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Transcript
Structure of the Cardiovascular System What you Should Know • The structure and function of arteries, capillaries and veins including the endothelium, central lumen, connective tissue, elastic fibres, smooth muscle and valves. • The role of vasoconstriction and vasodilation in controlling blood flow. What is the Cardiovascular system? • The cardiovascular system, also known as the circulatory system, is composed of blood, blood vessels and the heart. • The heart functions as a pump to move blood through the blood vessels of the body. • A circulatory system is essential for large, multi-cellular organisms, such as humans and animals, and provide at least five major functions that are necessary for life. The five major functions of the cardiovascular system are: • Transporting oxygen and removing carbon dioxide • Transporting nutrients and removing wastes • Fighting disease • Transporting hormones • Regulating body temperature Cardiovascular system • http://www.youtube.com/watch?v=CjNKbL _-cwA Components of the CVS • The CVS consists of a double pump (the heart) and a complex system of blood vessels. • Blood circulates from the heart through the arteries to the capillaries to the veins and back to the heart. • There is a decrease in blood pressure as the blood moves away from the heart. Blood Vessels There are 3 types of blood vessel: • Arteries (and arterioles) • Capillaries • Veins (and venules) Central lumen of blood vessels • The endothelium lining the central lumen of the blood vessels is surrounded by layers of tissue. • These surroundings layer differ in each type of blood vessel ARTERIES • Carry blood away from the heart • Endothelium – One cell thick • Elastic tissues & smooth muscles – Rebounds – Evens flow • Fibrous tissue – Tough – Resists stretch Arteries •Arteries carry blood away from the heart. •The largest arteries e.g. the Aorta, have thick elastic walls which can stretch to accommodate the surge of blood after each contraction of the heart. •Arteries branch many times, forming smaller and smaller vessels, the smallest of which are arterioles. • Contraction of the smooth muscle lining the walls of the arterioles allows them to open or close to varying degrees to adjust blood flow to different parts of the body e.g. during vasodilation and vasoconstriction VEINS • Carry blood towards the heart • Endothelium • Larger lumen than arteries • Thinner muscle layer & few elastic fibres – Blood at lower pressure • Fibrous tissue VEINS • Contain valves – Prevents backflow of blood • Situated between skeletal muscles – Muscle compresses vein when contracted – Blood “squirted” towards heart CAPILLARIES • Transport blood between arteries and veins • Form large networks (capillary beds) • Exchange of materials between blood and cells • Their walls are only one cell thick, allowing nutrients and waste to diffuse through with ease. Capillaries Arteriole Capillaries (capillary bed) Venule Blood vessels Tissue Fluid and the Lymphatic system What you Should Know • Tissue fluid supplies cells with glucose, oxygen and other substances. • Carbon dioxide and other metabolic wastes diffuse out of the cells and into the tissue fluid to be excreted. • Much of the tissue fluid returns to the blood. • Lymphatic vessels absorb excess tissue fluid and return the lymph fluid to the circulatory system. • Similarity of tissue fluid and blood plasma • Pressure filtration of fluids through capillary walls. Tissue Fluid • Blood consists of red and white blood cells, platelets and plasma • Plasma is a watery yellow liquid containing dissolved substances such as glucose, amino acids, respiratory gases, plasma proteins and useful ions • The blood plasma fluid which leaks into • the tissues is called tissue fluid Some of the tissue fluid returns to the blood at the venous end of the capillary Cells Lymphatic system tissue organs Blood plasma to tissue fluid • Blood pressure forces the fluid part of the blood • • • along with small soluble molecules out of the capillaries into the tissue fluid. Blood cells and large protein molecules are left behind The cells exchange molecules with the tissue fluid by diffusion down a concentration gradient Useful molecules such as food and oxygen diffuse into the cells whilst carbon dioxide and wastes diffuse out of them Lymph • The fluid that does not