Blood: Overview and Functions

Introduction to Blood

Blood is a vital connective tissue in the human body, responsible for the transport of essential substances and the maintenance of homeostasis.

• Transport Functions: Blood carries nutrients, wastes, hormones, and body heat throughout the body via blood vessels.

• Homeostatic Role: Blood helps regulate temperature, pH, and fluid balance.

Components of Blood

Main Components

Blood consists of two primary components: formed elements (living cells) and plasma (nonliving fluid matrix).

• Formed Elements: These are the cellular components, including erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

• Plasma: The liquid portion of blood, making up about 55% of total blood volume.

Separation of Blood

When blood is centrifuged, it separates into distinct layers:

• Plasma: Rises to the top (about 55% of blood volume).

• Erythrocytes: Sink to the bottom (about 45% of blood volume), a value known as the hematocrit.

• Buffy Coat: A thin, whitish layer between plasma and erythrocytes, containing leukocytes and platelets (less than 1% of blood volume).

Table: Composition of Blood

Physical Characteristics and Volume of Blood

Physical Properties

Blood exhibits several unique physical characteristics:

• Texture: Sticky, opaque fluid; heavier and thicker than water.

• Color: Varies with oxygen content—scarlet red when oxygen-rich, dull red or purple when oxygen-poor.

• Taste: Metallic and salty.

• pH: Slightly alkaline, ranging from 7.35 to 7.45.

• Temperature: Slightly higher than body temperature, at approximately 38°C (100.4°F).

Blood Volume

• Total Volume: About 5–6 liters (6 quarts) in a healthy adult.

• Percentage of Body Weight: Blood constitutes about 8% of total body weight.

Plasma

Composition of Plasma

Plasma is a straw-colored fluid that is approximately 90% water and contains a variety of dissolved substances.

• Nutrients: Glucose, amino acids, fatty acids, vitamins.

• Salts (Electrolytes): Sodium, potassium, calcium, magnesium, chloride, bicarbonate.

• Respiratory Gases: Oxygen and carbon dioxide.

• Hormones: Chemical messengers regulating physiological processes.

• Plasma Proteins: Albumin, globulins, fibrinogen.

• Waste Products: Urea, uric acid, creatinine, bilirubin.

Functions of Plasma Proteins

• Albumin: Maintains osmotic pressure and acts as a blood buffer.

• Clotting Proteins (e.g., Fibrinogen): Essential for blood clotting.

• Antibodies (Immunoglobulins): Provide immune defense against pathogens.

Dynamic Nature of Plasma

• Plasma composition changes as cells exchange substances with the blood.

• The liver produces more plasma proteins when levels drop.

• The respiratory and urinary systems help restore blood pH to normal if it becomes too acidic (acidosis) or too alkaline (alkalosis).

• Plasma also helps distribute body heat throughout the body.

Key Terms and Definitions

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

• Buffy Coat: The thin layer of leukocytes and platelets found between the plasma and erythrocytes after centrifugation.

• Plasma: The liquid, nonliving component of blood, containing water, proteins, and dissolved substances.

• Formed Elements: The cellular components of blood, including erythrocytes, leukocytes, and platelets.

Example: Clinical Application

When a patient experiences significant blood loss, a blood transfusion may be necessary to restore blood volume and maintain oxygen delivery to tissues. Understanding the components and properties of blood is essential for safe transfusion practices and for diagnosing blood disorders.