Blood: Structure, Functions, and Components (Chapter 17 Study Notes)

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Blood: Structure, Functions, and Components

Overview of Blood

Blood is a specialized connective tissue that serves as the internal transport system of the body. It consists of cellular elements suspended in a liquid matrix (plasma) and plays essential roles in transport, regulation, and protection.

Transport: Delivers oxygen, nutrients, and hormones; removes metabolic wastes.
Regulation: Maintains body temperature, pH, and fluid volume.
Protection: Prevents blood loss and defends against infection.

Functions of Blood

Blood performs three primary functions: transport, regulation, and protection.

Transport Functions

Oxygen and Nutrients: Blood delivers oxygen from the lungs and nutrients from the digestive tract to all body cells.
Metabolic Wastes: Transports waste products to the lungs (CO2) and kidneys (urea, creatinine) for elimination.
Hormones: Carries hormones from endocrine glands to target organs.

Regulatory Functions

Temperature: Absorbs and distributes heat throughout the body.
pH Balance: Maintains normal pH using buffers, including the "alkaline reserve" of bicarbonate ions.
Fluid Volume: Maintains adequate fluid volume in the circulatory system.

Protective Functions

Prevention of Blood Loss: Platelets and plasma proteins initiate clot formation.
Prevention of Infection: Blood carries agents of immunity, including antibodies, complement proteins, and white blood cells (WBCs).

Composition of Blood

Blood consists of plasma (liquid matrix) and formed elements (cells and cell fragments).

Plasma: Straw-colored, sticky fluid; ~90% water, contains over 100 dissolved solutes (nutrients, gases, hormones, wastes, proteins, ions).
Formed Elements: Includes red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).

Formed Elements

Red Blood Cells (RBCs): Also called erythrocytes; transport oxygen.
White Blood Cells (WBCs): Also called leukocytes; involved in immune defense.
Platelets: Cell fragments involved in clotting.

Tissue fibers (fibrin) only appear when blood clots.

Physical Characteristics and Volume of Blood

Viscosity: Blood is about five times more viscous than water, mainly due to RBCs.
Color: Scarlet red when oxygen-rich; dark red when oxygen-poor.
pH: 7.35–7.45 (slightly alkaline).
Volume: 5–6 L in males; 4–5 L in females (~8% of body weight).

Blood Sample Layers

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

Layer	Component	Percentage of Whole Blood
Plasma	Liquid matrix	~55%
Buffy coat	WBCs and platelets	<1%
Erythrocytes	RBCs	~45% (hematocrit)

Hematocrit is the percentage of whole blood volume made up of RBCs. Normal values: 47% ± 5% in males, 42% ± 5% in females.

Plasma Proteins

Albumin: ~60% of plasma proteins; functions as a blood buffer, carrier, and maintains osmotic pressure.
Globulins: ~36%; produced by the liver, bind to lipids, ions, and fat-soluble vitamins.
Fibrinogen: Involved in clotting; forms fibrin threads during coagulation.

Key Terms

Plasma: The liquid, extracellular matrix of blood.
Formed Elements: The cellular components of blood (RBCs, WBCs, platelets).
Hematocrit: The proportion of blood volume occupied by RBCs.
Albumin: The most abundant plasma protein, important for osmotic pressure and transport.

Example: Blood Sample Analysis

When analyzing a blood sample, the hematocrit value helps determine the oxygen-carrying capacity of the blood. A low hematocrit may indicate anemia, while a high value may suggest polycythemia.

Additional info: Blood is classified as a connective tissue because it contains cells suspended in an extracellular matrix (plasma). The presence of dissolved proteins and the ability to form fibers (fibrin during clotting) further support this classification.