Anatomy and Physiology Unit 4 - Circulatory & Lymphatic System PDF

UNIT Iv CIRCULATORY & LYMPHATIC SYSTEM  ANATOMY CHAPTER 4 CIRCULATORY & LYMPHATIC SYSTEM  INTRODUCTION The cardiovascular system is sometimes called the circulatory system. It consists of the heart, which is a muscular pumping organ, and a closed system of vessels calle...

UNIT Iv CIRCULATORY & LYMPHATIC SYSTEM  ANATOMY CHAPTER 4 CIRCULATORY & LYMPHATIC SYSTEM  INTRODUCTION The cardiovascular system is sometimes called the circulatory system. It consists of the heart, which is a muscular pumping organ, and a closed system of vessels called arteries, veins, and capillaries. Blood contained in the circulatory system is pumped by the heart around a closed circle or circuit of vessels as it passes again and again through the various circulations of the body.  CONNECTION BETWEEN HEART AND BLOOD VESSELS  HEART External Feature The human heart is a four-chambered, conical muscular organ, pyramid shaped, sized roughly like a person’s closed fist. It lies in the mediastinum, or middle region of the thorax, just behind the body of the sternum between the second through the sixth ribs. Approximately two-thirds of the heart’s mass is to the left of the midline of the body and one third to the right. Size and Shape of the Heart At birth, the heart is said to be transverse (wide) in type and appears large in proportion to the diameter of the chest cavity. Between puberty and 25 years of age the heart attains its adult shape and weight—about 300 grams is average for the male and 225 g for the female. Its approximate dimensions are length 12 cm, width 9 cm, and depth 6 cm. Coverings of the Heart Structure of the Heart Coverings The heart has its own covering, a fibroserous sac, called the pericardium. The pericardium enclosed the heart and the root of great vessels. The sac itself is made of tough white fibrous tissue but is lined with smooth, moist serous membrane. The fibrous sac attaches to the large blood vessels emerging from the top of the heart but not to the heart itself DIFFERENT LAYERS OF PERICARDIUM Paricardial Cavity Pericardium fits loosely around the heart, with a slight space between the visceral layer adhering to the heart and the parietal layer adhering to the inside of the fibrous sac. This space is called the pericardial space. It contains (10–15 mL) of lubricating fluid secreted by the serous membrane and called pericardial fluid.  Surfaces of the Heart ✓Sternocostal Surface ✓Diaphragmatic Surface ✓Base Apex It is formed by the left ventricle is directed downward and toward the left. It lies at the level of the fifth left intercostal space, 9 cm (3.5 inches) from the midline.  Structure of the Heart Layers (Walls) of the Heart ✓Epicardium The outer layer of the heart wall is called the epicardium, a name that literally means “on the heart.” The epicardium is actually the visceral layer of the serous pericardium. It has two different names: fibrous pericardium and serous pericardium. ✓Myocardium The bulk of the heart wall is the thick, contractile, middle layer of especially constructed and arranged cardiac muscle cells called the myocardium. ✓Endocardium The lining of the interior of the myocardial wall is a delicate layer of endothelial tissue known as the endocardium. Endothelium is the type of membranous tissue that lines the heart and blood vessels. Endothelium resembles simple squamous epithelium. LAYER OF THE HEART WALL Chambers of the Heart The interior of the heart is divided by vertical septa into four cavities, or heart chambers. The right and left atria, and right and left ventricles. The right atrium lies anterior to the left atrium and the right ventricle lies anterior to the left ventricle. The two chambers of the heart, the atria are often called the “receiving chambers” because they receive blood from vessels called veins. Veins are the large blood vessels that return blood from various tissues to the heart so that the blood can be pumped out to tissues again. The atria alternately relax and contract to receive blood and then push it into ventricles. Because the atria need not generate great pressure to move blood such a small distance, the myocardial wall of each atrium is not very thick. The term auricle (meaning “little ear”) refers to the ear like flap protruding from each atrium. Thus, the auricles are part of the atria. CHAMBERS OF HEART Right Atria It forms the right border and parts of the sternocostal surface and base of heart. At the junction between the right atrium and right auricle is a vertical groove, the sulcus terminalis, which in the inside forms the crista terminalis. Right Ventricle The right ventricle communicates with the right atrium through the AV orifice and with the pulmonary trunk through the orifice. It receives blood from right atrium and pumps it to the lungs through the pulmonary trunk. Left Atrium The left atrium consists of a main cavity and a left auricle, situated behind the right atrium. It forms the greater part of the base or the posterior surface of the heart and the part of left border, left surface and sternocostal surface. POSTERIOR VIEW OF HEART SHOWING BASE OF HEART Left Ventricle The left ventricle forms the apex, sternocostal surface, and left 2/3 of diaphragmatic surface. The walls of the left ventricle are three times thicker than right ventricle.  