Chapter 17: Blood – Structure, Function, and Composition

Blood: Overview and Functions

General Functions of Blood

Blood is a vital connective tissue that performs essential functions for the body, including transport, protection, and regulation. These functions are crucial for maintaining homeostasis and supporting life.

• Transport:

  • Transports gases such as oxygen (O2) and carbon dioxide (CO2).

  • Transports nutrients, wastes, hormones, and other substances throughout the body.

• Protection:

  • Prevents blood loss through clotting mechanisms.

  • Prevents infection by transporting immune cells and antibodies.

• Regulation:

  • Regulates fluid levels in tissues.

  • Maintains pH balance (normal range: 7.35–7.45).

  • Regulates body temperature by distributing heat.

Composition of Blood

Main Components

Blood is classified as a connective tissue and consists of two major components: plasma and formed elements.

• Plasma: The extracellular fluid (matrix) of blood, making up about 55% of total blood volume.

• Formed Elements: The cellular portion of blood, including:

  • Erythrocytes (red blood cells, RBCs)

  • Leukocytes (white blood cells, WBCs)

  • Platelets (cell fragments involved in clotting)

Blood composition can be analyzed using a hematocrit test, which measures the percentage of RBCs in blood.

Hematocrit

The hematocrit is the percent of blood volume that is composed of erythrocytes (RBCs). It is a key indicator of oxygen-carrying capacity.

• Normal hematocrit values:

  • Males: 45–55% PRBCs (packed red blood cells)

  • Females: 37–47% PRBCs

Physical Characteristics and Volume of Blood

Properties

Blood has distinct physical characteristics that are important for its function and clinical assessment.

• Appearance: Sticky, opaque fluid with a metallic taste.

• Color:

  • High O2 content: scarlet red

  • Low O2 content: dark red

• pH: 7.35–7.45 (slightly alkaline)

• Body weight: Blood makes up approximately 8% of total body weight.

• Average volume:

  • Males: 5–6 liters

  • Females: 4–5 liters

Blood Plasma

Composition and Function

Plasma is the liquid matrix of blood, responsible for transporting dissolved substances and maintaining osmotic balance.

• Percentage of blood: Plasma constitutes about 55% of blood volume.

• Serum: Plasma minus solids and clotting factors.

• Water content: Plasma is mostly water (about 90% by weight).

• Proteins:

  • Albumins: Maintain osmotic pressure.

  • Globulins: Transport and immune functions.

  • Fibrinogen: Involved in clotting.

  • Clotting factors, enzymes, and others

• Nutrients: Glucose, amino acids, lactic acid, lipids, and others.

• Electrolytes: Salts of sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), chloride (Cl), and others.

• Nitrogenous wastes: Urea and other metabolic byproducts.

• Hormones: Chemical messengers regulating body functions.

• Gases: Oxygen, carbon dioxide, and nitrogen.

Formed Elements of Blood

Types and Functions

The formed elements are the cellular components of blood, each with specialized functions.

• Erythrocytes (RBCs): Transport oxygen and carbon dioxide.

• Leukocytes (WBCs): Defend against pathogens and foreign substances.

• Platelets: Cell fragments that play a key role in blood clotting (hemostasis).

Example: A blood smear under a microscope shows erythrocytes as numerous, biconcave discs; leukocytes as larger, nucleated cells; and platelets as small fragments.

Summary Table: Major Components of Blood

Component	Percentage of Whole Blood	Main Function
Plasma	~55%	Transport of nutrients, wastes, hormones, and proteins
Buffy Coat (Leukocytes & Platelets)	Immune defense and clotting
Erythrocytes (RBCs)	~45%	Oxygen and carbon dioxide transport

Additional info: The notes above are expanded and clarified from the original slides and images, with academic context added for completeness and clarity.