Chapter 19: Blood – Structure, Functions, and Components

Blood: Structure and Overview

General Properties of Blood

Blood is a specialized fluid connective tissue that plays a vital role in maintaining homeostasis and supporting life. The average adult has about 5 liters of blood, which constitutes approximately 8% of total body weight. Blood circulates continuously through blood vessels, delivering essential substances and removing waste products.

• Blood composition: Blood consists of two main components: plasma (the liquid extracellular matrix) and formed elements (cells and cell fragments).

• Visible layers: When blood is centrifuged, it separates into three layers: plasma (top), buffy coat (middle, containing leukocytes and platelets), and erythrocytes (bottom).

Formed Elements of Blood

The formed elements are the cellular components suspended in plasma. They include:

• Erythrocytes: Also known as red blood cells (RBCs), responsible for oxygen transport.

• Leukocytes: White blood cells (WBCs) involved in immune defense.

• Platelets: Cell fragments essential for blood clotting.

Functions of Blood

Major Functions

Blood performs several critical functions necessary for survival:

• Exchanging gases: Transports oxygen and carbon dioxide between lungs and tissues.

• Distributing solutes: Plasma carries ions, nutrients, hormones, and waste products, helping regulate ion concentrations in tissues.

• Performing immune functions: Leukocytes and immune proteins are transported throughout the body to defend against pathogens.

• Maintaining body temperature: Blood absorbs and distributes heat generated by metabolic reactions.

• Functioning in blood clotting: Platelets and clotting proteins form blood clots to seal damaged vessels and prevent blood loss.

Additional Functions

• Preserving acid-base homeostasis: Blood maintains a stable pH (between 7.35 and 7.45) using buffering systems.

• Stabilizing blood pressure: Blood volume is a key factor in determining blood pressure.

Plasma: Composition and Proteins

Plasma Overview

Plasma is the liquid matrix of blood, making up about 55% of its volume. It consists mainly of water, dissolved solutes, and proteins.

• Plasma proteins: Form a colloid and account for about 9% of plasma volume.

Types of Plasma Proteins

• Albumin: A large protein synthesized in the liver, responsible for blood's colloid osmotic pressure. It draws water into blood by osmosis.

• Transport proteins: Bind to lipid-based molecules that are otherwise incompatible with water-based plasma, allowing their transport in blood.

• Clotting proteins: Stop bleeding from injured vessels by forming blood clots, with assistance from platelets.

Example: Centrifuged Blood Sample

When a blood sample is centrifuged, it separates into:

• Plasma (55%) – Contains water, plasma proteins, and small solutes.

• Buffy coat (1%) – Contains leukocytes and platelets.

• Erythrocytes (44%) – Red blood cells, responsible for oxygen transport.

Layer	Main Components	Percentage of Blood Volume
Plasma	Water, proteins, solutes	~55%
Buffy coat	Leukocytes, platelets	~1%
Erythrocytes	Red blood cells	~44%

Additional info: Plasma also contains dissolved gases, nutrients, hormones, and metabolic waste products.