2017
Phase 1 Revision Session Natalie May & Alex Webster 28 th March 2017
1. Anatomy 2. Cardiac Cycle, Contraction & Electrical Conduction 3. The normal ECG 4. Regulation (HR & BP) 5. Equations & Definitions 6. Blood 7. Histology 8. Embryology 9. Practice Qs
Anatomy Borders & Surfaces Coverings of the heart Chambers & valves Coronary arteries Aorta & its branches The Peer Teaching Society is not liable for false or misleading information
Borders & Surfaces 3 surfaces Sternocostal (anterior) Formed by right atrium & ventricle Diaphragmatic (inferior) Formed by right & left ventricle Little bit of the right atrium Base (posterior) Formed mainly by left atrium Borders Right - Formed by the right atrium Left - Auricle of left atrium and left ventricle inferiorly Inferior - Right Ventricle Apex - Left ventricle
Coverings of the Heart Pericardium 2 layers Fibrous Serous Fibrous = outer layer - 3 functions Tough connective tissue - protects heart Anchors heart to surrounding structures Prevents overfilling of heart Serous - deep to fibrous - 2 layers parietal - lines undersurface of fibrous pericardium attaches to great vessels, folds on itself to run on the external surface of the heart muscle - becomes visceral pericardium Slit like space between parietal and visceral layers is the pericardial cavity Pericardial fluid in cavity promotes frictionless gliding of the surfaces during cardiac activity
Myocardium & Endocardium Myocardium Middle contractile layer composed mainly of cardiac muscle fibres and connective tissue Connective tissue forms fibrous skeleton of the heart - reinforced in areas of high blood velocity - origin of great vessels and valves Electrically silent - limits spread of action potential Endocardium Inner layer white sheet of squamous epithelium Lines chambers and covers valves continuous with endothelium of blood vessels
Chambers of the Heart: Atria Superior chambers Internally: smooth posterior walls ridged anterior walls - pectinate muscle interatrial septum bares shallow depression - foramen ovale - fetal remnant of fossa ovalis Right atrium houses SA node - initiation of cardiac conduction Right atrium receives blood from 3 veins Superior vena cava (above diaphragm) Inferior vena cava (below diaphragm) Coronary sinus (drained from myocardium) Left atrium receives blood from 4 pulmonary veins These carry OXYGENATED blood
Chamber of the Heart: Ventricles Discharging chambers RV - > pulmonary trunk LV - > aorta Much more muscular than atria Left thicker than right Internally: Irregular ridges of muscle - trabeculae carnae Papillary muscles - anchored to AV valves via chordae tendinae - essential for valve function
Valves Atrioventricular (AV) valves Sit between the atria and ventricles Mitral - Left - 2 cusps Tricuspid - Right - 3 cusps Semilunar valves Aortic - Prevents backflow into LV Pulmonary - Prevents backflow into RV Both valves have 3 cusps
Coronary Circulation Right & Left coronary arteries RCA & LCA Originate at base of aorta at aortic sinuses Myocardium perfused during diastole RCA Follows coronary sulcus (AV groove) Supplies: Right atrium (RA) Both ventricles Portions of SA & AV node Marginal branches distal to RA supply surface of RV Continues posteriorly to form part/all of posterior descending artery (PDA) LCA 2 divisions Circumflex - runs to posterior surface to meet PDA Left anterior descending (LAD) - runs in anterior IV groove Supplies: LV, LA, IV septum Dominance The RCA forms the PDA in 90% of individuals Termed Right Dominance In 10% PDA formed by Circumflex Left Dominance
The Aorta & its Branches Ascending Aorta Runs deep to pulmonary trunk 2 branches - Left & right coronary arteries Aortic Arch Begins at sternal angle (T4) 3 branches (BCS) R- L Brachiocephalic (B) Passes under R clavicle branches into R Common Carotid & R Subclavian L Common Carotid (C) L Subclavian (S) 3 vessels issuing from aortic arch supply the head neck & upper limb Descending aorta Gives rise to the intercostal arteries Pierces diaphragm at T12 - Becomes abdominal aorta
Cardiac Cycle Phase 1: Ventricular filling Blood flows passively into LV through open AV valve Atria contract at end of phase 1 to force blood into LV Blood in ventricle at this point is the end diastolic volume EDV Phase 2: Ventricular systole Ventricles begin contracting LV pressure rises rapidly - close AV valves When LV pressure exceeds aortic pressure - SL valves open - blood enters aorta Phase 3: Ventricular diastole As LV ceases contracting pressure falls rapidly Blood in the aorta flows backwards towards the heart - closes the aortic valve This causes a brief rise in aortic pressure - dicrotic notch The Peer Teaching Society is not liable for false or misleading information
Cardiac Action Potentials Stage 0 - Rapid depolarisation Cardiac myocyte reaches threshold potential via intercalated disc of adjacent cardiac cell Voltage gated Na channels open > massive influx of Na into cell Stage 1 & 2 - Plateau Sodium channels close Voltage gated K channels open > K leaves cell Ca channels open > Ca enters This keeps membrane potential constant Stage 3 - Repolarisation Voltage gated Ca close K still diffuses out of cell Rapid repolarisation Stage 4 - Refractory period Absolute - membrane cannot respond at all - Na channels either already open or closed and inactivated Relative - The membrane will respond to a larger than normal stimulus The Peer Teaching Society is not liable for false or misleading information
Electrical Conduction Action potentials facilitate electrical conduction. Transmits signals generated in SA node, resulting in a wave of depolarisation via gap junctions at intercalated discs, causing contraction of heart muscle. Travels via AVN, bundle of His, bundle branches & purkinje fibres Signal stimulates contraction in Atria, followed by ventricles, with a delay (due to connective tissue insulation) between to avoid inefficient filling and backflow. Functional syncytium- propogates in all directions resulting in myocardium performing as a single contractile unit
The normal ECG PR Interval (start of P to start of R): 0.12-0.20sec QRS Complex : 0.08-0.10sec QT Interval: 0.4-0.43 sec RR Interval: 0.6-1.0sec SI- Lub, closing of mitral and tricuspid valves SII- Dub, closing of aortic and pulmonary valves Image from: http://hyperphysics.phy- astr.gsu.edu/hbase/biology/ecg.html
Equations Stroke Volume= End Diastolic Volume- End Systolic Volume Mean Arterial Pressure= (Cardiac output*systemic vascular resistance)+ central venous pressure Cardiac output [L/min]= stroke volume[l/beat]*hr [beats/min] Poiseuille s Law:
Frank- Starling s Law stroke volume will increase in response to an increase end diastolic volume when all other factors remain constant As a larger volume of blood flows into ventricles, blood stretches the wall of the heart (during diastole), which increases force of contraction and thus quantity of blood pumped into aorta (during systole).
