Chapter 20: The Cardiovascular System – Blood

Blood and Plasma

Functions of Blood

Blood is a vital connective tissue that performs several essential functions in the human body:

• Transport: Carries gases (oxygen and carbon dioxide), nutrients, hormones, and metabolic wastes throughout the body.

• Regulation: Maintains pH balance, body temperature, and fluid volume within tissues.

• Protection: Provides defense through clotting mechanisms and immune responses.

Composition of Blood

• Plasma: Makes up approximately 55% of total blood volume. It is a pale yellow fluid composed of about 90% water, dissolved proteins, electrolytes, nutrients, hormones, and waste products.

• Formed Elements: Comprise about 45% of blood and include red blood cells (RBCs), white blood cells (WBCs), and platelets.

Plasma Proteins

• Albumin: Maintains osmotic pressure, helping to regulate water movement between blood and tissues.

• Globulins: Function as antibodies (immunoglobulins) and transport proteins.

• Fibrinogen: Essential for blood clotting.

Serum

• Serum: Plasma without clotting proteins, especially fibrinogen. Used in diagnostic testing.

Red Blood Cells (Erythrocytes)

Structure and Function

• Biconcave Shape: Increases surface area for gas exchange and provides flexibility to navigate capillaries.

• No Nucleus: Mature RBCs lack a nucleus, maximizing space for hemoglobin.

• Hemoglobin: Each RBC is packed with hemoglobin, a protein that binds and transports oxygen and carbon dioxide.

Life Span and Removal

• Life Span: Approximately 120 days.

• Removal: Old or damaged RBCs are removed by the spleen and liver.

Hematocrit and Packed Cell Volume

• Hematocrit: The percentage of blood volume occupied by RBCs. Also called packed cell volume (PCV).

Blood Types

ABO Blood Group System

• Type A: Has A antigen on RBCs; produces anti-B antibodies.

• Type B: Has B antigen; produces anti-A antibodies.

• Type AB: Has both A and B antigens; produces no anti-A or anti-B antibodies (universal recipient).

• Type O: Has no A or B antigens; produces both anti-A and anti-B antibodies (universal donor).

Rh Blood Group System

• Rh Positive (Rh+): RBCs have the D antigen.

• Rh Negative (Rh−): RBCs lack the D antigen.

Transfusion Reactions

• Cross-Reaction: Transfusion of incompatible blood leads to agglutination (clumping) and hemolysis (destruction) of RBCs.

White Blood Cells (Leukocytes)

Types and Functions

• Granulocytes:

  • Neutrophils: Phagocytize bacteria; most abundant WBC.

  • Eosinophils: Combat parasitic infections and mediate allergic responses.

  • Basophils: Release histamine (inflammation) and heparin (anticoagulant).

• Agranulocytes:

  • Lymphocytes: Include B cells (produce antibodies), T cells (cell-mediated immunity), and NK (natural killer) cells.

  • Monocytes: Differentiate into macrophages for phagocytosis of pathogens and debris.

• Purpose: Defend the body against infection and foreign substances.

Platelets and Hemostasis

Platelets (Thrombocytes)

• Origin: Cell fragments derived from megakaryocytes in bone marrow.

• Function: Essential for blood clotting and repair of damaged blood vessels.

Hemostasis

Hemostasis is the process that stops bleeding and initiates tissue repair. It occurs in three main steps:

1. Vascular Spasm: Constriction of blood vessels to reduce blood flow.

2. Platelet Plug Formation: Platelets adhere to the site of injury and to each other, forming a temporary plug.

3. Coagulation: Formation of a stable fibrin clot to seal the break in the vessel wall.

Hematopoiesis

Blood Cell Formation

• Hematopoiesis: The process of forming all blood cells, occurring primarily in red bone marrow.

Erythropoiesis (Red Blood Cell Formation)

• Stages: Stem cell → proerythroblast → erythroblast → reticulocyte → erythrocyte.

• Regulation: Stimulated by erythropoietin (EPO), a hormone produced by the kidneys in response to low oxygen levels.

Leukopoiesis (White Blood Cell Formation)

• Process: Formation of WBCs from stem cells in bone marrow.

Megakaryocytes and Platelet Formation

• Megakaryocyte: Large bone marrow cell that fragments to release platelets into the bloodstream.

Laboratory Identification of Blood Cells

• Red Blood Cells (RBCs): Appear as pink, biconcave discs under the microscope.

• Platelets: Tiny purple fragments.

• Neutrophils: Multilobed nucleus.

• Eosinophils: Red cytoplasmic granules.

• Basophils: Dark blue cytoplasmic granules.

• Lymphocytes: Large, round nucleus with little cytoplasm.

• Monocytes: Kidney-shaped nucleus.

Summary Table: Blood Components and Functions

Component	Main Function	Key Features
Red Blood Cells (Erythrocytes)	Transport O2 and CO2	Biconcave, no nucleus, hemoglobin-rich
White Blood Cells (Leukocytes)	Immunity and defense	Granulocytes & agranulocytes; various nuclei shapes
Platelets (Thrombocytes)	Clotting	Cell fragments from megakaryocytes
Plasma	Transport, regulation	90% water, proteins, solutes

Key Equations and Definitions

• Hematocrit (Hct): $\text{Hematocrit} = \frac{\text{Volume of RBCs}}{\text{Total Blood Volume}} \times 100\%$

• Serum: Plasma minus clotting factors (especially fibrinogen).

• Hemoglobin: Protein in RBCs responsible for oxygen transport; each molecule contains four heme groups, each binding one O2 molecule.