Cardiovascular System Lecture 1 PDF

Cardiovascular system Components Heart Size of a closed fist Lies beneath the sternum in the mediastinum, between the second and sixth ribs Four chambers (two atria and two ventricles) Four valves (two atrioventricular [AV] and two semilunar va...

Cardiovascular system Components Heart Size of a closed fist Lies beneath the sternum in the mediastinum, between the second and sixth ribs Four chambers (two atria and two ventricles) Four valves (two atrioventricular [AV] and two semilunar valves) Surrounded by a sac called the pericardium Three-layers of wall: o Epicardium—outer layer o Myocardium—middle layer o Endocardium—inner layer Dr. Mahmoud Elsaid | Cardiovascular System 1 Heart chambers Atria Two upper chambers Separated by interatrial septum Receive blood returning to the heart Pump blood to the ventricles Ventricles Two lower chambers Separated by the interventricular septum Receive blood from the atria Heart valves Two AV valves and two semilunar valves Allow forward flow of blood through the heart and prevent backward flow Open and close in response to pressure changes caused by ventricular contraction and blood ejection AV valves separate the atria from the ventricles 1) Pulmonic valve—prevents backflow from the pulmonary artery into the right ventricle 2) Aortic valve—prevents backflow from the aorta into the left ventricle 3) Right AV valve (tricuspid) has three triangular cusps 4) Left AV valve (mitral or bicuspid) has two cusps The cardiovascular system consists of circuits: Pulmonary circuit provides blood flow between the heart and lungs Systemic circuit allows blood to flow to and from the rest of the body Coronary circuit provides blood to the heart The heart valves ensure that blood flows in one direction through the system Dr. Mahmoud Elsaid | Cardiovascular System 2  The heart's contraction Contraction of the heart causes the blood to move throughout the body The conduction system contains pacemaker cells, which have three unique characteristics: o Automaticity—ability to generate an electrical impulse automatically o Conductivity—ability to pass the impulse to the next cell o Contractility—ability to shorten the fibers in the heart when receiving the impulse Sinoatrial (SA) node Normal pacemaker of the heart Generates an impulse between 60 and 100 times per minute SA node's firing spreads an impulse throughout the right and left atria, resulting in atrial contraction Atrioventricular (AV) node Situated low in the septal wall of the right atrium Slows impulse conduction between the atria and ventricles Bundle of His: located in the interventricular septum and divided into branches right and left Purkinje fibers: begin within the apex of the heart and extend through the walls of the ventricles Dr. Mahmoud Elsaid | Cardiovascular System 3 Typical impulse conduction Generates in SA node → AV node → bundle of His → Purkinje fibers → spreads and tells the blood-filled ventricles to contract Dr. Mahmoud Elsaid | Cardiovascular System 4 Electricity and Action potential of the cardiac muscle Myocardial fibres have a resting membrane potential of approximately (- 90) mV. At resting membrane potential, the Na+ channels and Ca++ channels are closed. While some leakage of K+ through K+ channels The action potential in the stimulated cardiac myocyte is divided into 5 phases: Phase 0: Rapid depolarization rapid depolarization occurs due to rapid Na+ influx, so the inner membrane gradually becomes less negative When the membrane potential becomes (- 40) mV (reaches the threshold for initiating action potential), the Ca++ influxes lead to produce the rapidly rising phase of action potential depolarization. The membrane potential reaches to the +30 mV. Dr. Mahmoud Elsaid | Cardiovascular System 5 Phase 1: initial rapid repolarization. It is a short phase. The membrane potential in this phase reaches to (-10) mV. This phase occurs due to closure of Na+ channel and opening of K+ channel. Phase 2: plateau. It occurs due to slow influx of Ca++. Phase 3: repolarization. During this phase complete repolarization and the membrane reaches to approximately resting value. Phase 4: resting potential. The membrane potential is maintained at (-90)mV. Dr. Mahmoud Elsaid | Cardiovascular System 6  Action potential causes the release Ca++ into cytoplasm (from sarcoplasm) which causes the muscle contraction. All heart cells are electrically joint one to another by intercalated disc (gap junction), so one cardiac muscle generate action potential it just spread to the other. Dr. Mahmoud Elsaid | Cardiovascular System 7 Cardiac cycle It is the inclusive period of time from the start of one heartbeat to the initiation of the next. In each cardiac cycle, there are alternate contractions and relaxation of all chambers. Each contraction is called systole and each relaxation is called diastole. The events at single cardiac cycle: 1. Atrial systole: ▪ contraction of both atria (left and right) simultaneously leads to move blood (20%) from atria to the ventricles ▪ from right atrium to the right ventricle through tricuspid valve. ▪ from left atrium to the left ventricle through mitral valve 2. Atrial diastole: 3. Ventricular systole: In this period, the tricuspid and mitral valves are closed, and the blood forced into the blood vessels through semilunar valves from right ventricle to the pulmonary artery. from left ventricle to the aorta Dr. Mahmoud Elsaid | Cardiovascular System 8