

active skeletal muscles. In this section we will overview the important terminology used to describe amount of blood

leaving the heart – such as cardiac output, stroke volume and heart rate and how exercise and training affects these in the

immediate and long term. Getting the blood back to the heart is the job of the venous system and we have three

“pumping” mechanisms that helps accomplish this task. We will briefly overview these three mechanisms. And, we will

finish this section by taking a look at how the body redistributes blood flow from organs that are not essential to the

exercise to the working muscles that are in desperate need of the oxygen.

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Click the Cardiac Output, stroke volume and

heart rate button: Cardiac output is the amount of blood

pumped per minute by the heart. Increases in cardiac output

are closely related to how hard the athlete is working – which,

in turn is related to how much oxygen the working muscles are

demanding. The cardiac output is the most important

descriptor of heart function because it determines the quantity

of oxygen that can be transported by the blood. At rest cardiac

output is around 5 liters per minute and increase to around 30

liters per minute during exercise. Blood flowing into the right

atrium and ventricle must equal the flow through the lung and

flow into the left atrium and ventricle. If you roll the cursor

along the red bar under the graph you will see how cardiac

output increases. Cardiac output depends on two variables –

the stroke volume and the heart rate

Go to the next screen: Note that at rest there is hardly

any difference in cardiac output between the trained and

untrained. However, a trained runner has a higher cardiac

output capacity. The question is how is this higher cardiac

output accomplished. Remember that cardiac output

depends on two things – the heart rate and the stroke

volume. Stroke volume is the amount of blood the heart can

pump per beat. Let’s take a look at stroke volume and see

what happens to that under exercising conditions.

Go to the next screen: Maximal stroke volume can

exceed 200 mL per beat during exercise – at rest it typically

ranges from 50 to 110 mL/beat. Slide the cursor along the

red bar and watch what happens to both cardiac output and

stroke volume. Note how stroke volume reaches it’s highest

value about half way along the red bar. It turns out that

stroke volume reaches its highest value at around 40 to 60%

of VO2max. Also note, that cardiac output continues to

increase despite the fact that stroke volume does not change

any more as the athlete runs faster. When the athlete reaches

anaerobic threshold – or the point at which the aerobic

energy system can’t keep up with the energy demands and

the muscle cells can’t clear the hydrogen ions or lactate. The

athlete can still run a bit faster but the build up of hydrogen

ions will eventually cause the athlete to stop and recover.

There’s only one variable left – the heart rate. This variable is responsible for the increasing cardiac output even though

stroke volume does not increase after about 40 to 60% of VO2max.

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Go to the next screen: Maximum stroke volume can

be double in the athlete. Women generally have a lower

stroke volume than men do because they have smaller

hearts. The larger stroke volume is the result of the increase

in heart size that can be achieved through training. However,

there is also a genetic factor involved here – elite athletes

appear to have the genetically larger heart and this, in

addition to the training effect, provides them with an athletic

advantage. Keep in mind that any adjustment in structure of

the heart is a slow process that typically takes months – even

years to be completed and there is a genetic limit to its

stroke volume capacity.

Go to the next screen. Now let’s talk about heart rate

or the beating frequency of the heart. Heart rate increases

linearly with increasing workload or the volume of oxygen

consumed. Slide the cursor along the red bar to see the

relationship of heart rate to stroke volume and cardiac

output. Note that while stroke volume reaches its maximum

about half way along the red bar the heart rate keeps

increasing up to the anaerobic threshold. Cardiac output

stops increasing at this point as well. So, after about 40 to

60% of VO2 max the increase in cardiac output is due to the

increasing heart rate. Stroke volume is related to the size of

the heart and this is fixed depending on genetics and its

structural changes that can occur with training over a long

period of time. But heart rate is capable of instantly

changing over a wide range according to the oxygen needs

of the muscle cell. The beating frequency depends on the autonomic nervous system and hormone level – especially the

amount of adrenalin or epinephrine in the blood.

Go to the next screen: Heart rate is what is known as

its functional capacity. It allows rapid adjustment to the

needs of the moment and these adjustments occur in

milliseconds to seconds. The structural capacity of the heart

sets the maximal amount of blood it can pump in one beat.

The body can adjust this capacity to higher levels if needed

and does this by enlarging the heart size but this adjustment

occurs very, very slowly. This structural adjustment is an

important feature because it allows the heart to be kept at the

smaller size until the need for enlargement arises. Indeed the

heart will shrink back to its normal size once the athlete

stops high performance training.

