Blood: Structure, Function, and Composition

Functions of Blood

Blood is a vital connective tissue that performs several essential functions necessary for maintaining homeostasis in the human body.

• Transport:

  • Delivers oxygen (O2) and nutrients to body cells.

  • Transports metabolic wastes (e.g., carbon dioxide, urea) to lungs and kidneys for elimination.

  • Carries hormones from endocrine organs to target tissues.

• Regulation:

  • Maintains body temperature by absorbing and distributing heat.

  • Maintains normal pH using buffers, including the alkaline reserve of bicarbonate ions (HCO3-).

  • Maintains adequate fluid volume in the circulatory system.

• Protection:

  • Prevents blood loss via clot formation (plasma proteins and platelets).

  • Prevents infection by transporting agents of immunity:

    • Antibodies

    • Complement proteins

    • White blood cells (WBCs)

Composition of Blood

Blood is the only fluid tissue in the body and is classified as a specialized connective tissue. It consists of a nonliving fluid matrix called plasma and living cells known as formed elements.

• Plasma: The liquid, noncellular component.

• Formed Elements:

  • Erythrocytes (red blood cells, RBCs)

  • Leukocytes (white blood cells, WBCs)

  • Platelets (cell fragments involved in clotting)

Separation of Blood Components

When a tube of blood is centrifuged, it separates into three layers:

• Erythrocytes (bottom, ~45% of whole blood):

  • Measured as hematocrit (percentage of blood volume that is RBCs).

  • Normal values:

    • Males: 47% ± 5%

    • Females: 42% ± 5%

• Buffy coat (middle, <1%):

  • Contains WBCs and platelets.

  • Appears as a thin, whitish layer between RBCs and plasma.

• Plasma (top, ~55%):

  • Least dense component.

Physical Characteristics and Volume of Blood

Blood is a sticky, opaque fluid with a metallic taste. Its color and volume vary depending on oxygen content and individual characteristics.

• Color:

  • High O2: scarlet red

  • Low O2: dark red

• pH: 7.35–7.45 (slightly alkaline)

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

• Average volume:

  • Males: 5–6 L

  • Females: 4–5 L

Blood Plasma

Plasma is a straw-colored, sticky fluid that constitutes about 55% of blood volume. It is composed of approximately 90% water and over 100 dissolved solutes.

• Solutes include:

  • Nutrients (glucose, amino acids, fatty acids)

  • Gases (O2, CO2)

  • Hormones

  • Wastes (urea, creatinine)

  • Proteins (albumin, globulins, fibrinogen)

  • Inorganic ions (Na+, K+, Ca2+, Cl-, HCO3-)

• Plasma proteins (most abundant solutes):

  • Remain in blood; not taken up by cells.

  • Produced mostly by the liver.

  • Albumin (60% of plasma proteins):

    • Functions as a carrier for other molecules, a blood buffer, and contributes to plasma osmotic pressure.

Table: Composition of Plasma

The following table summarizes the main constituents of plasma and their functions:

Additional info: The table above is reconstructed from the slide image and standard textbook content for clarity and completeness.