The Circulatory System: Blood

Introduction

The circulatory system is essential for transporting substances throughout the body and maintaining homeostasis. Blood, as a major component, plays a critical role in transport, protection, and regulation within the human body.

Functions of the Circulatory System

• Transport: Blood carries oxygen, nutrients, hormones, and waste products to and from cells.

• Protection: Blood contains immune cells and proteins that defend against pathogens and prevent blood loss through clotting.

• Regulation: Blood helps regulate body temperature, pH, and fluid balance.

Components and General Properties of Blood

• Blood is a liquid connective tissue composed of plasma and formed elements.

• Volume: 4–6 liters in adults, about 8% of body weight.

• Temperature: 38°C (higher than body temperature).

• Viscosity: About 5 times more viscous than water, important for flow and resistance.

• pH: 7.35–7.45 (slightly alkaline).

Blood Composition

• Plasma: ~55% of blood volume; mostly water, proteins, and solutes.

• Formed Elements: ~45% erythrocytes (red blood cells), <1% leukocytes (white blood cells) and platelets.

Blood Plasma

• Water (92%): Solvent for carrying other substances.

• Proteins (7%): Albumin, globulins, and fibrinogen are the main types.

• Other Solutes (1%): Nutrients, wastes, hormones, and gases.

Plasma Proteins

• Albumin: Most abundant; maintains osmotic pressure and transports substances.

• Globulins: Involved in immune defense and transport.

• Fibrinogen: Essential for blood clotting.

Hematopoiesis: Blood Cell Production

Blood cells are continuously produced in the bone marrow through a process called hematopoiesis. This includes the formation of erythrocytes, leukocytes, and platelets from hematopoietic stem cells.

Erythrocytes (Red Blood Cells)

Structure and Function

• Main Functions: Transport oxygen from lungs to tissues and carbon dioxide from tissues to lungs.

• Shape: Biconcave disc, which increases surface area for gas exchange.

• No nucleus or mitochondria: Maximizes space for hemoglobin and prevents self-repair.

Hemoglobin

• Structure: Red, iron-containing protein that binds oxygen.

• Each iron atom binds one oxygen molecule.

• ATP Production: RBCs rely on anaerobic fermentation to avoid consuming the oxygen they transport.

Erythrocyte Life Cycle

• Lifespan: About 120 days.

• Production: Erythropoiesis occurs in the bone marrow, regulated by erythropoietin (mainly from the kidneys).

• Destruction: Old RBCs are removed by the spleen and liver; components are recycled.

Disorders: Sickle-Cell Anemia

• Cause: Mutation in the hemoglobin gene leads to abnormal, sickle-shaped RBCs.

• Effects: Reduced oxygen-carrying capacity, pain, and increased risk of clotting.

Leukocytes (White Blood Cells)

Structure and Function

• Main Role: Immune defense against pathogens.

• Location: Most reside in connective tissues; use bloodstream for transport.

Types of Leukocytes

• Granulocytes: Neutrophils, eosinophils, basophils (contain visible granules).

• Agranulocytes: Lymphocytes, monocytes (lack visible granules).

Leukocyte Life Cycle

• Leukopoiesis: Production of WBCs from hematopoietic stem cells in the bone marrow.

Leukocyte Disorders

• Leukemias: Cancers involving overproduction of abnormal WBCs.

• Infectious Mononucleosis: Viral disease causing excess lymphocytes.

Platelets and Control of Bleeding

Platelet Structure and Function

• Origin: Fragments of megakaryocytes in bone marrow.

• Function: Essential for blood clotting (hemostasis).

Hemostasis: Control of Bleeding

• Three Stages:

  1. Vascular spasm (vessel constriction)

  2. Platelet plug formation

  3. Coagulation (fibrin mesh traps cells)

Agglutination vs. Coagulation

• Coagulation: Blood cells and platelets stick together via fibrin (clotting protein).

• Agglutination: RBCs stick together due to antibodies binding to antigens.

Blood Types and Transfusion

Blood Typing

• Antigens: Cell surface markers that identify blood type.

• Antibodies: Proteins that recognize and bind foreign antigens, causing agglutination if incompatible blood is transfused.

ABO Blood Groups

Universal Donor and Recipient

• Universal Donor: Type O (no antigens on RBCs).

• Universal Recipient: Type AB (no antibodies in plasma).

Rh Factor

• Rh antigen: Presence (+) or absence (−) determines positive or negative blood type (e.g., A+ or A−).

• Clinical relevance: Rh incompatibility can cause problems in pregnancy if mother is Rh− and fetus is Rh+.

Summary Table: Main Components of Blood

Additional info: These notes provide a comprehensive overview of blood structure, function, and clinical relevance, suitable for college-level Anatomy & Physiology students.