Human Circulatory System: Anatomy and Physiology - Research Paper Example

? Human Circulatory system: Anatomy and Physiology First, middle initial, last) (Your college Human Circulatory system: Anatomy and Physiology The human circulatory system is made up of two systems, the cardiovascular system and the lymphatic system. The two systems work in cohort. The cardiovascular system consists mainly of heart, arteries, arterioles, capillaries, venules, and veins. The lymphatic system consists of the lymph, capillaries, the lymphatic vessels and the lymph nodes. It is estimated that if all the blood vessels were connected end to end, they would measure more than 60,000 miles. The amazing function of the heart is to pump blood though these 60,000 miles; blood which carries essential nutrients, hormones and oxygen to some 3 trillion cells of the body. The exchange of nutrients and wastes between blood and the cells occur at the capillaries. The human cardiovascular system is a closed system, which gets narrower as the vessels reach the cells and finally widens as the vessels reach the heart. Such a system allows a large surface area for exchange of nutrients and gases at the cell-capillary interface. Blood circulation is so important for the body that if the heart stops beating even for a few minutes, it can be fatal. The heart beats for an average 100,000 times a day pumping about 2000 gallons of blood per day throughout the body. It pumps the entire blood volume of an adult body of 5.6 liters for three times in a minute. Anatomically, heart is made up of cardiac muscles and consists of four chambers, separated by valves. The blood vessels are made up elastin membranes and smooth muscle cells (Clemente, 1985). The lymph node is the main functional unit of the lymphatic system. Here the immune system is primed and most of the immune cells are activated to fight the foreign substances in the body. The rudimentary heart is a pair of tubular vessels, the primitive aortae, and develops in the space of the pericardial area. These then develop a direct connection with the vessels of the yolk sac. Each vessel receives a vein called vitelline vein from the yolk sac, and prolong to form the dorsal aorta. A branch of the dorsal aorta forms the umbilical artery. Later the primitive aorta develops a ventral and a dorsal part connected by an arch. The two primitive aortae then fuse together to form a single tubular heart, whose posterior end receives two vitelline veins. The tubular heart beats rhythmically and pumps blood to the developing embryo. This is called vitelline circulation. There is a mesodermal tissue between the endothelial lining and outer wall of the heart, from which musculi papillares, chordae tendineae, and trabeculae later develop. The simple tubular heart later elongates and bends upon itself and forms an ‘S’ shaped structure. In due time constrictions in this structure divides it into five structures including primitive atrium and primitive ventricle. Constriction between the atrium and ventricle forms the atrial canal and form the site for atrioventricular valves. The sinus venosus forms behind the atrium whose right horn and transverse portion later form a part of the adult right atrium. The atrial canal forms into endocardial cushions and the septum intermedium, which forms the future right and left atrioventricular orifices (Clemente, 1985). Figure 1. Structure of human heart, and blood vessels. Sources: 1. http://kardiol.com/?p=4 2. http://www.accessexcellence.org/AE/AEC/CC/heart_anatomy.php The heart and the great vessels are contained in a conical fibro-serous sac, between the lungs. The heart is a hollow and muscular organ. It measures 12 cm in length and 8-9 cm in width, and weighs around 280 grams. It is divided into four chambers, the upper two are called left and right atrium, while the bottom two are called left and right ventricle. The atria are separated from ventricles by the coronary sulcus and the interatrial groove separates the two atria. Anterior longitudinal sulcus and the posterior longitudinal sulcus are two grooves separating the two ventricles. Right atrium can hold about 57cc of blood. It has a lager cavity called the sinus venarum and a smaller portion called auricula. The superior- and inferior- vena cava drain blood into the right atrium from upper and lower half of the body respectively. The coronary sinus, guarded by a valve, also opens into this atrium draining blood from the heart’s body itself. There is an atrioventricular opening guarded by the tricuspid valve between the right atrium and the right ventricle. The right ventricle has an opening for the pulmonary artery, guarded by the semilunar valves. The left atrium is smaller than the right atrium and consists of a larger principal cavity and a smaller auricula. Four pulmonary veins, without any valves, drain blood into the left atrium. The left atrioventricular opening is found between the left atrium and the left ventricle, guarded by the bicuspid or the mitral valve. The left ventricle, at the apex of the heart, is larger than the right ventricle. The aortic opening guarded by the aortic semilunar valves is also found in this chamber. Structurally, the heart is made up of muscle fibers and fibrous rings. Its walls are made up of outer epicardium, middle muscular myocardium and inner endocardium. The fibrous rings surround the artrioventricular openings and the arterial orifices. Interspersed between the muscle fibers of the heart are the