Blood: Internal Transport System

Overview of Blood

Blood is a vital, life-sustaining fluid that serves as the main transport medium of the cardiovascular system. It is classified as a connective tissue and is the only fluid tissue in the body.

• Functions of Blood: Transport, Regulation, and Protection

Functions of Blood

Transport

Blood is responsible for transporting essential substances throughout the body.

• Delivering oxygen (O2) and nutrients to body cells

• Transporting metabolic wastes (such as carbon dioxide and urea) to the lungs and kidneys for elimination

• Transporting hormones from endocrine organs to target tissues

Regulation

Blood helps maintain homeostasis by regulating several physiological parameters.

• Maintaining body temperature by absorbing and distributing heat

• Maintaining normal pH using buffers, including the alkaline reserve of bicarbonate ions

• Maintaining adequate fluid volume in the circulatory system

Protection

Blood plays a crucial role in protecting the body from blood loss and infection.

• Preventing blood loss: Plasma proteins and platelets initiate clot formation

• Preventing infection: Blood carries agents of immunity, including:

  • Antibodies

  • Complement proteins

  • White blood cells (leukocytes)

Composition of Blood

General Structure

Blood is a specialized connective tissue composed of a liquid matrix (plasma) and living cells (formed elements).

• Plasma: The nonliving, fluid matrix

• Formed elements: Living blood cells suspended in plasma, including:

  • Erythrocytes (red blood cells, RBCs)

  • Leukocytes (white blood cells, WBCs)

  • Platelets (cell fragments involved in clotting)

Major Components of Whole Blood

Blood can be separated into three main components by centrifugation:

Physical Characteristics and Volume

Blood has distinct physical properties and varies in color depending on oxygen content.

• Appearance: Sticky, opaque fluid with a metallic taste

• Color: Scarlet red when oxygen-rich; dark red when oxygen-poor

• pH: 7.35–7.45 (slightly alkaline)

• Volume: Approximately 8% of body weight

  • Males: 5–6 liters

  • Females: 4–5 liters

Key Terms

• Hematocrit: The percentage of blood volume occupied by erythrocytes (RBCs)

• Plasma: The liquid component of blood, containing water, proteins, electrolytes, nutrients, hormones, and waste products

• Formed elements: The cellular components of blood (RBCs, WBCs, and platelets)

Example: Clinical Application

Measurement of hematocrit is a common clinical test to assess the proportion of red blood cells in a patient's blood, which can help diagnose anemia or polycythemia.

Additional info: The study of blood and its disorders is called hematology. Blood composition and function are foundational topics in both anatomy and physiology, as they relate to oxygen delivery, immune defense, and homeostasis.