THE CIRCULATORY SYSTEMThe Cardiovascular System and the Lymphatic System
Two systems in OneMost of the cells in the human body are not indirect contact with the external environment, sothey rely on the circulatory system to act as atransport service for them. Two fluids movethrough the circulatory system: blood andlymph. The blood, heart, and blood vessels formthe Cardiovascular System. The lymph, lymphnodes and lymph vessels form the LymphaticSystem. The Cardiovascular System and theLymphatic System collectively make up theCirculatory System.
The Cardiovascular SystemThe cardiovascular system serves a number of important functions in the body. Most of these support other physiological systems. The major cardiovascular functions fall into five categories:1) Delivery2) Removal3) Transport4) Maintenance5) Prevention
-1) The cardiovascular system delivers oxygen andnutrients to, 2) and removes carbon dioxide andmetabolic waste products from, every cell in thebody. 3) It transports hormones from endocrineglands to their target receptors. 4) The systemmaintains body temperature, and the blood’sbuffering capabilities help control the body’s pH.The cardiovascular system maintains appropriatefluid levels to prevent dehydration and helps 5)prevent infection by invading organisms.
Structure and Function of the Cardiovascular SystemThe cardiovascular system responds immediately to the body’s many and everchanging needs. All bodily functions and virtually every cell in our body depend in some way on this system.Any system of circulation requires three components:1) A pump (the heart)2) A system of channels (the blood vessels)3) A fluid medium (the blood)
The HeartThe heart is a reddish-colored hollow organ thatlies in the thoracic cavity between the lungs andbehind the sternum, supported by the muscles ofthe diaphragm. It is conical in shape, with the tippointing downwards and to the left. Inside theheart there are four chambers: two atria in theupper part, separated by the interatrial wall, andthe two ventricles in the lower part, separated bythe interventricular wall. The right chambers, theright atrium and the right ventricle communicatewith one another through the tricuspid valve, andthe left chambers through the mitral valve.
-The walls of the heart have three layers: theendocardium, a very thin internal membrane; themyocardium, an intermediate layer of striatedmuscular tissue, which is thick in the ventricular wallareas and thin in the atrial walls; and thepericardium, a membrane that envelops the heart.The heart has two atria acting as receiving chambersand two ventricles acting as sending units. The heart isthe primary pump that circulates blood through theentire vascular system.
Blood Flow Through the HeartBlood that has coursed its way between the cells of the body, delivering oxygen and nutrients and picking up waste products, returns through the great veins – the superior vena cava and inferior vena cava – to the right atrium. This chamber receives all the body’s deoxygenated blood.From the right atrium, blood passes through the tricuspid valve into the right ventricle. This chamber pumps the blood through the pulmonary semilunar valve into the pulmonary artery, which carries the blood to the right and left lungs. Thus the right side of the heart is known as the pulmonary side, sending the blood that has circulated throughout the body into the lungs for reoxygenation.
-After receiving a fresh supply of oxygen, the blood exitsthe lungs through the pulmonary veins, which carry itback to the heart and into the left atrium. All freshlyoxygenated blood is received by this chamber. Fromthe left atrium, the blood passes through the bicuspid(mitral) valve into the left ventricle. Blood leaves theleft ventricle by passing through the aortic semilunarvalve into the aorta, which ultimately sends it out to allbody parts and systems. The left side of the heart isknown as the systemic side. It receives thereoxygenated blood from the lungs then sends it out tosupply all body tissues.
The MyocardiumCardiac muscle is collectively called the myocardium. Myocardial thickness varies directly with the stress placed on the heart chambers’ walls. The left ventricle is the most powerful of the four heart chambers. Through its contractions, the chamber must pump blood through the entire system route.The left ventricle’s tremendous power is reflected by the greater size of its muscular wall compared to the other heart chambers
-Although striated in appearance, the myocardiumdiffers from skeletal muscle in one important way.Cardiac muscle fibers are anatomicallyinterconnected end-to-end by dark stainingregions called intercalated disks. These disks havedesmosomes, which are structures that anchorthe individual cells together so they don’t pullapart during contraction, and gapjunctions, which allow rapid transmission of theimpulse signaling contraction. These featuresallow the myocardium in all four chambers to actas one large muscle fiber: All fibers contracttogether.
