Blood: Structure, Function, and Components – Study Notes for Anatomy & Physiology II

Blood

Introduction

Blood is a specialized connective tissue that plays a vital role in maintaining homeostasis and supporting life. It is the only liquid tissue in the human body and is essential for transportation, regulation, and protection.

Functions of Blood

Overview of Blood Functions

• Transportation: Blood transports oxygen, carbon dioxide, nutrients, wastes, ions, and hormones throughout the body.

• Maintenance of Homeostasis: Blood helps regulate body temperature, pH, and fluid/electrolyte balance.

• Protection: Blood protects the body by forming clots to prevent blood loss and by fighting infections through immune cells.

Composition of Blood

Main Components

Blood consists of two major components: plasma (liquid) and formed elements (cells and cell fragments).

• Plasma: The liquid matrix of blood, making up about 55% of total blood volume.

• Formed Elements: Cellular components, including:

  • Erythrocytes (red blood cells, RBCs): Responsible for oxygen and carbon dioxide transport.

  • Leukocytes (white blood cells, WBCs): Involved in immune defense.

  • Thrombocytes (platelets): Play a key role in blood clotting.

Components of Whole Blood

Separation and Proportions

When blood is centrifuged, it separates into distinct layers:

Component	Percentage of Whole Blood	Description
Plasma	~55%	Least dense; contains water, proteins, ions, hormones, gases
Buffy Coat	~1%	Leukocytes and platelets; thin, white layer
Erythrocytes	~45%	Most dense; red blood cells (hematocrit)

Hematocrit (Packed Cell Volume, PCV) is the percentage of blood volume occupied by erythrocytes. Normal values:

• Females: 37%–47%

• Males: 42%–52%

Physical Characteristics and Volume of Blood

Key Properties

• Appearance: Blood is a sticky, opaque fluid with a metallic taste.

• Color: Varies from scarlet (oxygen-rich) to dark red (oxygen-poor).

• pH: Maintained between 7.35–7.45 (slightly alkaline).

• Temperature: Approximately 38°C, slightly higher than average body temperature.

• Volume: Average blood volume is 5–6 L for males and 4–5 L for females.

Plasma

Composition and Function

• Water: Makes up about 90% of plasma; gives plasma its "straw" color.

• Solutes: Comprise 10% of plasma and include:

  • Ions: e.g., Na+

  • Hormones

  • Gases: e.g., CO2

  • Plasma proteins

Plasma Proteins

• Albumin: Maintains blood osmolarity and volume by drawing water into the bloodstream; also transports substances and buffers plasma pH.

• Globulins: Serve as transport molecules (alpha and beta globulins) and antibodies (gamma globulins).

• Fibrinogens: Essential for blood clotting.

Summary Table: Major Plasma Proteins

Protein	Main Function
Albumin	Osmotic pressure, transport, pH buffering
Globulins	Transport, immune defense (antibodies)
Fibrinogen	Blood clotting

Additional info: Plasma proteins are synthesized mainly in the liver and are crucial for maintaining blood volume and pressure.