Preload The amount of filling of the ventricles before contraction, aka the end-diastolic volume. Relates to the amount of stretch on the sarcomeres of the heart muscle before contraction Increased by anything that increases the stretch of the cardiac muscle and increases ventricular filling/venous return. e.g. aortic stenosis, ventricular systolic failure Decreased by anything that reduces cardiac muscle stretch and reduces venous return/ventricular filling. e.g. heart failure
Afterload Amount of blood left in the ventricle following contraction, AKA end systolic volume Increase in Afterload DECREASES stroke volume, INCREASES LVEDP The contraction is not as strong and more blood remains in the left ventricle. Decrease in Afterload INCREASES stroke volume, DECREASES LVEDP Reduction in ABP leads to a reduction in afterload so ventricles can eject more blood and less remains.
Regulation Homeostasis= regulation of a constant internal environment, despite external change. Reflex regulation occurs to allow supply of oxygenated blood to be provided to tissues in varying circumstances. Sensory monitoring- afferent communication via vagus nerve to hypothalamus and relevant brainstem nuclei Cardiac ianotropy: force of contractility of the heart Cardiac chronotropy: rate of contaction of the heart Baroreceptors- monitor pressure in arterial system and communicates with cardiac centre in medulla. Changes HR and force of contractility in response to feedback via parasympathetic and sympathetic innervation. Chemoreceptors- monitor levels of oxygen and carbon dioxide in blood, compensates with changes to rate & depth of respiration.
Blood Blood= fluid that transports nutrients, oxygen & metabolic waste. Cells are suspended in plasma (comprised of water, proteins (albumin!), & nutrients- glucose, amino acids & fatty acids) Albumin regulates osmotic pressure Cells mainly RBC (erythrocytes), WBC (leukocytes) & platelets (thrombocytes) Composition o RBC: Aneucleate, 120 days, Hb, O2/Co2 o WBC: immunity o Platelets: coagulation
Blood typing Blood typing: o Surface antigens o Inherited partially from both parents o ABO & Rh most important (out of 35 groupings) o Universal donor- O- o Universal acceptor- AB+
Histology Branching chains of cells Striations Nucleus in centre Lots of mitochondria Intercalated discs 1. 2. 3. Fascia adherens (anchoring junctions) Macula adherens (desmosomes) Gap junctions No fatigue or regeneration The Peer Teaching Society is not liable for false or misleading information
SBAs A 68- year- old male has a resting pulse rate of 40 beats/min. An ECG show atrial depolarisation occurring at a rate of 90/min and ventricular depolarisation occurring at a rate of 40/min. Ventricular depolarisation is not preceded by a P Wave. a) AV node b) Purkinje fibres in the left ventricle c) Right atrium d) Right ventricle e) Superior vena cava
SBAs A 28- year- old female complains of palpitations after drinking two cups of strong coffee. Which of the following sites is most likely to have been affected to cause this condition? a) AV node b) Purkinje fibres in the left ventricle c) Right atrium d) Sympathetic nerves e) Vagus nerve
SBAs A 62- year- old male with a long history of ischaemic heart disease complains of palpitations. His pulse is irregular and his ECG shows atrial depolarisation occurring at a rate of about 400/min with an irregularly irregular ventricular rate of about 90/min. Which of the following sites is most likely to have been affected to cause this condition? a) Aortic valve b) AV node c) Right atrium d) Right ventricle e) Superior vena cava
SBAs A 19- year- old medical student collapses whilst watching an eye operation. She is found to have a slow regular heart rate of 48 beats per minute and a blood pressure of 70/30mm Hg but rapidly recovers spontaneously. What is the most likely cause of the low blood pressure? a) Excessive discharge from parasympathetic nerves b) Excessive sympathetic activation c) Increased arterial afterload d) Reduced venous preload due to acute haemorrhage e) Severe right heart failure
SBAs A 63- year- old man has a blood pressure of 82/58mmHg six hours after an abdominal operation. He has a tachycardia and an increasingly swollen abdomen. What is the most likely cause of the low blood pressure a) Adrenal gland failure b) Drug effect c) Excessive discharge from parasympathetic nerves d) Reduced venous preload due to acute haemorrhage e) Severe right ventricular failure
SAQs 1. If BP = blood pressure, PVR = peripheral vascular resistance, and CO = cardiac output, write an equation that indicates the interrelationship between these parameters. 2. Which type of blood vessel is the major site of peripheral vascular resistance? 3. State 3 metabolic or physiological factors that reduce peripheral vascular resistance 4. Two anatomical sites where arterial baroreceptors are located 5. What is the normal systolic and diastolic BP for an adult male aged 30