Valves of the Heart The heart valves are mechanical devices that permit the flow of blood in one direction only. Four sets of valves are of importance to the normal functioning of the heart. Two of these, the AV valves, guard the openings between the atria and the ventricles (AV orifices). The AV valves are also called tricuspid/bicuspid valves. The other two heart valves, the semilunar (SL) valves, are located where the pulmonary artery and the aorta arise from the right and left ventricles, respectively. INTERIOR OF HEART BLOOD SUPPLY OF HEART AND VENOUS DRAINAGE OF HEART  CONDUCTION SYSTEM OF THE HEART The conducting system of the heart consists of specialized cardiac muscles present in the sinuatrial node, AV node, AV bundle and its left and right branches and the subendocardial plexus of purkinje fibers. ▪ The sinoatrial node (SA Node) ▪ Atrioventricular node (AV Node) ▪ Atrioventricular bundle ▪ Purkinje fibers Sinoatrial Node The sinoatrial node (SA node or pacemaker) located in the right atrial wall in the upper part of the sulcus terminalis near the opening of the superior vena cava. Atrioventricular Node The atrioventricular node (AV node) is a small mass of special cardiac muscle tissue, placed on the lower wall of the atrial septum just above the attachment of the septal cusp of the tricuspid valve. The atrioventricular bundle (AV bundle or bundle of His) is a bundle of special cardiac muscle fibers that originate in the AV node and extend by two branches down the two sides of the interventricular septum. Purkinje Fibers The Purkinje fibers form a subendocardial plexus. The LBB divides into anterior and posterior braches and the right bundle branch continues with the fibers of the Purkinje plexus. CONDUCTING SYSTEM OF THE HEART  ARTERIAL AND VENOUS SYSTEM The blood and lymphatic vessels that circulate through the body make up the vascular system, also known as the circulatory system. The arteries and veins transport blood throughout the body, providing the tissues with oxygen and nutrition and removing waste products from the cells. Lymphatic fluid is transported through lymph vessels (a clear, colourless fluid containing water and blood cells). By filtering and removing lymph from every area of the body, the lymphatic system aids in preserving and defending the body’s fluid environment.  ARTERIES OF THE BODY The major channels that transport oxygenated blood out from the heart are known as arteries (except for the pulmonary circuit, in which the arterial blood is deoxygenated). The systemic arteries are distributed like a ramified tree, with the main trunk, formed by the aorta, beginning at the left ventricle and the tiniest ramifications extending to the peripheral portions of the body and the enclosed organs. Types of Blood Vessels ✓Artery An artery is a vessel that carries blood away from the heart. After birth, all arteries except the pulmonary artery and its branches carry oxygenated blood. ✓Vein A vein is a vessel that carries blood toward the heart. All of the veins except the pulmonary veins contain deoxygenated blood. ✓Capillaries These are the microscopic vessels that carry blood from small arteries to small veins, that is, from arterioles to venules.  ARTERIES OF THE UPPER ARTERIES SUPPLING TO THE LIMB BRAIN ARTERIES OF THE HEAD AND NECK  LYMPHATIC TISSUE Lymph is formed from plasma (the liquid part of blood without the cells). The nutrients required to “feed” cells throughout the body are carried by the plasma. Plasma must ooze into the tissues through capillaries, the smallest blood arteries, in order to reach the cells. LYMPHATIC SYSTEM OF BODY Sections of the Lymphatic System There are two distinct sections of the lymphatic system. First section: ✓The bone marrow ✓The thymus Second Section Organs and tissues are a home for immune system cells. If immune system cells come into contact with invading organisms in any of the following locations, an immunological response may be set off. ✓Lymph nodes ✓The spleen ✓The tonsils and adenoids ✓Mucosa-associated lymphoid tissue VEINS USED FOR INTRAVENOUS INJECTIONS Forearm and arm ✓Cephalic vein ✓Basilic vein ✓Medial cubital vein ✓Medial basilic vein ✓Accessory cephalic vein ✓Cephalic vein Veins in hand Basilic vein Dorsal venous network VEINS USED FOR INTRAVENOUS Dorsal metacarpal vein INJECTIONS  CLINICAL ANATOMY The fossa ovalis is a shallow depression, which is the site of the foramen ovale in the fetus. Cardiac tamponade is acute heart failure due to compression of heart by a large or rapidly developing effusion. Pleural effusion presence of an abnormal amount of fluid and/or an abnormal character to fluid in the pericardial space. The pericardial space normally contains 15–50 mL of fluid. Vascular lesion of the heart can cause a variety of arrhythmias.  