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module 8 - circulatory system I the heart.docx

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Physiology 2130
Ingrid L.Stefanovic

Module 8 – Circulatory System I – The Heart Circulatory System I: The Heart Intro  Sits in your chest cavity btwn your lungs  During an avg life it will beat roughly 2.5 billion times o Have roughly 160 000 km of blood vessels that transport blood directly to almost every cell in your body  Four principle functions of the CV system: 1. Transports oxygen and nutrients to all cells of the body 2. Transports carbon dioxide and waste products from the cells 3. Helps regulate body temp and pH 4. Transports and distributes hormones and other substances within the body Anatomy – The Heart  Right atrium and ventricle – pump blood to the lungs  Left atrium and ventricle – pump blood to the rest of the body  The wall of the left ventricle is much thicker than the wall of the right ventricle o It must contract more forcefully to propel blood through the entire systemic circulation o The right ventricle only propels the blood to the nearby lungs so doesn’t need to contract as forcefully  Valves in the heart ensure the one-way flow of blood through the heart and prevents the blood from backing up into the atrium when the ventricles contract o Right atrioventricular (AV) valve/tricuspid valve  Right atrium  right ventricle o Left atrioventricular (AV) valve/bicuspid or mitral valve  Left atrium  left ventricle o Prevents the blood from backing up into the ventricle when it relaxes:  Pulmonary semilunar valve  Right ventricle  pulmonary artery  Aortic semilunar valve  Left ventricle  aorta o Chordae tendineae  Cords of collagen that attach to the valves at one end and to papillary muscles at the other  Prevents the AV valves rom being pushed into the atria when the pressure in the ventricles is high o Papillary muscles  Extensions of the ventricular muscles and are attached to the chordae tendineae  When the ventricles contract so do the papillary muscles and the AV valves are held in place and don’t fold backward into the atria 1 Module 8 – Circulatory System I – The Heart  Superior vena cava o Delivers blood to the heart from the head and upper limbs  Pulmonary artery o Blood leaving the right ventricle to the lungs  Aorta o Blood leaving the left ventricle to the entire body  Right atrium o Receives blood from the entire body o This blood is low in oxygen and high in carbon dioxide o Will then pump the blood into the right ventricle through the right AV/tricuspid valve  Left atrium o Receives blood from the lungs o This blood is right in oxygen and low in carbon dioxide o Will then pump blood into the left ventricle through the left AV/bicuspid valve  Right ventricle o Pumps blood into the pulmonary artery o Pulmonary artery then delivers this blood to the lungs for gas exchange  Left ventricle o Pumps blood into the aorta o The aorta then distributes the blood to the entire body Anatomy – Circulation Through the Heart 1. After flowing through the body, blood enters the heart at the right atrium 2. From right atrium it passes through the right AV valve and into the right ventricle 3. When the right ventricle contracts, it ejects the blood out of the heart through the pulmonary valve and into the pulmonary artery to the lungs 4. After passing through the lungs, removing CO2 and picking up O2, the blood returns through the pulmonary vein to the left atrium 5. Left atrium  left ventricle through the left AV valve 6. When the left ventricle contracts, blood is ejected through the aortic valve into the aorta and out to the body Myocardial Cells  Two types of myocardial cells (myo – muscle, cardio – heart)  Contractile cells – similar features to skeletal muscle  Nodal/conducting cells – similar features to nerve cells 2 Module 8 – Circulatory System I – The Heart Contractile Cells  Considered to be the ‘real’ muscle cells of the heart and form most of the walls of the atria and ventricles  Similar features and contract in almost the same way as skeletal muscle fibers  Contractile cells of the heart contain the same contractile proteins actin and myosin arranged in bundles of myofibrils surrounded by sarcoplasmic reticulum o Differ from skeletal muscle by having only one nucleus but far more mitochondria o 1/3 of their volume is taken up by mitochondria  These cells are extremely efficient at extracting oxygen; they extract ~80% of the oxygen from the passing blood (about 2x the amount of other cells)  The contractile cells are much shorter, are branched, and are joined together by special structures (intercalated discs) o Intercalated discs contain tight junctions that bind the cells tg, while gap junctions allow for the movement of ions and ion currents btwn the myocardial cells  Because of the gap junctions, the myocardial cells of the heart can conduct AP from cell to cell without the need for nerves 3 Module 8 – Circulatory System I – The Heart Nodal/Conducting Cells  These cells contract very weakly bc they contain very few contractile elements (myofibrils)  These special cells are able to spontaneously generate APs without the help of nervous input like regular neurons (self-excitability)  They can also rapidly conduct the APs to atrial and ventricular muscle o Therefore, these specialized cells provide a self-excitatory system for the heart to generate impulses and a transmission system for rapid conduction of the impulses through the heart Origin of Self-Excitability  Although nearly all of the cells in the heart can spontaneously generate APs, the sinoatrial node (SA node) is generally the site of origin o SA node is located in the upper posterior wall of the right atrium and is the first area to spontaneously depolarize, producing an AP  This is why the SA node is called the pacemaker of the heart o From here, the AP travels through the atria  AV node  Bundle of His  Purkinje fibers  ventricular muscle 1. SA node 2. AV node 3. Bundle of His 4. Bundle branches 5. Purkinje fibers Q: Why is the SA node called the pacemaker?  