Blood: Structure, Function, and Clinical Relevance

Blood: Structure, Function, and Clinical Relevance

Cardiovascular System Overview

The cardiovascular system is the primary transport system in the human body, consisting of the heart and blood vessels (arteries, veins, and capillaries). Blood flows through these vessels, delivering essential substances and removing waste products.

• Transportation: Oxygen, nutrients, hormones, and waste products.

• Regulation: pH, body temperature, and fluid volume.

• Protection: Clot formation and defense against infection.

Composition of Blood

Blood is the only fluid tissue in the body, composed of plasma and formed elements. It is more dense and viscous than water, with a pH of 7.35–7.45 and a temperature of approximately 38°C. Blood constitutes about 8% of body weight.

• Plasma: The liquid component, making up 55% of whole blood.

• Formed Elements: Erythrocytes (RBCs), leukocytes (WBCs), and platelets.

Blood Plasma

Plasma is the straw-colored, liquid portion of blood, consisting mainly of water and dissolved substances. It is crucial for maintaining osmotic pressure and transporting nutrients, hormones, and waste products.

• Water: 90% of plasma volume.

• Solutes: Electrolytes, plasma proteins (albumin, globulins, fibrinogen), nutrients, respiratory gases, hormones.

• Albumin: Most abundant plasma protein, major contributor to osmotic pressure.

Constituent	Description and Importance
Water	90% of plasma volume; dissolving and suspending medium for solutes; absorbs heat
Electrolytes	Maintain osmotic balance, pH, and membrane potential
Plasma proteins	Albumin (osmotic pressure), globulins (transport, immunity), fibrinogen (clotting)
Nutrients	Glucose, amino acids, fatty acids, vitamins
Respiratory gases	O2 and CO2
Hormones	Regulate physiological processes

Formed Elements of Blood

Formed elements are the cellular components of blood, including erythrocytes, leukocytes, and platelets. Most are short-lived and replenished by stem cells in the bone marrow.

• Erythrocytes (RBCs): 45% of whole blood; transport oxygen and carbon dioxide.

• Leukocytes (WBCs): <1% of whole blood; defense against disease.

• Platelets: <1% of whole blood; essential for clotting.

Cell Type	Description	Cells/μL	Life Span	Function
Erythrocytes	Biconcave, anucleate	4-6 million	100-120 days	Transport O2 and CO2
Leukocytes	Complete cells	4,800-10,800	Varies	Defense
Platelets	Cell fragments	150,000-400,000	5-10 days	Clotting

Leukocytes: Types and Functions

Leukocytes are the only complete cells among formed elements and are crucial for immune defense. They are classified as granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).

• Neutrophils: Phagocytize bacteria.

• Lymphocytes: T cells (attack virus-infected cells), B cells (produce antibodies).

• Monocytes: Differentiate into macrophages, phagocytic.

• Eosinophils: Kill parasitic worms, modulate allergic responses.

• Basophils: Release histamine, mediate inflammation.

Erythrocytes: Structure and Function

Erythrocytes (RBCs) are small, biconcave, anucleate cells specialized for gas transport. Their shape increases surface area for efficient gas exchange, and they contain hemoglobin, the oxygen-carrying protein.

• Biconcave shape: Maximizes surface area for gas exchange.

• No mitochondria: ATP production is anaerobic, preventing O2 consumption.

• Hemoglobin: 97% of cell volume (excluding water).

Hemoglobin Structure and Function

Hemoglobin is a protein composed of four globin chains (two alpha, two beta) and four heme groups, each containing an iron ion that binds oxygen. Hemoglobin also transports carbon dioxide as carbaminohemoglobin.

• Oxyhemoglobin: Oxygen-bound form, bright red.

• Deoxyhemoglobin: Oxygen released, dark red.

• Carbaminohemoglobin: CO2 bound to globin.

Hematopoiesis and Erythropoiesis

Hematopoiesis is the formation of all blood cells from hematopoietic stem cells in red bone marrow. Erythropoiesis is the specific process of RBC formation, regulated by erythropoietin (EPO) produced by the kidneys in response to hypoxia.

• Stem cell: Hemocytoblast.

• Committed cell: Proerythroblast.

• Developmental pathway: Basophilic erythroblast → polychromatic erythroblast → orthochromatic erythroblast → reticulocyte → erythrocyte.