APPLICATION AND IMPLICATION IN NURSING ✓To count the apical beat, one must place a stethoscope directly over the apex, that is, in the space between the fifth and sixth ribs (fifth intercostal space). ✓Oxygen therapy is required for patients with hypoxia. ✓The two common modes of delivery of oxygen are via face masks and nasal cannulas. ✓The normal boundaries of the heart are, however, influenced by such factors as age, body build, and state of contraction. ✓For patients with chronic conditions, cardiac nurses may monitor and assess heart conditions. They carry out or help with various treatments, like advanced cardiac life support or catheterization laboratory  PHYSIOLOGY CHAPTER 4 Circulatory & Lymphatic System  INTRODUCTION The human circulatory system consists of four major components; blood, blood vessels, the heart and the neural regulatory centres in the brain. The blood acts as a transporting medium, the blood vessels are the system of conduits which transport blood to all parts of the body, the heart acts as a pump and generate the major driving force of blood flow through the vessels, and the regulatory centres control the various activities of the circulatory system. Purpose of the Cardiovascular System The cardiovascular system is responsible for maintaining this blood flow. By the contractile action of the heart, a substantial amount of pressure is created within the circulatory system, that ensures adequate blood flow to different organs for efficient exchange of gases, fluid, electrolytes, large molecules The different components of between the blood and cells. human circulatory system  HEART Location The heart is a cone shaped hollow muscular organ. It is located in the thoracic cavity in the middle mediastinum between the lungs. It is oriented obliquely, more towards the left. The sternum and the ribs lie in the front and the thoracic spinal column lies behind the heart and protects heart from physical trauma. Structure ✓The heart has four chambers. The two small upper chambers are called left and right atria and two bigger lower chambers are called right and left ventricles. There is complete separation of all four chambers; two atria are separated by the interatrial septum and the two ventricles are separated by the interventricular septum. ✓Atrium receive the blood returning to heart through veins and get passively filled. ✓Ventricles pumps out the blood from heart into the arterial system. ✓Blood moves from atria to the ventricles through openings called atrioventricular orifices. ✓The openings between left ventricle and the aorta and the right ventricle and pulmonary artery are also guarded by two other valves called semilunar or SL valves. They prevent backflow of blood from arteries to the ventricles. POSITION OF THE HEART IN THORACIC CAVITY  Functions of the Heart Pumping Action of the Heart The heart is a muscular pump and occupies the central position in the human cardiovascular system Almost 90% of the heart is composed of cardiac muscle. The heart contains pacemaker tissues that generates electrical impulse which act as the stimulus for heart muscle contraction. For this reason, the fetal heart starts beat even without neural stimuli. The cardiac muscles of the ventricles contract to generate force, which major driving force for distributing the blood throughout the body and maintains blood pressure. Endocrine Function The heart muscles secrete certain proteins called cardiokine, the most important of which is called atrial natriuretic factor (ANF). This has natriuretic and vasodilator effect. They are secreted when atrial muscles are stretched by volume overload. STRUCTURE OF THE HEART  PRINCIPLES OF CIRCULATION Blood circulates through the heart in a definite way. The circulatory pattern of blood through heart shows that blood always flows from atrium to the ventricle. The right heart (comprising the right atrium and the right ventricle) is concerned with the flow of deoxygenated blood. The left heart (comprising the left atrium and the left ventricle) is concerned with the flow of oxygenated blood. The continuous flow of deoxygenated and oxygenated blood through the right and left heart respectively is called double circulation. Pulmonary Circulation The right atrium receives deoxygenated blood via superior and inferior vena cava. When the ventricles relax, the blood moves into the right ventricle. When the right ventricle contracts, the blood is pumped into the pulmonary artery. Next blood flows across the lungs, it releases carbon dioxide and receives oxygen from lung alveoli, and returns to the left atrium by pulmonary veins as oxygenated blood. This circulation from right ventricle to left atrium is called the Pulmonary or Lesser or Central circulation Systemic Circulation The oxygenated blood that returns to left atrium, moves to left ventricle and when the left ventricles contracts, the blood is forced into the aorta and circulate into the whole body, via systemic arteries and supply oxygen to all tissues. Finally, the blood returns back to the right atrium via the superior and inferior vena cava as deoxygenated blood and moves into right atrium. This circulatory pattern from left ventricle to right atrium is called the Systemic or Peripheral or Greater circulation.  