Blood: Structure, Functions, and Clinical Relevance

Functions of Blood

Blood is a major component of the cardiovascular system and is essential for maintaining homeostasis and protecting the body.

• Transportation: Blood transports oxygen, carbon dioxide, nutrients, hormones, and waste products throughout the body.

• Homeostasis: Blood helps regulate pH (normal range: 7.35–7.45), temperature, and concentrations of various substances.

• Protection: Blood contains cells and molecules of the immune system that defend against pathogens and injury.

Composition of Blood

The average adult human has approximately 5 liters (10.5 pints) of blood. Blood consists of two main components: plasma and formed elements.

• Plasma: The fluid extracellular matrix of blood, making up about 55% of total blood volume. Plasma is mostly water (about 90%) and contains dissolved substances such as gases, amino acids, proteins, carbohydrates, lipids, vitamins, hormones, electrolytes, and cellular wastes.

• Formed Elements: These include erythrocytes (RBCs), leukocytes (WBCs), and platelets.

Physical and Chemical Characteristics of Blood

Blood has unique physical and chemical properties that are important for its function.

• Viscosity: Blood is 3–4 times more viscous than water, which is important for its flow and function.

• pH: The normal pH of blood is tightly regulated between 7.35 and 7.45.

• Body Weight: Blood accounts for about 8% of total body weight. This value is affected by gender, body size, amount of adipose tissue, and changes in fluid composition.

Clinical Testing: Complete Blood Count (CBC)

A CBC is a common laboratory test that analyzes the components of blood.

• Procedure: A sample of whole blood is taken and spun down in a centrifuge to separate plasma from formed elements.

• Purpose: CBC provides information about the number and types of blood cells, which is useful for diagnosing various conditions.

Hematocrit (HCT)

Hematocrit is the percentage of red blood cells (RBCs) in whole blood and is an important clinical measurement.

• Normal Values:

  • Males: 40–54%

  • Females: 35–46%

• Low Hematocrit: May be caused by anemia, poor nutrition, or chemotherapy.

• High Hematocrit: May be caused by dehydration or excess production of RBCs.

Major Components of Blood

Blood is composed of plasma and formed elements.

• Plasma: The liquid portion containing dissolved substances.

• Formed Elements:

  • Red Blood Cells (RBCs): Transport oxygen and carbon dioxide.

  • White Blood Cells (WBCs): Defend against infection.

  • Platelets: Involved in blood clotting.

Factors Affecting Blood Volume

Blood volume varies among individuals and is influenced by several factors.

• Gender

• Body Size

• Amount of Adipose Tissue

• Variations in Fluid/Electrolyte Balance

Abnormal Hematocrit Values

Abnormal hematocrit values can indicate underlying health issues.

• Low Hematocrit:

  • Anemia

  • Nutrient deficiencies

  • Chemotherapy

• High Hematocrit:

  • Dehydration

  • Excess RBC production

Hematopoiesis

Hematopoiesis is the process of blood cell formation, which occurs in the red bone marrow.

• Stem Cells: All blood cells originate from hematopoietic stem cells (hemocytoblasts).

• Differentiation: Stem cells divide and are exposed to hematopoietic growth factors, which signal gene expression and cause differentiation into specific blood cell types.

Erythrocytes (Red Blood Cells)

Erythrocytes are the most abundant blood cells and are specialized for oxygen transport.

• Shape: Biconcave, which increases surface area for gas diffusion and allows flexibility.

• Lifespan: Approximately 120 days; old RBCs are removed by macrophages in the liver and spleen.

• Organelles: Mature RBCs lack organelles and carry only hemoglobin.

• Hemoglobin: Each RBC contains about 270 million hemoglobin molecules, which bind to oxygen and carbon dioxide.

Table: Major Components of Blood

Table: Normal Hematocrit Values

Key Equations

• Hematocrit Calculation:

• Blood pH Range:

Example: CBC Interpretation

A patient with a hematocrit of 30% may be diagnosed with anemia, while a hematocrit of 55% may suggest dehydration or polycythemia.

Additional info: The notes infer the importance of blood composition in clinical diagnosis and the role of hematopoiesis in maintaining healthy blood cell levels.