Forwards run the length of each fiber.

Forwards run the length of each fiber.

Forwards and backwards to the right and are at the same level of the fifth to eight dorsal vertebrae.

The apex of the heart points downwards and forwards to the left and corresponds to the space between the fifth and sixth ribs. However, in thin people, the hearts apex may be pointing more downwards than to the left.Its atrial border corresponds to a line drawn across the sternum on a level with the upper border of the third costal cartilage. Its apex corresponds to a line drawn across the lower end of the same bone. Its upper surface is rounded and convex, directed upwards and forwards, which is formed mainly by the right ventricle and a part of the left ventricle. The back surface of the heart is flattened and rests upon the diaphragm.

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Of its two borders, the right is the longest and thinnest, the left is shorter but thicker and round. The muscles that make up the heart are known as cardiac muscles. Cardiac muscle only exists in the heart, not like skeletel muscle which is found in many parts of the body. Cardiac muscle fibers possess striations that are typical of skeletel muscle.

However, they only respond to the autonomic nervous system and electrical commands that are generated from the heart. Skeletel muscle may have many nuclei, but cardiac muscle only has one nucleus. As well, cardiac muscle is very small compared to the larger skeletel muscle. As fitting with its duty, cardiac muscle has many mitochondria to convert food into energy faster than other muscles. Cardiac muscles communicate between junctions that are laid down between the muscles.

They are called intercalated disks. Along certain points of the disks, cell membranes fuse together. The electrical current required to cause the muscles to contract pass through the cells easily and the adjoining cells will respond as well due to the intercalated disks.

The cardiac muscle is really a large number of cells working together that function to act as a single cell. There are many proteins that give cardiac, as well as other muscles, to contract. Thin bundles of protein called myofibrils run the length of each fiber. Within the myofibrils are filaments (tiny threads of protein) that are arranged in a repeating pattern called a sarcomere.

The filaments in each sacromere are made up of the proteins actin and myosin. Two clusters of actin are set in each end of the sacromere stretch towards the centre but do not touch. There are continuos threads of myosin located at the end of the sarcomere. The contraction can occur because of the region where the actin and myosin over lap each other. Small hooks on the myosin binds to the actin filaments and pull towards the centre of the sarcomere. This happens through the rapid ratchet-like actions of the myosin and actin pulling together.

When the sarcomere pulls together, the fiber contracts and so does the muscle. In order for this to occur again, the sarcomere must be stretched out, which is caused by the blood re-entering the heart, expanding it.In an adult, the heart measures about five inches in length, three and a half inches in the broadest part of its horizontal diameter, and two and a half inches in its posterior. The average weight in the males is from ten to twelve ounces.

In the female, the average weight is eight to ten ounces. The heart will continue to grow in size up to old age. This growth is more obvious in men than in women. The heart is subdivided by a muscle called the septum into two halves, which are named right and left according to their position. A muscle divides each half into two cavities. The upper cavity on each side is called the atria or auricle, and the lower side is called the ventricle. The right atrium and ventricle form the venous side of the heart.

Dark venous blood is pumped into the right atrium from the entire body by the superior vena cava(SVC) and inferior vena cava (IVC), and the coronary sinus. From the right atrium, the blood passes into the right ventricle and from the right ventricle, through the pulmonary artery into the le #”, l, l G( T Q X GTimes New Roman Symbol ArialTimes New Roman Navraj GrewalThe Circulatory SystemFebruary 1997In all organisms, circulation involves all of the fluids of the body and the continual movement of them between the various tissues in the body. It is a process of taking in material that is required for metabolism and delivering it to the cells of the body. The by-products then to be delivered out of the body into the environment. Invertebrate animals have many different liquids, cells and methods of circulation. There are two kinds of circulation paths: open circulatory system and closed circulatory system.

