The Cardiovascular system. The heart All respiring tissues in the body require oxygen in order to produce energy The heart is the pumping mechanism that.

The Cardiovascular system

The heart

• All respiring tissues in the body require oxygen in order to produce energy

• The heart is the pumping mechanism that pushes the blood rich of oxygen around the body

• An efficient muscular pump made out of specialist cardiac tissue that works continuously

• The blood vessels act as an extensive network of pipes carrying blood around the body

• The cardiovascular system consists of the heart, blood and blood vessels which, together with the respiratory system, transport oxygen to working muscles and remote carbon dioxide.

Structure of the heart

• Double pump- the left side carries oxygen rich blood to working muscles and the right carries blood low in oxygen to the lungs for re-saturation

• Blood low in oxygen returns from the body to the right atrium via the vena cave. At the same time oxygen rich blood returns to the left atrium from the lungs via the pulmonary veins.

• Atrio ventricular Valves (AV) control blood flow into the ventricle and prevent back flow into the ventricles and prevent back flow of blood(Bicuspid on the left and Tricuspid on the right).

• Semi- lunar valves control blood flow from the ventricles through the pulmonary artery and aorta

The cardiac cycle

• This refers to the electrical and mechanical events that take place in the heart during one complete heart beat

• Typically at rest, one complete heartbeat will occur every 0.8 seconds and occurs approximately 72 times per minute

• During this time the heart will first relax and fill with blood (DIASTOLE)- and then contract, forcing blood from one part of the heart to another or forcing blood out of the heart altogether (SYSTOLE).

Diastole & Systole• atrial diastole- atria fill with blood. AV valves are closed to

prevent blood flow into ventricles• Ventricular diastole- rising pressure in the atria causes the AV

valves to open and the ventricles to fill with blood. Semi lunar valves are closed.

• Atrial systole- atria contract, forcing blood into the ventricles. AV valves open to allow blood into ventricles.

• Ventricular systole- ventricles contract, increasing pressure with them, and forcing blood into the aorta and pulmonary artery. AV valves are closed to stop back flow of blood.

DIASTOLE takes approx. 0.5 secondsSYSTOLE takes approx. 0.3 seconds

Pathways of bloodLungsPulmonary veinLeft atriumBicuspid valveLeft ventricleSemi-lunar valveAortaBodyVena cavaRight atriumTricuspid valve Right ventricle Semi-lunar valvePulmonary arteryREAPEAT

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Conduction system of the heart

• Cardiac tissue is extremely specialised• It is described as MYOGENIC, which means that

it can generate its own electrical impulses and does not require stimulation by the brain

• It also possesses an intricate network of nerves• Together, these two factors ensure an efficient

flow of blood through the heart and around the body

Conduction• Cardiac impulse originates from the SINOATRIAL NODE (SA node) located in the

muscular wall of the right atrium and acts as the hearts intrinsic pacemaker• Once the SA node has emitted the electrical impulse, it rapidly spreads

throughout both aria, creating a wave of excitation and causing them both to contract

• The impulse then arrives and activates another specialised area of cardiac tissue known as the ATRIOVENTRICULAR NODE (AV node)

• The AV node initially delays the transmission of the cardiac impulse from spreading to the ventricles (for 0.1 second). This is to enable the atria to contract before ventricular contraction begins

• After this short delay, the impulse is sent down the septum of the heart via the bundle of HIS and throughout the muscular walls of the ventricles via the Purkinje fibres

• Both ventricles now contract, forcing the blood out of the heart and around the body

Control of the heart rate

• Heart is myogenic and generates its own impulses from the SA node; but rate at which cardiac impulses are fired can be altered and controlled by mechanisms external to the heart

• During exercise, heart rate must increase and SA node must fire impulses more rapidly in order to meet the body’s demands for oxygen

• Able to do this through 2 main regulatory mechanisms: neural control mechanism & hormonal control mechanism

NEURAL CONTROL MECHANISM

• The cardiac control centre (CCC) regulates heart rate and it is situated in the medulla oblongata

• the CCC is under involuntary control and is made up of 2 components:

- The sympathetic nervous system which is responsible for increasing heart rate- The parasympathetic nervous system which is

the heart’s braking system, returning heart rate back to normal resting levels

The ccc receives information from various sensory receptors around the body including:• Mechanoreceptors: detect changes in muscle

activity. An increase in muscle activity results in an increase in HR

• Chemoreceptors- detect changes in pH as a result of build up of CO2, lactic acid and O2 in the blood. Increase leads to more acidic blood and raised HR

• Baroreceptors-detect increases in blood flow and therefore blood pressure within the vessels. If stretch receptors in the vena cava are stimulated, then the CCC causes an increase in HR

Exercise & the CCC

• During strenuous exercise, CCC responds to information from the various receptors and, in turn, stimulates the SA node via the sympathetic nervous system- causes HR and SV to increase

• Once exercise stops, reduction in sympathetic stimulation of SA node. Parasympathetic stimulation by the Vagus nerve takes over, causing a decrease in HR

• The greater the parasympathetic stimulation of the SA node, the quicker the HR will return to normal.

