Blood: Structure, Function, and Clinical Aspects

Overview of Blood

Blood is a specialized connective tissue that plays a vital role in transporting substances throughout the body. It is essential for maintaining homeostasis, defending against disease, and distributing heat.

• Transports nutrients, wastes, hormones, and body heat via blood vessels.

• Composed of formed elements (cells and cell fragments) and plasma (fluid matrix).

Components of Blood

• Formed Elements: Living cells (erythrocytes, leukocytes, platelets).

• Plasma: Nonliving, straw-colored fluid matrix (about 55% of blood volume).

When blood is centrifuged:

• Plasma rises to the top (55%).

• Erythrocytes (red blood cells) sink to the bottom (45%, known as the hematocrit).

• Buffy coat (thin, whitish layer) between plasma and erythrocytes contains leukocytes and platelets (<1%).

Physical Characteristics and Volume

• Sticky, opaque fluid; heavier and thicker than water.

• Color varies with oxygen content: scarlet red (oxygen-rich), dull red/purple (oxygen-poor).

• Metallic, salty taste.

• pH: Slightly alkaline (7.35–7.45).

• Temperature: ~38°C (100.4°F), slightly higher than body temperature.

• Volume: 5–6 liters (6 quarts) in healthy adults; ~8% of body weight.

Plasma

Composition and Functions

Plasma is the liquid component of blood, making up about 55% of its volume.

• 90% water.

• Contains dissolved substances: nutrients, salts (electrolytes), respiratory gases (CO2, O2), hormones, plasma proteins, waste products (e.g., urea).

Plasma Proteins

• Albumin: Maintains osmotic pressure and acts as a blood buffer.

• Clotting proteins: Essential for blood coagulation.

• Antibodies: Provide immune defense against pathogens.

Most plasma proteins are synthesized by the liver. Plasma composition changes as substances are exchanged with body cells. The liver and respiratory/urinary systems help maintain plasma homeostasis, including pH balance.

Formed Elements

Erythrocytes (Red Blood Cells, RBCs)

• Main function: Transport oxygen via hemoglobin.

• Anucleate (no nucleus), few organelles, lack mitochondria (produce ATP anaerobically).

• Biconcave disc shape increases surface area for gas exchange.

• Normal count: ~5 million/mm3 of blood.

Hemoglobin

• Iron-containing protein that binds oxygen.

• Each molecule binds up to 4 O2 molecules.

• Each RBC contains ~250 million hemoglobin molecules.

• Normal blood: 12–18 g hemoglobin per 100 mL.

Homeostatic Imbalances of RBCs

• Anemia: Decreased oxygen-carrying capacity due to low RBC count or abnormal hemoglobin.

• Sickle Cell Anemia (SCA): Caused by abnormal hemoglobin shape.

• Polycythemia: Excessive RBCs (from bone marrow cancer or high altitude); increases blood viscosity and slows flow.

Leukocytes (White Blood Cells, WBCs)

• Crucial for defense against disease.

• Complete cells with nucleus and organelles.

• Can move in/out of blood vessels (diapedesis), respond to chemical signals (positive chemotaxis), and move by amoeboid motion.

• Normal count: 4,800–10,800/mm3.

Homeostatic Imbalances of WBCs

• Leukocytosis: High WBC count (normal in infection; pathological in leukemia/mononucleosis).

• Leukopenia: Low WBC count (often drug-induced).

• Leukemia: Cancerous bone marrow produces many immature WBCs.

Types of Leukocytes

• Granulocytes: Contain cytoplasmic granules; lobed nuclei.

• Agranulocytes: Lack visible granules; nuclei are spherical, oval, or kidney-shaped.

Mnemonic: Never Let Monkeys Eat Bananas (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils)

Granulocytes

• Neutrophils: Most numerous; multilobed nucleus; increase during infection.

• Eosinophils: Kill parasites; involved in allergies.

• Basophils: Rarest; release histamine (inflammation) and heparin (anticoagulant).

Agranulocytes

• Lymphocytes: Large nucleus; reside in lymphatic tissues; immune response.

• Monocytes: Largest WBC; U/kidney-shaped nucleus; become macrophages in tissues.

Platelets

• Fragments of megakaryocytes.

• Essential for blood clotting.

• Normal count: ~300,000/mm3.

