Blood: Structure, Composition, and Function (Chapter 19 Study Notes)

Chapter 19: Blood

Introduction to Blood

Blood is a specialized connective tissue that plays a vital role in the transport of substances, regulation of physiological processes, and protection against disease. It consists of cellular elements suspended in a liquid extracellular matrix called plasma.

• Functions of Blood: Transport of gases, nutrients, wastes, hormones; regulation of pH, temperature, and fluid balance; protection via immune responses and clotting.

• Main Components: Plasma (liquid matrix) and formed elements (cells and cell fragments).

The Three Visible Layers of Blood

Blood Composition and Centrifugation

When a blood sample is centrifuged, it separates into three distinct layers based on density:

• Plasma: The top, pale yellow layer, making up about 55% of blood volume. It is mostly water with dissolved proteins, nutrients, and wastes.

• Buffy Coat: The thin, middle layer (about 1% of blood volume) containing leukocytes (white blood cells) and platelets.

• Erythrocytes: The bottom, red layer (about 44% of blood volume), composed of red blood cells.

Example: In a centrifuged tube, the plasma is on top, the buffy coat is a thin white layer in the middle, and erythrocytes settle at the bottom.

Plasma: Components and Functions

Major Plasma Components

Plasma is the liquid portion of blood, serving as a transport medium for many substances. Its composition is as follows:

Plasma Component	Function
Water	90% of plasma volume; solvent that dissolves and transports many solutes through the body
Plasma Proteins (9%)	Multiple functions (see below)
Albumin	Maintains osmotic pressure
Immune proteins	Produced by leukocytes; function in immunity
Transport proteins	Bind and transport hydrophobic compounds through the blood
Clotting proteins	Stop blood loss from damaged vessels
Other Solutes (1%)	Multiple functions (see below)
Glucose, amino acids	Nutrition; used for protein synthesis
Ions	Electrolyte and acid-base homeostasis
Dissolved gases (O2, CO2)	Oxygen delivered to tissues; carbon dioxide delivered to lungs for exhalation
Wastes	Delivered to the appropriate organ for excretion

• Osmotic Pressure: Maintained primarily by albumin, helps keep water in the bloodstream.

• Immune Proteins: Also called antibodies or immunoglobulins, these help defend against pathogens.

• Clotting Proteins: Such as fibrinogen, are essential for blood coagulation.

Formed Elements of Blood

Overview of Cellular Components

The formed elements of blood include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (thrombocytes). Each type has specialized functions:

• Erythrocytes: Most numerous; transport oxygen and carbon dioxide via hemoglobin.

• Leukocytes: Involved in immune defense; several classes exist (e.g., neutrophils, lymphocytes).

• Platelets: Cell fragments essential for blood clotting (hemostasis).

Example: In a blood smear, erythrocytes appear as biconcave discs, leukocytes are larger with nuclei, and platelets are small fragments.

Key Terms

• Plasma: The liquid, non-cellular part of blood.

• Formed Elements: The cellular components of blood (erythrocytes, leukocytes, platelets).

• Buffy Coat: The thin layer of leukocytes and platelets in centrifuged blood.

Summary Table: Blood Composition

Layer	Percentage of Blood Volume	Main Components
Plasma	~55%	Water, proteins, nutrients, wastes, hormones
Buffy Coat	~1%	Leukocytes, platelets
Erythrocytes	~44%	Red blood cells

Additional info: The percentages of each layer can vary depending on hydration, health status, and individual differences.