All vertebrates use the closed system. This means that the fluid is carried through the body in a network of vessels. In contrast, the open system does not use this method, rather the fluid passes freely throughout the tissues. In the closed system, the two fluids are blood and lymph. They are each in their own intricate network. Blood uses the cardiovascular system and lymph uses the lymphatic system.

The highest amount of complexity is reached in the human body. All organisms take molecules from their environment and use them in metabolism to produce their energy. The by-products are then removed into the environment.

An organism creates a difference between the internal environment of the animal in relation to the external environment. With cells, either as single organisms, (ie, ameba) or as parts of a multicellular animal (ie, human beings), there is a requirement of taking molecules in by direct diffusion through a cell wall or by vacuoles. The process of cyclosis, streaming of fluid cytoplasm) distributes the metabolic product. The molecules are conveyed between cells and throughout the body of multicellular organisms in the circulatory fluid called the blood. It travels through the body in blood vessels which is pumped through by the heart.

There is a major role in homeostasis (the constancy of the internal environment) played by the lymph and blood. This occurs, because the fluids distribute substances to parts of the body when and where required by taking it away from other areas where there may be a surplus which may be harmful. There are many invertebrate animals that live in the water and the supplying of fluid is not extremely important.For land dwelling creatures however, the fluid that reaches the tissues is supplied by drinking water. It is absorbed in the alimentary canal and passed into the bloodstream. Fluid can leave the blood with food or other organic compounds that are dissolved in the stream.

From the blood they all pass into the tissue needing the material. This material is then returned in the form of lymph. Lymph passes through in lymphatic channels, which provides the lymphatic circulation.However, in many invertebrates, circulating fluid is not restricted to blood or lymphatic vessels.

Both of the functions of the circulating and tissue fluid are combined as a single fluid. This is known as hemolymph. The internal circulatory system transports important gases and nutrients around the body of an organism. It removes unwanted products of metabolism from the tissues and carries it to the excretory organs.

For the blood and fluid to be moved or pumped throughout the body, an organism requires an organ to pump the fluid.The heart is “the viscus of cardiac muscle that maintains the circulation of the blood”. It is divided into four cavities, two atria and two ventricles. The left atrium receives oxygenated blood from the lungs. The blood is then passed to the left ventricle, which forces it through the aorta, through the arteries to supply the tissues of the body. The right atrium receives the blood after it has passed through the tissues and has delivered much of its oxygen and organic molecules. The blood then passes through the right ventricle into the lungs where it gets oxygenated.

In the heart, there are four major valves, the left atrioventricular valve, also known as the mitral or bicuspid valve, the right atrioventricular valve, (tricuspid), aortic valve, and the pulmonary valve. The heart tissue itself is nourished by the blood in the coronary arteries. The heart is placed below the alimentary canal in front of the centre of the chest. It is located in it’s own body cavity. The two atria are pointed upwungs. Once the blood becomes oxygenated by its passage through the lungs, it returns to the left side of the heart by the pulmonary veins which open into the left atrium. From the left atrium, the blood passes into the left ventricle where it is distributed by the aorta and its subdivisions through the entire body.

The right atrium is a little longer than the left. Its walls are also somewhat thinner than the left. The right atrium is capable of containing about two ounces of fluid. It consists of two parts, a principle cavity and an appendix auriculae.

The sinus is a large quadrilateral-shaped cavity located between the IVC and the SVC. Its walls are very thin and are connected on the lower surface with the right ventricle and with the left atrium. The rest of the right atrium is unattached. The appendix auricle is a small conical muscular pouch. It projects from the sinus forwards and to the left side, where it overlaps the root of the pulmonary artery.There are four main openings into the right atrium; the SVC, IVC, coronary sinus, and the atriculo-ventricular opening.

The larger IVC returns blood from the lower half of the body and opens into the lowest part of the right atrium, near the septum. The smaller SVC returns blood from the upper half of the body and opens into the upper and front part of the right atrium. The coronary sinus opens into the right atrium between the IVC and auriculo-ventricular opening.

