Blood: Structure, Function, and Components

Overview of Blood in Human Anatomy & Physiology

Blood is a specialized connective tissue that plays a vital role in maintaining homeostasis within the human body. It consists of cellular elements suspended in a liquid matrix and is essential for transport, regulation, and protection.

Functions of Blood

Transport

• Oxygen and Nutrients: Blood delivers oxygen (O2) and nutrients to body cells.

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

• Hormone Distribution: Carries hormones from endocrine glands to target organs.

Regulation

• Temperature: Maintains body temperature by absorbing and distributing heat.

• pH Balance: Maintains normal pH using buffers, including the "alkaline reserve" of bicarbonate ions.

• Fluid Volume: Maintains adequate fluid volume in the circulatory system.

Protection

• Prevention of Blood Loss: Plasma proteins and platelets initiate clot formation to prevent excessive bleeding.

• Immunity: Blood carries agents of immunity, including antibodies, complement proteins, and white blood cells (leukocytes).

Blood Composition

Plasma and Formed Elements

Blood is composed of plasma (the liquid matrix) and formed elements (cells and cell fragments).

• Plasma: Straw-colored, sticky fluid making up about 55% of blood volume. Contains water, proteins, nutrients, hormones, gases, and waste products.

• Formed Elements: Includes erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (cell fragments).

Formed Elements

• Erythrocytes (RBCs): Specialized for oxygen transport; lack nuclei and most organelles.

• Leukocytes (WBCs): Complete cells with nuclei; function in immune defense.

• Platelets: Cell fragments involved in blood clotting.

Additional info: Tissue fibers (fibrin) only appear when blood clots, providing structural support to the clot.

Physical Characteristics of Blood

• Viscosity: Blood is about five times more viscous than water due to the presence of formed elements.

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

• pH: Maintained between 7.35 and 7.45.

• Volume: Average adult male: 5–6 L; female: 4–5 L.

Plasma Components

• Water: ~90% of plasma volume.

• Solutes: Over 100 dissolved substances, including nutrients, gases, hormones, wastes, proteins, and inorganic ions.

• Plasma Proteins: Most abundant solutes (~8% of plasma by weight). Major types include:

  • Albumin: ~60% of plasma proteins; functions as a blood buffer, carrier, and maintains osmotic pressure.

  • Globulins: ~36%; bind to lipids, ions, and fat-soluble vitamins.

  • Fibrinogen: Involved in clot formation.

Summary Table: Blood Components

Key Terms

• Hematocrit: Percentage of blood volume occupied by erythrocytes; normal values are ~45% (47% ± 5% in males, 42% ± 5% in females).

• Buffy Coat: Thin layer between plasma and RBCs in centrifuged blood; contains WBCs and platelets.

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

• Plasma: Liquid extracellular matrix of blood.

Example: Blood Sample Layers

• After centrifugation, a blood sample separates into three layers:

  • Bottom: RBCs (hematocrit)

  • Middle: Buffy coat (WBCs and platelets)

  • Top: Plasma

Conclusion

Blood is a complex tissue essential for transport, regulation, and protection in the human body. Understanding its composition and functions is foundational for further study in anatomy and physiology.