Blood: Structure, Function, and Disorders

Blood: Structure, Function, and Disorders

Introduction to Blood

Blood is a specialized connective tissue essential for transporting substances, regulating physiological processes, and protecting the body against disease. It consists of a liquid matrix called plasma and various formed elements, including red blood cells, white blood cells, and platelets.

Blood Composition and Volume

• Plasma: The liquid fraction of blood, making up about 55% of total blood volume. It contains water, proteins, nutrients, gases, and waste products.

• Formed Elements: Suspended in plasma, these include erythrocytes (RBCs), leukocytes (WBCs), and platelets (thrombocytes).

• Normal Blood Volumes: Plasma: 2.6 L; Formed elements: 2.4 L; Whole blood: 4–6 L (7–9% of body weight).

Blood pH and Donation

• pH: Blood is slightly alkaline, with a pH of 7.35–7.45. A decrease toward neutral is called acidosis.

• Donation: About 14 million units are donated annually. Plasma expanders can temporarily maintain volume after hemorrhage, but stored blood is viable for only 6 weeks.

Blood Plasma

Plasma is the liquid portion of blood minus the formed elements. It is composed mainly of water (91%) and dissolved substances such as nutrients, salts, gases, and proteins.

• Plasma Proteins:

  • Albumins: Maintain osmotic pressure and water balance.

  • Globulins: Include antibodies for immune defense.

  • Fibrinogen and Prothrombin: Essential for blood clotting.

• Serum: Plasma minus clotting factors; contains antibodies.

Formed Elements of Blood

• Red Blood Cells (Erythrocytes): 4.2–6.2 million/mm3

• White Blood Cells (Leukocytes): 5,000–10,000/mm3

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

• Granular Leukocytes: Neutrophils, eosinophils, basophils

• Agranular Leukocytes: Lymphocytes, monocytes

Hematopoiesis

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

Mechanisms of Blood Disease

• Blood diseases often result from failure of myeloid or lymphoid tissues due to toxins, radiation, genetic defects, nutritional deficiencies, or cancers (e.g., leukemia).

• Aspiration biopsy cytology (ABC) is used to diagnose blood diseases by examining blood-forming tissues.

Red Blood Cells (Erythrocytes)

Structure and Function

RBCs are biconcave disks with flexible membranes, lacking nuclei and most organelles, which maximizes space for hemoglobin (Hb). Their primary function is to transport oxygen and carbon dioxide.

• Hemoglobin (Hb): The red pigment that binds O2 (as oxyhemoglobin) and CO2 (as carbaminohemoglobin).

• Lifespan: About 120 days.

RBC Count and Hematocrit

The complete blood cell count (CBC) measures various blood constituents, including RBCs. Hematocrit (PCV) is the percentage of blood volume occupied by RBCs.

RBC Abnormalities

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

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

Blood Types and Transfusion

ABO System

• 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-positive: Rh antigen present

• Rh-negative: No Rh antigen; anti-Rh antibodies form only after exposure

• Erythroblastosis fetalis: Hemolytic disease of the newborn due to Rh incompatibility

Red Blood Cell Disorders

Polycythemia

• Excessive RBC production, often due to bone marrow cancer

• Symptoms: Increased blood viscosity, slow flow, hypertension, risk of clotting

• Treatment: Blood removal, chemotherapy, irradiation

Anemia

Anemia is characterized by low RBC count or abnormal hemoglobin, resulting in reduced oxygen-carrying capacity. Normal Hb: 12–14 g/100 mL; anemia: <9 g/100 mL.

• Symptoms: Fatigue, pallor, weakness, increased heart and respiratory rates

Types of Anemia

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

• Aplastic: Bone marrow failure (toxins, radiation, drugs)

• Pernicious: Vitamin B12 deficiency, macrocytic RBCs, CNS symptoms

• Folate Deficiency: Low folate, common in malnutrition/alcoholism

• Iron Deficiency: Microcytic, hypochromic RBCs, low hematocrit

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

Sickle Cell Anemia

• Genetic disorder causing abnormal hemoglobin (HbS)

• RBCs sickle under low O2, leading to hemolysis, pain crises, and organ damage

Thalassemia

• Inherited hemolytic anemia, common in Mediterranean populations

• Microcytic, short-lived RBCs; severe forms require marrow/stem cell transplantation

Hemolytic Disease of the Newborn (Erythroblastosis Fetalis)

