What is Blood Composition?
Blood is a liquid connective tissue that carries oxygen, nutrients, hormones and waste throughout the body. It is made of a fluid part called plasma and living formed elements suspended within it.
Blood is about 55% plasma (water, proteins, salts) and 45% formed elements: red blood cells (oxygen transport), white blood cells (immunity) and platelets (clotting).
Step-by-step worked examples
A blood sample is centrifuged. If it contains 55% plasma, what percentage is formed elements?
Whole blood = plasma + formed elements = 100% Formed elements = 100% − 55% = 45% So 45% is red blood cells, white blood cells and platelets.
A healthy adult has about 5,000,000 red blood cells per microliter (µL) of blood. If a 5-liter blood volume is assumed, roughly how many RBCs are there in total?
5 liters = 5,000,000 µL RBCs per µL ≈ 5,000,000 Total RBCs ≈ 5,000,000 × 5,000,000 = 2.5 × 10¹³ cells This matches the commonly cited figure of ~25 trillion red blood cells in an adult.
A patient's white blood cell count is 12,000 cells/µL. The normal range is 4,500–11,000 cells/µL. What does this suggest?
Compare 12,000 to the normal range 4,500–11,000 12,000 > 11,000, so the count is above normal This condition is called leukocytosis and often signals infection or inflammation.
Flashcards
Quick quiz
Q1.What percentage of whole blood is typically plasma?
Q2.Which blood cell type carries oxygen?
Q3.Which formed element is responsible for immune defense?
Q4.What is the main function of platelets?
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Common mistakes
Thinking blood is a single uniform liquid. — Correct: Blood is a mixture of liquid plasma and living formed elements suspended in it.
Believing white blood cells are the most numerous blood cell. — Correct: Red blood cells vastly outnumber white blood cells — about 700 RBCs for every WBC.
Assuming platelets are complete cells. — Correct: Platelets are small cell fragments budded off from larger cells called megakaryocytes.
Confusing plasma with serum. — Correct: Serum is plasma with the clotting proteins (like fibrinogen) removed after clotting occurs.
FAQ
What is blood composition?
Blood composition refers to the proportion of plasma (about 55%) and formed elements — red blood cells, white blood cells and platelets (about 45%) — that make up whole blood.
What are examples of blood cells?
The three main blood cells are red blood cells (erythrocytes), white blood cells (leukocytes, including neutrophils and lymphocytes) and platelets (thrombocytes).
How is blood composition measured?
A complete blood count (CBC) test measures red cell count, white cell count, platelet count and hematocrit (percentage of blood volume that is red blood cells).
Why is plasma important in blood?
Plasma carries water, nutrients, hormones, proteins and waste, and serves as the transport medium that keeps blood cells suspended and circulating.