The Cardiac Conduction System Cardiac muscle has the unique ability to generate its own electrical signal, called autoconduction, that allows it to contract rhythmically without neural stimulation. With neither neural nor hormonal stimulation, the intrinsic heart rate averages 70 to 80 beats (contractions) per minute.There are four components of the cardiac conduction system:1) Sinoatrial (SA) node2) Atrioventricular (AV) node3) Atrioventricular (AV) bundle (bundle of His)4) Purkinje fibers
-The impulse for heart contraction is initiated in thesinoatrial (SA) node, a group of specialized cardiacmuscle fibers located in the posterior wall of the rightatrium. Because this tissue generates theimpulse, typically at the frequency of about 60 to 80beats per minute, the SA node is known as the heart’spacemaker, and the beating rate it establishes is calledthe sinus rhythm. The electrical impulse generated bythe SA node spreads through both atria and reachesthe atrioventriular (AV) node. Located in the right atrialwall near the center of the heart. As the impulsespreads through the atria, they are signaled tocontract, which they do almost immediately.
- The AV node conducts the impulse from theatria into the ventricles. The impulse isdelayed by about 0.13 s as it passes throughthe AV node, then it enters the AV bundle.This delay allows the atria to fully contractbefore the ventricles do, maximizingventricular filling. The AV bundle travels alongthe ventricular septum and then sends rightand left bundle branches into both ventricles.
-These branches send the impulse toward the apexof the heart, then outward. Each bundle branchsubdivides into many smaller ones that spreadthroughout the entire ventricular wall. Theseterminal branches of the AV bundle are thePurkinje fibers. They transmit the impulsethrough the ventricles approximately six timesfaster than through the rest of the cardiacconduction system. This rapid conduction allowsall parts of the ventricles to contract at about thesame time.
Extrinsic Control of Heart Activity Although the heart initiates its own electrical impulses (intrinsic control), their timing and effects can be altered. Under normal conditions, this is accomplished primarily through three extrinsic systems:1) The parasympathetic nervous system2) The sympathetic nervous system3) The endocrine system (hormones)
-The parasympathetic system, a branch of theautonomic nervous system, acts on the heartthrough the vagus nerve (cranial nerve X). Atrest, parasympathetic system activitypredominates in a state referred to as vagal tone.The vagus nerve has a depressant effect on theheart – it slows impulse conduction and thusdecreases the heart rate. It also decreases theforce of cardiac contraction.
-The sympathetic nervous system, the otherbranch of the autonomic nervous system, hasopposite effects. Sympathetic stimulationincreases impulse conduction speed and thusheart rate (up to 250 beats per minute).Sympathetic input also increases thecontraction force. The sympathetic nervoussystem predominates during times of physicaland emotional stress, when the body’sdemands are higher.
-The endocrine system exerts its effectsthrough the hormones released by theadrenal medulla: norepinephrine andepinephrine. Like the sympathetic nervoussystem, these hormones stimulate theheart, increasing its rate. In fact, release ofthese hormones is triggered by sympatheticstimulation during times of stress and theiractions prolong the sympathetic response.
The Cardiac CycleThe cardiac cycle consists of all heartchambers undergoing a relaxation phase(diastole) and a contraction phase (systole).During diastole, the chambers fill with blood.During systole, the chambers contract andexpel their contents. The diastolic phase islonger than the systolic phase. The pressurethat blood exerts on the arterial walls duringthe two phases is called diastolic and systolicblood pressure.
The Arterial SystemThe arterial system consists of a network ofblood vessels called arteries, which start fromthe heart and extend throughout thebody, carrying by means of the arterial bloodthe oxygen that is essential for the cells tofunction. The further these vessels are fromthe heart, the narrower they become. Arteriesare the largest, most muscular, and mostelastic vessels, and they always carry bloodaway from the heart to the arterioles.
-The largest and most important artery in thebody is the aorta, which leaves the leftventricle upwards, and describes a curveknown as the aortic arch, after which it startsto descend. From the aortic arch, arteriesbranch off .
Pulmonary Arterial SystemThe pulmonary artery leaves the rightventricle and then divides into twobranches, the right and left pulmonaryarteries, which enter the lungs and spread outto form a similar structure to the bronchialtree, ending as alveolar capillaries
The Venous SystemThe venous system consists of a network of bloodvessels that approximately parallels the structureof the arterial network but runs in the oppositedirection. Veins start as venules, getting larger ontheir way back to the heart. They collect thedeoxygenated blood, loaded with wastesubstances (venous blood), to the right-handchambers of the heart, where it is passed on tothe pulmonary vessel to be oxygenated andconverted into arterial blood.
-The walls of the veins are less elastic andmuscular than those of the arteries, becausethe blood circulating through them is beingdrawn in by the suction effect of the heart.Inside the veins, any backflow of blood isprevented by a system of valves.