return to the blood is now referred to as lymph and is collected by the lymphatic system. • Lymph travels under low pressure to enter the main circulation near the heart • The lymphatic system has no pump • The contraction of skeletal muscles squeezes lymph along the vessels • Lymph travels through vessels in one direction only due to the presence of valves Comparison of contents of plasma and tissue fluid Plasma Tissue fluid More protein Less protein Red blood cells & white blood cells White blood cells Less fat More fat Summary Tissue Fluid and Lymphatic System Lymph passes into lymphatic system Lymph vessel Blood leaving in venule low pressure Some tissue fluid enters capillary by osmosis Some tissue fluid enters lymphatic system Blood arriving in the arteriole high pressure capillary Some plasma forced out of capillary Respiring cell Tissue fluid Structure and Function of the Heart Cardiac Function and Cardiac Output What You Should Know • Definition of cardiac output and its calculation. • Description of the cardiac cycle to include the functions of atrial systole, ventricular systole, diastole. • Effect of pressure changes on atrioventricular (AV) and semi lunar (SL) valves. Heart Rate (HR) • Number of times heart beats in one minute • Normal values around 72bpm • Normal range is between 60-90 Cardiac Output Cardiac Output is the volume of blood pumped by each ventricle per minute and is the function of two factors: • Heart rate (beats per minute) • Stroke volume (the volume of blood ejected by each ventricle during each contraction) • The left and right ventricles pump the same volume of blood through the aorta and pulmonary artery. CO = HR x SV Cardiac Output (CO) • The volume of blood pumped by each ventricle per minute, measured in litres • Calculated as follows • CO = HR x SV • Normal values are around 5 litres/min Stroke Volume (SV) • Volume of blood ejected by each ventricle during contraction • Normal values are around 70ml At rest: HR = 72bpm SV = 70ml i.e. CO = 72 x 70 = 5040 ml/min = 5 litres/min • Cardiac Output varies between individuals and depends on their physical fitness and level of activity. • For example, the heart of a highly trained athlete can pump 30-35 l/min • Most non-athletes can only achieve a max of 20 litres. Some typical values for cardiac output at varying levels of activity Activity Level Heart rate (bpm) Stroke Volume (ml) Cardiac Output (l/min) Rest 72 70 5 Mild 100 110 11 Moderate 120 112 13.4 Heavy (athlete) 200 150 30 Structure of the Heart Revision on Circulatory System • Double system • The pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to heart • The systemic circuit carries oxygenated blood from the left ventricle to the aorta and then the rest of the body and returns deoxygenated from the body to the heart The cardiac cycle • Each heartbeat is called a cardiac cycle and consists of the following • Atria contract simultaneously • Ventricles contract simultaneously • All chambers relax • Lasts about 0.8 secs (0.3systole,0.5 diastole) Two phases of the cardiac cycle • Systole: contraction of the heart (Atrial first, the ventricular), blood forced out of chambers • Diastole: relaxation of the heart, chambers fill with blood • The opening and closing of the AV and SL valves are responsible for the heart sounds heard with a stethoscope. THE CARDIAC CYCLE 1. Atrial & ventricular diastole 2. Atrial systole, ventricular diastole 3. Ventricular systole, atrial diastole ATRIAL & VENTRICULAR DIASTOLE • Blood enters atria from vena cava & pulmonary vein • AV valves open ATRIAL SYSTOLE & VENTRICULAR DIASTOLE • Both atria contract • Blood forced into relaxed ventricles • AV valves still open • Ring of muscle around entrance to each atrium closed • Prevents backflow of blood into veins ATRIAL DIASTOLE & VENTRICULAR SYSTOLE • About 0.1 secs after atrial systole • Ventricles contract • Blood forced into arteries through the open semi-lunar valves • AV valves close • Prevents backflow of blood to atria ATRIAL DIASTOLE & VENTRICULAR SYSTOLE • Semi-lunar valves close when pressure in arteries exceeds pressure in ventricles • Cardiac cycle begins again! PRESSURE CHANGES THE CONDUCTING SYSTEM OF THE HEART What You Should Know • • • • • • • • • • The structure and function of cardiac conducting system including nervous and hormonal control. Control of contraction and timing by cells of the