It returns blood from the cardiac muscle of the heart and is protected by a semicircular fold of the lining of the atrium called the coronary valve. The auriculo-ventricular opening is the large oval opening of communication between the right atrium and ventricle. There are two main valves located within the right atrium, the Eustachian valve and the coronary valve. The Eustachian valve is located between the anterior margin of the IVC and the auricule-ventricular orifice. It is semilunar in form. The coronary valve is a semicircular fold of the lining membrane of the right atrium, protecting the orifice of the coronary sinus.The right ventricle is triangular-shaped and extends from the right atrium to near the apex.

Its anterior surface is rounded and convex and forms the larger part of the front of the heart. Its posterior surface is flattened, rests on the diaphragm muscle, and forms only a small part of this surface. Its inner wall is formed by the partition between the two ventricles, the septum, and bulges into the cavity of the right ventricle. Superiorly, the ventricle forms a conical structure called the infundibulum from which the pulmonary artery arises. The walls of the right ventricle are thinner than those of the left ventricle. The thickest part of the wall is at the base and it gradually becomes thinner towards the apex.

The cavity can contain up to two ounces of fluid. There are two openings in the right ventricle the auriculo-ventricular opening and the opening of the pulmonary artery. The auriculo-ventricular opening is the large oval opening between the right atrium and the right ventricle.

The opening is about an inch in diameter. It is surrounded by a fibrous ring, covered by the lining membrane of the heart (endocardium), and is larger than the opening between the left atrium and the left ventricle. It is protected by the tricuspid valve. The opening of the pulmonary artery is round and is situated at the top of the conus arteriosus, close to the septum. It is on the left side and is in front of the auriculo-ventricular opening. It is protected by the semilunar valves. There are two main valves associated with the right ventricle; the tricuspid valve and the semilunar valve consists of three segments of a triangular shape formed by the lining membrane of the heart (endocardium).

They are strengthened by a layer of fibrous tissue and muscular fibers. These segments are connected by their bases to the auriculo-ventricular orifice and by their sides with one another, so as to form a continuous membrane which is attached around the margin of the auriculo-ventricular opening. Their free margin and ventricular surfaces are attached to many delicate tendinous cords called chordae tendinae. As seen in the diagram to the right.

The central part of each valve segment is thick and strong while the lateral margins are thin and indented. The chordae tendinae are connected with the adjacent margins of the main segment of the valves. The semilunar valves guard the opening of the pulmonary artery. They consist of three semicircular folds formed by the endothelial lining of the heart and are strengthened by fibrous tissue. When blood flow is in the direction of the opening of the valve it causes the valve to open. This occurs when the heart pumps the blood (systole). During rest of the heart (diastole), the current of blood along the pulmonary artery is thrown back by its elastic walls, these valves become immediately expanded and close the entrance of the tube.

The valves are attached by their border to the wall of the artery at its connection with the ventricle. The free flap of each valve is thicker than the rest of the valve and is strengthened by a bundle of tendinous fibers. The left atrium is smaller but thicker than the right atrium. It consists of two parts; a principle cavity/sinus and an appendix auriculae. The sinus is cubed in shape and is covered in the front by the pulmonary artery and the aorta. In the heart , it is separated from the right atrium by the septum auricularum.

Behind the sinus on each side, it receives the pulmonary veins. The appendix auriculae in the left atrium is narrower and more curved than the same auriculae in the right atrium. Its borders are more deeply pressed in, causing a folded appearance. Its direction is forwards towards the right side, overlapping the root of the pulmonary artery. There are two main openings in the left atrium. The openings of the four pulmonary veins and the atrial-ventricular opening.

Two of the four pulmonary veins open into the right side of the atrium and two open into the left side. The two veins on the left exit into the atrium through a common opening. None of the pulmonary veins have valves. This is because there is less pressure on the veins than in the arteries. The atrial-ventricular opening is the large oval opening of blood flow between the atrium and the ventricle. It is smaller than the same opening between the right atrium and ventricle.The left ventricle is longer and more cone shaped than the right ventricle.