-The two major veins of the body are thesuperior and inferior venae cavae, two largeintrathoracic veins; the former receives thevenous circulation from the upperextremities, the head and the neck, and thelatter receives the blood from the lowerextremities and the abdominal and thoraciccavities
Factors that Help Venous Blood return to the Heart• By the time blood has passed from the capillaries into the venous system the pressure has dropped significantly. The average blood pressure in the venous system is only 2 mmHg (millimeters of mercury) as compared to an average of 100 mmHg in the arterial system. The low venous pressure is barely adequate to drive blood back to the heart, particularly from the legs. Other mechanisms are needed to aid in the return of blood to the heart. The flow of venous blood back to the heart is increased by (1) the sympathetic nervous system, (2) the skeletal muscle pump, and (3) the respiratory pump.
-• Veins are enervated by sympathetic motor neurons. Sympathetic input causes vasoconstriction, which increases pressure, which drives blood back to the heart. When the body needs to mobilize more blood for physical activity, the sympathetic nervous system induces vasoconstriction of veins.
-• The action of the skeletal muscle pump. Veins pass between skeletal muscles. The contraction of skeletal muscle squeezes the vein, thus increasing blood pressure in that section of the vein. Pressure causes the upstream valve (furthest from the heart) to close and the downstream valve (the one closest to the heart) to open. Repeated cycles of contraction and relaxation, as occurs in the leg muscles while walking, effectively pumps blood back to the heart.
-• While the contraction of skeletal muscle in the legs drives venous blood out of the lower limbs, the act of breathing helps to drive venous blood out of the abdominal cavity. As air is inspired, the diaphragm descends and abdominal pressure increases. The increasing pressure squeezes veins and moves blood back toward the heart. The rhythmic movement of venous blood caused by the act of breathing is called the respiratory pump. Gravity helps the backflow of blood from the areas above the heart (head, neck, shoulders…)
Capillaries• Capillaries are the smallest and most numerous of blood vessels. Capillaries function as the site of exchange of nutrients and wastes between blood and tissues. The anatomy of capillaries is well suited to the task of efficient exchange. Capillary walls are composed of a single layer of epithelial cells surrounded by a basement layer of connective tissue. The thin nature of the walls facilitates efficient diffusion of oxygen and carbon dioxide. Most capillaries also have pores between cells that allow for bulk transport of fluid and dissolved substances from the blood into the tissues and vice versa.
-• Although capillaries are extremely numerous (40 billion in the body), collectively they hold only about 5% of the total blood volume at any one time. This is because most capillaries are closed most of the time. Precapillary sphincters, which are bands of smooth muscle that wrap around arterioles, control the amount of blood flowing in a particular capillary bed. Contraction of the sphincter shuts off blood flow to a capillary bed, while relaxation of the sphincter allows blood to flow.
The Blood• The average adult has about five liters of blood coursing through blood vessels, delivering essential elements, and removing harmful wastes. Without blood, the human body would stop working.• Blood is the fluid of life, transporting oxygen from the lungs to body tissue and carbon dioxide from body tissue to the lungs. Blood is the fluid of growth, transporting nourishment from digestion and hormones from glands throughout the body. Blood is the fluid of health, transporting disease fighting substances to the tissue and waste to the kidneys.• Blood contains red blood cells (erythrocytes) and white blood cells (leukocytes), which are responsible for nourishing and cleansing the body. Since the cells are alive, they too need nourishment.
-• Vitamins and Minerals keep the blood healthy. The blood cells have a definite life cycle, just as all living organisms do. Approximately 55 percent of blood is plasma, a straw-colored clear liquid. The liquid plasma carries the solid cells and the platelets ( thrombocytes), which help blood clot. Without blood platelets, we would bleed to death.• When the human body loses a little bit of blood through a minor wound, the platelets cause the blood to clot so that the bleeding stops. Because new blood is always being made inside of our bones, the body can replace the lost blood. When the human body loses a lot of blood through a major wound, that blood has to be replaced through a blood transfusion from other people.• But everybodys blood is not the same. There are four different blood types.
The Lymphatic System• The lymphatic system consists of organs, ducts, and nodes. It transports a watery clear fluid called lymph.• This fluid distributes immune cells and other factors throughout the body. It also interacts with the blood circulatory system to drain fluid from cells and tissues.• The lymphatic system contains immune cells called lymphocytes, which protect the body against antigens (viruses, bacteria, etc.) that invade the body.
FunctionsMain functions of the lymphatic system:• "to collect and return interstitial fluid, including plasma protein to the blood, and thus help maintain fluid balance,• to defend the body against disease by producing lymphocytes,• to absorb lipids from the intestine and transport them to the blood."