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What is Blood?

Blood is a red fluid that circulates through the body’s blood vessels. It consists of blood cells and plasma. The primary organ responsible for producing blood cells is the bone marrow, located within the cavities of bones. The average blood volume in the body is about 4,000 to 5,000 milliliters (ml), depending on factors like sex and body weight. This can be estimated with the following formula:

Estimated Blood Volume (ml) = Body Weight (kg) x 70 (ml)

Components of Blood

  1. Blood Cells
    Blood cells make up approximately 45% of total blood volume.
    • Red Blood Cells: These cells are shaped like doughnuts with a central indentation, created in the bone marrow. They contain hemoglobin, which gives them their red color. Red blood cells serve as “couriers,” delivering oxygen and essential nutrients throughout the body while also transporting waste products like carbon dioxide to the lungs for exhalation. The lifespan of red blood cells is about 120 days.
  2. Plasma
    Plasma accounts for around 55% of total blood volume.
    • Plasma is the liquid portion of blood, primarily composed of water. It includes proteins such as albumin, globulins, and immunoglobulins. Plasma acts as a medium that allows various blood cells to circulate in the body, driven by the rhythmic contractions of the heart. Additionally, it contains antibodies and clotting factors that help prevent excessive bleeding.
  3. Buffy Coat
    • White Blood Cells: These function like “defenders,” protecting the body by attacking pathogens or foreign substances that invade. They also produce antibodies, which act as “shields” against infections. There are several types of white blood cells, some of which can directly destroy pathogens, with an average lifespan of about 20 days, depending on the type.
    • Platelets: These small cell fragments are produced in the bone marrow and are essential for blood clotting. They work together to form a strong mesh that seals wounds in blood vessels, similar to using plaster to fix a hole in a wall. This prevents significant blood loss during injuries and helps stop bleeding in patients with low platelet counts. Platelets typically last about 10 days.