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 medium), the heart (the pump), and blood vessels (the conducting passageways).

• Blood: Specialized connective tissue with cells suspended in a fluid matrix.

• Heart: Muscular organ that pumps blood throughout the body.

• Blood Vessels: Arteries, veins, and capillaries that transport blood.

Components and Functions of Blood

Main Components of Blood

Blood is composed of plasma (the liquid portion) and formed elements (cells and cell fragments). It is a specialized connective tissue that performs vital functions for the body.

• Plasma: Makes up 46–63% of blood volume; consists of 92% water, plasma proteins, and other solutes.

• Formed Elements: Include red blood cells (RBCs), white blood cells (WBCs), and platelets.

Functions of Blood

• Transports dissolved gases (O2, CO2), nutrients, hormones, and metabolic wastes.

• Regulates pH and ion composition of interstitial fluids.

• Restricts fluid losses at injury sites (clotting).

• Defends against toxins and pathogens (immune response).

• Stabilizes body temperature by redistributing heat.

Physical Characteristics of Blood

• Temperature: Slightly higher than body temperature.

• Viscosity: Thicker than water due to formed elements and plasma proteins.

• pH: Slightly alkaline (7.35–7.45).

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

Plasma Composition

• Water: 92% of plasma; acts as a solvent and medium for transport.

• Plasma Proteins (mostly synthesized by the liver):

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

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

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

• Other Solutes: Organic nutrients, wastes, and electrolytes.

Formed Elements

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

• White Blood Cells (Leukocytes): Defend against pathogens and remove debris.

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

Red Blood Cells (RBCs)

Structure and Function

Red blood cells (erythrocytes) are the most abundant formed elements, making up 99.9% of all blood cells. Their primary function is to transport oxygen and carbon dioxide via the protein hemoglobin.

• Biconcave disc shape: Increases surface area for gas exchange and allows flexibility in capillaries.

• Anucleate: Mature RBCs lack nuclei, mitochondria, and ribosomes, making them specialized for gas transport but unable to divide or repair themselves.

• Rouleaux formation: RBCs can stack like coins to pass through narrow capillaries efficiently.

Hemoglobin Structure and Function

Hemoglobin (Hb) is a complex protein responsible for oxygen and carbon dioxide transport in RBCs.

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

• Oxygen binds reversibly to the iron in heme, forming oxyhemoglobin.

• Carbon dioxide binds to the polypeptide chains, forming carbaminohemoglobin.

RBC Life Cycle and Erythropoiesis

RBCs have a lifespan of about 120 days. They are produced in the red bone marrow through a process called erythropoiesis.

• Hemocytoblasts (hematopoietic stem cells) differentiate into myeloid stem cells, which give rise to RBCs.

• Stages: Proerythroblast → Basophilic erythroblast → Polychromatophilic erythroblast → Normoblast (nucleus ejected) → Reticulocyte → Mature RBC.

Regulation of Erythropoiesis

• Erythropoietin (EPO): Hormone produced by kidneys and liver in response to hypoxia; stimulates RBC production.

• Requires amino acids, iron, vitamin B12, and folic acid.

• Blood doping: Artificially increasing RBC count to enhance oxygen delivery (dangerous and unethical in sports).

RBC Recycling and Disorders

• Old or damaged RBCs are phagocytized by macrophages in the spleen, liver, and bone marrow.

• Hemoglobin is broken down: heme → biliverdin → bilirubin (excreted in bile); iron is recycled via transferrin, ferritin, and hemosiderin.

• Anemia: Condition of low RBC count or hemoglobin, leading to reduced oxygen delivery.

• Jaundice: Yellowing of skin/eyes due to excess bilirubin.

Blood Types and Transfusion Compatibility

ABO and Rh Blood Groups

Blood type is determined by the presence or absence of specific surface antigens (agglutinogens) on RBCs. The two main groups are ABO and Rh.