However, the maximal beating frequency of the heart does

not change with training. This is fixed and depends on the

age of the athlete – the older the athlete the lower the maximal heart rate. Training does reduce the heart rate at sub

maximal workloads, though, because the heart becomes stronger and can pump out more blood per beat. Indeed, a slow

heart rate in a healthy individual indicates an efficient circulatory system.

I want to make one more comment about the max heart rate – while there is no training an athlete can do that will

increase the max heart rate, there is a gradually lowering of the max heart rate with advancing age. The max heart rate

loses about 1 beat per min each year. This becomes a relevant issue for you if you are training older runners and you are

using heart rate as an estimate of training stress.

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Go to the next screen: Let’s just quickly review what

we just discussed. The amount of blood the heart pumps in

one minute is called the cardiac output. Cardiac output

depends on two variables – the heart rate and the stroke

volume. Stroke volume is the amount of blood pumped out

per beat and the heart rate is the number of beats the heart

makes in 1 minute. Both stroke volume and heart rate

increase during exercise increasing the cardiac output.

Stroke volume increases up to about 40 – 60% of VO2max.

After that it plateaus. Heart rate varies in direct relationship

to the workload – and plays a larger role in increasing

cardiac output.

The stroke volume can increase with training but there is a

genetic limit and it takes a long time for this to occur

because it involves a structural change.

There is an upper limit to the maximum heart rate and this does not change with training. However, during rest and

exercise the trained athlete has a lower heart rate for the same workload than an untrained athlete. And finally, the athlete

has a higher cardiac output than the untrained. Some of this is due to genetics – that is the elite athlete has a naturally

larger heart – and some of it is due to the structural changes that occur with training.

Click the Return Button

Click the Venous Return Button: The left side of

the heart can only pump out as much blood as it receives

from the right side. The venous system is responsible for

keeping pace with the cardiac output needs. The blood in the

venous system is under very low pressure and the walls of

the veins have a very low compliance which simply means

that they easily expand to depending on the amount of blood

they need to hold. If the cells need a higher cardiac output

the blood must be sent back to the heart at a higher rate of

speed. There are three mechanisms that increase the venous

return to the heart.

One mechanism is a muscle pump and a respiratory pump

and the veins will constrict. This reduces the amount of

blood that is in the veins forcing it out of the veins towards

the heart. As the muscle contract their veins are compressed and the blood within them is forced toward the heart. Note

the valves in the veins that stop the backflow of blood until the next muscle contraction.

The second mechanism is the respiratory pump, As the chest wall expands due to the brain signals to the respiratory

muscles to contract this lowers the pressure in the chest cavity allowing the veins in the chest cavity to fill with blood.

When the athlete breathes out the pressure in the chest cavity increases sending the venous blood towards the heart.

And finally, the veins will constrict and this reduces the volume of the venous system squeezing the blood toward the

heart.

Click the Return Button

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Click the Redistribution of Blood Flow Button: Another important adjustment the body makes to ensure that

there is adequate blood flow to the working muscles is to

redistribute the blood flow from organs that don’t need it to

the working muscles. The redistribution of blood flow is due

to a constriction of the muscles surrounding the arterioles

supplying the capillary beds organs that are not essential and

a vasodilation of arterioles of the capillary beds supplying

the active muscles.

Let’s take a look at the result of the vasoconstriction and

vasodilation phenomenons. At rest, for example, the heart

pumps around 5 to 6 liters of blood through the bloods

vessels. This is all that is needed to deliver the necessary

oxygen and nutrients to the cells.

As you analyze how that blood flow or 5 to 6 liters is distributed to the organs note that 24% of it or about 1.5 liters goes

to the liver and gut, 19% or 1.1 liters goes to the kidneys and around 20% goes to the muscles. The brain needs at least 2

liters of blood and at rest this represent 35% of the cardiac output.

During exercise, things changes quite a bit – because cardiac output increases dramatically in order to meet the oxygen

demands of the working muscles. In this example cardiac output is 25 liters per minute. 100% of that goes through the

lungs and then 100% of the blood passing through the lungs it is sent to the from the left ventricle where it is then sent

out to the body’s organs. But note that the blood flow through the gut, liver and kidney has dropped dramatically. The

muscles are now receiving around 90% of the blood flow or 22 liters per minute – the brain still needs its 2 liters – in fact

under exercising conditions it needs a bit more – around 2.5 liters. When the weather is hot then more blood will be

diverted to the skin for cooling purposes. This blood flow to the skin is drawn away from the working muscles and this

explains why athletic performances are often lower in hotter weather.

All these factors – the muscle pump, the respiratory pump, venoconstriction and the redistribution of blood flow, in

conjunction with the increased extraction of oxygen under exercising conditions all increase blood circulation.

Click the Return Button

Click the Home Menu Button

You have now completed this module