the circulatory system
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THE CIRCULATORY SYSTEM
The Cardiovascular System and the Lymphatic System
Two systems in One
Most of the cells in the human body are not in direct contact with the external environment, so they rely on the circulatory system to act as a transport service for them. Two fluids move through the circulatory system: blood and lymph. The blood, heart, and blood vessels form the Cardiovascular System. The lymph, lymph nodes and lymph vessels form the Lymphatic System. The Cardiovascular System and the Lymphatic System collectively make up the Circulatory System.
Two in One
The Cardiovascular System
The 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
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1) The cardiovascular system delivers oxygen and nutrients to, 2) and removes carbon dioxide and metabolic waste products from, every cell in the body. 3) It transports hormones from endocrine glands to their target receptors. 4) The system maintains body temperature, and the blood’s buffering capabilities help control the body’s pH. The cardiovascular system maintains appropriate fluid levels to prevent dehydration and helps 5) prevent infection by invading organisms.
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Structure and Function of the Cardiovascular System
The 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 Heart
The heart is a reddish-colored hollow organ that lies in the thoracic cavity between the lungs and behind the sternum, supported by the muscles of the diaphragm. It is conical in shape, with the tip pointing downwards and to the left. Inside the heart there are four chambers: two atria in the upper part, separated by the interatrial wall, and the two ventricles in the lower part, separated by the interventricular wall. The right chambers, the right atrium and the right ventricle communicate with one another through the tricuspid valve, and the left chambers through the mitral valve.
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The walls of the heart have three layers: the endocardium, a very thin internal membrane; the myocardium, an intermediate layer of striated muscular tissue, which is thick in the ventricular wall areas and thin in the atrial walls; and the pericardium, a membrane that envelops the heart.
The heart has two atria acting as receiving chambers and two ventricles acting as sending units. The heart is the primary pump that circulates blood through the entire vascular system.
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Blood Flow Through the Heart
Blood 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.
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After receiving a fresh supply of oxygen, the blood exits the lungs through the pulmonary veins, which carry it back to the heart and into the left atrium. All freshly oxygenated blood is received by this chamber. From the left atrium, the blood passes through the bicuspid (mitral) valve into the left ventricle. Blood leaves the left ventricle by passing through the aortic semilunar valve into the aorta, which ultimately sends it out to all body parts and systems. The left side of the heart is known as the systemic side. It receives the reoxygenated blood from the lungs then sends it out to supply all body tissues.
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The Myocardium
Cardiac 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
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Although striated in appearance, the myocardium differs from skeletal muscle in one important way. Cardiac muscle fibers are anatomically interconnected end-to-end by dark staining regions called intercalated disks. These disks have desmosomes, which are structures that anchor the individual cells together so they don’t pull apart during contraction, and gap junctions, which allow rapid transmission of the impulse signaling contraction. These features allow the myocardium in all four chambers to act as one large muscle fiber: All fibers contract together.
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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
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The impulse for heart contraction is initiated in the sinoatrial (SA) node, a group of specialized cardiac muscle fibers located in the posterior wall of the right atrium. Because this tissue generates the impulse, typically at the frequency of about 60 to 80 beats per minute, the SA node is known as the heart’s pacemaker, and the beating rate it establishes is called the sinus rhythm. The electrical impulse generated by the SA node spreads through both atria and reaches the atrioventriular (AV) node. Located in the right atrial wall near the center of the heart. As the impulse spreads through the atria, they are signaled to contract, which they do almost immediately.
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The AV node conducts the impulse from the atria into the ventricles. The impulse is delayed by about 0.13 s as it passes through the AV node, then it enters the AV bundle. This delay allows the atria to fully contract before the ventricles do, maximizing ventricular filling. The AV bundle travels along the ventricular septum and then sends right and left bundle branches into both ventricles.
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These branches send the impulse toward the apex of the heart, then outward. Each bundle branch subdivides into many smaller ones that spread throughout the entire ventricular wall. These terminal branches of the AV bundle are the Purkinje fibers. They transmit the impulse through the ventricles approximately six times faster than through the rest of the cardiac conduction system. This rapid conduction allows all parts of the ventricles to contract at about the same 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)
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The parasympathetic system, a branch of the autonomic nervous system, acts on the heart through the vagus nerve (cranial nerve X). At rest, parasympathetic system activity predominates in a state referred to as vagal tone. The vagus nerve has a depressant effect on the heart – it slows impulse conduction and thus decreases the heart rate. It also decreases the force of cardiac contraction.
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The sympathetic nervous system, the other branch of the autonomic nervous system, has opposite effects. Sympathetic stimulation increases impulse conduction speed and thus heart rate (up to 250 beats per minute). Sympathetic input also increases the contraction force. The sympathetic nervous system predominates during times of physical and emotional stress, when the body’s demands are higher.
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The endocrine system exerts its effects through the hormones released by the adrenal medulla: norepinephrine and epinephrine. Like the sympathetic nervous system, these hormones stimulate the heart, increasing its rate. In fact, release of these hormones is triggered by sympathetic stimulation during times of stress and their actions prolong the sympathetic response.
The Cardiac Cycle
The cardiac cycle consists of all heart chambers undergoing a relaxation phase (diastole) and a contraction phase (systole). During diastole, the chambers fill with blood. During systole, the chambers contract and expel their contents. The diastolic phase is longer than the systolic phase. The pressure that blood exerts on the arterial walls during the two phases is called diastolic and systolic blood pressure.
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The Vascular System
The vascular system is composed of a series of vessels that transport blood from the heart to the tissues and back:
• Arteries• Arterioles• Capillaries• Venules• Veins
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The Arterial System
The arterial system consists of a network of blood vessels called arteries, which start from the heart and extend throughout the body, carrying by means of the arterial blood the oxygen that is essential for the cells to function. The further these vessels are from the heart, the narrower they become. Arteries are the largest, most muscular, and most elastic vessels, and they always carry blood away from the heart to the arterioles.
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The largest and most important artery in the body is the aorta, which leaves the left ventricle upwards, and describes a curve known as the aortic arch, after which it starts to descend. From the aortic arch, arteries branch off .
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Pulmonary Arterial System
The pulmonary artery leaves the right ventricle and then divides into two branches, the right and left pulmonary arteries, which enter the lungs and spread out to form a similar structure to the bronchial tree, ending as alveolar capillaries
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The Venous System
The venous system consists of a network of blood vessels that approximately parallels the structure of the arterial network but runs in the opposite direction. Veins start as venules, getting larger on their way back to the heart. They collect the deoxygenated blood, loaded with waste substances (venous blood), to the right-hand chambers of the heart, where it is passed on to the pulmonary vessel to be oxygenated and converted into arterial blood.
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The walls of the veins are less elastic and muscular than those of the arteries, because the blood circulating through them is being drawn in by the suction effect of the heart. Inside the veins, any backflow of blood is prevented by a system of valves.
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The two major veins of the body are the superior and inferior venae cavae, two large intrathoracic veins; the former receives the venous circulation from the upper extremities, the head and the neck, and the latter receives the blood from the lower extremities and the abdominal and thoracic cavities
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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.
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• 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.
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• 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.
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• 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.
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• 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.
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-• 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 everybody's 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.
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Functions
Main 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."
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