The Cardiovascular System: The Basics


The Cardiovascular System 

Blood Vessels 

  • The human cardiovascular system is a closed system 
  • The amount of smooth muscle in the blood vessels determines the contractile tone, diameter and flow of the blood vessels 
  • Elasticity accommodates changes in flow
  • High pressure system 
  • The renal bed has the highest resistance to flow in the human body 

How do we investigate cardiac function?

  • PET scans are used to visualise a 3D image of the external structure of the heart and architecture of the blood vessels 
  • Ultrasound is used to measure functional volumes via real time imaging which allows you to see valves opening and closing as well as contraction and relaxation of the heart 
  • ECG is used to monitor electrical behaviour of the heart which underpins the contractile function

Lead II ECG form

  • P wave - depolarisation of the heart 
  • QRS complex - depolarisation of ventricles 
  • T wave - repolarisation of ventricles 
  • Lead II involves placing electrodes on the right arm and left leg 

12-lead ECG form 

  • Amplitude and direction of wave form changes in a 12-lead ECG 
  • 10 leads are required for a 12-lead ECG 
  • A lead is an electrical axis 

The Cardiac Ventricular Action Potential 

  • More than 90% of cardiomyocytes are ventricular myocytes 
  • Striated appearance due to actin and myosin 
  • Contraction induces shortening of filaments along their horizontal axis 
  • Gap junctions located in intercalated discs allow action potentials to propagate through the myocytes therefore cardiomyocytes are electrically coupled
  • A long action potential is fundamental to maintaining a discrete cycle of contraction 
  • The refractory period prevents action potentials from summating 
  • Phase 0 - Resting Membrane Potential 
  • Phase 1 - Depolarisation due to inward movement of sodium ions
  • Phase 2 - Calcium influx 
  • Phase 3 - Potassium efflux induces the refractory period 

Ionic Currents during Ventricular Action Potential 

  • Sodium, calcium and potassium ion channels 
  • Sodium and calcium exchanger 
  • L-type voltage-gated calcium channels
  • Longer lasting and short transient potassium currents 

Action Potentials in Pacemaker Cells 

  • The pacemaker cells include those of the Sinoatrial Node because they initiate their own action potentials although the Atrioventricular Node also has some ability to initiate electrical activity 
  • The intrinsic rhythm of the AVN and Purkinje fibres is slower so that the SAN overrides these structures and is the main source of action potential generation 
  • Therefore the AVN acts as a reserve pacemaker in case there is damage to the SAN 
  • Phase 0 - T-type calcium channels open 
  • Phase 1 - L-type calcium channels open 
  • Phase 2 - Potassium channels open 
  • Phase 3 - Slow sodium leak 

Regulation of the Heart Rate

  • Stimulation of the vagus nerve (parasympathetic) slows the heart rate
  • Stimulation of the adrengeric receptors (sympathetic) releases adrenaline and increases the heart rate 

Cardiac Arrythmias 

  • Cardiac arrythmia is a term to describe any irregularity of the heart rate or rhythm 
  • Abnormal automaticity (ectopic pacemaker activity) could be caused by localised ischemia (inadequate blood


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