It forms a small part of the left side of the front surface of the heart and a large par of the back surface. It also forms the apex of the heart because it extends beyond the right ventricle. Its walls are almost twice as thick as those of the right ventricle. They are thickest in the broadest part of the ventricle, becoming gradually thinner towards the base and also towards the apex.

This is the thinnest part of the left ventricle.There are two main openings in the left ventricle, the atrial-ventricular opening and the aortic opening. The atrial-ventricular opening is located behind and to the left side of the aortic opening. The opening is a slightly smaller than the same opening between the right atrium and right ventricle. Its position is the center of the sternum. It is surrounded by a very dense fibrous ring and is covered by the lining membrane of the heart and is protected by the mitral valve. The circular aortic opening is located in front of and to the right side of the atrial-ventricular opening.

It is separated by one of the segments of the mitral valve. The opening is protected by the semilunar valves. The semilunar valves have no chordae tendonae, therefore they are simpler in structure than the atrial-ventricular valves. There are two valves located within the left ventricle; the mitral valve and the semilunar valve.

The mitral valve is attached to the circumference of the atrial-ventricular opening in the same way that the tricuspid valve is attached on the opposite side of the heart. The valve contains a few muscular fibers and is strengthened by fibrous tissue. It is formed by the lining of the heart, the endocardium. It is larger, thicker, and stronger than the tricuspid and consists of two segments of differing size. The mitral valves are connected to many chordae tendonae. Their attachment is the same as on the right side except they are thicker, stronger and there are not as many.

The semilunar valves surround the aortic opening. They are similar in structure and attachment to those of the pulmonary artery. However, they are larger, thicker, and stronger than those of the right side.

Between each valve and the cylinder of the aorta is a deep depression called the sinuses of Valsalva. The depressions are larger than those at the root of the pulmonary artery.The heart and its vessels are surrounded by a membranous sac called the pericardium. The pericardial sac is composed of two layers. The parietal pericardium and the visceral pericardium with the space in-between the two called the pericardial cavity. The parietal pericardium is made up of mostly compact fibrocollagenous tissue along with elastic tissue. It is a membrane of loose irregular connective tissue that is lined internally by a mesothelium which is simple squamous epithelium.

The visceral pericardium forms the internal lining of the pericardium and comes over the outer surface of the heart. This reflection forms the outer layer of the epicardium. The visceral epicardium is also composed of fibrocollagenous tissue with elastic tissue, but is smooth mesothelium. The pericardial cavity is located between the parietal and visceral pericardium and contains small amounts of serous fluid.The heart tissue itself can be subdivided into three layers: (from the outside in) epicardium, myocardium, and endocardium. The epicardium is the outermost layer of the heart and consists of a loose connective tissue of fibroblasts, collagen fibers, and adipose tissue.

It contains a stroma which houses coronary arteries and veins that are surrounded by a layer of fat. These coronary branches penetrate the myocardium. The myocardium contains the main muscle mass of the heart and is mostly made up of striated muscle cells. Each of the cardiac muscle cells contain one central elongated nucleus with some central euchromatin and some peripheral heterochromatin.

The two atria have a very thin myocardial layer which increases in thickness as you go from the atria to the right ventricle and into the left ventricle. The outer surface of the myocardium, next to the epicardium, is not composed of smooth muscle but is very smooth in texture. The inner surface of the myocardium is rough and is raised into trabeculations.

The ventricular papillary muscles, which are for the attachment of the chordae tendinae, are extensions of the myocardium even though they are covered by the endocardium. The outer layer of the myocardium is superficial bulbospiral and swirls around the ventricle in a clockwise fashion. The middle layer is circular muscles that are the ventricular constrictors.

