Blood: Composition, Function, and Physiology

Blood: Composition, Function, and Physiology

Introduction to Blood

Blood is a specialized liquid connective tissue that plays a vital role in the transport of substances, protection against disease, and regulation of homeostasis. It constitutes about 8% of total body mass, with males typically having 5-6 liters and females 4-5 liters.

• Definition: Blood is the only liquid connective tissue, lacking collagen and elastic fibers.

• Main Components: Formed elements (cells and cell fragments) and plasma (non-living fluid matrix).

• Functions:

  • Transport of oxygen, nutrients, wastes, and hormones

  • Protection against blood loss (clotting) and infection (immune system)

  • Regulation of body temperature, pH (buffering), and fluid balance

Blood Composition

Blood consists of plasma and formed elements. The formed elements include erythrocytes (RBCs), leukocytes (WBCs), and platelets.

• Plasma: The liquid portion (~55% of blood volume), composed of water, proteins, nutrients, electrolytes, wastes, hormones, and gases.

• Formed Elements:

  • Erythrocytes (RBCs): Oxygen-transporting cells

  • Leukocytes (WBCs): Immune function and defense

  • Platelets: Cell fragments involved in blood clotting

Physical Properties of Blood

• Hematocrit: Percentage of blood volume that is RBCs (about 45%)

• Buffy Coat: Layer containing WBCs and platelets (about 1%)

• Plasma: Remainder of blood volume (about 55%)

Blood Plasma

Plasma is the liquid matrix of blood, consisting of approximately 92% water and 8% solutes.

• Serum: Plasma without clotting proteins (fibrinogen)

• Major Plasma Proteins:

  • Albumins: Smallest and most abundant; contribute to viscosity, osmolarity, and influence blood pressure and fluid balance

  • Globulins: (Alpha, beta, gamma) Provide immune functions and transport proteins

  • Fibrinogen: Precursor to fibrin, essential for clot formation

Erythrocytes and Oxygen Transport

Structure and Function of Erythrocytes

Erythrocytes (red blood cells) are specialized for oxygen transport. They are biconcave discs, which increases surface area for gas exchange and flexibility for movement through capillaries.

• Diameter: ~7.5 μm

• Features: Lack nuclei and organelles; do not divide; renewed by bone marrow

• Function: Transport oxygen from lungs to tissues and carbon dioxide from tissues to lungs

Hemoglobin Structure and Function

Hemoglobin is the protein responsible for oxygen transport in erythrocytes.

• Structure: Four polypeptide chains (2 alpha, 2 beta), each bound to a heme group containing one iron atom

• Oxygen Binding: Each iron atom binds reversibly to one O2 molecule

• Equation: $\text{Hb} + O_2 \rightleftharpoons \text{HbO}_2$

Erythropoiesis (Production of RBCs)

Erythropoiesis is the process of RBC formation, occurring in the red bone marrow.

• Stages:

  1. Hemopoietic stem cells (HSCs) differentiate into colony-forming units (CFUs)

  2. CFUs become erythroblasts, which synthesize hemoglobin

  3. Nucleus is discarded, forming reticulocytes

  4. Reticulocytes mature into erythrocytes

• Regulation: Erythropoietin (EPO) from kidneys stimulates erythropoiesis in response to hypoxemia

• Negative Feedback: Drop in RBC count → kidney releases EPO → increased RBC production

Iron Metabolism

Iron is essential for hemoglobin synthesis and erythropoiesis.

• Forms: Ferric (Fe3+) and ferrous (Fe2+) ions

• Transport: Iron binds to transferrin in plasma and is stored as ferritin in the liver

Erythrocyte Death and Anemia

RBCs have a lifespan of about 120 days. Their breakdown occurs in the spleen and liver.

• Anemia: Decreased oxygen-carrying capacity of blood

• Causes: Kidney failure, dietary deficiency, hemorrhage, hemolysis, abnormal hemoglobin (e.g., sickle cell anemia)

• Symptoms: Pale skin, shortness of breath, edema, reduced blood pressure, increased heart rate

Leukocytes and Immune Function

Types and Functions of Leukocytes

Leukocytes (white blood cells) are crucial for immune defense. They are less abundant than RBCs and possess nuclei and organelles.

• Granulocytes:

  • Neutrophils: Phagocytize bacteria; most abundant

  • Eosinophils: Attack parasites; phagocytize antigen-antibody complexes

  • Basophils: Release histamine (inflammation) and heparin (anticoagulant)

• Agranulocytes:

  • Lymphocytes: Immune response; produce antibodies; destroy cancer cells

  • Monocytes: Differentiate into macrophages; phagocytize pathogens and debris

Leukopoiesis (Production of WBCs)

Leukopoiesis is the formation of WBCs from hemopoietic stem cells in bone marrow.

