Blood: Composition and Overview

Introduction to Blood

Blood is a specialized connective tissue essential for the transport of gases, nutrients, wastes, and hormones throughout the body. It also plays critical roles in regulation and protection.

• Distribution: Transports oxygen, nutrients, hormones, and metabolic wastes.

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

• Protection: Prevents blood loss (hemostasis) and infection (immune response).

Blood Circulation Pathways

Blood circulates through two main circuits:

• Systemic Circuit: Delivers oxygenated blood from the heart to tissues and returns deoxygenated blood back to the heart.

• Pulmonary Circuit: Carries deoxygenated blood from the heart to the lungs for gas exchange and returns oxygenated blood to the heart.

Blood Composition

Major Components of Blood

Blood consists of a liquid matrix called plasma and formed elements (cells and cell fragments) suspended within it.

• Plasma: Non-living fluid matrix (~55% of blood volume).

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

Physical Characteristics and Volume

• Color: Scarlet (high O2) to dark red (low O2).

• pH: 7.35–7.45 (slightly alkaline).

• Viscosity: Thicker than water due to formed elements.

• Volume: 5–6 L in males, 4–5 L in females (~8% of body weight).

Hematocrit

The hematocrit is the percentage of blood volume occupied by erythrocytes (RBCs):

• Males: 47% ± 5%

• Females: 42% ± 5%

Blood Plasma

Composition of Plasma

Plasma is about 90% water and contains over 100 dissolved solutes:

• Nutrients: Glucose, amino acids, lipids

• Gases: O2, CO2

• Hormones

• Wastes: Urea, creatinine

• Electrolytes: Na+, K+, Ca2+, Cl-

• Plasma Proteins: Most abundant solutes, produced mainly by the liver

Formed Elements

Types of Formed Elements

• Erythrocytes (RBCs): Most abundant, transport O2

• Leukocytes (WBCs): Defense against disease

• Platelets: Cell fragments involved in clotting

Erythrocytes (Red Blood Cells)

Erythrocytes are biconcave, anucleate cells specialized for gas transport.

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

• Contents: Filled with hemoglobin (Hb), lack mitochondria and organelles.

• Function: Transport O2 and a small amount of CO2.

Hemoglobin Structure and Function

Hemoglobin is the protein responsible for oxygen transport in the blood.

• Structure: Four polypeptide chains (2 alpha, 2 beta) each with a heme group containing iron.

• Oxygen Binding: Each iron atom binds one O2; each Hb molecule can carry four O2 molecules.

• Forms: Oxyhemoglobin (O2-bound), deoxyhemoglobin (O2-released), carbaminohemoglobin (CO2-bound).

Hematopoiesis and Erythropoiesis

Hematopoiesis is the process of blood cell formation, occurring in red bone marrow. Erythropoiesis is the specific formation of erythrocytes.

• Stem Cells: Hematopoietic stem cells (hemocytoblasts) give rise to all formed elements.

• Stages of Erythropoiesis: Myeloid stem cell → proerythroblast → erythroblast stages → reticulocyte → erythrocyte.

• Regulation: Controlled by erythropoietin (EPO), mainly produced by the kidneys in response to hypoxia.

Fate and Destruction of Erythrocytes

RBCs have a lifespan of 100–120 days. Old RBCs are removed by macrophages in the spleen, liver, and bone marrow. Hemoglobin is broken down and recycled.

• Iron: Recycled and stored as ferritin or hemosiderin.

• Heme: Degraded to bilirubin, excreted in bile.

• Globin: Broken down to amino acids.

Erythrocyte Disorders

• Anemia: Low O2-carrying capacity due to blood loss, low RBC production, or high RBC destruction.

• Polycythemia: Excess RBCs increase blood viscosity, can be primary (vera) or secondary (high altitude, blood doping).

Leukocytes (White Blood Cells)

Types and Functions of Leukocytes

Leukocytes are crucial for defense against pathogens. They are classified as granulocytes or agranulocytes based on the presence of cytoplasmic granules.

Granulocytes

• Neutrophils: Most abundant, multilobed nucleus, phagocytic.

• Eosinophils: Bilobed nucleus, red granules, attack parasites.

• Basophils: Large, dark granules, release histamine.

Agranulocytes

• Lymphocytes: Large, round nucleus, crucial for adaptive immunity (T and B cells).

• Monocytes: Largest WBC, kidney-shaped nucleus, become macrophages in tissues.

Leukopoiesis

Leukopoiesis is the formation of WBCs from hematopoietic stem cells, regulated by interleukins and colony-stimulating factors (CSFs).

Leukocyte Disorders

• Leukopenia: Abnormally low WBC count, often drug-induced.

• Leukemia: Cancerous overproduction of abnormal WBCs, crowding out normal cells.

• Infectious Mononucleosis: Viral disease causing high numbers of atypical lymphocytes.

Platelets and Hemostasis

Platelets

Platelets are cell fragments derived from megakaryocytes, essential for blood clotting.

• Normal Count: 150,000–400,000/μl blood.

• Function: Form temporary platelet plugs in vessel injury.

Hemostasis: Stoppage of Bleeding

Hemostasis is a rapid, localized response to blood vessel injury, involving three steps:

1. Vascular Spasm: Vasoconstriction of damaged vessel.

2. Platelet Plug Formation: Platelets adhere to exposed collagen, become activated, and aggregate.

3. Coagulation: Fibrin mesh reinforces the plug, forming a stable clot.

Coagulation Cascade

Coagulation involves a cascade of reactions leading to the conversion of fibrinogen to fibrin. There are three main phases:

1. Formation of prothrombin activator

2. Conversion of prothrombin to thrombin

3. Conversion of fibrinogen to fibrin

Clot Retraction and Fibrinolysis

• Clot Retraction: Platelets contract, pulling wound edges together.

• Fibrinolysis: Plasmin digests fibrin, removing the clot after healing.

Hemostasis Disorders

• Thromboembolic Disorders: Unwanted clot formation (thrombus, embolus).

• Bleeding Disorders: Thrombocytopenia (low platelets), hemophilia (clotting factor deficiency).

Blood Transfusions and Blood Groups

Blood Groups and Typing

Blood groups are determined by antigens (agglutinogens) on RBC membranes. The most clinically important are the ABO and Rh systems.

• ABO System: Types A, B, AB, O based on presence/absence of A and B antigens.

• Rh System: Rh+ (antigen present) or Rh– (antigen absent).

Transfusion Reactions

If mismatched blood is transfused, the recipient's antibodies attack donor RBCs, causing agglutination and hemolysis, which can be fatal.

Hemolytic Disease of the Newborn

Occurs when an Rh– mother carries an Rh+ fetus, leading to maternal anti-Rh antibodies attacking fetal RBCs in subsequent pregnancies. Prevented by RhoGAM administration.

Diagnostic Blood Tests

• Hematocrit: Measures RBC percentage; used to diagnose anemia.

• Blood Glucose: Screens for diabetes.

• WBC Count: Detects infection, allergy, or leukemia.

• Platelet Count: Assesses hemostasis.

• Complete Blood Count (CBC): Comprehensive evaluation of formed elements.

Developmental Aspects of Blood

• Fetal blood cells form in yolk sac, liver, and spleen before red bone marrow becomes primary site.

• Hemoglobin F (fetal) has higher O2 affinity than adult hemoglobin.

• Aging increases risk of blood disorders, often secondary to other organ dysfunction.