circulatory system 2020.pdf

 The circulatory system is composed of the heart and blood vessels, including arteries, veins, and capillaries. The bodies actually have two circulatory systems: - The pulmonary circulation is a short loop from the heart to the lungs and back again. - The systemic circulation sends blood from...

 The circulatory system is composed of the heart and blood vessels, including arteries, veins, and capillaries. The bodies actually have two circulatory systems: - The pulmonary circulation is a short loop from the heart to the lungs and back again. - The systemic circulation sends blood from the heart to all the other parts of the bodies and back again.  Blood vessels There are three types of blood vessels: Arteries: carry oxygenated blood from the heart to the rest of the body. Arteries are strong and stretchy, which helps push blood through the circulatory system. Their elastic walls help keep blood pressure consistent. Arteries branch into smaller arterioles. Veins: these carry deoxygenated blood back to the heart and increase in size as they get closer to the heart. Veins have thinner walls than arteries. Capillaries: they connect the smallest arteries to the smallest veins. They have very thin walls, which allow them to exchange compounds with surrounding tissues, such as carbon dioxide, water, oxygen, waste, and nutrients.  Basic anatomy of the heart The heart is the key organ in the circulatory system. The heart is a hollow, muscular organ, its main function is to propel blood throughout the body. It usually beats from 60 to 100 times per minute, but can go much faster when necessary. The heart is located under the rib cage -- 2/3 of it is to the left of the breastbone (sternum) -- and between the lungs and above the diaphragm. It is about the size of a closed fist, weighs about 200 to 425 grams and is somewhat cone-shaped. Chambers of the Heart The heart contains 4 chambers: the right atrium, left atrium, right ventricle, and left ventricle. The atria are smaller than the ventricles and have thinner, less muscular walls than the ventricles. The atria act as receiving chambers for blood, so they are connected to the veins that carry blood to the heart. The ventricles are the larger, stronger pumping chambers that send blood out of the heart. The ventricles are connected to the arteries that carry blood away from the heart. The chambers on the right side of the heart are smaller and have less myocardium in their heart wall when compared to the left side of the heart. This difference in size between the sides of the heart is related to their functions and the size of the 2 circulatory loops. The right side of the heart maintains pulmonary circulation to the nearby lungs while the left side of the heart pumps blood all the way to the extremities of the body in the systemic circulatory loop Septum of the heart The left atria and left ventricle are separated from the right atria and right ventricle by the septum. The septum of the heart is the dividing wall between the right and left sides of the heart. That portion of the septum that separates the two upper chambers (the right and left atria) of the heart is termed the atrial (or interatrial) septum while the portion of the septum that lies between the two lower chambers (the right and left ventricles) of the heart is called the ventricular (or interventricular) septum. The word "septum" is borrowed from the Latin "septum" meaning a "dividing wall or enclosure The interventricular septum separates the two ventricles. The lower and much larger part of the interventricular septum is termed the muscular interventricular septum and is composed of muscle of similar thickness to that of the left ventricular free wall. The uppermost portion of the septum, termed the membranous interventricular septum, also forms a portion of the right atrial wall.  The Heart Valves The heart has four valves that regulate blood flow through the heart in one direction: Two atrioventricular (AV) valves separate the atria from the ventricles: The right-sided tricuspid valve is three leaflet structure. The tricuspid valve regulates blood flow between the right atrium and right ventricle ( deoxygenated blood). The left-sided mitral valve has only two leaflets. The mitral valve lets oxygen-rich blood from the lungs pass from the left atrium into the left ventricle. Two semilunar valves separate the ventricles from their respective outflow tract: Both pulmonary and aortic valves are composed of three-leaflets or cusps (three cusps). The aortic valve opens the way for oxygen-rich blood to pass from the left ventricle into the aorta, the body’s largest artery. The pulmonary valve controls blood flow from the right ventricle into the pulmonary arteries, which carry blood to the lungs to pick up oxygen.  wall of the heart Pericardium The heart sits within a fluid-filled cavity called the pericardial cavity. The walls and lining of the pericardial cavity are a special membrane known as the pericardium. Pericardium is a type of serous membrane that produces serous fluid to lubricate the heart and prevent friction between the ever beating heart and its surrounding organs. Besides lubrication, the pericardium serves to hold the heart in position and maintain a hollow space for the heart to expand into when it is full. The pericardium has 2 layers—a visceral layer that covers the outside of the heart and a parietal layer that forms a sac around the outside of the pericardial cavity.  