HEMODYNAMIC PRINCIPLES ✓The blood flow is a function of pressure gradient and resistance. ✓This is similar to the ohms law calculated for current flow. More the pressure gradient more the flow and vice versa and more the resistance, less the flow. ✓The pressure gradient is the difference between the mean arterial pressure (MAP) which is the pressure at the beginning of the aorta (about 100 mmHg) and the central venous pressure (CVP) which is the pressure near the terminal portion of the inferior vena cava (about 0 mmHg). ✓The flow represents the amount of blood ejected from the heart which is cardiac output. ✓The resistance denotes the resistance offered by the systemic arterioles. ✓Flow (cardiac output) = Pressure gradient × 1/Resistance Or, Pressure gradient (MAP – CVP) = Cardiac output × Systemic vascular resistance  CORONARY CIRCULATION (BLOOD SUPPLY TO THE HEART) The heart is the most metabolically active organ, with the highest oxygen consumption per mass. The coronary circulation, which is responsible for delivering blood to the myocardium and accounts for about 5% of cardiac output, meets this demand for oxygen. The heart or more specifically the heart muscles are supplied by two main arteries that come out as the first two branches from the aorta. They are known as left and right coronary arteries. These arteries subdivide into smaller coronary arteries which supplies blood to the myocardium ✓Coronary Blood Vessels The left and right coronary arteries are the first branches of the aorta, and they originate at the base of the ascending aorta. The right coronary artery follows the coronary sulcus (sulcus, which demarcates the junction of the atria and ventricle) and gives off branches that supply the right atrium, ventricles, parts of the conducting system and descends posteriorly between the two ventricles as the posterior inter ventricular branch, supplying the interventricular septum. ✓Driving Pressure in Coronary Circulation In coronary circulation the blood flows from the aorta to different coronary arteries, and then back to the right atrium. Therefore, the pressure difference between the right atrium and the aorta is the major determining factor of the coronary blood flow.  Factors Affecting Coronary Blood Flow ✓Hypoxia induced increased coronary blood flow ✓Neural factors induced increased coronary blood flow  PULMONARY CIRCULATION The lungs have three circulations—pulmonary, bronchial and lymphatic. ✓Pulmonary Circulation Pulmonary artery arises from the right ventricle and divides into right and left pulmonary arteries which convey deoxygenated blood to the right and left lung, respectively. ✓Bronchial Circulation Bronchial circulation constitutes two left and one right bronchial arteries that arise from the descending thoracic aorta. They supply oxygenated blood to lung tissues and large and small bronchi of the lungs. ✓Lymphatic Circulation Lungs are richly supplied by lymphatics. Lymphatics are present in the walls of the terminal bronchioles and in all the supportive tissues of the lungs. HEART MUSCLES ✓The cardiac muscle cells are striated, involuntary, short and branched structures. The cells have one, centrally placed nucleus and a sarcoplasmic reticulum (SR) associated with the T tubules. The T tubules are extensions of the muscle membrane that moves deep into the cell and provides extracellular calcium to the muscle cells during contraction. ✓The muscle cells are interconnected with each other. The connection between two adjacent muscles appears as dark stained areas under microscope and known as intercalated disks. Respiratory gas exchange HEART MUSCLES  CONDUCTING SYSTEM OF THE HEART Conducting system include the following type of tissues: ✓Sinoatrial Node (SA Node) This is a small nodal structure located in the right atrial wall near the opening of the superior vena cava. ✓Internodal Fibres Internodal fibres are composed of three branches; the anterior, middle and the posterior internodal tracts. ✓Atrioventricular Node (AV Node) This is a small bundle of muscle fibres at the base of the right atrium, just above the junction of the atria and ventricles. ✓His – Purkenji System This is a bundle system which mainly conducts impulse from AV node to the ventricles. It is composed of three different fibre system. Pacemaker Potential The pacemaker potential is the gradual change in the resting membrane potential of the pacemaker cells of heart which leads to the generation of action potential. EFFECTS OF SYMPATHETIC AND PARASYMPATHETIC STIMULATION ON Electrocardiogram The heart muscles are always subjected to electrical stimulation. The body tissues and fluids are conductor of electricity; therefore, the ongoing electrical activities of the heart can be recorded by placing suitable electrodes on the skin surface. This recording, called an electrocardiogram (ECG).  