The inner layer, which is deep bulbospiral, swirls around the ventricle in a counterclockwise fashion. The layer underneath the myocardium is known as the enodcardium. It contains a continuous smooth endothelial layer that covers all the inner surfaces of the heart, including the valves. The outer layer of the endocardium, underneath the myocardium, is irregularly arranged collagenous fibers that may contain Purkinje fibers/cells. The inner part of the endocardium contains more regularly arranged collagen and elastic fibers than the outer layer. Some myofibroblasts are present in the endocardium which is thicker in the atria than in the ventricles.

There is a subendothelial component of the endocardium underneath the endothelium. The component contains fibroblasts, scattered smooth muscle cells, elastic fibers, collagen fibers, and an amorphous ground substance that contains glycoproteins and proteoglycans.The valves of the heart are attached to the cardiac skeleton and consist of chondroid, which is a material resembling cartilage. The base of each valve is supported by a fibrocollagenous ring. Each valve also has a dense fibrocollagenous central plate that is covered by simple squamous epithelium. Chordae tendonae connect with the valves at the edge of each cusp as well as underneath each cusp at one end and they attach to papillary muscles in the ventricles at the other end.

Endocardial endothelium completely covers the papillary muscles, valves, and the chordae tendonae. The junctions between the cusps of each valve are known as commissures.In order for the heart to maintain the nearly 60.

000 miles of circulatory path, the heart itself must ‘feed’ itself with a supply of blood. The arteries that provide the nutrients are known as the coronary arteries. These arteries encircle the entire heart and originate just above the aortic valve.

They drain the blood from the pockets formed by the cusps of the valves. The left coronary artery divides into the anterior descending artery. It carries blood down the front of the heart to both ventricles. As well to the circumflex artery which winds around the back of the heart to feed the left ventricle and atrium. There is a right coronary artery that curves around the heart, sending the marginal artery along the front for the right ventricle and atrium. The next artery is the posterior descending artery. It travels down the back of the of the heart, sending smaller arteries into each ventricle.

The cardiac veins carry the blood back to the coronary sinus. It drains into the right atrium. Many heart diseases and problems can be attributed to a problem with the coronary arteries. It occurs when the blood flow through the arteries (coronary) can be blocked by such things as blood clots (thrombosis), causing a heart attack if total blockage occurs. Atherosclerosis, which partial blocking may lead to angina pectoris coronary spasm.

This temporarily cuts off blood supply. In some cases however, problems with the coronary arteries can be looked after. Specially designed catheters are used in cases of blocked arteries. With a percutaneous transluminal coronary angioplasty (PTCA) a catheter with a small balloon is feed into the target artery. It is then inflated to increase the area of flow for the blood.

Other ways used to remove the plaque on the arteries is to use different catheters with different settings. A laser may be used to burn the plaque away, or using a shearing tool to scrape the plaque off. Instead of trying to open a blocked artery, another method of ‘fixing’ the problem is to bypass the clog and insert an alternate method for which the blood can get to the tissue. This process is called a coronary bypass surgery. In this procedure, veins are taken out from the leg, the saphenous vein, or from arteries near the collarbone and one end is attached to the source of oxygenated blood and the other end just after the clog.

The conducting system of the heart consists of four main components; the sinuatrial node (SA), the atrioventricular node (AV), the bundle of His, and the Purkinje fibers or cells. All the parts of this conducting system are composed of modified cardiac muscle cells. The SA node is located in the right atrium, at the point where the superior vena cava enters. The small muscle fibers of the SA node contain a central nodal artery and desmosomes.

