Chapter 17A: Blood

Blood: Internal Transport System

Overview

Blood is the life-sustaining transport fluid of the cardiovascular system. It is essential for delivering nutrients, gases, and hormones throughout the body, as well as for removing waste products. Understanding blood's anatomy and physiology is crucial for clinical practice, such as advising patients on preventing blood clots during hospital stays.

Functions of Blood

Main Functions

• Transport

• Regulation

• Protection

Transport

• Delivering oxygen (O2) and nutrients to body cells

• Transporting metabolic wastes to lungs and kidneys for elimination

• Transporting hormones from endocrine organs to target organs

Regulation

• 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

• Preventing blood loss through clot formation (plasma proteins and platelets)

• Preventing infection by transporting agents of immunity:

  • Antibodies

  • Complement proteins

  • White blood cells

Composition of Blood

General Composition

Blood is the only fluid tissue in the body and is classified as a connective tissue. Its matrix is a nonliving fluid called plasma, and the living blood cells are called formed elements.

• Formed elements include:

  • Erythrocytes (red blood cells, RBCs)

  • Leukocytes (white blood cells, WBCs)

  • Platelets

Major Components of Whole Blood

When blood is centrifuged, it separates into three layers:

Component	Location in Tube	Percentage of Whole Blood	Description
Plasma	Top	~55%	Least dense; contains water, proteins, nutrients, hormones, wastes
Buffy coat	Middle (thin layer)	<1%	Leukocytes and platelets
Erythrocytes	Bottom	~45%	Most dense; red blood cells

Hematocrit is the percentage of blood volume that is RBCs. Normal values:

• Males: 47% ± 5%

• Females: 42% ± 5%

Physical Characteristics and Volume

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

• Color varies with O2 content:

  • High O2: scarlet red

  • Low O2: dark red

• pH: 7.35–7.45

• Makes up ~8% of body weight

• Average volume:

  • Males: 5–6 L

  • Females: 4–5 L

Blood Plasma

Composition and Function

• Straw-colored, sticky fluid

• About 90% water

• Over 100 dissolved solutes:

  • Nutrients, gases, hormones, wastes, proteins, inorganic ions

• Plasma proteins are the most abundant solutes:

  • Remain in blood; not taken up by cells

  • Produced mostly by the liver

  • Albumin makes up 60% of plasma proteins:

    • Functions as a carrier of other molecules

    • Acts as a blood buffer

    • Contributes to plasma osmotic pressure

Example: Clinical Relevance

Understanding the composition and function of blood is essential for diagnosing and treating disorders such as anemia, clotting disorders, and immune deficiencies.