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Blood Composition And Functions

Discover the fascinating world of blood composition and its incredible functions, unlocking the secrets behind this vital fluid that keeps us alive.

USMLE Guide: Blood Composition and Functions


In this guide, we will provide a comprehensive overview of blood composition and its essential functions. Understanding the components of blood and their roles is crucial for medical professionals, especially for those preparing for the USMLE exams. This guide aims to simplify complex concepts related to blood composition and functions, ensuring a thorough preparation for the examination.

Table of Contents

  1. Blood Composition
    • Plasma
    • Formed Elements
  2. Plasma
    • Proteins
    • Electrolytes
    • Hormones
    • Waste Products
  3. Formed Elements
    • Red Blood Cells (Erythrocytes)
    • White Blood Cells (Leukocytes)
    • Platelets (Thrombocytes)
  4. Blood Functions
    • Transport of Oxygen and Carbon Dioxide
    • Immune Response
    • Blood Clotting
    • Regulation of Body Temperature
    • Maintenance of pH Balance

1. Blood Composition


Plasma is the liquid component of blood, constituting approximately 55% of the total blood volume. It is a yellowish fluid that carries various substances throughout the body.

Formed Elements

Formed elements are the cellular components of blood, accounting for approximately 45% of the total blood volume. They include red blood cells, white blood cells, and platelets.

2. Plasma


Plasma contains several proteins, including albumin, globulins, and fibrinogen. Albumin helps maintain colloidal osmotic pressure, while globulins play a vital role in the immune response. Fibrinogen is essential for blood clotting.


Plasma contains various electrolytes, including sodium, potassium, calcium, and chloride ions. These electrolytes help maintain the osmotic balance, pH balance, and participate in nerve and muscle function.


Several hormones, such as insulin and thyroid hormones, circulate in the plasma, playing crucial roles in regulating various body functions.

Waste Products

Plasma carries waste products, such as urea and carbon dioxide, which are transported to the kidneys and lungs for elimination.

3. Formed Elements

Red Blood Cells (Erythrocytes)

Red blood cells are the most abundant formed elements. They contain hemoglobin, which binds to oxygen and facilitates its transport to tissues. Erythrocytes also play a role in removing carbon dioxide, a waste product of cellular respiration.

White Blood Cells (Leukocytes)

White blood cells are responsible for the body's immune response. They defend against pathogens, foreign substances, and abnormal cells. There are different types of leukocytes, including neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

Platelets (Thrombocytes)

Platelets are essential for blood clotting. When a blood vessel is damaged, platelets aggregate and initiate clot formation to prevent excessive bleeding.

4. Blood Functions

Transport of Oxygen and Carbon Dioxide

Red blood cells transport oxygen from the lungs to all body tissues and carry carbon dioxide, a waste product, back to the lungs for elimination.

Immune Response

White blood cells play a vital role in the immune response by recognizing and eliminating pathogens, foreign substances, and abnormal cells.

Blood Clotting

Platelets and clotting factors present in plasma work together to form a clot when a blood vessel is damaged. This prevents excessive bleeding and aids in wound healing.

Regulation of Body Temperature

Blood helps regulate body temperature by redistributing heat throughout the body, maintaining a stable internal environment.

Maintenance of pH Balance

Blood acts as a buffer system, preventing drastic changes in pH by neutralizing acids and bases, thereby maintaining the body's pH balance.

Remember to review the details of each section and practice with related usmle questions to reinforce your understanding. Good luck with your exam preparation!

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