purkinje’s fibers. The artrioventricular bundle of His is the only connection between the atrium and the ventricles forming the atrioventricular node and the sinoatrial node. These nodes send systolic contractions from atria to ventricles (Clemente, 1985). Now a days image based models are available, which can give detailed anatomical structure and fiber orientation of the heart (Deng, Jiao, Ye, & Xia, 2012). Functionally, the heart pumps blood to all parts of the body by the arteries. This pumping action occurs by regular contractions, about 70/min; each contraction followed by a rest period constitutes the cardiac cycle. Each cardiac cycle, lasting about 0.8 second, has three phases: (1) atrial systole, a short simultaneous contraction of the atria, (2) ventricular systole, a more prolonged contraction of both ventricles, and (3) a period of rest for the whole heart. The venous openings followed immediately by the atrial contraction forces the blood into the ventricles through the atrioventricular openings. Passage of blood back into the atria is prevented by the closing of the tricuspid and the bicuspid valves when the ventricles contract. When the pressure in the ventricles get higher than that in the aorta and the pulmonary artery, the valves of these blood vessels open and the blood from the right ventricle flow into the pulmonary artery from the right ventricle and into the aorta from the left ventricle. When the ventricles relax, the pressure in the respective vessels closes the semilunar valves and prevents blood form reentering into the ventricles. During the rest period, the tricuspid and the bicuspid valves also relax and blood flows from veins into the atria due to negative intrathoracic pressure (Clemente, 1985). Anatomically, all blood vessels are tubular structures. They are made up of 3 main layers: (1) tunica adventitia, the outermost strong covering consisting of connective tissue, collagen and elastin; it helps the vessel to anchor to the adjacent tissue, (2) tunica media, made up of smooth muscles and elastic fibers. It is thicker in arteries than veins. When these muscles contract, the lumen becomes narrow (vasoconstriction) and gets wider (vasodilation) upon relaxation, and (3) tunica intima, made up of smooth endothelial cells and elastic membrane. Elastic lamina lies between each layer. The walls of the larger vessels have their own blood supply too! They have their own arteries and veins called vasa vasorum (vessels of vessels). The veins, but not the arteries, contain valves in them to prevent backflow of the blood. These valves are bicuspid and made up of elastic tissue. Physiologically, high upstream blood pressure causes the blood in arteries to move forward and also press outward on the arterial wall. This is called hydrostatic pressure. The bigger arteries stretch when the pressure increases, but recoils back to prevent further drop in pressure. Blood flows due to pressure gradient, from high pressure to low pressure, so this drop in pressure is important for proper blood flow in the body by the arteries. Veins, on the other hand can expand to hold large blood volume. Veins carry about 64% of the body’s blood. Blood flow in the veins is more complex, compared to the arteries. This is because, the blood pressure is low in them, flow rate depends on muscle contraction and relaxation of the legs, blood has to move against the gravity, veins collapse in absence of blood, and they have to carry large volume of blood. Since most of the veins are embedded in muscles, muscle contraction helps the blood to move forward to the heart (Clemente, 1985). Like nerve cells, heart muscle cells are able to generate action potential (AP), but they last a bit longer in the heart. Heart muscle cells at different region possess different AP. AP in the heart has five stages, 0 to 4. Phase 0 is the immediate depolarization, which makes the voltage positive. Here the permeability to sodium ions increase and that to potassium ions decrease. Little repolarization begins as soon as the permeability to sodium ions decrease, which constitute phase 1. The membrane potential then becomes and remains steady at 0 millivolts. This is phase 2 and is termed as the plateau of the AP. At this point the rate of inward flow of calcium ion equals that of outbound potassium ions. Since the permeability to the calcium ions begin to decrease and that to potassium ions begin to increase, the voltage does not remain zero. Repolarization begins again and this marks the phase 3. Until the next AP initiation, the voltage decreases to its original value and remains there. This is stage 4, where the muscle cell is unstimulated. These APs initiate contractions in the heart, making it a pump. Cardiac muscles communicate to each other though gap junctions. Cardiomyocyte or cardiac muscle cells are made up of hundreds of myofibrils. These fibrils contain action and myosin proteins in them. Myosin contains a myosin head. Each AP causes the cardiomyocytes to release calcium ions from the reserve. These calcium ions allow myosin heads to bind to the actin molecules and pulls them causing the cell to contract (Oracle ThinkQuest). The lymphatic system consists of the lymph, capillary network present in various tissues and organs, the vessels which carry lymph from capillaries to the large veins in the neck, and the lymph nodes or glands present in the pathways of the vessels where the lymph is filtered and lymphocytes are added to the lymph. Lymph has the