Blood: Structure, Functions, and Components

Overview of Blood

Blood is a specialized fluid connective tissue that circulates continuously through the heart and blood vessels, playing a vital role in maintaining homeostasis. The average adult body contains about 5 liters of blood, which accounts for approximately 8% of total body weight.

• Plasma: The liquid extracellular matrix of blood, making up the majority of its volume.

• Formed Elements: Cellular components suspended in plasma, including:

  • Erythrocytes (Red Blood Cells, RBCs): Responsible for oxygen transport.

  • Leukocytes (White Blood Cells, WBCs): Involved in immune defense.

  • Platelets: Cell fragments essential for blood clotting.

Blood Composition and Centrifugation

When blood is centrifuged, its components separate into three distinct layers, each with specific functions and characteristics.

• Plasma (Top Layer): Constitutes about 55% of total blood volume; contains water, proteins, and solutes.

• Buffy Coat (Middle Layer): About 1% of blood volume; consists of leukocytes and platelets.

• Erythrocytes (Bottom Layer): About 44% of blood volume; contains red blood cells.

• Hematocrit: The percentage of blood volume composed of erythrocytes. Normal values vary by sex and age.

Functions of Blood

Blood performs several essential functions necessary for life and homeostasis:

• Exchanging Gases: Oxygen is transported from the lungs to tissues primarily by erythrocytes, while carbon dioxide is carried from tissues to the lungs by erythrocytes and plasma.

• Distributing Solutes: Blood carries nutrients, hormones, ions, and metabolic wastes throughout the body.

• Performing Immune Functions: Leukocytes and immune proteins use blood as a transport medium to reach all body tissues and defend against pathogens.

• Maintaining Body Temperature: Blood absorbs and distributes heat produced by cellular metabolism, helping to regulate tissue temperature.

• Sealing Damaged Vessels by Forming Blood Clots: Platelets and clotting proteins form clots to seal injured vessels and prevent blood loss.

• Preserving Acid-Base Homeostasis: Blood maintains pH within the narrow range of 7.35–7.45, crucial for enzymatic and cellular functions. This is achieved through buffer systems present in plasma.

• Stabilizing Blood Pressure: Blood volume is a primary determinant of blood pressure; maintaining adequate volume is essential for stable pressure and tissue perfusion.

Visual Representation: Layers of Blood

When blood is centrifuged, the following layers are observed:

Key Terms and Definitions

• Plasma: The liquid portion of blood, primarily water, containing dissolved proteins, nutrients, and waste products.

• Formed Elements: The cellular components of blood, including erythrocytes, leukocytes, and platelets.

• Hematocrit: The proportion of blood volume occupied by red blood cells, expressed as a percentage.

• Buffy Coat: The thin layer of leukocytes and platelets found between plasma and erythrocytes after centrifugation.

Example: Clinical Application

A complete blood count (CBC) is a common laboratory test that measures the proportions of plasma, buffy coat, and erythrocytes, providing valuable information about a patient's health, such as anemia, infection, or clotting disorders.

Additional info: The study notes above expand on the brief points in the slides, providing definitions, context, and a table for visual understanding. These notes are suitable for college-level Anatomy & Physiology students preparing for exams or reviewing the chapter on blood.