Blood: Composition, Functions, and Cellular Elements

Learning Objectives

• Understand the characteristics and functions of blood.

• Describe the composition of plasma.

• Describe the characteristics and functions of the RBCs and WBCs found in blood.

• Understand the ABO blood groups and blood typing.

• Understand the process of hemostasis.

Key Terms

• Whole blood, Plasma, Formed elements, Hematocrit, Buffy coat

• Albumin, Globulin, Fibrinogen, Electrolyte

• Erythrocyte, Hemoglobin, Oxyhemoglobin, Deoxyhemoglobin, Carbaminohemoglobin

• Erythropoiesis, Erythropoietin

• Agglutination, Hemolysis, Thrombin, Fibrin, Fibrinolysis, Embolus, Thrombus

• Leukocyte, Neutrophil, Eosinophil, Basophil, Monocyte, Lymphocyte

• Platelets, Thrombocytopoiesis, Megakaryocyte

Functions of Blood

Overview

Blood is a specialized connective tissue that performs vital functions necessary for homeostasis and survival.

• Transportation: Transports oxygen (O2), carbon dioxide (CO2), nutrients, hormones, and waste products throughout the body.

• Regulation: Maintains pH and ion composition (e.g., calcium), and regulates fluid balance.

• Prevention of Blood Loss: Blood clotting (coagulation) mechanisms prevent excessive bleeding after injury.

• Defense: Protects against toxins and pathogens via immune cells and antibodies.

• Stabilization of Body Temperature: Distributes heat throughout the body and helps maintain thermal homeostasis.

Composition of Blood

Major Components

Blood consists of two main components: plasma (the liquid matrix) and formed elements (cells and cell fragments).

• Plasma: Makes up 46–63% of whole blood; primarily water, ions, proteins, and other solutes.

• Formed Elements: Comprise 37–54% of whole blood; include red blood cells (RBCs), white blood cells (WBCs), and platelets.

Physical Properties

• Viscosity: Blood is about 5 times more viscous than water.

• Equivalent Salt Concentration: 0.9% NaCl (isotonic to body fluids).

• Percent of Body Weight: Blood accounts for approximately 7% of total body weight.

• Volume: Typical values: Females 4–5 L, Males 5–6 L.

Centrifugation of Blood

When blood is centrifuged, it separates into three layers:

• Plasma: Top layer; mostly water, proteins, and solutes.

• Buffy Coat: Middle thin layer; contains WBCs and platelets.

• Formed Elements: Bottom layer; primarily RBCs.

Hematocrit is calculated as:

Plasma Composition

Plasma is the fluid matrix of blood and contains:

• Water: ~92% of plasma; solvent for carrying other substances.

• Plasma Proteins (7%):

  • Albumins (60%): Main transport proteins; maintain osmotic pressure.

  • Globulins (35%): Include antibodies (immunoglobulins) and transport globulins.

  • Fibrinogen (4%): Precursor to fibrin, essential for blood clotting.

  • Regulatory Proteins (<1%): Enzymes, hormones, and other regulatory molecules.

• Other Solutes (1%):

  • Electrolytes: Ions such as Na+, K+, Ca2+, Cl-.

  • Organic Nutrients: Glucose, amino acids, lipids, etc.

Red Blood Cells (Erythrocytes)

Structure and Function

Red blood cells (RBCs) are specialized for the transport of oxygen and carbon dioxide.

• Shape: Biconcave disc (~8 μm diameter), which increases surface area for gas exchange.

• Flexibility: Can bend and flex to pass through small capillaries.

• Stacking: Form stacks called rouleaux to facilitate smooth flow through vessels.

• Lifespan: Approximately 120 days; lack nucleus and ribosomes, so cannot repair themselves.

• Function: Transport O2 from lungs to tissues and CO2 from tissues to lungs.

Hemoglobin

Hemoglobin is the protein in RBCs responsible for oxygen and carbon dioxide transport.

• Structure: Composed of four protein chains (2 alpha, 2 beta) and four heme groups, each containing an iron ion.

• Oxyhemoglobin: Hemoglobin with O2 bound to iron in heme.

• Deoxyhemoglobin: Hemoglobin without O2 bound; iron is not strongly bound to heme.

• Carbaminohemoglobin: CO2 bound to amino acids in the globin chains.

Function: Each hemoglobin molecule can carry up to four O2 molecules.

Summary Table: Major Components of Blood

Example: Hematocrit Calculation

If a blood sample has 45 mL of packed RBCs in 100 mL of whole blood:

Additional info: The notes above provide a foundation for understanding blood's role in physiology, including its composition, the structure and function of erythrocytes, and the importance of plasma proteins. Further study would include white blood cells, platelets, blood typing, and hemostasis mechanisms.