Compose a 1750 words assignment on red blood cells and their journey. Needs to be plagiarism free! The journey of red blood cells (erythrocytes) usually begins in the circulatory system. During their passage through the blood vessels, the erythrocytes undergo several important changes in shape and structure, particularly when needed to pass through the narrow passages in the circulatory system.An erythrocyte is a biconcave disc of a diameter ranging between 6-8 microns, with a mean thickness of 2.5 micrometers at the periphery and approximately 1 micrometer towards the center of the cell (Guyton & Hall, 2006). It is clear that transporting oxygen from the lung to various other organs is the function of the RBCs during their journey in the human circulatory system.Importantly, this oxygen available for the functioning of organs is stored into the hemoglobin of RBC and when it reaches its particular destination, oxygen is liberated from hemoglobin and it moves through the cellular passive diffusion. Furthermore, carbon dioxide (CO2) that is released after cellular activities is then again fused with hemoglobin of RBC. This deoxygenated blood due to the presence of CO2 into the hemoglobin then enters into the heart and is finally diffused to the lungs. It was reported that in some lower animals (some invertebrates), Hb is present as a free protein in plasma and it is not bound to the RBCs like in human beings. The total life span of each erythrocyte in the circulation is 120 days (Dean, 2005). But during this period if any of them gets damaged, then they could be eliminated from the circulatory system with the help of macrophages which is usually present in the bone marrow, spleen, or in the liver (Premkumar, 2004). &nbsp.It is reported, a normal man has an average of 5,200,000 red blood cells (RBC) per cubic millimeter and a normal woman has an average of 4,700,000 RBCs per cubic millimeter (Guyton & Hall, 2006). It is also demonstrated that around 3 million red blood cells (RBC) enter the circulation each second (Starr & McMillan, 2012). Hemoglobin is usually concentrated in the red blood cells (RBC). the metabolic limit of the hemoglobin-forming mechanism of the body allows only a maximum concentration of 34grams of Haemoglobin in every 100 milliliters of cells (Guyton & Hall, 2006). In any normal and healthy individual, the hemoglobin (Hb) concentration remains at this maximum permitted level.