Blood and the Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system is essential for transporting substances throughout the body and maintaining homeostasis. It consists of three main components:

• Blood: The fluid connective tissue that transports nutrients, gases, wastes, and hormones.

• Heart: The muscular pump that circulates blood.

• Blood Vessels: The network of tubes (arteries, veins, capillaries) that carry blood to and from all body tissues.

Components and Functions of Blood

Composition of Blood

Blood is a specialized connective tissue composed of cells and cell fragments (formed elements) suspended in a fluid matrix (plasma).

• Plasma: The liquid component, making up 46-63% of blood volume. It is about 92% water and contains proteins, nutrients, hormones, electrolytes, and wastes.

• Formed Elements: The cellular portion, including red blood cells (RBCs), white blood cells (WBCs), and platelets.

Functions of Blood

• Transport: Carries dissolved gases (O2, CO2), nutrients, hormones, and metabolic wastes.

• Regulation: Maintains pH, ion composition, and fluid balance in tissues.

• Restriction of Fluid Loss: Clotting mechanisms prevent excessive bleeding after injury.

• Defense: WBCs and antibodies protect against toxins and pathogens.

• Stabilization of Body Temperature: Distributes heat generated by muscles.

Physical Characteristics of Blood

• Temperature: Approximately 38°C (100.4°F).

• Viscosity: About 5 times more viscous than water.

• pH: Slightly alkaline (7.35–7.45).

• Volume: About 7% of body weight in kilograms.

Plasma and Plasma Proteins

• Albumins: Most abundant; contribute to osmotic pressure and transport substances.

• Globulins: Include antibodies (immunoglobulins) and transport proteins.

• Fibrinogen: Soluble protein involved in clotting; converted to insoluble fibrin during coagulation.

Most plasma proteins are synthesized by the liver, except immunoglobulins, which are produced by WBCs.

Formed Elements

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

• White Blood Cells (Leukocytes): Defend against infection and disease.

• Platelets (Thrombocytes): Cell fragments involved in clotting.

Hematopoiesis is the process of producing formed elements, primarily in red bone marrow.

Red Blood Cells (RBCs)

Structure and Function

• RBCs are biconcave discs, increasing surface area for gas exchange and allowing flexibility in capillaries.

• They lack nuclei, mitochondria, and ribosomes, making them unable to divide or repair themselves.

• Contain hemoglobin, a protein responsible for oxygen and carbon dioxide transport.

Hemoglobin (Hb)

• Composed of four polypeptide chains (2 alpha, 2 beta), each with a heme group containing iron.

• Iron binds reversibly to oxygen, forming oxyhemoglobin ().

• Hemoglobin also binds carbon dioxide (as carbaminohemoglobin) at different sites than oxygen.

RBC Life Cycle and Erythropoiesis

• RBCs live about 120 days; about 1% are replaced daily.

• Erythropoiesis is the formation of RBCs from stem cells (hemocytoblasts) in red bone marrow.

• Regulated by the hormone erythropoietin (EPO), secreted by kidneys and liver in response to hypoxia.

• Essential nutrients: amino acids, iron, vitamin B12, B6, and folic acid.

Hemoglobin Recycling

• Old RBCs are phagocytized in the spleen, liver, and bone marrow.

• Heme is converted to biliverdin (green), then bilirubin (yellow), which is excreted in bile.

• Iron is recycled, transported by transferrin, and stored as ferritin or hemosiderin.

• Excessive hemolysis can cause hemoglobinuria (Hb in urine) or hematuria (RBCs in urine).

Blood Types

ABO and Rh Blood Groups

• Antigens (agglutinogens) on RBC surfaces determine blood type.

• Four main ABO types:

  • Type A: A antigen, anti-B antibodies

  • Type B: B antigen, anti-A antibodies

  • Type AB: A and B antigens, no anti-A or anti-B antibodies

  • Type O: No A or B antigens, both anti-A and anti-B antibodies

• Rh (D) antigen: Presence (+) or absence (−) determines Rh status.

