Blood Physiology: Iron Metabolism, Leukocytes, and Coagulation

Iron Metabolism

The body contains a total of approximately 4.5 grams of iron. Of this total:

• 4% corresponds to Hemoglobin (Hb).
• 15–30% is stored in the reticuloendothelial system, primarily as myoglobin.

White Blood Cells (Leukocytes)

Leukocytes are mobile units of the systemic organism. They provide protection and form part of the bone marrow and lymphatic tissue.

Normal count ranges between 4,000 to 11,000 leukocytes per microliter. Granulocytes are the most common type (ca. 65%), including neutrophils, eosinophils, and basophils.

Principal Functions of Leukocytes

1. Granulocytes and Monocytes: Protect the body from invading microorganisms (MO) through phagocytosis.
2. Lymphocytes and Plasma Cells: Generally act within the immune system.

Eosinophils

Eosinophils constitute about 2% of leukocytes in the blood. They are weak phagocytes, primarily active against parasitic infections.

They accumulate in tissues that have suffered an allergic reaction via chemotaxis. Mast cells and basophils release eosinophil chemotactic factor, which permits eosinophil migration.

Basophils

Basophils release several key substances:

• Heparin: Prevents blood coagulation.
• Bradykinin and Serotonin.

They play a very important role, along with mast cells, in allergic reactions. Immunoglobulin E (IgE) tends to bind to both mast cells and basophils.

Lymphocytes

Lymphocytes vary in size. Most nuclei lack cytoplasmic granules.

• B Lymphocytes: Responsible for humoral immunity. Upon encountering an antigen, they become plasma cells, which synthesize and secrete antibodies (gammaglobulins).
• T Lymphocytes: Responsible for cellular immunity.

Platelets (Thrombocytes)

Platelets are round or oval discs, approximately 1 to 4 µm in diameter. They originate from megakaryocytes and lack a nucleus.

Characteristics and Function

Platelets contain several key components necessary for function:

• Contractile Proteins: Actin, myosin, and thrombosthenin.
• Organelles: Golgi reticulum remnants synthesize enzymes and store large amounts of Ca²⁺. Mitochondria produce ATP and ADP.
• Enzyme System: Produces Prostaglandin (involved in hormones, vascular, and local tissue reactions).
• Proteins: Fibrin-stabilizing protein and Growth Factor Protein.

Platelet Membrane Components

• Glycoproteins: Allow adhesion to damaged endothelium.
• Phospholipids: Provide diverse active sites crucial for the process of blood clotting.

Blood Coagulation (Hemostasis)

Coagulation occurs rapidly: 15–20 seconds following intense trauma, or 1–2 minutes following a minor injury. Substances activating the traumatized vascular wall and proteins from platelets initiate the blood clotting process.

Mechanisms of Blood Coagulation: The Coagulation Cascade

The cascade involves numerous clotting factors and results in the following sequence:

1. Formation of Factor Prothrombin Activator (FX).
2. Prothrombin converts to Thrombin.
3. Fibrinogen converts to Fibrin (the stable clot).

Extrinsic Pathway

Stimulus: Damage to the vascular wall or explosive extravascular tissue damage.

This is the rapid coagulation pathway (approximately 15 seconds).

Intrinsic Pathway

Stimulus: Trauma to the blood itself or exposure of blood to collagen in the wall of an injured blood vessel.

This is the slower coagulation pathway (approximately 1 to 6 minutes).