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Structure of the heart and Cardiac Cycle

Structure of the heart and Cardiac Cycle. Structure of the heart. DEFINITIONS/EXPLANATIONS. SYSTOLE Period of contraction of heart DIASTOLE Period of relaxation of heart SINO-ATRIAL NODE (SAN) Pace maker of the heart, initiates cardiac impulse Found in region of right atrium

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Structure of the heart and Cardiac Cycle

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  1. Structure of the heart and Cardiac Cycle

  2. Structure of the heart

  3. DEFINITIONS/EXPLANATIONS SYSTOLE • Period of contraction of heart DIASTOLE • Period of relaxation of heart SINO-ATRIAL NODE (SAN) • Pace maker of the heart, initiates cardiac impulse • Found in region of right atrium • A specialised area of cardiac muscle fibres.

  4. DEFINITIONS/EXPLANATIONS ATRIO-VENTRICULAR NODE (AVN) • Second node in right atrium responsible for passing on cardiac impulse BUNDLE OF HIS • Found in the septum and conducts the cardiac impulse to the apex (tip) of the heart PURKINJE FIBRES • Send impulses to ventricles to contract

  5. SEQUENCE OF EVENTS • Originates in the SA Node (sinoatrial node). • Sends a wave of excitation through the atria causing them to contract. • This then stimulates the AV node (atrioventricular node). • Sends a wave through the bundle of his and the purkingjie fibres causing the ventricles to contract.

  6. SEQUENCE OF EVENTS 5. This is called VENTRICULAR SYSTOLE. 6. Once systole has finished DIASTOLE follows • All this takes place in a fraction of a second

  7. Cardiac Cycle • Refers to the electrical and mechanical movements of the heart. • Rest one complete heartbeat takes 0.8 seconds.

  8. CONTROL OF HEART RATE • The timing of the cardiac contractions is altered by two extrinsic factors: • Neural control • Hormonal control • One intrinsic factor can also alter HR • Neural control is the most important control mechanism

  9. NEURAL CONTROL • The SA node is controlled by the autonomic nervous system • Two nerves stimulate this node; • Sympathetic cardiac accelerator nerve (speeds up HR) • Parasympathetic vagus nerve (slows down HR) • Overall control of both nerves is with the cardiac control centre in the medulla of the brain.

  10. STIMULATION OF CARDIAC CONTROL CENTRE Stimulated by: • Muscle receptors in muscle and joints at the start of exercise • Chemoreceptors in the muscle that sense chemical changes in muscle and blood (changes in O2, CO2 and pH levels) • Emotional excitement • Changes in blood pressure detected by baroreceptors in aortic and carotid arteries

  11. HORMONAL CONTROL • Adrenaline (secreted from adrenal glands in the kidney) stimulates the SA node • This causes an increase in HR • Prepares body for the ‘flight or fight’ scenario • Adrenaline also increases the strength of contraction produced by the myocardium (heart muscle)

  12. INTRINSIC CONTROL • Warm muscle conducts nerve impulses faster • Thus HR of a warm heart increases • A drop in temperature will reduce HR • Increased venous return stretches heart which stimulates SA node and thus increases HR and SV • This is called ‘Starling’s Law of the heart’

  13. Changes of Cardiac output • Increases why? • What does this enable to happen? • What happens to heart rate why? • What happens to stroke volume why?

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