Its pacemaker potential has the fastest spontaneous depolarization rate compared to other areas of the heart 4 Module 8 – Circulatory System I – The Heart SA Node Potential  Na+ moving into the cell, down their concentration gradient o Na+ permeability is slightly higher here in the heart than in other cells o This will make the inside of the cell more positive (depolarized) over time  Ca++ are similar to Na+ as they are also trying to move into the cell and will also depolarize the cell  Have not yet created an AP  Although the movement of both Na+ and Ca++ into the cell causes a depolarization, the main cause of the spontaneous AP is the movement of K+ o The K+ permeability of the SA node cells decreases over time (less K+ leak out) o Since the Na+/K+ pump is always pumping K+ into the cell, both these factors will cause these cells to depolarize  Because Na+ and Ca++ are flowing into the cell and K+ build up inside, the membrane potential of the SA nodal cells depolarizes from -60mV to -40mV (the threshold of these cells) o Consequently, the SA nodal cells do not have a stable ‘resting’ membrane potential like neurons or muscle cells o This slow depolarization is completely spontaneous and is called the pacemaker potential o The pacemaker potential is responsible for setting the pace of the heartbeat and any alteration to it will affect the heart rate  Once the membrane potential depolarizes to threshold (-40mV), special voltage-gated Ca++ channels will open  Ca++ will rapidly flow in, producing the depolarization phase of the SA node AP  These Ca++ channels will begin closing at roughly the same time as voltage- gated K+ channels begin to open, allow K+ out to repolarize the cel  Once the cell has return to its lowest value (-60 mV), the pacemaker potential will begin depolarizing the cell and the sequence will repeat  This influx of Ca++ is important during the contraction of the heart 5 Module 8 – Circulatory System I – The Heart Q: The pacemaker potential is caused by:  A small amount of Na+ leaking into the cell  A small amount of Ca++ leaking into the cell  A decrease in the movement of K+ out of the cell (decrease in permeability) Myocardial Cells – Conducting System of the Heart  From the SA node, the AP spreads throughout the atrial muscle, causing it to contract  From the atria, the AP travels to the ventricles o However, the atria are electrically isolated from the ventricle by a fibrous tissue, therefore the AP cannot jump directly down to the ventricles o The AP must first travel through the AV node  each branch of the Bundle of His down to the apex of the heart  Purkinje Fibers  rapidly distributes AP to ventricular muscle which then contracts  SA node  atrial muscle  AV node  Bundle of His  Purkinje fibers  ventricular muscle  SA node has one of the slowest conduction speed, the AP speeds up through the atrial muscle to ensure that this muscle contracts simultaneously  The AP and contraction of the muscle moves top down since the SA node is at the top of the heart o This ensures that the blood is forced down into the ventricles  The AV node slows the conduction speed in order to ensure that the atria have finished contracting before the ventricles contract o The AP must now reach the base of the heart rapidly and does so through the Bundle of His which conducts the AP at a very fast rate  It is important for the AP to reach the apex of the heart to contract first so the blood can be forced up and out through the valves at the top of the ventrucles  The Purkinje fibers then spread the AP throughout the ventricular muscle so it contracts the apex upward Q: Correct statements about the conducting system in the heart:  The conduction of the AP through the AV node is slowed in order to ensure that the atria have fully contracted prior to ventricular contraction  The Purkinje fibers conduct the AP at the fastest rate compared to other regions of the heart Electrocardiogram (ECG)  Body fluids a good conductors of electricity and bc the heart sits in the middle of this conducting fluid, when the AP passes through various parts of the heart, the electrical current can spread to the surface of the body  If electrodes are placed on the skin around the heart, electrical potentials generated can be recorded as a electrocardiogram (ECG) 6 Module 8 – Circulatory System I – The Heart  P wave – depolarization of the atrial muscle leading to their contraction  QRS complex – depolarization of the ventricular muscle just prior to contraction  T wave – repolarization of the ventricular muscle as it relaxes  **There is no wave associated with the repolarization of the atrial muscle o this event does occur but is obscured by the much larger QRS Cardiac Cycle  Two primary phases: o Systole – period of isovolumic contraction  period of ejection o Diastole – period of isovolumic relaxation  passive/active ventricular filling  In order for blood to flow, there must be a pressure gradient from high  low btwn two areas 1. Ventricular contraction causes the AV valves to close, which signals the beginning of ventricular systole. The semilunar valves were closed during the previous diastole and remain closed during this period 7 Module 8 – Circulatory System I – The Heart 2. Continued ventricular contraction increase
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