3.1.2 (8.5) The Human Heart (OCR A-level Biology)

  • The human heart is part of a double circulatory system

  • This means that blood passes through the heart twice

    • Deoxygenated blood from organs goes through heart, goes to lungs, blood becomes oxygenated, then passes through heart again and goes to other organs

    • Cycle repeats

LABELLING THE HEART

  • Sides of the heart are flipped in diagrams, Left side of heart is displayed as the right and vice versa

  • RIGHT SIDE

    • Blood arrives through Vena Cava (vein) through either the superior (from brain) and inferior (from other organs) and enters Right Atrium, blood then passes through Tricuspid (or Atrioventricular) valve into Right Ventricle and then passes through Semi-lunar valve and exits through Pulmonary artery

  • LEFT SIDE

    • Blood arrives through Pulmonary vein and enters Left Atrium, blood then passes through Bicuspid (or Atrioventricular) valve into Left Ventricle and then passes through Semi-lunar valve and exits through Aorta

  • METHODS TO REMEMBER

    • To remember Atrium + Ventricle placement

      • A+V stacked makes a diamond, meaning the Atrium should be on top and the ventricle on the bottom

    • To remember blood flow of veins + Arteries

      • Vein contains ‘in’ meaning blood is flowing in to the heart

      • Artery starts with ‘A’ for blood flowing away from the heart

    • To remember which sides of the heart are which

      • If you were to hold a diagram of a heart against yourself with the heart facing away from you, the left side of the diagram is on your own left

      • You are looking at the diagram as if it is in your own chest

    • To remember valves

      • Tricuspid and Bicuspid can be replaced by the term atrioventricular valve if you cant remember which sides they belong on

      • Tricuspid = right

      • Bicuspid = left

THE CARDAIC CYCLE

  1. ATRIAL SYSTOLE

    • Contraction of atria

    • Pressure increase in atrias, causes AV vales to open and blood flows into ventricles

  2. VENTRICULAR SYSTOLE

    • Contraction of ventricle

    • Pressure increases in ventricles, causes SL valves to open and blood to be forced out through arteries

  3. DIASTOLE

    • Relaxation of Atria + Ventricles

    • Blood flows passively into the heart

ELECTRICAL RHYTHM OF THE HEART

  1. Wave of electrical excitation begins at the Sino-Atrial node (SAN)

    • This causes Atria to contract and a heartbeat to be initiated

    • Layer of non-conducting tissues prevents this wave from immediately reaching ventricles

  2. Wave of electrical excitation picked by Atrio-Ventricular node (AVN)

    • Imposes a slightly delay before stimulating bundle of his (conducting tissues) which is made up of purkinje fibres

  3. Bundle of His separates into two branches and conducts the wave of electrical excitation to the hearts apex (bottom of heart)

    • Triggers contraction of ventricles to efficiently empty ventricles

ELECTROCARDIAGRAMS

  • BRADYCARDIA

    • Heartbeats are too slow

    • Common in fit people

    • Abnormal treated by pacemaker to keep heartbeat steady

  • TACHYCARDIA

    • Heartbeats are too rapid

    • Normal during exercise

    • Abnormal treated by medication or in severe cases, surgery

  • ECTOPIC HEARTBEAT

    • Heartbeats outside of normal rhythm

  • ATRIAL FIBRILATION

    • Rapid, incomplete contraction of atria

    • Less effective pumping of blood

    • Example of Arrhythmia (abnormal heart rhythm)