The muscle fibers do not contain intercalated discs. The AV node is located in the medial wall, in front of the opening of the coronary sinus and above the tricuspid ring. Its small muscle fibers are more regularly arranged than those of the SA node. The AV node contains a rich nerve and blood supply. The bundle of His has a right (single bundle) and a left (branched bundle) bundle branch located underneath the endocardium. It is similar to the other components of the nerve system. The Purkinje fibers/cells can be found in clusters of about six cells which are located under the endocardium in the ventricles. The cytoplasm of Purkinje fibers appears pale under the microscope and contains many glycogen granules.In the diagram below, we can see what electrical impulses pass along the Purkinje fibers during a single heart beat. The stages can be seen on an electrocardiogram (ECG). The current that flows through the atria produces a peak in electrical impulse. This is known as the P wave. As the impulse crosses the AV node, the ECG becomes level. This is the PR segment. The impulse then passes the muscular ventricals, and a larger peak occurs, known as the QRS complex. When the electrical system recovers for the next impulse, the T wave appears. We will see how ECGs can give important information about the health of ones heart. It can be seen in the following diagram of the ECGs of various persons with a normal ECG and ones with abnormal ECGs.The principle function of the heart and circulatory system is to provide oxygen and nutrients and to remove metabolic waste products from tissues and organs of the body, as mentioned before. The heart is the pump that provides the energy necessary for transporting the blood through the circulatory system in order to let the exchange of oxygen, carbon dioxide, and other molecules through the thin-walled capillaries. The contraction of the heart produces changes in pressures and flows in the heart chambers and blood vessels. The mechanical events of the cardiac cycle can be divided into four periods: late diastole, atrial systole, ventricular systole, and early diastole.In late diastole, the mitral and tricuspid valves are open and the pulmonary and aortic valves are closed. Blood flows into the heart throughout diastole thus filling the atria and ventricles. The rate of filling declines as the ventricles become swollen, and the cusps of the atrioventricular valves start to close. The pressure in the ventricles remains low throughout late diastole.In atrial systole, contraction of the atria forces more blood into the ventricles, but approximately 70 percent of the ventricular filling occurs during diastole. Contraction of the atrial muscle that surrounds the openings of the superior and inferior vena cava and pulmonary veins, narrows their openings and the blood moving towards the heart tends to keep blood in the heart. However, there is some regurgitation of blood into the veins during atrial systole. At the start of ventricular systole, the AV valves close. The muscles of the ventricles initially contract relatively little. Intraventricular pressure rises sharply as the muscles squeezes the blood in the ventricle. This period of isovolumetric ventricular contraction lasts about 0.05 seconds until the pressures in the ventricles exceed the pressure in the aorta and in the pulmonary artery and the aortic and pulmonary valves (semilunar valves) open. During this isovolumetric contraction, the AV valves bulge into the atria, causing a sharp rise in atrial pressure. When the semilunar valves open, the phase of ventricular ejection begins. Ejection is initially rapid, but slows down as systole continues. The intraventricular pressure rises to a maximum and then declines before ventricular systole ends. Late in systole, the aortic pressure is higher than the ventricular pressure, but for a short period, momentum keeps the blood moving forward. The AV valves are pulled down by the contractions of the ventricular muscle, and the atrial pressure drops.In early diastole, after the ventricular muscle if fully contracted, the already falling ventricular pressure drops even more rapidly. This is the period known as protodiastole and it lasts about 0.04 seconds. It ends when the momentum of the ejected blood is overcome and the semilunar valves close. After the valves are closed, pressure continues to drop rapidly during the period of isovolumetric relaxation. Isovolumetric relaxation ends when the ventricular pressure falls below the atrial pressure and the AV valves open, allowing the ventricles to fill. Filling is rapid at first, then slows as the next cardiac contraction is about to happen. Atrial pressure continues to rise after the end of ventricular systole until the AV valves open. At this time it drops and slowly rises again until the next atrial systole.Blood vessels are of great importance to the circulatory system, since it is in them that the life fluid is carries. Since there is