sino-atrial node (SAN) and atrio-ventricular node (AVN). Interpretation of electrocardiograms (ECG). The medulla regulates the rate of the SAN through the antagonistic action of the autonomic nervous system (ANS). Sympathetic accelerator nerves release adrenaline (epinephrine) and slowing parasympathetic nerves release acetylcholine. Blood pressure changes, in response to cardiac cycle, and its measurement. Blood pressure changes in the aorta during the cardiac cycle. Measurement of blood pressure using a sphygmomanometer. A typical reading for a young adult is 120/70 mmHg. Hypertension is a major risk factor for many diseases including coronary heart disease. The Conducting System of the Heart • The heart beat starts in the heart itself • It is regulated by the nervous system and hormones 1. Sino-atrial (SA) node 2. Atrio-ventricular (AV) node 3. Bundle of His 4. Right & left bundle branches 5. Purkinje fibres • The muscle cells of the heart are myogenic as they contract spontaneously • The sinoatrial node (SAN) also known as the pacemaker is situated in the wall of the right atrium • It ensures that both atria contract simultaneously • It sends out a wave of excitation (electrical impulses) which is carried through the muscular walls of both atria • Results in atrial systole THE AV NODE • (2)The AVN then picks up the impulse from SAN • (3)Passes to Bundle of His • Travels down the septum • (4)Passes to the bundle branches (right & left) • (5)Passes to Purkinje fibres • Ventricles contract from bottom up SAN Bundle of His AVN septum Regulation of the Heart Autonomic Nervous System and the Heart • The autonomic nervous system regulates the functions of our internal organs such as the heart, stomach and intestines. • The autonomic nervous system functions involuntary. For example, we do not notice when blood vessels change size or when our heart beats faster. • The control centre for regulation of heart rate is located in the medulla of the brain • The autonomic nervous system consists of 2 opposing (antagonistic) branches – Sympathetic pathway – Parasympathetic pathway Sympathetic Nerve • An increase in the number of nerve impulses at the SAN via the sympathetic nerve increases heart rate • Sympathetic accelerator nerves release norepinephrine (noradrenaline) Parasympathetic Nerve • An increase in the number of nerve impulses at the SAN via the parasympathetic nerve decreases heart rate • parasympathetic nerves release acetylcholine. Hormonal Regulation of the Heart • Under certain circumstances e.g. stress or exercise the sympathetic nervous system causes the adrenal glands to produce adrenaline which travels in the blood to act on the SAN, which generates impulses at a higher rate, increasing heart rate ELECTROCARDIOGRAM • ECG (electrocardiogram) is a test that measures the electrical activity of the heart. • The signals that make the heart's muscle fibres contract come from the sinoatrial node. • In an ECG test, the electrical impulses made while the heart is beating are recorded and shown on a piece of paper. • This is known as an electrocardiogram, and records any problems with the heart's rhythm, and the conduction of the heart beat through the heart which may be affected by underlying heart disease. • Electrodes placed on body • Three waves ELECTROCARDIOGRAM • P wave – Electrical excitation from SA node spreading across atria • QRS wave – Wave of excitation passing over the ventricles • T wave – Electrical recovery of the ventricles ABNORMAL ECG’S • Atrial flutter • Rapid contraction of the atria • Atria contract 3 times for every ventricular contraction ABNORMAL ECG • Ventricular tachycardia • Ventricles beat rapidly and independently of the atria ABNORMAL ECG’S • Ventricular fibrillation • Unco-ordinated electrical activity • Pumping cannot take place • Fatal if not corrected • Defibrillation Blood Pressure • Force exerted by blood against walls of blood vessels • Measure 2 values – systolic BP (pressure during ventricular contraction) and diastolic BP (pressure during ventricular relaxation) • Measured using sphygmomanometer in mm Hg • Normal values are around 120/70 Hypertension • Hypertension is a major risk factor for many diseases including coronary heart disease. Measurement of BP Both systolic and diastolic BP can be measured by an inflatable instrument called a sphygmomanometer which is wrapped around the upper arm.