Blood: Structure, Function, and Composition

Blood: Structure, Function, and Composition

17.1 Functions of Blood

Blood is a vital connective tissue that performs several essential functions necessary for maintaining homeostasis in the human body. These functions can be grouped into three main categories: transport, regulation, and protection.

• Transport Functions:

  • Delivers oxygen from the lungs and nutrients from the digestive tract to all body tissues.

  • Removes metabolic wastes (such as carbon dioxide and urea) from cells for elimination.

  • Transports hormones from endocrine organs to target tissues.

• Regulation Functions:

  • Maintains body temperature by absorbing and distributing heat.

  • Helps maintain a constant blood pH through buffers.

  • Ensures adequate fluid volume in the circulatory system.

• Protective Functions:

  • Prevents blood loss through hemostasis (clot formation).

  • Prevents infection by transporting immune cells and antibodies.

17.2 Composition of Blood

Blood is a specialized body fluid composed of two main components: formed elements and plasma. Each component plays a unique role in the overall function of blood.

• Formed Elements:

  • Include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (thrombocytes).

  • Account for approximately 45% of whole blood volume (measured as the hematocrit).

• Plasma:

  • Straw-colored, viscous fluid making up about 55% of whole blood.

  • Composed of 90% water and 10% solutes (including nutrients, respiratory gases, electrolytes, hormones, and proteins).

• Physical Properties:

  • Blood is a viscous, slightly alkaline fluid (pH ~7.4).

  • Represents about 8% of total body weight.

  • Average adult blood volume is approximately 5 liters.

Major Components of Whole Blood

The major components of whole blood can be separated by centrifugation, as illustrated below:

Component	Percentage of Whole Blood	Description
Plasma	~55%	Least dense; contains water, proteins, nutrients, hormones, and waste products.
Buffy Coat	<1%	Contains leukocytes (WBCs) and platelets.
Erythrocytes	~45% (hematocrit)	Most dense; responsible for oxygen transport.

Key Terms

• Hematocrit: The percentage of blood volume occupied by erythrocytes (red blood cells).

• Plasma: The liquid, non-cellular portion of blood.

• Formed Elements: The cellular components of blood (RBCs, WBCs, platelets).

Example:

If a blood sample is centrifuged, the erythrocytes settle at the bottom (most dense), the buffy coat forms a thin middle layer, and plasma remains at the top (least dense).

Additional info: The composition and function of blood are foundational concepts in anatomy and physiology, as blood is essential for the transport of substances, regulation of homeostasis, and protection against disease.