a need of a transport system early on in life, within the embryo, these organs are among the first to develop. Blood vessels consist of a closed system of tubes which transport blood all over the body and back to the heart. Arteries take blood away from the heart under high pressure. The pressure is exerted by the pumping of the heart itself. The blood is forced into these tubes, which are elastic, causing them to recoil. This sends blood in pulsing waves. Since there is high pressure and pulsating movements in the artery, it must be strong for the fast and efficient delivery of blood.The wall of the artery has three main layers. The inner surface is smoother endothelium covered by a surface of elastic tissues. The two form what is called the tunica intima. The tunica is thicker in arteries and consists of smooth muscle cells with elastic fibers. The outer layer is called the tunica advetitia. It is the strongest of the layers. It is made up if collagenous and elastic fibers. This layer helps the artery from over-expanding. There is also a small blood vessel, vasa vasorum, in this layer to provide walls of the larger arteries with nourishment.When there is the transition between artery and arteriole, it is very gradual. It is when there is a thinning of the vessel wall and a decrease in the size of the passageway. A single layer of circular and/or spiral smooth muscle fibers now make up the tunica media. The tunica adventitia consists of tissue elements.As the arterioles get smaller in size, the three coats become less and less definite. As capillaries come together, there are small venules that form. There function is to collect the blood from the capillary bed, which is the network of capillaries. The venules are made up of small amounts of collagenous tissue. As venules increase in size, the seem to have the same characteristics for their walls as those of arteries.The functions of veins is to conduct blood from the outer tissues to the heart. There is an endothelial lining that is surrounded by the tunica media. However, this all contains much less muscle and elastic tissue that is found in the arteries. Since there is a low blood pressure exerted in the vein, a mechanism is required to ensure that there is no back-flow of blood. So, valves are used in order to keep the flow in one direction.The valves is the vein are formed by semilunar folds in the tunica intima. As blood flows towards the heart, the flaps of the valve are flattened against the inner wall, leaving an open passage for the blood to flow in. There are more valves in veins on the extremities of the body, since that is where there is the least amount of blood pressure. Veins seem to follow a parallel course to that of the arteries but there are more veins than arteries. Veins also have a greater capacity than arteries but with thinner walls. About 60% of the blood in the body is contained in the regular systematic circulation, with 40% in the veins. The pulmonary trunk is the stem of the pulmonary arteries. It arises from the top surface of the right ventricle. It rises 4-5 centimeters above the surface before it divides into the right and left pulmonary arteries. These arteries go to the lungs. The right and left pulmonary arteries are short but have a large diameter. The walls are versatile enough to serve the stroke volume of the right ventricle. The pulmonary artery has to operate under high pressure in order to function properly to handle the large amounts of deoxygenated blood that comes out of the right ventricle and to the lungs. In contrast, the pulmonary vein only works with blood under low pressure, as they return oxygen rich blood to the left atrium. The blood itself is made up of many specialized cells that are suspended in a liquid medium which is plasma. The circulating blood is always supplying oxygen and nutrients as well as other metabolic molecules. Blood also takes material away from the cells that can be toxic such as carbon dioxide, or ‘good’ things such as chemical messengers. Blood itself is red in color and is denser and more viscous than water. It gets it red color from the hemoglobin in it. Hemoglobin is an iron containing protein. It brightens in color when it is carrying oxygen, known then as oxyhemoglobin, and darkens when oxygen is removed (deoxyhemoglobin). Red cells of blood makes up 45% of the blood and all the other cells such as white cells and platelets make up 1%.The rest is plasma. Plasma contains 90% water and it is freely exchangeable with other body cells. The main difference between plasma and the extracellular fluids is of the tissues is the high protein content of the plasma. The plasma also contains lipids, salts, glucose, amino acids, vitamins, hormones and waste products. The total amount of blood varies with age, sex, weight, body build as well as a few other factors. A rough average is about 60 milliliters per kilogram of body weight. Blood cell formation is known as hematopoiesis. It takes place