Dietary Requirements and RBC Life Cycle

RBC production requires amino acids, lipids, carbohydrates, iron, vitamin B12, and folic acid. Iron is stored as ferritin and hemosiderin, transported by transferrin. RBCs have a lifespan of 100–120 days and are removed by the spleen.

• Hemoglobin breakdown: Heme → bilirubin (excreted as stercobilin), iron recycled.

• Globin: Amino acids recycled.

Blood Disorders

Anemia

Anemia is characterized by abnormally low oxygen-carrying capacity due to decreased RBC production, increased RBC destruction, or blood loss. Symptoms include fatigue, pallor, shortness of breath, and chills.

• Iron deficiency anemia: Insufficient iron for hemoglobin synthesis.

• Pernicious anemia: Vitamin B12 deficiency.

• Hemolytic anemia: RBC destruction (e.g., sickle-cell anemia, thalassemia).

Polycythemia

Polycythemia is an abnormal excess of RBCs, increasing blood viscosity and causing sluggish blood flow. It may result from bone marrow cancer (polycythemia vera) or conditions stimulating EPO production (secondary polycythemia).

Platelets and Hemostasis

Platelets are cytoplasmic fragments of megakaryocytes, essential for clotting. They degenerate in about 10 days and are produced in bone marrow and lungs.

• Normal count: 150,000–400,000 platelets/μL.

• Contain: Serotonin, Ca2+, enzymes, ADP, PDGF.

Hemostasis: Steps and Clotting Factors

Hemostasis is the process of stopping bleeding, involving three principal steps: vascular spasm, platelet plug formation, and coagulation. Platelets and clotting factors play critical roles in each step.

• Vascular spasm: Vasoconstriction in response to vessel damage.

• Platelet plug formation: Platelets adhere to exposed collagen, become activated, and aggregate.

• Coagulation: Reinforces plug with fibrin mesh, transforming blood from liquid to gel.

Factor Number	Factor Name	Source	Function
I	Fibrinogen	Liver	Converted to fibrin
II	Prothrombin	Liver	Converted to thrombin
III	Tissue factor	Tissue cells	Activates extrinsic pathway
IV	Calcium ions	Plasma	Required for all stages
V-XIII	Various	Liver, platelets	Clotting cascade

Fibrinolytic System and Anticoagulants

The fibrinolytic system removes clots after repair. Plasminogen is converted to plasmin, which digests fibrin. Anticoagulants (e.g., antithrombin III, heparin) inhibit clotting to prevent excessive or inappropriate clot formation.

• Clot retraction: Platelets contract, pulling vessel edges together.

• Repair: PDGF and VEGF stimulate tissue regeneration.

• Fibrinolysis: Plasmin digests fibrin, dissolving the clot.

Hemostasis Disorders

Disorders include thromboembolic conditions (thrombus, embolus), bleeding disorders (thrombocytopenia, hemophilia), and disseminated intravascular coagulation (DIC).

• Thrombus: Clot in unbroken vessel.

• Embolus: Freely floating clot.

• Thrombocytopenia: Low platelet count.

• Hemophilia: Hereditary deficiency of clotting factors.

• DIC: Widespread clotting and bleeding.

Blood Types and Transfusion Reactions

Blood types are determined by antigens (agglutinogens) on RBC membranes. The ABO and Rh systems are clinically significant. Transfusion of incompatible blood can cause agglutination and hemolysis, leading to severe reactions.

• ABO system: Presence/absence of A and B antigens.

• Rh system: Presence/absence of D antigen (Rh+ or Rh–).

• Transfusion reactions: Recipient antibodies attack donor RBCs.

Blood Group	Antigens	Antibodies	Blood Received
AB	A, B	None	A, B, AB, O
A	A	Anti-B	A, O
B	B	Anti-A	B, O
O	None	Anti-A, Anti-B	O

Rh Blood Groups and Hemolytic Disease of the Newborn

Rh incompatibility can cause hemolytic disease of the newborn (erythroblastosis fetalis) if an Rh– mother carries an Rh+ fetus. RhoGAM prevents sensitization.

• Rh+: D antigen present.

• Rh–: D antigen absent; anti-Rh antibodies form after exposure.

Developmental Aspects of Blood

Blood cells develop from mesenchymal blood islands in the embryo. Fetal blood cells form in the yolk sac, liver, and spleen, with hemoglobin F having higher O2 affinity than adult hemoglobin A. Blood diseases of aging include chronic leukemias, anemias, and clotting disorders.