Hormonal control of HR• Immediately prior to competition there is an “anticipatory” rise in

HR largely due to the hormone adrenaline.• Adrenaline is released by the adrenal glands into the blood stream

during times of stress. Prepares body for impending exercise by increasing HR and strength of ventricular contraction, consequently forms a part of the sympathetic system.

• During exercise, adrenaline & noradrenaline can aid the body’s response to exercise by:

- Increasing HR and rate of respiration- Vasoconstriction, which increases blood pressure, helping blood reach working

muscles- Increasing blood glucose levels by stimulating the break down of glycogen in

the liver. This helps fuel muscular contraction

Cardiac dynamics & performance• The performance of the heart is dependent upon 2 variables; stroke

volume and heart rate• STROKE VOLUME is the volume of blood pumped out of the heart

per beat. 75ml of blood is ejected from the left ventricle per beat, but is significantly higher in a trained athlete

Stroke volume is determined by:- Venous return- the greater the volume of blood returning to the

right atrium, the greater the stroke volume since more blood is available to be pumped out.

- Elasticity of cardiac fibres refers to the degree of stretch of cardiac fibres just before contraction. The greater the stretch of the cardiac fibres ( as a result increase in venous return) the greater the force of contraction. This is also known as starlings law.

HEART RATE

• Number of complete cardiac cycles per minute.• That is the number of times the left ventricle

ejects blood into the aorta every minute.• Average resting heart rate is 72 bets per minute,

but this varies depending on the level of fitness• Elite performers will have resting heart rate much

lower than this. A decrease in resting heart rate is known as BRADYCARDIA.

CARDIAC OUTPUT

• Cardiac output (Q) is the volume of blood ejected from the heart every minute. Measured in litres per minute

• It is the product of SV x HR• If there is an increase in either SV or HR (or

both) then cardiac output will increase.

Cardiac dynamics during exercise

• During exercise the body’s muscles to work faster in order to deliver sufficient oxygen to working muscles and to remove waste products.

• Heart rate- typically, HR increase linearly with exercise intensity, so that the harder you work, the higher your heart rate will be.

Short term effects of exercise

• Heart rate increase due to the increased demand for oxygen and removal of waste

• Cardiac output increases as HR increases• During maximal exercise SV starts to decrease

slightly as there is a reduced plasma volume due to loss of fluids as sweat. Venous return therefore drops and SV follows.

• Ejection fraction drops. This is the percentage of total possible blood that is ejected from the ventricles

How does HR respond before, during and following exercise

• Immediately prior- “anticipatory” rise due to stimulation of sympathetic nervous system and release of adrenaline

• During 400m- heart rate rapidly rises due to proprioceptors, mechanoreceptors detect movement. Chemoreceptors detect increase in CO2 and blood acidity. CCC stimulates sympathetic to release adrenaline and the SA node is stimulated and HR increase.

• During recovery, increased HR raises blood pressure. Baroreceptors detect increase in BP and stimulate parasympathetic NS (Vagus nerves) to decrease HR. sympathetic nervous stimulation decreases.

How is it possible for a trained and an untrained individual to have the same cardiac output for a given workload?

• An untrained person will have a high HR and low SV

• A trained person will have a low HR and large SV

• This can only occur at sub maximal workloads. At high workloads untrained individuals will not be able to increase their HR sufficiently to match the CO achieved by the trained individual

Long term effects of training

• Cardiac output (at rest) does not change• Resting stroke volume increases due to

hypertrophy. This is sometimes known as “athletes heart”. The heart can pump out more blood per beat sue to this extra strength

• Bradycardia- decrease in resting HR due to an increase in SV.

CIRCULATION• Blood is transported around the body by a continuous

network of blood vessels, which make up the vascular system. There are 2 circulatory networks working together to form a double circuit:

- Systemic circulation: this is the transport of oxygenated blood from the left ventricle to working muscles and the return of deoxygenated blood back tot eh heart once oxygen has been extracted

- Pulmonary circulation: this is the transport of deoxygenated blood to the lungs where it is re-saturated with oxygen and returned to the left side of the heart via the pulmonary vein

• this system ensures that blood is continually re-saturated with oxygen and delivered to the working muscles, whilst CO2 is transported and expelled from the body

Vessels of circulation

• Arteries & arterioles- carry oxygen rich blood away from the heart. As arteries get further away from the heart they get smaller.

• Capillaries- blood from arterioles will eventually reach an extensive network of capillaries where gas exchange takes place

• Veins & venules- carry blood low in oxygen and high in CO2 from capillaries towards the heart

ARTERIES & ARTERIOLES

CHARACTERISTICS:• Carry blood under high pressure• Walls made of elastic fibres that enable them to stretch

and withstand the pumping action of the heart• During exercise the elastic fibres within the walls will

vasoconstrict reducing their diameter and reducing blood flow to inactive organs such as the digestive system. Arterioles supplying blood to working muscles vasodilate. This allows more blood, and therefore more oxygen to reach these exercising tissues. This mechanism of blood redistribution is known as the vascualar shunt.