Blood Transfusions and Compatibility

• Transfusion reactions (cross-reactions) occur if donor and recipient blood types are incompatible, causing agglutination and hemolysis.

• Type O− is the universal donor; AB+ is the universal recipient.

• Compatibility testing (cross-matching) is essential before transfusions.

Hemolytic Disease of the Newborn (HDN)

• Occurs when an Rh− mother is sensitized to Rh+ fetal blood, producing anti-Rh antibodies that can attack a subsequent Rh+ fetus.

• Prevention: Administration of anti-Rh immunoglobulin (RhoGAM) to the mother.

White Blood Cells (WBCs)

Types and Functions

• Leukocytes are nucleated cells involved in defense against pathogens, removal of toxins, and attacking abnormal cells.

• Most WBCs are found in connective tissues and lymphatic organs; only a small fraction circulates in blood.

Classification of WBCs

• Granular Leukocytes:

  • Neutrophils: 50–70%; phagocytic, first responders to infection, form pus.

  • Eosinophils: 2–4%; attack parasites, involved in allergic responses.

  • Basophils: <1%; release histamine (vasodilation) and heparin (anticoagulant).

• Agranular Leukocytes:

  • Monocytes: 2–8%; become macrophages, phagocytize large particles.

  • Lymphocytes: 20–40%; include T cells (cell-mediated immunity), B cells (antibody production), and NK cells (immune surveillance).

WBC Production and Regulation

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

• Regulated by colony-stimulating factors (CSFs) and exposure to antigens.

• WBC disorders include leukopenia (low count), leukocytosis (high count), and leukemia (cancer of WBCs).

Platelets

Structure, Function, and Production

• Platelets (thrombocytes) are cell fragments involved in blood clotting.

• Circulate for 9–12 days; removed by phagocytes in the spleen.

• Functions:

  • Release clotting chemicals

  • Form temporary platelet plugs

  • Reduce the size of vessel breaks

• Produced by megakaryocytes in red bone marrow (thrombocytopoiesis), stimulated by thrombopoietin (TPO), interleukin-6 (IL-6), and multi-CSF.

Hemostasis

Phases of Hemostasis

Hemostasis is the process of stopping bleeding and involves three phases:

1. Vascular Phase: Vascular spasm constricts blood vessels; endothelial cells release factors for repair and become sticky.

2. Platelet Phase: Platelets adhere to exposed collagen, aggregate to form a plug, and release chemicals (ADP, thromboxane A2, serotonin, clotting factors, PDGF, Ca2+).

3. Coagulation Phase: Cascade of reactions involving clotting factors leads to the conversion of fibrinogen to fibrin, forming a stable clot.

Coagulation Pathways

• Extrinsic Pathway: Triggered by tissue factor from damaged cells; activates Factor X.

• Intrinsic Pathway: Triggered by exposure of blood to collagen; activates Factor X via a series of steps.

• Common Pathway: Factor X activates prothrombin activator, which converts prothrombin to thrombin. Thrombin converts fibrinogen to fibrin.

Key Equations:

Regulation of Clotting

• Positive feedback by thrombin accelerates clotting.

• Anticoagulants (antithrombin-III, heparin, thrombomodulin, prostacyclin) prevent excessive clotting.

• Calcium ions and vitamin K are essential for clotting factor synthesis and function.

Clot Retraction and Fibrinolysis

• Clot Retraction: Platelets contract, pulling torn vessel edges together for repair.

• Fibrinolysis: Plasminogen is converted to plasmin (by thrombin and t-PA), which digests fibrin and dissolves the clot.

Clinical Correlations

• Thrombocytopenia: Low platelet count, leading to bleeding risk.

• Hemophilia: Inherited deficiency of clotting factors, causing excessive bleeding.

• Thrombophilia: Increased tendency to form clots (e.g., deep vein thrombosis, pulmonary embolism).