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For example, the heart and the blood vessels of the cardiovascular system circulate blood and transport oxygen and nutrients to all the body cells. Levels of Organization: Molecules form cells. Cells form tissues, and tissues form organs. Organs that fulfill related functions are called organ systems. Technically, blood is a transport liquid pumped by the heart (or an equivalent structure) to all parts of the body, after which it is returned to the heart to repeat the process. Blood is both a tissue and a fluid. It is a tissue because it is a collection of similar specialized cells that serve particular functions.
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Red blood cells (also called erythrocytes) make up about 40% of the blood's volume. Red blood cells contain hemoglobin, a protein that gives blood its red color and enables it to carry oxygen from the lungs and deliver it to all body tissues. The five main types of blood cells are basophils, neutrophils, eosinophils, monocytes, and lymphocytes. Basophils are the cells mainly responsible for allergic reactions. They defend against parasites and bacteria by excreting two chemicals: heparin and histamine. Heparin is a blood-thinning substance. Red cells contain a special protein called hemoglobin, which helps carry oxygen from the lungs to the rest of the body and then returns carbon dioxide from the body to the lungs so it can be exhaled. Blood appears red because of the large number of red blood cells, which get their color from the hemoglobin. In addition to erythropoietin, red blood cell production requires adequate supplies of substrates, mainly iron, vitamin B12, folate, and heme. RBCs survive about 120 days.
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