Applied Physiology Lecture 1: The Heart PDF

College of health and medical technique/ anaesthesia Department. 2nd stage / applied physiology lecture.1  The heart The cardiovascular system acts as a transport system for the tissues and has the following functions: Supply of oxygen and removal of CO2 Delivery of nutrients and removal of met...

College of health and medical technique/ anaesthesia Department. 2nd stage / applied physiology lecture.1  The heart The cardiovascular system acts as a transport system for the tissues and has the following functions: Supply of oxygen and removal of CO2 Delivery of nutrients and removal of metabolic waste products Delivery of hormones and vasoactive substances to target cells The heart is the driving force behind this system, and can be considered a transducer that converts chemical energy into mechanical energy. It consists of a right-sided low-pressure pump and a left-sided high-pressure pump. Each of these pumps is composed of an atrium and a ventricle. The atria prime the ventricles, which in turn eject the cardiac output (CO) into either the pulmonary or the systemic circulation.  Electrical activity of the heart 1. The heart is primarily made up of muscle tissue. A network of nerve fibers coordinates the contraction and relaxation of the cardiac muscle tissue to obtain an efficient, wave-like pumping action of the heart 2. Myocardial contraction results from a change in voltage across the cell membrane (depolarisation), which leads to an action potential. Although contraction may happen spontaneously, it is normally in response to an electrical impulse. 3. The heart has dual and opposing nerve supplies: parasympathetic (acetylcholine transmitter, slows heart rate) and sympathetic (catecholamine transmitter, increases heart rate and force of contraction). 4. Both the parasympathetic and the sympathetic nerve supplies originate in cardiovascular control centers in the medulla 5. The parasympathetic supply is via the right and left vagus nerves. The right vagus mainly slows depolarization of the SA node, and the left vagus slows conduction through the AV node. 6. Cardiac muscle operates as a functional syncytium. It is not a true syncytium (a mass of protoplasm with many nuclei forming one cell). Cardiac muscle functions as a syncytium due to the presence of low-resistance connections between adjacent cells, and when an action potential is generated, the atria or the ventricles contract together. 7. Nerve action potentials last only 1 ms and are produced by changes in membrane permeability to sodium and potassium ions. However, cardiac action potentials last much longer (250 ms) because changes in membrane calcium permeability produce a prolonged plateau phase. Types of action potential 1. Two different types of AP are found in the heart: fast and slow responses. Fast response tissues depend on the opening of voltage-gated Na+ channels to initiate depolarization.  These tissues include the atria and ventricles, along with the specialized infra nodal conducting system (bundle of His, fascicles and bundle branches, terminal Purkinje fibers). 2. Slow-response AP are normally produced by the pacemaker cells in the sinoatrial (SA) node and the atrioventricular (AV) node., is initiated by movement of Ca2+ through long-lasting (L-type) voltage-gated Ca2+ channels. Cardiac Conduction System This pathway is made up of 5 elements: 1. The Sino-atrial (SA) node 2. The atrio-ventricular (AV) node 3. The bundle of His 4. The left and right bundle branches 5. The Purkinje fibres  The SA node is the natural pacemaker of the heart. You may have heard of permanent pacemakers (PPMs) and temporary pacing wires (TPWs) which are used when the SA node has ceased to function properly.  The SA node releases electrical stimuli at a regular rate, the rate is dictated by the needs of the body. Each stimulus passes through the myocardial cells of the atria creating a wave of contraction which spreads rapidly through both atria.  The term used for the release (discharge) of an electrical stimulus is "depolarisation", and the term for recharging is "repolarisation".  So, the 3 stages of a single heart beat are:  Atrial depolarisation  Ventricular depolarisation  Atrial and ventricular repolarisation.  EXCITATION _CONTRACTION COUPLING 1. This term describes the events initially triggered by an action potential and which culminate in contraction of a myofibril. The strength of cardiac muscle contraction is highly dependent on the calcium concentration in the ECF at the end of action potential plateau the calcium ion flow into the cell decrease and the intra cellular calcium is actively pumped back into the sarcoplasmic reticulum and the muscle until the next action potential 2. The contraction of the heart follows from the spontaneous generation of an impulse (automaticity), Exhibit automaticity (ability to depolarize spontaneously) and rhythmicity (ability to maintain a regular discharge rate). 3. Heart exhibits three main “electrical” characteristics that are regulated by the autonomic nervous system: chronotropic (rhythm of automaticity), bathmotropy (cellular excitability), and dromotropy (impulse conduction). Pathologic abnormalities (both congenital and acquired), as well as a wide variety of drugs, can alter cellular automaticity, excitability, and the velocity and path of impulse conduction. A definition of basic terms and concepts 1. Membrane potential: Determined by the relative permeability of the cell membrane to specific ions, and forces both chemical (concentration gradient) and electrostatic (imparted by ion charge) that drive movement of ions across the membrane. 2. Conductance: An expression of how easily an ion flows across a membrane either through active pumps located in the membrane or ion channels; when specific ion channels open, conductance (g) for that ion increases. 3.RESTING MEMBRANCE POTENTIAL (RMP). Is the electrical potential across the cell membrane during diastolic and is about -90 mv. 4. Threshold potential: The membrane potential at which inward currents exceed outward currents as the result of voltage-gating of Na+ and Ca2+ channels, and depolarization becomes self-sustained. At this point, the action potential is initiated. 5. Excitability describes the ability of cardiac tissue to depolarize to a given electrical stimulus. its dependent on the difference between RMP and threshold potential (TP) and thus changes in RMP will alter myocardial excitability when RMP decreases (become more negative) this difference becomes greater and the heart becomes less excitable. similarly, excitability  is increase as the difference between RMP and TP decrease various factors affect excitability including (catecholamines, B-blocker, local anaesthetic agents, plasma electrolyte concentration). 6. REFRACTORINESS During rapid depolarization and the early part of repolarisation the cell cannot be depolarized to produce another action potential regardless of stimulus strength. the sodium and calcium channels are inactivated and repolarisation must occur before they can open again this is called absolute refractory period The fast-response AP is described by the following phases: The fast-response AP of the cardiac muscle cell can be divided into five distinct phases Phase 0 – initial rapid depolarization /upstroke Phase 1 – early rapid repolarisation Phase 2 – prolonged plateau phase Phase 3 – final rapid repolarisation Phase 4 – resting membrane potential (RMP)