Hematopoiesis (Blood Cell Formation)

Hematopoiesis is the process of forming new blood cells, occurring primarily in red bone marrow.

• All blood cells derive from a common stem cell: hemocytoblast.

• Hemocytoblasts differentiate into:

  • Lymphoid stem cells (produce lymphocytes).

  • Myeloid stem cells (produce all other formed elements).

Red Blood Cell Formation

• RBCs are anucleate and cannot divide or synthesize proteins.

• Life span: 100–120 days; removed by phagocytes in spleen/liver.

• Replacement via division of hemocytoblasts in red bone marrow.

• Reticulocytes: Immature RBCs that mature in the bloodstream.

• Production regulated by erythropoietin (Epo) (mainly from kidneys in response to low O2).

White Blood Cell and Platelet Formation

• Regulated by hormones:

  • Colony Stimulating Factors (CSFs) and interleukins stimulate leukocyte production.

  • Thrombopoietin stimulates platelet production from megakaryocytes.

Hemostasis (Prevention of Blood Loss)

Hemostasis is the process that stops bleeding after blood vessel injury, involving three main phases:

1. Vascular Spasms: Vasoconstriction narrows the vessel, reducing blood loss.

2. Platelet Plug Formation: Platelets adhere to exposed collagen fibers, become sticky, and form a plug.

3. Coagulation (Blood Clotting): Complex cascade involving tissue factor (TF), PF3, clotting factors, and calcium ions. Prothrombin activator converts prothrombin to thrombin, which then converts fibrinogen to fibrin, forming a meshwork (clot).

Clotting typically occurs within 3–6 minutes. The clot retracts as the vessel heals and is eventually dissolved by plasmin.

Disorders of Hemostasis

• Thrombus: Clot in an unbroken vessel; can be dangerous if it blocks circulation.

• Embolus: Thrombus that breaks free and travels; may cause blockages elsewhere (e.g., brain, lungs).

• Thrombocytopenia: Low platelet count; leads to spontaneous bleeding and petechiae.

• Hemophilia: Hereditary disorder; missing clotting factors cause prolonged bleeding.

Blood Groups and Transfusions

Blood Loss and Transfusions

• Loss of 15–30%: Pallor and weakness.

• Loss >30%: Shock, potentially fatal.

• Transfusions treat severe blood loss, anemia, or thrombocytopenia.

Blood Group Antigens and Antibodies

• Antigens: Genetically determined proteins on RBC membranes; recognized as foreign by the immune system.

• Antibodies: Proteins that bind foreign antigens, causing agglutination and lysis.

• Blood is typed by mixing with anti-A and anti-B sera; agglutination indicates presence of specific antigens.

ABO Blood Group System

Rh Blood Group System

• Named after Rhesus monkey antigen (agglutinogen D).

• Rh-positive: Rh antigen present (most Americans).

• Rh-negative: No Rh antigen; anti-Rh antibodies form only after exposure to Rh+ blood.

Rh incompatibility can cause hemolytic disease of the newborn if an Rh-negative mother carries an Rh-positive fetus. RhoGAM® can prevent maternal sensitization.

Blood Typing and Cross-Matching

• Blood samples mixed with anti-A and anti-B sera to determine type.

• Cross-matching tests donor and recipient compatibility to prevent transfusion reactions.

Developmental Aspects of Blood

• Fetal liver and spleen produce blood cells early; bone marrow takes over by the seventh month.

• Congenital blood defects include hemolytic anemias and hemophilia.

• Maternal-fetal blood incompatibility can cause fetal cyanosis.

• Fetal hemoglobin differs from adult hemoglobin; infants may develop physiologic jaundice due to immature liver function.

• Leukemias are most common in the very young and very old; older adults are also at risk for anemia and clotting disorders.

Key Equations and Concepts

• Hematocrit: Proportion of RBCs in blood volume.

• Hemoglobin Oxygen Capacity: Each hemoglobin molecule binds 4 O2 molecules.

• Blood pH: Maintained between 7.35 and 7.45.

Example: Blood Transfusion Compatibility

• A patient with type O blood can only receive type O blood but can donate to any ABO type (universal donor).

• A patient with type AB blood can receive from any ABO type (universal recipient).

Additional info: The above notes expand on the original content by providing definitions, clinical context, and tables for clarity. Equations are included for hematocrit and oxygen-carrying capacity, and the ABO/Rh compatibility is summarized for practical application.