CARDIAC CYCLE In a single heartbeat, the heart undergoes different mechanical events, which includes contraction and relaxation of its atria and ventricles. The contraction is called systole and the relaxation is called diastole. Atrial Systole and Diastole The impulse for cardiac muscle excitation is initiated at the SA node. It is located in the left atrium. So atrial excitation and contraction is the first event of cardiac cycle. Given a cardiac cycle time of 0.8 sec, the duration of atrial systole is only 0.1 sec. During atrial systole, the blood from the atria is forced into the ventricles through the AV valves. Atrial systole is followed by atrial relaxation or diastole which is about 0.7 sec. Ventricle Systole After excitation of the atria, the impulse from the SA node pass to AV node by internodal fibres and further down to the ventricular muscles via the His Purkenji system and cause excitation (depolarization) of ventricular muscle. This leads to ventricular contraction or systole. Isovolumetric Contraction With the onset of ventricular systole, the pressure within the ventricle rises and to prevent backflow of blood from ventricles to the atria, the AV valves close. This produces the first heart sound. Rapid Ejection Phase As the name implies, in this phase, blood is ejected rapidly from ventricles into the arteries. After isovolumetric phase, ventricular contraction continues, there is further rise in intraventricular pressure. Slow Ejection Phase With rapid ejection, as much of the blood volume is ejected out, the intraventricular pressure falls, and now, blood continues to be ejected slowly for a longer period. This phase is therefore called slow ejection phase. However, the volume of blood ejected in this phase is less than the volume ejected in rapid ejection phase. Ventricular Diastole Given a cardiac cycle time of 0.8 sec, the ventricular diastolic time is 0.5 sec and it comprised of five different phases, each marked with different events. These phases are as follow: ✓Protodiastole ✓Isometric Relaxation Phase ✓1st Rapid Filling Phase ✓Slow Filling or Diastasis ✓Last rapid Filling Phase  HEART SOUNDS Mechanical activities during cardiac cycle produces four different sounds within the heart that are termed as heart sounds, each having a different characteristic feature and significance. These sounds are studied to detect cardiac problems. 1. First Heart Sound This is produced by the vibration caused by the closure of the AV valves at the onset of ventricular systole during isovolumetric contraction. It sounds like the spoken word LUBB. It can be auscultated over the mitral and tricuspid area by a stethoscope. 2. Second Heart Sound This is produced by the vibration caused by the closure of the semilunar valves at the onset of ventricular diastole just at the end of Protodiastole phase. It sounds like the spoken word DUBB. It is best auscultated over the aortic and pulmonary area. 3. Third Heart Sound It is produced during the first rapid filling phase of ventricular diastole, due to rushing of blood from atria to the ventricles. 4. Fourth Heart Sound It is produced by the vibration caused by the second rapid rush of blood from atria to the ventricles during last rapid filling phase of ventricular diastole. It is normally not audible, but can be heard when atrial pressure is high or when the ventricles are stiff and noncompliant, as occur in ventricular hypertrophy. CARDIAC OUTPUT AND STROKE VOLUME Cardiac output is the total amount of blood ejected from each ventricle per minute. The normal cardiac output is about 5 litters. Cardiac output is of immense physiological importance. It determines the amount of the blood flowing in the circulatory system and the blood supply to the organs. Factors Affecting Cardiac Output Two major physiological determinants that directly affect cardiac output are the stroke volume and heart rate. The amount of blood ejected from each ventricle in each beat is called stroke volume (SV) and the heart rate is the number of heart beats per minute. Therefore, multiplying the stroke volume with the number of heart beat per minute, gives the total amount of blood ejected from each ventricle of the heart in one minute, which is cardiac output (CO).  