Cardiovascular System


Cardiovascular System

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Conduction System Diagram

Conduction system - electrical impulse movement per heartbeat

Heart is myogenic - generates + controls input

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Conduction System Process

1. SA node in right atrial wall generates electical impulse + fires through atria wall, causing them to contract - known as pacemaker as firing rate determines heart rate

2. AV node collects impulse + delays it for around 0.1 seconds to allow atria to finish contracting, then releases impulse to bundle of his

3. Bundle of his in septum splits impulse in 2, ready to be seperately distributed through each ventricle

4. Bundle of his branches carry impulse to base of each ventricle

5. Purkinje fibres distribute the impulse through ventrical walls, causing them to contract

Once electrical impulse journey is complete, atria + ventricles relax + heart refills with blood

This process is one heartbeat

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Cardiac Cycle

The mechanical events of one heartbeat - movement of blood through heart

Systole - contraction of heart chambers

Diastole - relaxation of heart chambers

EDV - ventricle volume when full

ESV - ventricle volume after systole


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Cardiac Cycle Process

1. Both atria fill with blood + AV valves close

2. Atrial blood pressure rises above ventricular pressure

3. Rising blood pressure forces AV valves to open + blood passes passively into both ventricles + semilunar valves close

4. Both atria contract, forcing remaining atrial blood to move actively into ventricles

5. AV valves close

6. Both ventricles contract, increasing ventricular pressure

7. Semilunar valves forced open + AV valves still closed

8. Blood forced out of body + into muscles

9. Diastole of next cardiac cycle begins

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Conduction System Control of Cardiac Cycle

No electrical impulse - causes diastole + cardiac muscle relaxes so SL valves close, atria fill with blood opening the AV valves + blood starts to enter ventricles

SA node fires and electrical impulse through atria walls to AV node + AV node delays impulse - causes atrial systole as atrial muscle contracts so AV valves forced open + blood is pushed into ventricles until atria finish contracting

Bundle of his splits + passes impulse through 2 branches to purkinje fibres in bothe venricle walls - causes ventricular systole as ventricular muscle cntracts so AV valves close + blood is pushed into the arteries, forcing SL valves open until ventricles finish contracting

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Heart Rate, Stroke Volume + Cardiac Output

Heart Rate (HR):

  • number of cardiac cycles completed in one minute (BPM)
  • lower HR - more efficient cardiac muscle
  • maximal heart rate estimated by subtracting age from 220
  • bracycardia - when HR less than 60 BPM
  • tachycardia - when HR higher than 100 BPM

Stroke Volume (SV):

  • volume of blood ejected from left ventricle per beat (ml)
  • occurs during ventricular systole
  • dependant on venous return + ventricular elasticity

Cardiac Output (Q):

  • the volume of blood ejected from left ventricle per minute (L/min)
  • caridac output = heart rate x stroke volume (Q = HR x SV)
  • cardiac hypertrophy makes cardiac muscle more efficient as greater volume of blood ejected per beat
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Heart Rate, Stroke Volume + Cardiac Output Table

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Control of HR in Exercise - Neural


  • located in muscles, aorta + carotid arteries
  • detect chemical changes in blood stream
  • detect decreased pH (increased lactic acid) + increase CO2


  • located in muscles, tendons + joints
  • inform of increased motor activity during exercise (movement)


  • located in blood vessel walls
  • informs of increased blood pressure in blood vessels
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Control of HR in Exercise - Hormonal + Intrinsic

Hormonal Control - Adrenaline:

  • released from adrenal glands
  • increased force of ventricular contraction (SV)
  • increased spread of electrical activity through heart (HR)
  • sympathetic nervous system stimulated

Intrinsic Control:

  • increased temperature - decreased blood viscocity so increased speed of nerve impulse
  • increased VR - increased stretch on right atrium causes SA node to increase firing rate and therefore increase stroke volume
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Control of HR in Exercise - CCC

Cardiac control centre is medulla oblongata in brain

1. Control mechanisms send information to CCC

2. If increase in HR required, sympathetic nervous system activated, releasing adrenaline, noradrenaline + sending stimulation to SA node via accelerator nerve

3. If decrease in HR required, parasympathetic nervous system actioned to inhibit effects via vagus nerve

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Blood Vessels - Arteries + Arterioles

Transports oxygenated blood from heart to muscles + organs

The main artery is aorta + subdivides into arterioles

Have large layer of small muscle + elastic tissue to:

  • allow vasoconstricton + vasodilation
  • regulates blood flow
  • controls blood pressure

Pre-capillary sphincters:

  • ring of smooth muscle muscle surrounding entry of capillary bed
  • dilate + constrict to control blood flow through capillary bed
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Blood Vessels - Veins + Venules

Transport oxygenated blood from muscles + organs back to heart

Venules leaving capillary bed reconnect to form veins

Main vein is vena cava + carried blood back to atria

Have small layer of smooth muscle allowing them to vasoconstrict + vasoldilate to maintain slow blood flow towards heart

Veins have one-way pocket valves to prevent backflow of blood

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Blood Vessels - Capillaries


  • bring blood into close contact with muscle + oxygen cells for gaseous exchange
  • capillary walls - composed of single layer of cells thin enough for gas, nutrient + waste exchange
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Distribution of Q at Rest + During Exercise

During maximal exericse cardiac output distributed to skeletal muscle instead of body organs

This redistribution is known as vascular shunt mechanism

Occurs due to:

  • 1. vasodilation of arterioles supplying muscles
  • 2. vasodilation of pre-capillary sphincters supplying muscles
  • 3. vasoconstriction of arterioles supplying organs
  • 4. vasoconstriction of pre-capillary sphincters supplying organs
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Control/Regulation of Distribution of Blood

Baroreceptors send information about increased blood pressure during exercise in blood vessels supplying muscles

Chemoreceptors send information about decreased pH during exercise to blood supplying muscles

Information sent to Vasomotor Control Centre (VCC)

After recieving information either:

  • increased sympathetic nerve activity causes vasoconstriction
  • decreased sympathetic nerve activity causes vasoldilation
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Venous Return

Venous return - the return of blood to the heart through the venules + veins to the right atrium

During exercise - VR = SV so need to maintain high VR during exercise as done by the 5 mechanisms

During recovery:

  • cardiac output remains high but blood pressure too low to return blood from muscles back to heart
  • causes dizziness + heavy legs known as blood pooling
  • active cool down prevents this, maintaing venous return via muscle pump + respiratory pump
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Venous Return Mechanisms

Pocket Valves - prevent backflow of blood

Muscle Pump - contracting muscles squeeze blood back to heart

Smooth Muscles - middle layer of veins contracts + relaxes

Respiratory Pump - increases pressure in abdomen, squeezes large veins forcing blood back to the heart

Gravity - blood from upper body aided by gravity

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