Blood: Structure, Functions, and Components

Blood: Structure, Functions, and Components

Functions of Blood

Blood is a vital connective tissue that performs essential functions necessary for maintaining homeostasis and overall health. Its functions can be grouped into three main categories: transport, regulation, and protection.

• Transport Functions:

  • Delivers oxygen from the lungs and nutrients from the digestive tract to body tissues.

  • Removes metabolic wastes (such as carbon dioxide and urea) from cells for elimination.

  • Transports hormones from endocrine glands to target organs.

• Regulation Functions:

  • Maintains body temperature by absorbing and distributing heat.

  • Helps maintain a constant blood pH through buffers.

  • Ensures adequate fluid volume in the circulatory system.

• Protective Functions:

  • Prevents blood loss through hemostasis (clotting mechanisms).

  • Defends against infection via immune cells and antibodies.

Major Components of Whole Blood

Blood consists of two main components: plasma and formed elements. The separation of these components can be visualized after centrifugation of a blood sample.

• Plasma: Makes up about 55% of whole blood and is the least dense component.

• Buffy Coat: Contains leukocytes (white blood cells) and platelets; accounts for less than 1% of blood volume.

• Erythrocytes: Red blood cells; comprise about 45% of blood volume (measured as hematocrit), and are the most dense component.

Hematocrit is the percentage of blood volume occupied by erythrocytes. It is a key indicator of oxygen-carrying capacity.

Composition of Blood

Blood is a viscous, slightly alkaline fluid that represents about 8% of total body weight. The average blood volume in a normal adult is approximately 5 liters.

• Formed Elements: Include erythrocytes, leukocytes, and platelets.

• Plasma: The liquid matrix of blood, making up 55% of whole blood.

Plasma: Structure and Contents

Plasma is a straw-colored, viscous fluid that is 90% water. The remaining 10% consists of various solutes essential for physiological functions.

• Water: Serves as the solvent and medium for transport.

• Solutes:

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

  • Respiratory Gases: Oxygen and carbon dioxide.

  • Electrolytes: Sodium, potassium, calcium, chloride, bicarbonate.

  • Hormones: Steroid and thyroid hormones carried by plasma proteins.

  • Proteins: Albumin, globulins, and fibrinogen (see below for details).

Plasma Proteins

• Albumin: Most abundant plasma protein; maintains osmotic pressure and transports molecules.

• Globulins: Include antibodies and transport proteins.

• Fibrinogen: Essential for blood clotting.

Summary Table: Major Components of Plasma

Constituent	Description and Importance
Water	90% of plasma volume; solvent for carrying other substances.
Electrolytes	Maintain osmotic balance and pH; include sodium, potassium, calcium, chloride, bicarbonate.
Plasma Proteins	Albumin (osmotic pressure, transport), Globulins (antibodies, transport), Fibrinogen (clotting).
Nutrients	Glucose, amino acids, fatty acids, vitamins; provide energy and building blocks.
Respiratory Gases	Oxygen (mostly bound to hemoglobin), carbon dioxide (dissolved or as bicarbonate).
Hormones	Regulate physiological processes; transported by plasma proteins.

Additional info: Plasma also contains nonprotein nitrogenous substances (e.g., urea, uric acid, creatinine) as metabolic by-products.