Factors affecting stroke volume Myocardial Contractility Contractility refers to the contracting capacity of the ventricular muscles. More the power of contraction, more blood will be ejected from the heart with each stroke and more will be the stroke volume and this will increase the cardiac output. ✓Neural factors ✓Hormonal factors ✓Ionic factors ✓Drugs Venous Return When the venous return is more, the ventricles get more filled with blood during diastole. This increases the end diastolic volume. More ventricular filling stretches the ventricles with more elongation of ventricular muscle fibres. As the ventricular muscle fibres get stretched, they contract with more force, this ejects more blood from ventricles, or in other words, the stroke volume is increased.  HEART RATE Heart rate refers to the number of times the heart beats in one minute. The normal range of heart rate varies between 60–90 beats/min with an average of 75 beats/min. Heart rate above 100 beats/min is known as tachycardia and heart rate below 60 beats/min is known as bradycardia. The normal heart rate follows a rhythmic pattern and any change in the rhythmic pattern is known as arrhythmia Conditions Decreasing HR ❑Athletes have decreased heart rate due to increased capacity of the heart ❑Emotional conditions like shock depression grief decreases heart rate Heart rate increases during inspiration and decreases during expiration this phenomena is known as sinus arrhythmia  BLOOD PRESSURE Definition The term blood pressure in cardiovascular physiology indicates the arterial blood pressure, i.e., the pressure within the arteries. It is defined as the lateral pressure exerted by the moving column of blood on the wall of the arteries ▪ Systolic Blood Pressure This is the maximum pressure in the arteries obtained during systole. The normal range of systolic pressure is 90–140 mm of Hg. ▪ Diastolic Blood Pressure This is the minimum pressure in the arteries obtained at the end of diastole. The normal range of diastolic pressure is 60–90 mm of Hg. ▪ Pulse Pressure This is the obtained as the difference between systolic and diastolic blood pressure. The normal pulse pressure is about 40 mm of Hg. ▪ Mean Arterial Blood Pressure Mean arterial blood pressure indicates the pressure in the arterial system during the whole range of systolic and diastolic phase of the heart. PULSE With each stroke volume, as the blood is forced into the aorta and subsequently in the arteries a pressure wave that travels along the arteries. As the arterial walls contain elastic tissues, the wave causes alternate expansion and contraction of the arterial wall, which is called pulse.  Examination of the Pulse Some common areas where arterial pulsations can be felt are as follows: Temporal arterial pulse, Carotid arterial pulse, Radial arterial pulse, Femoral arterial pulse, Popliteal arterial pulse. COMMON PERIPHERAL SITES OF PULSE ASSESSMENT CARDIOVASCULAR HOMEOSTASIS IN EXERCISE ✓Redistribution of Blood Flow in the Body with Increase in the Skeletal Muscle Blood Flow During strenuous exercise muscle blood flow increase up to 20 times. This is called exercise hyperaemia. ✓Increased Cardiac Activity During exercise, the peripheral stimuli from the exercising muscles and proprioceptors present in the muscles and joints are conveyed to the medullary sympathetic vasomotor center, which in turn increases all cardiac activities ✓Decrease in Blood Volume Blood volume is decreased in exercise because with increase in hydrostatic pressure more filtration of fluid occurs in the capillaries.  DISORDERS OF THE HEART Acute Pericarditis This is inflammation of the pericardium, sometimes due to infection, radiation therapy, or connective tissue disease, causing pain and friction rub. Cardiac Tamponade Compression of the heart by an abnormal accumulation of fluid in the pericardial cavity, interfering with ventricular filling; may result from pericarditis. Cardiomyopathy This is disease of the myocardium. There may thinning of the heart wall, or thickening of the interventricular septum which can cause dilation and failure of the heart. ✓Infective Endocarditis Inflammation of the endocardium, usually due to infection, by streptococcus and staphylococcus bacteria. ✓Myocardial Ischemia This is inadequate blood flow to the myocardium, usually because of coronary atherosclerosis; can lead to myocardial infarction. ✓Septal Defects Abnormal openings in the interatrial or interventricular septum, resulting in blood flow between the atrium, or blood from the left ventricle returning to the right ventricle. ✓Cardiac Failure This is the inability of the heart to pump blood in sufficient need to meet the body’s demand. Various cause of cardiac failure include; decreased cardiac contractility due to reduced coronary blood flow, valvular damage or dysfunction, external compression of the heart due to pericardial effusion, cardiomyopathy and hypertension. ✓Hypertension High blood pressure, is commonly considered to be a chronic high resting blood pressure with systolic above 140 mmHg and diastolic above 90 mmHg. Hypertension is the major cause of heart failure, stroke, and kidney failure. ✓Aneurysm An aneurysm is a weak point in a blood vessel or in the heart wall that forms a thin-walled, bulging sac that pulsates with each beat of the heart and may eventually rupture. ✓Circulatory Shock Cardiovascular homeostasis is all about maintaining adequate blood flow to different organs according to their needs. to maintain supply of nutrients and removal of wastes to and from the cells. The failure to maintain adequate flow is referred to a circulatory shock.  