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What are the 3 major components of blood and what gives blood its color?

Blood is a complex bodily fluid that transports oxygen, nutrients, hormones, and waste throughout the circulatory system. It consists of various components that each serve important functions in the human body. The three major components of blood are red blood cells, white blood cells, and platelets, which are all suspended in a liquid matrix known as plasma. The red color of blood comes primarily from the iron-containing protein hemoglobin that is abundant in red blood cells.


Plasma is the liquid component of blood, representing about 55% of total blood volume. It is mostly water (92% by volume) along with proteins, nutrients, minerals, hormones, carbon dioxide, and dissolved waste products. Plasma provides a transport medium for cells and helps to regulate pH, temperature, and osmotic pressure in the body. Major plasma proteins include:

  • Albumin – helps maintain oncotic pressure to prevent fluid from leaving the blood vessels
  • Globulins – assist in immune function and transporting lipids
  • Fibrinogen – involved in blood clotting
  • Prothrombin – coagulation factor that is converted to thrombin during clotting

Other plasma constituents include glucose, fats, sodium, potassium, calcium, chloride, bicarbonate, magnesium, and hormones like insulin and cortisol. Plasma itself is transparent yellow in color due to proteins within it.

Red Blood Cells

Red blood cells (RBCs), also called erythrocytes, are by far the most abundant cell type in blood. A single drop of blood contains millions of RBCs, accounting for roughly 45% of total blood volume. RBCs are specialized for transporting oxygen and appear red under the microscope due to the hundreds of millions of hemoglobin proteins within each cell. Hemoglobin binds reversibly to oxygen molecules through its iron-containing heme groups. This gives blood its distinct red color and allows hemoglobin to deliver oxygen from the lungs to tissues throughout the body. Mature RBCs lack a nucleus and organelles to provide more space for hemoglobin storage.

Some key facts about red blood cells:

  • Diameter: ~7-8 μm
  • Lifespan: ~120 days
  • Produced in bone marrow at a rate of ~2 million per second
  • Abundant antioxidants to handle oxidative stress
  • Unique biconcave disc shape provides a high surface area to volume ratio
  • Highly flexible and deformable to pass through narrow capillaries

The number of RBCs per unit volume is measured as the hematocrit, which ranges between 36-50% in healthy women and 41-53% in healthy men. RBC production decreases at high altitude due to lower oxygen availability. Certain conditions like anemia and blood loss can significantly reduce RBC numbers and hematocrit.

White Blood Cells

White blood cells (WBCs), also called leukocytes, are immune system cells that defend the body against infection and foreign materials. Unlike RBCs, WBCs contain DNA and organelles. There are several types of WBCs, each with distinct functions:

Type Function
Neutrophils Phagocytose and destroy bacteria and foreign particles
Eosinophils Respond to parasites and allergens
Basophils Involved in inflammatory responses
Lymphocytes B cells and T cells involved in the adaptive immune response
Monocytes Differentiate into macrophages and dendritic cells

Although WBCs make up just 1% of total blood volume, they are highly important for immune defense. The normal WBC count ranges between 4,500-11,000 per microliter of blood. Counts outside this range can indicate infection, inflammation, or blood disorders. WBCs become elevated in the body’s response to pathogens. Chemotherapy also frequently lowers WBC counts, requiring isolation to avoid infection.


Platelets, or thrombocytes, are small cell fragments produced by megakaryocytes in the bone marrow. While they do not contain a nucleus, platelets play a major role in blood clotting and wound healing. Some key facts about platelets:

  • Diameter: 2-3 μm
  • Count: 150,000-450,000 per microliter of blood
  • Lifespan: ~7-10 days
  • Clumping triggers coagulation cascade
  • Contain granules to release clotting factors
  • Represent less than 1% of blood volume

When a blood vessel is damaged, platelets rush to the wound site and stick together to plug the leak. This platelet plug is reinforced by a fibrous mesh of fibrin threads during coagulation. Anticoagulant drugs like aspirin work by reducing platelet function. Abnormally low platelet counts, called thrombocytopenia, prevent normal clotting and increase risk of excessive bleeding.

What Gives Blood its Red Color?

The red color of blood comes from the hundreds of millions of red blood cells flowing through the body’s blood vessels. RBCs get their color primarily from the iron-containing protein hemoglobin, which represents about 35% of the total red cell volume. Hemoglobin consists of 4 protein subunits, and each subunit contains 1 heme group with an iron atom at its center. It’s this iron atom that binds reversibly to oxygen molecules. Iron can exhibit oxidation states of Fe2+ and Fe3+ which influence hemoglobin’s bright red hue.

In its oxygenated form, hemoglobin takes on a vivid red color. This is because the Fe2+ form strongly absorbs green and blue light, leaving behind predominantly red wavelengths. In deoxygenated blood, hemoglobin takes on a more purple-red color as the iron shifts towards its Fe3+ state. This is why oxygen-rich arterial blood appears brighter cherry red, while deoxygenated venous blood has a darker maroon color.

The small amount of plasma and platelets in blood contribute little to its overall color. However, plasma absorption of certain wavelengths gives it a yellowish tinge. Conditions affecting red blood cells can significantly alter blood color. For example:

  • Anemia – Fewer RBCs causes blood to appear pale and diluted
  • Polycythemia – High RBC count makes blood deep red and thick
  • Methemoglobinemia – Hemoglobin oxidation makes blood appear chocolate brown

Understanding what contributes to normal blood color helps hematologists diagnose underlying problems when significant color changes occur.


In summary, the three major components of blood are plasma, red blood cells, and white blood cells, with platelets representing a smaller fraction. Plasma is the straw-colored liquid that carries blood cells and dissolved proteins, sugars, fats, and electrolytes. Red blood cells specialize in oxygen transport through the iron-containing hemoglobin that gives blood its distinct red hue. White blood cells fight infection and adapt immune responses to foreign material. Platelets help stop bleeding by aggregating at wound sites and initiating the coagulation cascade. Any changes in the ratio or function of these blood components can signify disease states or underlying health conditions.