Overview of the Circulatory System


Overview of the Circulatory System

Overview of the Circulatory System

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What does the cardiovascular system do?

  • Tisusue perfusion
    • Immune system
    • Hormones
    • Waste
    • Temperature control
    • pH control
    • Carbon dioxide
    • Oxygen
    • Nutrients
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Cardiovascular System


Blood Vessels

Blood Vessels

  • Tunica intima
    • Endothelium
  • Tunica media
    • Smooth muscle
    • Elastic fibres
  • Tunica adventitia
    • Collagen fibres
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Pressure and Pulsation from Heart to Tissue

Why does pressure and pulsation decrease?

  • Pressure falls because of resistance to flow
  • Resistance is inversely proportional to vessel diameter
  • Blood vessels get smaller as they get further from the heart, so resistance increases and pressure energy is lose
  • Flow is less pulsatile due to increased blood vessel compliance
  • Blood vessel walls get thinner as they get further from the heart
  • The thinner the wall the more stretchy (compliant) the vessel is
  • Result is the pulsation is damped by movement of blood vessel wall
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Blood Vessel Size

Heart Contraction

Foetal Circulation

  • Maternal circulation supplies nutrients and oxygen
  • Lungs not required for gaseous exchange: bypass via ductus arteriosus
  • Digstion not required: bypassed via ductus venosus
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Foetal Circulation

Cardiac Output

  • Cardiac output = stroke volume x heart rate
    • Stroke volume = volume ejected in 1 contraction cycle
    • Heart rate = number of contraction cycle per unit of time
  • Resting cardiac output: 4-7 l/min (depends on body size)
  • Continuously adjusted
    • sleep reduced by 10%
    • standing reduced by 20%
    • stress increased by 25%
    • pregnancy increased by 40%
    • heavy exercise increased by 500%
  • Changes in carbon monoxide usually involve changes in heart rate and stroke volume
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Stroke Volume

  • Stroke volume is affected by
    • diastolic stretch - depends on venous filling pressure
    • force of contraction
    • arterial pressure

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Diastolic Stretch

  • Frank-Starling's law of the heart: the volume entering at the beginning of a contraction cycle is the volume ejected at the end of the cycle

Venous Return

  • Blood returns to the heart via the vena cava
  • The rate of venous return determines cardiac output
  • The circulation is a closed look so venous return = cardiac output
  • Factors affecting venous return
    • skeletal muscle activity
    • gravity
    • sympathetic nervous activity
    • breathing or compression of thoracic vena cava
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Skeletal Muscle Pump

  • Veins contain non-return valves
  • Contraction displaces blood into next compartment
  • Mechanism very important for extremeties 

Overcoming Gravity

  • Changes in posture impact upon forces affecting venous return

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Gravity - Postural Hypotension

  • Standing causes pooling of blood in the feet and lower legs
  • Without compensation would lead to precipitous fall in blood pressure
  • Compensation mechanisms
    • baroreceptor reflex: vasoconstriction of veins
    • local myogenic response
    • skeletal muscle pumping
    • breathing
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Autonomic Sympathetic Effect on Venous Return

  • Activation of the sympathetic nervous sytem has multiple effects on cardiovascular system function
  • In terms of affecting venous return...
    • veins are normally very compliant (stretchy)
    • activation of the sympathetic nervous system causes a decrease in venocompliance
    • a decrease in compliance leads to an increase in venous pressure and hence venous return
    • increased venous return leads to increased cardiac output
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Control of Venous Return - Breathing

  • Breathing cycle affects intra-thoracic pressure and thus venous return

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