wall of the heart (Cont.) The wall of the heart consists of three layers of tissue: Endocardium (inner layer) — lines the inside of the heart and protects the valves and chambers Myocardium (middle layer) — the muscles of the heart. Epicardium (outer layer)—it is also known as visceral pericardium as it forms the inner layer of the pericardium. It is protective layer mostly made of connective tissue. These layers are covered in a thin protective coating called the pericardium.  Physiological Aspects Normally, the heart rate is 60-100 beats/minute & the heart rhythm is regular. Sino-atrial node (SAN): It is the normal pace-maker of the heart. It is situated in the upper part of right atrium anterior to superior vena cava. Sympathetic stimulation accelerates while parasympathetic (vagal) stimulation slows the SAN. The impulses spread from the SAN to stimulate both atria causing atrial contraction (represented by P wave). Atrio-ventricular node (AVN): Impulses pass from atria to ventricles through the AVN in which there is delay of contraction to allow atria to contract before ventricles. Sympathetic stimulation accelerates while vagal stimulation slows the conduction in AVN. The passage of impulses from the atria to the ventricles is represented by P-R interval. Normally the AVN allows passage of impulses from the atria to the ventricles but not the reverse (i.e. no retrograde conduction).  Impulses pass from the AVN to bundle of His in the interventricular septum, then along the 2 bundle branches (where bundle of His divides into right and left pathways, called bundle branches), to stimulate the right and left ventricles. & finally Purkinje fibers to terminate in ventricular myocardium causing ventricular contraction ( represented by QRS complex). The ventricles are supplied by the sympathetic system but not the vagus nerve. Conductive system : consists of: Sinoatrial node Atrioventricular node (AVN) Bundle of His (atrioventricular (AV) bundle) Right & left bundle branches Purkinje fibers  Normal cardiac rhythm results from electrical impulses that begin in a special group of cells that form the sinoatrial node (SAN), which is located in the right atrium. The SAN acts as the heart’s natural pacemaker. Impulses spread from the sinus node to the right and left atria causing them to contract at the same time. The impulses then travel to the atrioventricular node (AVN) to the Bundle of His (AV Bundle). From the AV Bundle, the impulses travel through conducting system (located in the septum), this conducting system splits to form the right and left bundle branches which split to the purkinje fibres. How the heart works ? Right side The right atrium receives deoxygenated blood from the body through veins called the superior and inferior vena cava (the largest veins in the body). The right atrium contracts and blood passes to the right ventricle. Once the right ventricle is full, it contracts and pumps the blood through to the lungs via the pulmonary artery, where it picks up oxygen and offloads carbon dioxide. Left side Newly oxygenated blood returns to the left atrium via the pulmonary vein. The left atrium contracts, pushing blood into the left ventricle. Once the left ventricle is full, it contracts and pushes the blood back out to the body via the aorta. Heart rate: It is the number of heart beats per minute. Normally: 60-100/minute. It is controlled by autonomic nervous system: Sympathetic stimulation increases the heart rate. Parasympathetic stimulation decreases the heart rate. Stroke volume is the amount of the blood pumped by ventricle with each beat. It depends on: - Preload volume load or diastolic load. - Afterload pressure load or systolic load. - Contractility strength of cardiac muscle. Preload: It is the degree of stretch of myocardium prior to contraction. Within limits, the stroke volume is directly related to preload. Starling’s law: within limits, the greater the length of myocardial fibers the more powerful the contraction. Afterload: It is the resistance facing the ventricle during contraction. The stroke volume is inversely related to afterload. Contractility: It is the force of contraction of cardiac muscle. The stroke volume is directly related to contractility.  Preload = volume of blood received by the heart. Basically, preload is stretch. The amount of the volume being returned to the heart. preload is the end diastolic volume that stretches the right or left ventricle of the heart to its greatest dimensions under variable physiologic demand. It is the amount of ventricular stretch at the end of diastole. Some people remember this by using analogy of a balloon – blow air into the balloon and it stretches; the more air you blow in, the greater the stretch.  Afterload = pressure or resistance the heart has to overcome to eject blood. Afterload is squeeze. The amount of resistance that the heart has to overcome in order to eject blood. Afterload is the pressure the heart must work against to eject blood during systole (ventricular contraction).... As aortic/pulmonary pressure increases, the afterload on the left/right ventricle increase respectively. It is known as the systemic vascular resistance. It is the amount of resistance the heart must overcome to open the aortic valve and push the blood volume out into the systemic circulation. If you think about balloon analogy; afterload is represented by the knot at the end of the balloon. To get the air out, the balloon must work against that knot. Cardiac output: It is volume of blood pumped by each ventricle in one minute. Cardiac output (COP) = heart rate (HR) x stroke volume (SV) Normally 5 liters/minute in the average. It depends on - Heart rate - Preload - Afterload - contractility