Blood: Structure, Function, and Components – Study Notes for Anatomy & Physiology

Blood: Structure, Function, and Components

Overview of Blood

Blood is a specialized connective tissue that constitutes approximately 8% of total body weight. It is essential for the transport of substances throughout the body and plays a critical role in maintaining homeostasis.

• Definition: Blood is a fluid connective tissue composed of plasma and formed elements (cells and cell fragments).

• Main Components:

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

  • Formed Elements: Cellular components, including erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

• Physical Properties: Blood is denser and more viscous than water, with a slightly alkaline pH (7.35–7.45).

Functions of Blood

Blood performs several vital functions necessary for survival and homeostasis:

• Exchange of Gases: Transports oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs.

• Transport of Nutrients and Wastes: Delivers nutrients (e.g., glucose, amino acids) and removes metabolic wastes (e.g., urea, creatinine).

• Regulation of Molecules: Distributes hormones and other regulatory molecules throughout the body.

• Osmoregulation: Maintains osmotic balance by distributing solutes and regulating water content.

• Immune Function: Contains leukocytes and antibodies that defend against pathogens.

• Thermoregulation: Helps maintain body temperature by distributing heat.

• Clot Formation: Platelets and clotting factors prevent excessive blood loss after injury.

• Acid-Base Homeostasis: Buffers in blood help maintain pH balance.

• Stabilization of Blood Pressure: Maintains adequate blood volume and pressure.

Blood Composition

Blood consists of two main components: plasma and formed elements. When centrifuged, blood separates into three visible layers:

• Plasma (Top Layer): Straw-colored liquid, about 55% of blood volume.

• Buffy Coat (Middle Layer): Thin, whitish layer containing leukocytes and platelets (

  • Erythrocytes (Bottom Layer): Red blood cells, about 45% of blood volume (hematocrit).

Table: Main Components of Blood

Component	Percentage of Blood Volume	Main Constituents
Plasma	~55%	Water, plasma proteins, small solutes (ions, nutrients, wastes)
Buffy Coat	<1%	Leukocytes (WBCs), Platelets
Erythrocytes	~45%	Red blood cells

Plasma

Plasma is the liquid matrix of blood, serving as the medium for transporting substances.

• Composition:

  • ~91% water

  • ~7% plasma proteins (albumins, globulins, fibrinogen)

  • ~2% other solutes (electrolytes, nutrients, gases, waste products)

• Functions of Plasma Proteins:

  • Albumins: Maintain osmotic pressure and transport substances.

  • Globulins: Transport lipids, hormones, and act as antibodies (immunoglobulins).

  • Fibrinogen: Essential for blood clotting.

Formed Elements

The formed elements of blood include erythrocytes, leukocytes, and platelets.

• Erythrocytes (Red Blood Cells): Specialized for oxygen and carbon dioxide transport.

• Leukocytes (White Blood Cells): Involved in immune defense.

• Platelets (Thrombocytes): Cell fragments important for blood clotting.

Exchange of Substances in Capillaries

Exchanges between blood and other tissues occur primarily in capillaries, across their thin walls.

• Capillary Structure: Composed of a single layer of endothelial cells and a basement membrane.

• Gas Exchange Example: In pulmonary capillaries, oxygen diffuses from alveolar air into the blood, while carbon dioxide diffuses from blood into alveolar air for exhalation.

• Other Exchanges: Nutrients, wastes, and hormones also cross capillary walls to reach tissues or enter the bloodstream.

Example: Pulmonary Capillary Gas Exchange

• Oxygen (O2) is loaded from alveolar air into the blood.

• Carbon dioxide (CO2) is unloaded from the blood into the alveolar air.

Additional info: Capillaries are typically so narrow that erythrocytes must pass through them in single file, maximizing the efficiency of gas and nutrient exchange.