composition similar to blood plasma and is transparent, slightly yellowish in color, watery fluid with specific gravity of 1.015. Apart from constituents of plasma, lymph has larger proteins, lymphocytes, foreign particles, and frequently RBCs. The capillaries consist of single layer of endothelial cells and are bathed in intercellular tissue fluids. Lymph is made by physical process like filtration, diffusion, osmosis, and active secretion by the endothelial cells of the capillaries. They are present in almost all tissues, like skin, subcutaneous tissue, joint capsules, pericardial cavity, salivary gland, liver, spleen, nasal cavity, bronchia, heart, kidneys, ovary, prostate, testes, the alimentary canal etc. The lymphatic vessels collect lymph from the capillaries. They are made up of three layers; the outer coat is elastic and transparent, inner coat has smooth muscles, and the external coat has connective tissue. These vessels have semilunar valves like those present in veins, but are more in number and their number increases as the vessels approach the lymph nodes, and areas of neck. The lymph glands are small bean shaped bodies, interspersed in the path of lymphatic vessels, such that the lymph passes through them on their way to blood. A lymph node is made up of several lymphoid lobules, which are the structural and functional units of the lymph nodes. The capsule of the lymph node is collagenous and has a subcapsular sinus. Lymph carrying macrophages, dendritic cells, and lymphocytes from the sinus drains into the node cortex and the medulla. Activated B cells mature into plasma in the cortex, which have nodules with germinal centers. T cells are processed in the medulla of the lymph nodes. Afferent lymphatic vessels bring lymph into the node and efferent vessels exit the node (Clemente, 1985). Recent evidence shows that lymphatic endothelial cells play an important role in the trafficking of lymphocytes and aid in the regulation of tolerance and immunity. They do this by the presence of cell adhesion molecules on their surface (Tewalt, Cohen, Rouhani, & Engelhard, 2012). Figure 2. The Structure of a lymph node and the circulatory system Source: 1. http://www.acm.uiuc.edu/sigbio/project/updated-lymphatic/lymph7.html 2. http://www.healingdaily.com/exercise/effects-of-rebounding-on-the-lymphatic- system.htm Functions of the lymphatic system includes following: Collection of wastes from the intercellular fluid Filtering lymph at the nodes Filtering blood of bacteria, viruses, and foreign particles, by the spleen Fight infection by raising an immune reaction The abnormal physiology leading to diseases of the circulatory system are numerous and a complete description is beyond the scope of this paper. However, certain disease types can be mentioned here. As far as cardiovascular system is concerned, there are six types of diseases. They are 1) Ischemic heart diseases, which is due to poor or absence of blood circulation in the body of the heart itself. 2) Stroke, abnormal circulation of blood in the brain. 3) Peripheral vascular diseases, usually the lower extremities.4) Heart failure, when the pump cannot keep up with body’s need of blood. 5) Rheumatic heart disease where a bacterial infection in the childhood affects the valves of the heart and have to be replaced, and 6) Congenital heart disease, where a birth defect leads to abnormal structure of heart (PHAC, 2010). Lymphedema is the most common disease of the lymphatic system where the organs of the system get inflamed. Obesity, with higher body mass index has been attributed to the lymphedema of the lower extremity (Greene, Grant, & Slavin, 2012). Other diseases of the system include lymphangioma, protein-losing enteropathy, intestinal lymphangiectasia, complex vascular malformations, infectious diseases, and lymphangioleiomyomatosis (Lee, Bergan, & Rockson, 2001). The human circulatory system encompasses the cardiovascular system and the lymphatic system. The cardiovascular system provides nutrition and oxygen to the tissues and organs of the body, while the lymphatic system gathers all the wastes and foreign substance from the interstitial spaces and processes them for their disposal. The two systems work in close collaboration with each other. The cardiovascular system is a closed system while the lymphatic system is an open system. Both the systems can have acquired and inherent anatomical as well as physiological problems, some of them being fatal. Reference List Clemente, C. (1985). Gray's Anatomy of the Human Body (30th Ed.). Philadelphia: Lea & Febiger. Deng, D., Jiao, P., Ye, X., & Xia, L. (2012). An image-based model of the whole human heart with detailed anatomical structure and fiber orientation. Comput Math Methods Med. 2012:891070. doi: 10.1155/2012/891070 Greene, A.K., Grant, F.D., & Slavin, S.A. (2012). Lower-extremity lymphedema and elevated body- mass index. N Engl J Med. 366(22):2136-7. Lee, B., Bergan, J., & Rockson, S.G. (Eds.). (2001). Lymphedema, London: Springer-Verlag. Oracle ThinkQuest. (n/a). The cardiac Action Potential. Retrieved from http://library.thinkquest.org/C003758/Function/The%20Cardiac%20Action%20Potential.htm. PHAC. (2010). Six Types of Cardiovascular Disease. Retrieved from http://www.phac-aspc.gc.ca/cd-mc/cvd-mcv/cvd-mcv-eng.php Tewalt, E.F., Cohen, J.N., Rouhani, S.J., & Engelhard, V.H. (2012). Lymphatic endothelial cells- key players in regulation of tolerance and immunity. Front Immunol. 3:305.doi: 10.3389/fimmu.2012.00305. Read More