Blood: Structure, Function, and Clinical Relevance

Blood: Structure, Function, and Clinical Relevance

Overview of Blood

Blood is a specialized liquid connective tissue that interacts with all other body systems. It plays essential roles in transportation, regulation, and protection within the human body.

• Transportation: Carries gases (O2, CO2), nutrients, and metabolic wastes.

• Regulation: Maintains pH, temperature, and water content of tissues.

• Protection: Provides immune defense and enables clotting to prevent blood loss.

Physical Characteristics and Components of Blood

• Viscosity: Blood is more viscous than water and appears opaque.

• Color: Ranges from bright scarlet (high O2) to brick red (low O2).

• pH: Maintained between 7.35 and 7.45.

• Temperature: Approximately 38°C.

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

Component	Percentage of Blood Volume	Description
Red Blood Cells (RBCs)	44%	Carry oxygen and carbon dioxide
White Blood Cells (WBCs) & Platelets (Buffy coat)	1%	Immune defense and clotting
Plasma (Matrix)	55%	Liquid portion; contains water, proteins, solutes

• Plasma Composition: ~90% water, ~9% proteins (albumins, globulins, fibrinogen, regulatory proteins), ~1% solutes (nutrients, wastes, gases).

Red Blood Cells (Erythrocytes)

Red blood cells are specialized for oxygen transport and have unique structural adaptations.

• Structure: Anucleate, biconcave discs (increased surface area for gas exchange).

• Count: ~5.4 million/μL of blood.

• Lack of Organelles: Maximizes space for hemoglobin (Hb).

• Lifespan: Functional for up to 120 days; removed by phagocytosis in the spleen and liver.

Hemoglobin (Hb) Molecule

• Composed of 4 polypeptide chains (globins), each with a heme (iron) group.

• Each heme binds one O2 molecule.

Recycling: After RBC destruction, heme and amino acids are retained; waste products are excreted via the digestive and urinary systems.

Hematopoiesis and Erythropoiesis

Blood cells are produced through hematopoiesis, primarily in red bone marrow.

• Hematopoietic Stem Cells: Can differentiate into any formed element (RBCs, WBCs, platelets).

• Hematopoiesis: Formation of blood cells, stimulated by hormones.

• Erythropoiesis: Production of RBCs, regulated to balance production and destruction.

• Regulation: Erythropoietin (from kidneys) increases RBC production in response to hypoxia (low O2).

• Requirements: Adequate iron, amino acids, and B vitamins are necessary for effective erythropoiesis.

Anemia

• Definition: Reduced oxygen-carrying capacity of blood due to low Hb or RBC count.

• Iron-deficiency anemia: Most common; insufficient iron for Hb synthesis.

• Pernicious anemia: Decreased RBC production due to vitamin B12 deficiency.

White Blood Cells (Leukocytes)

WBCs are crucial for immune defense and are classified based on the presence or absence of granules.

• Function: Combat inflammation and infection; most leave circulation to act in tissues.

• Phagocytosis: Many WBCs engulf and destroy pathogens.

• Normal Count: 5,000–10,000/μL.

• Leukocytosis: Elevated WBC count (often in infection).

• Leukopenia: Decreased WBC count (risk of infection).

• Leukemia: Cancerous overproduction of WBCs in bone marrow, interfering with RBC production.

Classification of Leukocytes

Type	Granules	Nucleus	Main Function
Neutrophils	Present (granulocyte)	3–5 lobed	Phagocytosis of bacteria
Eosinophils	Present (granulocyte)	Bi-lobed	Combat parasitic worms
Basophils	Present (granulocyte)	Lobed (S-shaped)	Release histamine (inflammation)
Monocytes	Absent (agranulocyte)	U-shaped	Become macrophages (phagocytosis)
Lymphocytes	Absent (agranulocyte)	Large, round	B cells (antibodies), T cells (cell-mediated immunity)

Leukopoiesis

• Hematopoietic Stem Cells: Differentiate into lymphoid (lymphocytes) and myeloid (other WBCs, RBCs) lines.

• Bone Marrow Transplant: Used to treat anemia, leukemia, and other blood disorders.

• Cord Blood Transplants: Lower risk of rejection; stem cells often from the host.

Blood Groups and Typing

Blood groups are determined by antigens on RBC membranes and antibodies in plasma. Compatibility is essential for safe transfusions.

• Agglutination: Clumping occurs when incompatible antigens and antibodies bind.

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

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

• Cross-matching: Uses antisera to identify blood type and ensure compatibility.

Rh Factor and Incompatibility

• Rh-: RBCs lack Rh antigen.

• Rh+: RBCs possess Rh antigen.

• Normal blood does not contain anti-Rh antibodies unless exposed to Rh+ blood.

Hemolytic Disease of the Newborn

• Occurs when an Rh- mother carries an Rh+ fetus.

• First pregnancy: mother becomes sensitized to Rh antigen.

• Second pregnancy: maternal antibodies attack fetal RBCs, causing hemolysis.

Platelets (Thrombocytes)

• Structure: Disc-shaped, anucleate cell fragments derived from megakaryocytes.

• Lifespan: 7–10 days; recycled by spleen and liver.

• Function: Contain proteins essential for blood clotting.

• Normal Count: 250,000–400,000/μL.

Hemostasis (Prevention of Blood Loss)

Hemostasis is a sequential process that stops bleeding following vessel injury.

1. Vascular Spasm: Immediate vasoconstriction to reduce blood flow.

2. Platelet Plug Formation: Platelets adhere to exposed collagen and each other, forming a temporary plug.

3. Coagulation: Formation of a stable clot via activation of clotting factors and conversion of fibrinogen to fibrin.

4. Clot Retraction: The clot contracts to reduce its size and bring wound edges together.

5. Thrombolysis: Clot is dissolved after healing is complete.

Clotting Cascade

1. Formation of Prothrombinase:

   • Extrinsic Pathway: Rapid; initiated by tissue factor from outside the vessel.

   • Intrinsic Pathway: Slower; initiated by factors within the blood or vessel wall.

2. Conversion of Prothrombin to Thrombin:

   • Prothrombinase catalyzes this reaction.

   • Equation: $\text{Prothrombin} \xrightarrow{\text{Prothrombinase}} \text{Thrombin}$

3. Conversion of Fibrinogen to Fibrin:

   • Thrombin catalyzes this reaction.

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

   • Fibrin strands form the structural basis of the clot.

Summary Table: Steps of Hemostasis

Step	Description
Vascular Spasm	Vessel constricts to reduce blood flow
Platelet Plug Formation	Platelets adhere to exposed collagen and each other
Coagulation	Clot forms via fibrin meshwork
Clot Retraction	Clot contracts, wound edges drawn together
Thrombolysis	Clot is dissolved after healing

Example: When a blood vessel is injured, the hemostatic process rapidly prevents excessive blood loss, allowing tissue repair to occur.

Additional info: The balance between clot formation and dissolution is critical; disorders can lead to excessive bleeding (hemophilia) or unwanted clotting (thrombosis).