Chapter 19: Blood – Structure, Composition, and Functions

Blood and the Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system is essential for transporting substances throughout the body and maintaining homeostasis. It consists of three main components:

• The heart: A muscular pump that propels blood.

• Blood vessels: A series of conducting hoses that distribute blood.

• Blood: A fluid connective tissue that serves as the transport medium.

Components and Functions of Blood

Definition and General Properties

Blood is a specialized connective tissue composed of cells suspended in a fluid matrix called plasma. It performs several vital functions:

• Transport: Dissolved gases (O2, CO2), nutrients, hormones, and metabolic wastes.

• Regulation: pH and ion composition of interstitial fluids.

• Restriction: Limits fluid loss at injury sites.

• Defense: Protects against toxins and pathogens.

• Stabilization: Maintains body temperature.

Physical Characteristics of Blood

• Temperature: 38°C (100.4°F)

• Viscosity: High, due to cellular and protein content.

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

• Volume: Approximately 7% of body weight in liters. For a 75-kg (165-lb) person, this is about 5.4 liters (5.4 quarts).

Composition of Whole Blood

Whole blood consists of two main components:

• Plasma: The fluid portion.

• Formed elements: Cells and cell fragments.

Fractionation is the process of separating whole blood into plasma and formed elements for analysis or medical use.

Plasma

Plasma makes up about 55% of blood volume and is primarily water (over 90%). It also contains dissolved plasma proteins and other solutes. Plasma is similar in composition to interstitial fluid because water, ions, and small solutes are exchanged across capillary walls.

Table: Composition of Whole Blood

Component	Percentage of Blood Volume	Main Constituents
Plasma	~55%	Water (92%), Plasma Proteins (7%), Other Solutes (1%)
Formed Elements	~45%	Red Blood Cells (99.9%), White Blood Cells & Platelets (<0.1%)

Plasma Proteins

Plasma proteins are crucial for maintaining osmotic pressure, transporting substances, and immune defense. The major types include:

• Albumins (60%): Major contributors to plasma osmolarity; transport fatty acids, thyroid hormones, and some steroid hormones.

• Globulins (35%): Include antibodies (immunoglobulins) for immune defense and transport globulins (hormone-binding proteins, metalloproteins, apolipoproteins, and steroid-binding proteins).

• Fibrinogen (4%): Soluble protein involved in blood clotting; converted to insoluble fibrin during clot formation.

• Other plasma proteins (1%): Enzymes, hormones, and other regulatory proteins.

Origins of Plasma Proteins

• More than 90% are synthesized in the liver (albumins, fibrinogen, most globulins, and proenzymes).

• Antibodies are produced by plasma cells.

• Peptide hormones are made by endocrine organs.

Formed Elements

The formed elements of blood include:

• Red blood cells (erythrocytes): Transport oxygen and carbon dioxide.

• White blood cells (leukocytes): Defend against pathogens.

• Platelets (thrombocytes): Cell fragments involved in clotting.

Hemopoiesis is the process of producing formed elements, primarily occurring in red bone marrow.

Example:

A blood sample from a healthy adult typically separates into plasma (top layer, pale yellow) and formed elements (bottom layer, red), with a thin "buffy coat" of white blood cells and platelets in between.

Additional info: Plasma proteins play a key role in maintaining blood volume and pressure, and their concentrations can change in response to disease or dehydration.