• Type A: Surface antigen A; anti-B antibodies in plasma.

• Type B: Surface antigen B; anti-A antibodies in plasma.

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

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

Rh Factor

• Rh positive (Rh+): Rh antigen present; no anti-Rh antibodies.

• Rh negative (Rh–): Rh antigen absent; can develop anti-Rh antibodies if exposed to Rh+ blood.

Hemolytic Disease of the Newborn (HDN)

Occurs when an Rh– mother carries an Rh+ fetus. Sensitization during delivery can lead to maternal anti-Rh antibodies attacking fetal RBCs in subsequent pregnancies, causing anemia and jaundice in the newborn. RhoGAM is used to prevent sensitization.

Transfusion Reactions and Compatibility Testing

• Cross-reaction (transfusion reaction): Occurs if incompatible blood is transfused; antibodies attack donor RBCs, causing agglutination and hemolysis.

• Compatibility testing: Includes blood typing and cross-match testing to prevent reactions.

• Type O–: Universal donor; Type AB+: Universal recipient.

White Blood Cells (WBCs)

Types and Functions

White blood cells (leukocytes) are involved in defending the body against pathogens, removing toxins and wastes, and attacking abnormal or damaged cells. They are classified as granular or agranular based on the presence of cytoplasmic granules.

• Granular Leukocytes:

  • Neutrophils: Phagocytic, first responders to infection; multilobed nucleus.

  • Eosinophils: Attack parasites, involved in allergic responses.

  • Basophils: Release histamine and heparin, enhance inflammation.

• Agranular Leukocytes:

  • Monocytes: Become macrophages in tissues; phagocytize large particles.

  • Lymphocytes: Specific immunity; include T cells, B cells, and natural killer (NK) cells.

WBC Production and Regulation

• Leukopoiesis: Production of WBCs from hemocytoblasts in bone marrow.

• Colony-stimulating factors (CSFs): Hormones that regulate WBC populations.

• Lymphocytopoiesis: Production of lymphocytes in lymphatic tissues and organs.

Platelets

Structure, Function, and Production

Platelets (thrombocytes) are small cell fragments involved in hemostasis (blood clotting). They circulate for 9–12 days and are removed by phagocytes, mainly in the spleen.

• Functions:

  • Release clotting chemicals.

  • Form temporary platelet plugs.

  • Reduce the size of vessel breaks.

• Thrombocytopoiesis: Platelet production from megakaryocytes in red bone marrow, stimulated by thrombopoietin (TPO), interleukin-6 (IL-6), and multi-CSF.

Hemostasis (Prevention of Blood Loss)

Phases of Hemostasis

Hemostasis is the process of stopping bleeding and involves three phases: vascular, platelet, and coagulation.

1. Vascular Phase: Vascular spasm constricts the vessel to reduce blood loss; endothelial cells release factors for repair and become sticky.

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

3. Coagulation Phase: Involves a cascade of reactions leading to the conversion of fibrinogen to fibrin, forming a stable blood clot. Pathways include extrinsic, intrinsic, and common pathways.

Clot Retraction and Fibrinolysis

• Clot retraction: Platelets contract to pull the edges of the wound together, reducing bleeding and stabilizing the injury site.

• Fibrinolysis: Gradual breakdown and removal of the clot by plasmin, which is formed from plasminogen by thrombin and tissue plasminogen activator (t-PA).

Regulation of Clotting

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

• Calcium ions and vitamin K are essential for the synthesis and function of clotting factors.

Clinical Correlations

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

• Hemophilia: Inherited bleeding disorder due to deficiency of clotting factors.

• Thrombophilia: Increased tendency to form clots.

• Deep vein thrombosis (DVT) and pulmonary embolism: Dangerous clotting events in veins and lungs, respectively.

Summary Table: Main Components of Blood

Additional info: This guide integrates textbook content with academic context to provide a comprehensive overview of blood structure, function, and clinical relevance for Anatomy & Physiology students.