CAPILLARIES

• Walls are just one cell thick which means the diffusion distance for O2 and CO2 is very short

• Narrow diameter of capillaries further enhances gas exchange as blood cells can only travel in single file and, therefore, blood flow is very slow. This maximises the diffusion of nutrients across the cell walls.

• Layer of moisture around capillaries that aids gas exchange

• The large number of capillaries surrounding the tissues also provides a large surface area for exchange of nutrients into and out of the blood

VEINS & VENULES

• Walls are less elastic than arteries but do contain a very thin layer of involuntary muscle, which when stimulated can help return blood back to the heart

• Carry blood under low pressure as they have a larger diameter

• Valves in veins ensure blood only travels in one direction towards the heart(no back flow)

• At rest veins act as a reservoir of blood, containing about 70% of the blood at any one time. This partly accounts for the dramatic increase in cardiac output when we start exercise

VENOUS RETURN• Volume of blood that returns to the right side of the heart• Stroke volume is dependent on venous return. Increase in volume of

blood returning to the heart will result in an increase in SV and also cardiac output

• Various mechanisms are responsible for improving flow of blood back to the heart and enhance SV:

Skeletal pump- contraction of the muscles during exercise squeezes and pumps blood back towards the heart

Smooth muscle within walls of veins- works in conjunction with the muscular pump to squeeze blood back to the heart

Respiratory pump- increased rate and depth of breathing during exercise increases pressure within the abdomen during inspiration compressing the veins and squeezing blood back to the heart

Gravity- assists the flow of blood from the upper extremities of the body into the superior vena cava

Importance of venous return during cool down

• Venous return is essential in maintaining cardiac output during exercise

• By completing a cool down, venous return can be maintained, preventing “pooling” of blood in the veins. The cool down has the effect of maintaining the muscle pump and cardiac output

• A reduced cardiac output following exercise can reduce blood flow to the brain and increase the likelihood of dizziness or even fainting

Redistribution of blood during exercise- VASCUALR SHUNT

• Occurs thorough vasomotor control and the action of the sympathetic nervous system

• Blood is diverted away from non-essential organs through vasoconstriction and redirected to working muscles through vasodilation

• At rest, only about 20% of total cardiac output is directed to working muscles while during maximal exercise working muscles may receives up to 90% of total blood flow

Vascular shunt during exercise

Redirection of blood flow is important because..:1. It increases oxygen supply to working muscles2. It provides working muscles with the necessary

fuels to contract (glucose & fatty acids)3. It removes CO2 and lactic acid from muscles4. It helps maintain body temperature and rids the

body of excess heat produced during exercise

Vasomotor control

• Vascular shunt is regulated by vasomotor control centre located in the medulla oblongata of the brain

• Exercise results in an increase in CO2 and lactic acid in the blood which is detected by chemoreceptors

• Chemoreceptors inform the vasomotor centre, stimulates the sympathetic nervous system to vasoconstrict arterioles supplying non-essential organs. At the same time vasodilation of arterioles supplying working muscles results in increased blood supply to them.

Blood pressure and blood velocity

• Blood pressure is the force exerted by the blood against the walls of the blood vessels. Blood pressure is determined by 2 factors:

1. Cardiac output 2. Resistance to flow- this dependent on blood

viscosity, blood vessel length and blood vessel radius. The smaller the cross sectional area of the vessel, the faster the flow.

Increases in blood pressure

• Blood pressure increases when either CO2 or resistance increases

• Steady state aerobic exercise- systolic pressure increase as a result of increased CO2, while diastolic pressure remains constant. In well trained individuals, diastolic pressure may even decrease as a result of increased arteriole dilation. This increased pressure ensures sufficient blood gets to working muscles.

• During high intensity isometric or anaerobic work, both systolic and diastolic pressures increase. Result of muscles squeezing the veins, increasing peripheral resistance.

BLOOD VELOCITY

• Blood velocity is related to cross sectional area• The smaller the area, the faster the blood flow• Therefore, blood flows fastest in arteries, then

arterioles and is slowest in capillaries as capillaries have the greatest cross sectional area

• The cross sectional area of venules is less than capillaries, therefore velocity of blood flow increases. The blood velocity in veins is also assisted by the skeletal and respiratory pumps

• Slow blood velocity allows gas exchange to happen

BLOOD PRESSURE

• Blood pressure is related to resistance to flow and cardiac output

• Resistance to flow is due to the friction between the blood and the walls of the blood vessels

• Blood pressure is highest in the arteries and steadily drops as the blood passes through arterioles, capillaries, venules and is lowest in the veins