The Heart - Control of Heart Rate PDF

The Heart Control of Heart Rate  Describe the position and function of the conducting system (SA and AV Intended nodes, AV bundle, bundle of His and Purkinje fibres) Learning Describe the regulation of heart rate Outcomes Describe a normal ECG readi...

The Heart Control of Heart Rate  Describe the position and function of the conducting system (SA and AV Intended nodes, AV bundle, bundle of His and Purkinje fibres) Learning Describe the regulation of heart rate Outcomes Describe a normal ECG reading (PQRST) and relate to the conducting pathway Explain the cardiac cycle including systole and diastole Define cardiac output, stroke volume, end diastolic volume and end systolic volume Control of Heart Rate Cardiac Muscle Controls the regular beating of the heart Loading… Control of heart rate involves the brain and autonomic nervous system. Cardiac Muscle Controls the Regular Beating of the Heart Cardiac (heart) muscle is “myogenic” – it can contract and relax without receiving signals from nerves. This pattern of contraction controls the regular heartbeat  Conducting System of the Loading… Heart Describe the position and function of the conducting system Process starts in the sinoatrial node (SAN) found in the wall of the right atrium SAN is like a pacemaker – it sets the rhythm of the heartbeat by sending out regular waves of electrical activity to the atrial walls It causes the right and left atria to contract at the same time A band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles, instead these waves of electrical activity are transferred from the SAN to the atrioventricular node (AVN) The AVN is responsible for passing the waves of electrical activity on to the bundle of His, but there is a slight delay before the AVN reacts Regulation of the heartbeat Control of Heart Rate involves the Brain and Autonomic Nervous System The SAN generates electrical impulses that cause the cardiac muscles to contract The rate at which the SAN fire is unconsciously controlled by the medulla oblongata The rate needs to alter to respond to internal stimuli Stimuli are detected by pressure receptors and chemical (chemo) receptors There are pressure receptors (baroreceptors) in the aorta and carotid arteries (in the neck). They are stimulated by high and low blood pressure There are chemoreceptors in the aorta, carotid arteries and in the medulla. They monitor the oxygen level in the blood and carbon dioxide and pH Electrical impulses from receptors are sent to the medulla along sensory neurones. The medulla processes the information and sends impulses to the SAN along sympathetic or parasympathetic neurones (which are part of the autonomic nervous system) Stimuli Detected by Receptors Cause Heart Rate to Speed up or Slow Down Normal ECG Loading…  ECG related to conducting system The P wave represents atrial depolarization, which spreads from the SA node through contractile fibers in both atria. The QRS complex represents rapid ventricular depolarization, as the action potential spreads through ventricular contractile fibers. The T wave indicates ventricular repolarization and occurs just as the ventricles are starting to relax. The T wave is smaller and wider than the QRS complex because repolarization occurs more slowly than depolarization. During the plateau period of steady depolarization, the ECG tracing is flat. The Cardiac Cycle Pumps Blood Around the Body The cardiac cycle is an ongoing sequence of contraction and relaxation of the atria and ventricles that keeps blood continuously circulating around the body. The volume of the atria dna ventricles changes as they contract and relax. Pressure changes also occur due to the changes in chamber volume (decreasing the volume of a chamber by contraction will increase the pressure in a chamber. The cardiac cycle can be simplified into separate stages.  The period of contraction that the heart undergoes while it pumps blood into Define systole circulation is called systole. The period of relaxation that occurs as the and diastole chambers fill with blood is called diastole.  Cardiac output (CO) is the volume of blood ejected from the left ventricle (or the right ventricle) into the aorta (or pulmonary trunk) each minute. Define cardiac Cardiac output equals the stroke volume (SV), output and the volume of blood ejected by the ventricle during each contraction, multiplied by the stroke volume heart rate (HR), the number of heartbeats per minute: CO=SV×HR(mL/min)(mL/beat)(beats/min) Define end diastolic volume and end systolic volume