• Cell Lines:

  • Myeloblasts → neutrophils, eosinophils, basophils

  • Monoblasts → monocytes

  • Lymphoblasts → lymphocytes

• Lifespan: Granulocytes (5 days), monocytes (years as macrophages), lymphocytes (decades)

Platelets and Hemostasis

Platelet Structure and Function

Platelets are cytoplasmic fragments of megakaryocytes, essential for blood clotting.

• Functions:

  • Secrete vasoconstrictors to reduce blood loss

  • Form platelet plugs to seal small breaks

  • Release clotting factors (procoagulants)

  • Initiate clot-dissolving enzymes

  • Attract immune cells to sites of inflammation

  • Phagocytize bacteria

  • Stimulate vessel repair via growth factors

Hemostasis: Steps in Blood Clotting

Hemostasis is the process of stopping bleeding, involving several steps:

1. Vascular Spasm: Vasoconstriction reduces blood flow and vessel diameter

2. Platelet Plug Formation: Platelets adhere to exposed collagen, aggregate, and release granules

3. Coagulation: Clotting factors lead to conversion of fibrinogen to fibrin, forming a mesh that traps RBCs

   • $\text{Fibrinogen} \xrightarrow{\text{Thrombin}} \text{Fibrin}$

4. Clot Retraction: Clot contracts, serum is squeezed out

5. Thrombolysis: Plasminogen is activated to plasmin, which dissolves the clot

Intrinsic vs. Extrinsic Pathways

• Intrinsic Pathway: Initiated by damage inside the vessel

• Extrinsic Pathway: Initiated by external trauma to the vessel wall

Blood Types and Transfusion Compatibility

ABO and Rh Blood Groups

Blood types are determined by the presence or absence of specific antigens (agglutinogens) on erythrocytes and antibodies (agglutinins) in plasma.

• ABO System:

  • Type A: A antigen, anti-B antibody

  • Type B: B antigen, anti-A antibody

  • Type AB: A and B antigens, no anti-A or anti-B antibodies

  • Type O: No antigens, both anti-A and anti-B antibodies

• Rh System:

  • Rh positive: D antigen present

  • Rh negative: D antigen absent

  • Anti-D antibodies form only after exposure to Rh+ blood

Table: ABO and Rh Blood Types

Blood Type	Antigen(s) on RBC	Antibody in Plasma	Can Receive From	Can Donate To
A+	A, D	Anti-B	A+, A-, O+, O-	A+, AB+
B+	B, D	Anti-A	B+, B-, O+, O-	B+, AB+
AB+	A, B, D	None	All types	AB+
O+	D	Anti-A, Anti-B	O+, O-	O+, A+, B+, AB+
A-	A	Anti-B, Anti-D	A-, O-	A-, A+, AB-, AB+
B-	B	Anti-A, Anti-D	B-, O-	B-, B+, AB-, AB+
AB-	A, B	Anti-D	AB-, A-, B-, O-	AB-, AB+
O-	None	Anti-A, Anti-B, Anti-D	O-	All types

Additional info: Table entries inferred and expanded for completeness.

Transfusion Reactions

Transfusion reactions occur when incompatible blood is transfused, leading to agglutination and destruction of erythrocytes.

• Agglutination: Antibodies bind to antigens on RBCs, causing clumping

• Consequences: Hemolysis, anemia, kidney failure, and potentially fatal outcomes

Rh Factor and Pregnancy

Rh incompatibility can cause hemolytic disease of the newborn if an Rh-negative mother carries an Rh-positive fetus.

• First Pregnancy: Usually not affected; antibodies form after exposure

• Subsequent Pregnancies: Maternal antibodies may attack fetal RBCs, causing anemia

• Prevention: Rh immune globulin injections prevent antibody formation

Table: Rh Factor and Pregnancy Precautions

Mother	Baby	Precaution
Rh positive	Rh positive	None
Rh positive	Rh negative	None
Rh negative	Rh positive	Rh immune globulin injections
Rh negative	Rh negative	None

Additional info: Table entries inferred and expanded for clarity.

Summary

• Blood is a complex tissue essential for transport, protection, and regulation.

• Its components include plasma, erythrocytes, leukocytes, and platelets, each with specialized functions.

• Understanding blood types and compatibility is crucial for safe transfusions and pregnancy management.