STRUCTURE AND FUNCTIONS OF THE VASCULAR SYSTEM Structure of the Blood Vessels The three major categories of blood vessels in the circulatory system are the arteries, veins and the capillaries. There are different categories of arteries and veins. All types of arteries and veins have the same basic three-layered structure; an inner tunica intima, middle tunica media and outer tunica adventitia. Function of the Blood Vessels Function of Arteries The main function of the arteries is to carry blood away from heart. There are different arteries each type has a specific function. The largest arteries are aorta and large arteries. They arise from heart and when the ventricles contract, the heart is ejected into these arteries. Therefore, these arteries have to withstand great force. ✓The content of elastic tissues in their walls make these arteries distensible. With each heart beat as blood is pumped out in these vessels, the wall of the vessels expands to accommodate the blood during systolic flow without much rise in pressure. ✓In the vascular tree, the branching of large arteries form smaller arteries, which by further branching produce arterioles and metarterioles with progressively smaller diameter. The smaller arteries and arterioles and meta-arterioles have smooth muscles within their walls which can constrict and alter the flow of blood through these arteries. Functions of Capillaries Four different forces, called starlings forces that act across the capillaries, determine the net movement of substances, to and from the cell to blood and vice versa. These forces are: ❑Hydrostatic pressure of the capillaries contributed by the blood pressure (favours filtration) ❑Osmotic pressure of the capillaries contributed by the plasma proteins (favours reabsorption) ❑Hydrostatic pressure in the interstitial space (favours reabsorption). ❑Osmotic pressure in the interstitial space (favours filtration)  Functions of Veins They have large diameter and thin wall and lack elastic tissues. They are supplied with valves which permits one way flow of blood through the veins and assists the blood to flow towards the heart. Small veins that arise by the unification of the capillaries are called venules. Venules are supplied by nerve and by contraction they provide post capillary resistance, which can increase hydrostatic pressure in the capillaries and increase filtration. Venules unite to form veins and veins unite to form large veins and vena cava, which returns blood to heart. LYMPHATIC CIRCULATION The lymphatic circulation is concerned with the flow of lymph and consists lymph, lymph vessels, and lymph nodes, lymph organs and lymphoid tissues. Lymph Lymph is a clear watery fluid, similar in composition to plasma, but does not contain plasma proteins. During capillary exchange, at arterial end, fluid move out of the capillaries by filtration. Lymph Vessels The smallest lymph vessels are the lymph capillaries. They arise as are blind-end tubes present in the interstitial spaces (Fig. 26). Like the blood capillaries, they have an endothelial cell layer, but their walls are more permeable. Thoracic Duct It opens into the left subclavian vein in the root of the neck. It drains lymph from both legs, the pelvic and abdominal cavities, the left half of the thorax, head and neck and the left arm. Right Lymphatic Duct It drains lymph from the right half of the thorax, head and neck and the right arm into the right subclavian vein. Lymph Nodes These are oval shaped nodular structure located along the length of lymph vessels. They contain reticular and lymphatic tissue. The lymphatic tissue contains immune and defence cells, including lymphocytes and macrophages. Lymph Organs Spleen and thymus are considered as lymph organs. Spleen store blood, performs phagocytic action and removes ageing or damaged cells from the bloodstream. The T-and B-lymphocytes present in spleen act against antigens. Lymphoid Tissues Lymphoid tissues are also known as mucosa-associated lymphoid tissue (MALT) which are present throughout the gastrointestinal tract, in the respiratory tract and in the genitourinary tract, all systems of the body exposed to the external environment.