Blood: Structure, Function, and Disorders

Blood: Structure, Function, and Disorders

Overview of Blood Composition and Volume

Blood is a specialized connective tissue essential for transporting substances, regulating physiological processes, and protecting the body. It consists of a liquid matrix called plasma and cellular components known as formed elements.

• Plasma: The extracellular, liquid portion of blood, making up about 55% of total blood volume.

• Formed Elements: Suspended cells and cell fragments, including erythrocytes (RBCs), leukocytes (WBCs), and platelets (thrombocytes).

• Normal Blood Volume: 4–6 liters in adults, accounting for 7–9% of total body weight.

Blood pH and Donation

Blood is slightly alkaline, with a pH range of 7.35–7.45. Deviations toward neutrality (acidosis) can be harmful. Blood donations are critical for medical care, but stored blood has a limited shelf life (about 6 weeks).

Blood Plasma

Plasma is the liquid fraction of blood after removal of formed elements. It is primarily water (91%) with dissolved proteins, nutrients, gases, and waste products.

• Major Plasma Proteins:

  • Albumins: Maintain osmotic pressure and water balance.

  • Globulins: Include antibodies for immune defense.

  • Fibrinogen & Prothrombin: Essential for blood clotting.

• Serum: Plasma without clotting factors; contains antibodies.

Formed Elements of Blood

The formed elements include three main types: erythrocytes (RBCs), leukocytes (WBCs), and platelets (thrombocytes). Each type has distinct functions and subtypes.

• RBCs (Erythrocytes): 4.2–6.2 million/mm3

• WBCs (Leukocytes): 5,000–10,000/mm3

• Platelets (Thrombocytes): 150,000–400,000/mm3

Hematopoiesis: Formation of Blood Cells

Hematopoiesis is the process of blood cell formation, occurring primarily in red bone marrow (myeloid tissue) and lymphoid tissues (lymph nodes, thymus, spleen). Myeloid tissue produces all blood cells except some lymphocytes and monocytes, which are formed in lymphoid tissue.

Red Blood Cells (Erythrocytes)

RBCs are specialized for oxygen and carbon dioxide transport. Their biconcave shape increases surface area for gas exchange, and the absence of a nucleus allows more space for hemoglobin (Hb).

• Life Span: ~120 days

• Hemoglobin: Iron-containing protein that binds O2 and CO2

Functions of RBCs

• Oxygen Transport: O2 binds to hemoglobin forming oxyhemoglobin.

• Carbon Dioxide Transport: CO2 binds to hemoglobin (carbaminohemoglobin) or is converted to bicarbonate.

• Acid-Base Balance: RBCs help maintain pH homeostasis.

Complete Blood Count (CBC) and Hematocrit

The CBC is a comprehensive test measuring RBCs, WBCs, platelets, hemoglobin, and hematocrit (percentage of blood volume occupied by RBCs). Hematocrit is used to assess anemia or polycythemia.

RBC Abnormalities

RBCs are classified by size and hemoglobin content:

• Size: Normocytes (normal), microcytic (small), macrocytic (large)

• Hemoglobin Content: Normochromic (normal), hypochromic (low), hyperchromic (high)

Blood Types and Transfusion Compatibility

Blood types are determined by antigens on RBCs and antibodies in plasma. The ABO and Rh systems are most clinically significant.

• Type A: A antigens, anti-B antibodies

• Type B: B antigens, anti-A antibodies

• Type AB: A and B antigens, no antibodies (universal recipient)

• Type O: No antigens, anti-A and anti-B antibodies (universal donor)

• Rh System: Rh+ (antigen present), Rh– (antigen absent)

Red Blood Cell Disorders

Polycythemia

Polycythemia is an abnormal increase in RBCs, often due to bone marrow cancer. It leads to increased blood viscosity, slow flow, and risk of clotting or hemorrhage.

Anemia

Anemia is characterized by low RBC count or hemoglobin, resulting in reduced oxygen-carrying capacity. Symptoms include fatigue, pallor, and increased heart/respiratory rates.