in hematopoietic tissue. In the embryo however, it occurs in the liver. The hematopoietic tissue is located in the bone marrow. Bone marrow is a mixture of developing as well as mature blood cells. All blood cells are born from primordial cells called multipotent hematopoietic stem cells. Through dividing and differentiating, these cells give rise to the major blood cells: red cells, phagocytic cells, megakaryocytes and lymphocytes. Arteries pierce the wall of the bone from the outside and enter the marrow. It then divides into fine branches combine into a large venous sac called the sinusoids. In the sinusoids, blood flows very slowly. The blood that had been developing in the bone marrow can enter the circulatory system by penetrating the walls of the sinusoids. There are also components of blood that allow it to clot. Clotting is the solidification of blood where there is an injured blood vessel. When there is an injured blood vessel, platelets in the blood gather at the site of injury and stick to the wall of the vessel. When there is minor damage to the vessel, ruptured platelets seal the leak. When a more serious break occurs, the actual clotting process takes over. First, the ruptured platelets and the vessel wall release an enzyme known as thromboplastin. This enzyme begins a series of reactions. The result is the changing of prothrombin to a plasma protein thrombin. This enzyme then converts the soluble plasma fibrinogen into the insoluble form of fibrin. The fibrin acts as a net and traps red blood cells and platelets to form the clot. The newly formed clot stops the bleeding and contracts and hardens. The wound is later repaired by the growth of cells that replace the destroyed ones. A disease that is attributed with the inability for clot formation is known as hemophilia. It is caused by not having enough platelets and vitamin K in the body. Vitamin K is needed for the synthesis of prothrombin. There is not just one ‘kind’ of blood. There exists the ABO blood typing. It is the variations on the molecular scale in one kind of self marker on red blood cells. The self markers are known as antibodies. People with one type of marker are said to belong to that group, either A or B if there is only one type of marker. Some people have both markers and are said to be AB blood type, while others have no markers and are type O. If someone is type A, the antibodies from that type will ignore other type A blood but attack type B. It is the same vise versa. However, type AB blood’s markers can tolerate A, B or AB blood. Type O blood has markers against all other forms. When there is a mixture of incorrect blood typing, such as A mixed with B, something called agglutination will occur. Antibodies act against foreign cells and cause clumping. If this occurs in a body, the clumps can destroy the smaller vessels and can lead to damage of tissue or death.The lymphatic system is a has very important role to play in the body. Many people say that the lymphatic system is not a part of the circulatory system, but since it is connected to the circulatory system it will be in this case. The role of the lymphatic system is to return fluid from the body tissues to the bloodstream. This fluid when in intracellular spaces is known as interstitial fluid. It bathes and nourishes body tissue. If there was no method to remove this fluid, the surrounding tissue would become swollen and eventually burst. Some of the interstitial fluid may go back into the capillaries that have low pressure, but most of it returns through the lymphatic system. The fluid is then called lymph. It is chemically much the same as plasma. Lymph however contains half as many proteins as plasma, since larger proteins cannot seep through the capillaries. The lymphatic system also serves as a line of defense against diseases. These harmful particles are filtered out by small masses of tissue called lymph nodes. These nodes which can vary in size from 1/25 to 1 inch. The nodes are in bunches in certain parts of the body. They are found in great numbers in the neck and armpits, above the groin, as well as close to large organs and blood vessels. In the lymph nodes are macrophages, meaning ‘big eaters’. They absorb harmful matter and tissue. The lymphatic vessels on which these are located are like blood vessels. The lymph flows from small branches to larger ones. The lymph from the upper right quarter of the body reaches the thoracic duct, which is the largest vessel of its kind. Lymph then flows upwards through the duct and into a blood vessel near the junction of the neck and right shoulder. With this evidence of the importance of the circulatory system, I am sure that there is no doubt that it is a very important part of our bodies. Without we would not be able to function as ‘large’ organism with out needs for nourishment of food and air. With this ingenious system in place, we are able to live and thrive.

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