The Circulatory System: Blood

Introduction

The circulatory system is essential for transporting substances throughout the body, maintaining homeostasis, and protecting against disease. Blood, a specialized connective tissue, plays a central role in these processes by circulating through the heart and blood vessels.

Functions of the Circulatory System

Overview of Circulatory System Functions

• Transport: Delivers oxygen, nutrients, hormones, and removes metabolic wastes.

• Protection: Defends against pathogens and prevents blood loss through clotting.

• Regulation: Maintains pH, temperature, and fluid balance.

Components and General Properties of Blood

Blood Composition and Physical Properties

• Volume: 4–6 liters in adults.

• Percentage of Body Weight: ~8%.

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

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

• pH: 7.35–7.45 (slightly alkaline).

Blood is composed of plasma (about 55%) and formed elements (about 45%). Formed elements include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

Blood Plasma

Composition and Function of Plasma

• Water: 92% of plasma, serves as a solvent and medium for transport.

• Proteins: 7% (albumin, globulins, fibrinogen) – crucial for osmotic balance, transport, and clotting.

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

Major Plasma Proteins

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

• Globulins: Involved in immune defense and transport.

• Fibrinogen: Essential for blood clotting.

How Blood is Produced

Hematopoiesis

Blood cells are continuously produced in the red bone marrow through a process called hematopoiesis. Each day, the body produces approximately 400 billion platelets, 200 billion erythrocytes, and 10 billion leukocytes.

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.

• Lack of Nucleus: Mature erythrocytes lack a nucleus and most organelles, maximizing space for hemoglobin but preventing self-repair.

Hemoglobin

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

• Oxygen Binding: Each iron atom in hemoglobin can bind one oxygen molecule.

• ATP Production: RBCs rely on anaerobic fermentation for ATP, preventing consumption of transported oxygen.

Life Cycle of Erythrocytes

• Lifespan: About 120 days.

• Production: Erythropoiesis occurs in the red bone marrow from hematopoietic stem cells.

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

Blood Types

ABO and Rh Blood Groups

• Antigens: Surface markers on RBCs that determine blood type.

• Antibodies: Proteins in plasma that react against foreign antigens.

• Agglutination: Clumping of RBCs when incompatible blood types mix, due to antibody-antigen reaction.

The Rh antigen is either present (+) or absent (−). Rh-negative individuals only develop antibodies if exposed to Rh-positive blood.

Leukocytes (White Blood Cells)

Structure and Function

• Main Role: Defense against infection and foreign substances.

• 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 bone marrow.

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

• Vascular Spasm: Constriction of blood vessel to reduce blood flow.

• Platelet Plug Formation: Platelets adhere to exposed collagen and each other.

• Coagulation: Fibrin forms a mesh that traps blood cells, forming a clot.

Clinical Aspects: Blood Transfusion and Disorders

Blood Transfusion

• Whole Blood: Used in cases of rapid, substantial blood loss.

• Packed Red Blood Cells (PRBCs): Used to restore oxygen-carrying capacity; plasma and WBCs removed.

• Universal Donor: Type O (no antigens).

• Universal Recipient: Type AB (no antibodies).

Blood Disorders

• Sickle-Cell Anemia: Genetic disorder causing abnormal hemoglobin and sickle-shaped RBCs.

• Leukemias: Cancers involving overproduction of abnormal WBCs.

• Infectious Mononucleosis: Viral disease causing excess lymphocytes.

Summary Table: Formed Elements of Blood

Additional info: This guide covers the essential structure, function, and clinical relevance of blood as part of the cardiovascular system, suitable for college-level Anatomy & Physiology students.