• Hemorrhagic Anemia: Due to blood loss (acute or chronic)

• Aplastic Anemia: Bone marrow failure

• Pernicious Anemia: Vitamin B12 deficiency

• Folate Deficiency Anemia: Low folate (B9)

• Iron Deficiency Anemia: Low iron, microcytic and hypochromic RBCs

• Hemolytic Anemias: Increased RBC destruction (e.g., sickle cell, thalassemia)

Leukocytes (White Blood Cells)

WBCs are divided into granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). They play key roles in immune defense, inflammation, and phagocytosis.

• Leukopenia: Low WBC count (<5,000/mm3)

• Leukocytosis: High WBC count (>10,000/mm3)

White Blood Cell Disorders

• Leukemia: Cancer of WBCs, classified as acute/chronic and lymphoid/myeloid.

• Multiple Myeloma: Cancer of plasma cells (B lymphocytes), causing bone lesions and anemia.

• Infectious Mononucleosis: Viral infection (Epstein-Barr virus) causing atypical lymphocytosis.

Platelets and Blood Clotting

Platelets (thrombocytes) are cell fragments essential for hemostasis. They form a platelet plug at injury sites and release factors that initiate the clotting cascade.

• Clotting Mechanism:

  1. Damaged tissue releases clotting factors.

  2. Prothrombin activator (with Ca2+) converts prothrombin to thrombin.

  3. Thrombin converts fibrinogen to fibrin, forming a mesh that traps cells and forms a clot.

• Clotting Disorders: Hemophilia (factor VIII deficiency), thrombocytopenia (low platelets), vitamin K deficiency.

Summary Table: Classes of Blood Cells and Functions

Body Cell	Function
Erythrocyte	Oxygen and carbon dioxide transport
Neutrophil	Immune defense (phagocytosis)
Eosinophil	Defense against parasites
Basophil	Inflammatory response and heparin secretion
B lymphocyte	Antibody production; precursor of plasma cells
T lymphocyte	Cellular immune response
Monocyte	Immune defense (phagocytosis)
Thrombocyte	Blood clotting

Summary Table: Blood Typing

Blood Type (ABO and Rh)	Antigens Present	Antibodies Present	Percent of General Population
O+	Rh	anti-A, anti-B	35%
O–	None	anti-A, anti-B, anti-Rh?	7%
A+	A, Rh	anti-B	35%
A–	A	anti-B, anti-Rh?	7%
B+	B, Rh	anti-A	8%
B–	B	anti-A, anti-Rh?	2%
AB+	A, B, Rh	None	4%
AB–	A, B	anti-Rh?	2%

Summary Table: Laboratory Results for Types of Anemia

Anemia	Folate Content	Hemoglobin	Hematocrit	Iron Content	RBC Size (Volume)	Vitamin B12 Content
Aplastic anemia	Normal	Low to normal	Low to normal	Normal to high	Normal to slightly high	Normal
Pernicious anemia	Normal	Low	Low	Normal	High	Low
Hemorrhagic anemia	Normal	Low	Low	High	Normal	Normal
Acute blood-loss anemia	Normal	Low	Low	Normal	Slightly low	Normal
Chronic blood-loss anemia	Normal	Low	Low	Low	Low to normal	Normal
Folate deficiency anemia	Low	Low	Low	Normal	Low	Normal
Iron deficiency anemia	Normal	Low	Low	Low	Low	Normal
Hemolytic anemia (sickle cell anemia and thalassemia)	Normal	Low	Low	Normal to high	Low	Normal

Key Equations

• Hematocrit (Hct): $\text{Hct} = \frac{\text{Volume of RBCs}}{\text{Total Blood Volume}} \times 100$

• Oxygen Carrying Capacity: $\text{O}_2 \text{ capacity} = \text{Hb concentration} \times 1.34$

Additional info:

• Blood disorders can be diagnosed using laboratory tests such as CBC, blood smears, and specific biochemical markers.

• Blood doping and erythropoietin (EPO) use can artificially increase RBC count but pose health risks.