Introduction
The average human adult has about five to six liters of blood. Blood comprises about seven to eight percent of the total body weight. Blood carries nutrients and oxygen to tissues and removes waste products. It also has immune cells and platelets that perform varied functions.
Blood adapts to the body's needs during rest and function. During an infection, the blood delivers more immune cells to the infection site, where they work together to fight harmful invaders. According to the World Health Organization (WHO), about 120 million blood units are donated each year worldwide.
What Is the Basic Blood Composition?
Blood is a specialized body fluid with plasma, red blood cells (RBCs), white blood cells (WBCs), and platelets. These components transport oxygen and nutrients to the lungs and tissues, form blood clots to prevent excess blood loss, carry cells and antibodies that fight infection, and bring waste products to the kidneys and liver. Furthermore, blood also helps in regulating body temperature.
A blood sample can be separated into its components by centrifugation (a method of separating molecules by spinning them in solution at high speed around an axis). During the process, the test tube containing blood is left to stand for some time, and the blood separates into three layers. It is because the denser components sink to the bottom, and fluid remains at the top. The plasma (straw-colored fluid) denotes the top layer and forms about 60 percent of blood. Plasma (about 90 percent water) contains proteins (albumin, clotting factors, antibodies, enzymes, and hormones), glucose, and fat particles. The middle layer consists of WBCs and platelets (also called thrombocytes). Finally, the bottom red layer comprises RBCs.
What Are the Functions of the Blood Components?
All of the blood cells originate in the bone marrow as stem cells.
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Plasma (which comprises about 55 percent of blood) transports blood cells along with nutrients, antibodies (defense proteins), clotting factors, and hormones. Hence, antibodies and clotting factors from the plasma can treat various immune deficiencies and bleeding disorders.
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RBCs are the most abundant blood cell (about 40 to 45 percent of the blood volume). The RBC production is controlled by erythropoietin (a hormone produced mainly by the kidneys). After about seven days of maturation, RBCs are released into the blood. On average, an RBC survives only 120 days (a human body has an average of 4 to 6 million RBCs). The RBC lacks a nucleus allowing it to store more hemoglobin (Hb, the oxygen-binding protein). It also enables the RBC to transport more oxygen (as Hb binds oxygen). The shape of an RBC is a biconcave disk with a flattened center. The concavity increases its surface area for oxygen diffusion. Blood appears red because of RBCs, and the color is due to Hb. Hb carries oxygen from the lungs to the organs and returns carbon dioxide.
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WBCs have different shapes and sizes. The various WBCs (neutrophils, eosinophils, basophils, monocytes, and lymphocytes) have a role in the immune response. They circulate in the blood until they receive a signal of injury or inflammation. Subsequently, the WBCs squeeze through the blood vessel wall. As a result, they migrate to the signal source and help in healing.
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Platelets are irregularly shaped cell fragments that circulate in the blood until their activation to form a blood clot. The stem cells develop into megakaryocytes (platelet precursors) that develop into platelets in the bloodstream. If they encounter damaged blood vessel walls, they stick to the damaged area and form a blood clot. Finally, the spleen removes them from circulation after about nine days (their life span).
What Are the Applied Aspects of the Blood Components?
Blood cells develop in the bone marrow through a highly regulated process (hematopoiesis). These stem cells are present in the blood, bone marrow, and umbilical cords of newborns. Stem cells from all three cell sources can treat leukemia, lymphoma, bone marrow failure, and various immune disorders.
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If a patient has anemia (a low Hb level), it leads to pallor (paleness of the skin). Other symptoms are tiredness and shortness of breath because of a lack of oxygen. Individuals with low levels of WBCs (the normal range is 4000 to 11000) may have infections. The risk for different types of infections depends upon the deficiency of the particular WBC. For example, the primary function of macrophages is bacterial defense. Hence, macrophage deficiency leads to recurrent bacterial infections. In contrast, T cells (a type of lymphocyte) fight viral infections, and a loss of their function can increase susceptibility to viral infections.
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The average range of platelets in an individual is 150,000 and 400,000. If the number drops below this range (thrombocytopenia), uncontrolled bleeding becomes a risk. Thrombocytopenia can be due to a bone marrow disorder (leukemia) or an immune system issue. On the other hand, a rise above the upper limit (thrombocythemia) indicates a risk of uncontrolled blood clotting. Thrombocythemia can hamper adequate blood supply to the heart and brain, causing heart attacks and strokes. In rheumatoid arthritis and leukemia, the spleen becomes overactive and removes excess platelets (leading to increased bleeding). However, antiplatelet therapies are available to prevent such potentially fatal events.
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A complete blood count (CBC) test gives information on blood cells (especially the RBCs and their percentage) or protein content (hemoglobin), WBCs, and platelets. The CBC can diagnose anemia, infection, and other blood disorders.
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The platelet count and plasma clotting tests can evaluate bleeding and clotting disorders. Further, a blood smear (used to view blood cells under a microscope) is also helpful. RBCs appear regular and round with a pale center in a usual blood smear. Therefore, variations in their size or shape may suggest a blood disorder.
Conclusion
Blood is rightly called "red gold" because of the value of its composition. Blood transfusion is required for various health conditions, including anemia, pregnancy and childbirth, severe trauma, and surgical procedures. It plays a pertinent role in regulating normal body function and homeostasis. Blood circulation is an essential function since it supplies vital organs with oxygen and nutrients. Poor blood circulation can harm the heart, kidneys, and brain and lead to fatal consequences. Hence, regular blood workups and tests can aid doctors in diagnosing blood diseases and conditions.
