Cardiovascular System

Introduction

The cardiovascular system is responsible for the transport of nutrients, gases, hormones, and waste products throughout the body. Blood, a key component of this system, consists of several cellular elements with specialized functions. Understanding the structure and function of blood cells is essential for comprehending physiological and pathological processes.

Elements of Blood

Red Blood Cells (RBCs or Erythrocytes)

Red blood cells are the most abundant cellular component of blood, primarily responsible for oxygen transport. They are produced in the bone marrow through a process called erythropoiesis.

• Structure: RBCs are biconcave discs that lack nuclei and mitochondria, optimizing their surface area for gas exchange.

• Function: Their main role is oxygen transportation from the lungs to tissues and carbon dioxide removal.

• Hemoglobin: Each RBC contains approximately 280 million hemoglobin proteins. Hemoglobin is a tetramer, with each subunit containing a heme group that binds iron and oxygen.

• Affinity: Carbon monoxide (CO) binds to heme iron with 200–250 times greater affinity than oxygen (O2), making CO poisoning dangerous.

Additional info: The lack of organelles in mature RBCs allows more room for hemoglobin, but limits their lifespan to about 120 days.

Application: Iron-Deficiency Anemia

Iron-deficiency anemia is a common disorder resulting from insufficient iron, leading to reduced hemoglobin synthesis and impaired oxygen transport.

• Erythropoietin (EPO): A glycoprotein hormone released by the kidneys that regulates RBC and hemoglobin production in the bone marrow.

• Iron Transport: Iron from storage, heme recycling, and diet travels in blood attached to transferrin.

• Iron Uptake: Erythrocytes acquire iron via receptor-mediated endocytosis.

• Causes: Inadequate iron intake, increased demand (e.g., pregnancy), or malabsorption (e.g., celiac disease).

• Effects: Leads to reduced hemoglobin synthesis, resulting in microcytic (small) and hypochromic (pale) RBCs.

Additional info: Symptoms include fatigue, pallor, and shortness of breath.

Recombinant EPO

Recombinant erythropoietin (EPO) is used therapeutically to treat anemia associated with chronic kidney disease, cancer, or surgery.

• Clinical Use: Increases RBC production in patients with low counts.

• Space Physiology: Astronauts experience low RBC counts in microgravity, requiring EPO therapy.

• Sports: EPO is misused for performance enhancement in endurance athletes, monitored by the World Anti-Doping Agency.

Additional info: EPO abuse can increase the risk of thrombosis due to elevated hematocrit.

White Blood Cells (WBCs or Leukocytes)

White blood cells are produced in the bone marrow (leukopoiesis) and are essential for the body's defense against infection and disease.

• Function: Key players in the immune system, involved in extravasation (movement out of blood vessels).

• Classification:

  • Granulocytes: Contain granules in the cytoplasm. Includes neutrophils, eosinophils, and basophils.

  • Agranulocytes: Lack granules. Includes monocytes and lymphocytes.

Neutrophils

Neutrophils are the most abundant WBCs (50–70%) and act as first responders to infection.

• Function: Phagocytosis and degranulation to destroy pathogens.

• NETs (Neutrophil Extracellular Traps): Release fibers that immobilize and kill pathogens, facilitating phagocytosis.

• Role: "Warriors" that kill and die, causing tissue damage and forming pus.

Additional info: Elevated neutrophil counts are seen in bacterial infections.

Eosinophils

Eosinophils (1–3% of WBCs) have a bilobed nucleus and act by degranulation.

• Function: Defense against parasites (especially helminths).

• Role in Allergy: Release enzymes (histaminase, arylsulfatase) to degrade histamine and leukotrienes, modulating allergic and inflammatory responses.

Additional info: Eosinophilia is seen in parasitic infections and allergic conditions.

Basophils

Basophils (0.5–1% of WBCs) are involved in allergic and inflammatory reactions.

• Function: Degranulation releases histamine (vasodilator), leukotrienes (increase vascular permeability), and heparin (anticoagulant).

Additional info: Basophils are similar in function to mast cells found in tissues.

Monocytes

Monocytes (2–10% of WBCs) are large phagocytic cells.

• Function: "Big eaters" that digest microbes, apoptotic cells, and debris; clean up after infection and tissue injury.

• Differentiation: Upon entering tissues, monocytes become macrophages.

Additional info: Macrophages also play a role in antigen presentation to lymphocytes.

Lymphocytes

Lymphocytes (20–40% of WBCs) are central to specific (adaptive) immunity and immunological memory.

• B cells: Produce antibodies for opsonization and neutralization of pathogens.

• T cells:

  • Cytotoxic T cells (CD8+): Recognize and kill infected or cancerous cells.

  • Helper T cells (CD4+): Activate B cells and cytotoxic T cells.

  • Regulatory T cells (Tregs): Suppress immune responses to prevent autoimmunity.

Additional info: Lymphocyte counts increase in viral infections.

Platelets (Thrombocytes)

Platelets are cell fragments derived from megakaryocytes in the bone marrow and are essential for blood clotting.

• Function: Form the initial mass of the clot and promote thrombin production, which converts fibrinogen to fibrin.

Additional info: Platelet disorders can lead to bleeding or thrombotic conditions.

Hematopoiesis — Bone Marrow

Introduction

Hematopoiesis is the process of forming new blood cells in the bone marrow, which is located in the interior of bones.

• Red marrow: Active site of hematopoiesis.

• Yellow marrow: Primarily fat storage.

• Age-related changes: In children, almost all marrow is red; in adults, only flat bones (hip, vertebrae, sternum, ribs) contain red marrow.

• Clinical relevance: Hip bone is the preferred site for bone marrow aspiration and biopsy.

Additional info: Bone marrow failure can lead to pancytopenia (deficiency of all blood cell types).

Application — Leukemia

Leukemia is a group of cancers affecting hematopoietic stem cells, resulting in the inability to produce healthy blood cells.

• Effects on RBCs: Anemia, fatigue.

• Effects on WBCs: Increased risk of infections due to dysfunctional immune cells.

• Effects on Platelets: Bleeding and bruising due to low platelet counts.

• Treatment: Bone marrow transplantation may be required.

Additional info: Leukemia is classified as acute or chronic, and as lymphoid or myeloid, depending on the cell lineage affected.