• Caused by maternal-fetal blood incompatibility (ABO or Rh)

• Symptoms: Jaundice, anemia, organ damage; prevention with RhoGAM

White Blood Cells (Leukocytes)

Types and Functions

• Granulocytes:

  • Neutrophils: Most numerous, phagocytic, increase in bacterial infections

  • Eosinophils: Defend against parasites, involved in allergies

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

• Agranulocytes:

  • Lymphocytes: B cells (antibody production), T cells (cellular immunity)

  • Monocytes: Largest WBCs, become macrophages in tissues

WBC Disorders

• Leukopenia: Low WBC count (<5,000/mm3), seen in immune disorders (e.g., AIDS)

• Leukocytosis: High WBC count (>10,000/mm3), common in infections and leukemia

• Differential WBC Count: Measures proportions of each WBC type

Leukemias and Blood Cancers

• Lymphoid Neoplasms: From B/T lymphocyte precursors

• Myeloid Neoplasms: From granulocyte, monocyte, RBC, or platelet precursors

• Multiple Myeloma: Cancer of plasma cells, causes bone lesions and anemia

• Leukemia Types:

  • Chronic Lymphocytic Leukemia (CLL): Older adults, slow progression

  • Acute Lymphocytic Leukemia (ALL): Children, rapid onset, high cure rate in children

  • Chronic Myeloid Leukemia (CML): Adults, slow progression, treatable with Gleevec

  • Acute Myeloid Leukemia (AML): Adults, rapid progression, poor prognosis

Infectious Mononucleosis

• Viral infection (Epstein-Barr virus), common in young adults

• Symptoms: Fever, fatigue, sore throat, lymphadenopathy

Platelets and Blood Clotting

Platelet Function

• Platelets (thrombocytes) are essential for hemostasis (stopping bleeding).

• They form a platelet plug at injury sites and release clotting factors.

Clotting Mechanism

• Damaged tissues release clotting factors, forming prothrombin activator.

• Prothrombin activator and calcium convert prothrombin to thrombin.

• Thrombin converts fibrinogen to fibrin, forming a mesh that traps blood cells to form a clot.

$\text{Prothrombin} + \text{Prothrombin Activator} + \text{Ca}^{2+} \rightarrow \text{Thrombin}$ $\text{Thrombin} + \text{Fibrinogen} \rightarrow \text{Fibrin (clot)}$

Clotting Disorders

• Thrombus: Stationary clot

• Embolus: Circulating clot

• Hemophilia: X-linked disorder, lack of factor VIII, causes severe bleeding

• Thrombocytopenia: Low platelet count, causes bleeding and purpura

• Vitamin K Deficiency: Impairs synthesis of clotting factors

Summary Table: Classes of Blood Cells

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 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	Low	Normal to high	Normal to slightly high	Normal
Pernicious anemia	Low to 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	Normal
Folate deficiency anemia	Low	Low	Low	Normal	High	Normal
Iron deficiency anemia	Normal	Low	Low	Low	Low	Normal
Hemolytic anemia (sickle cell, 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 Transport: $\text{Hb} + \text{O}_2 \rightarrow \text{Oxyhemoglobin}$

• CO2 Transport: $\text{Hb} + \text{CO}_2 \rightarrow \text{Carbaminohemoglobin}$

Clinical Applications

• Cardiac Blood Tests: Enzyme levels (CK, LDH, SGOT) and troponins help diagnose myocardial infarction.

• Complete Blood Cell Count (CBC): Provides information on RBC, WBC, platelet counts, hemoglobin, hematocrit, and more.

Health and Well-Being: Blood Doping

Blood doping involves increasing RBC count to enhance athletic performance, often by transfusion or using erythropoietin (EPO). It is considered unsafe and unethical due to risks of high hematocrit, blood viscosity, and cardiovascular complications.

Review Questions

1. Blood pH is between 7.35 and 7.45. This makes the blood: B. Slightly alkaline

2. The formed element that functions in oxygen and carbon dioxide transport is the: A. Erythrocyte

3. During periods of chronic blood loss, the body helps maintain homeostasis by producing: C. Normocytic RBCs

4. If you have type A blood, type ____ antigen is on the RBC and the plasma contains _____ antibodies. C. A, anti-B

5. _____ anemia results from a deficiency of vitamin B12. D. Pernicious

6. _____ leukemia results from cancerous transformation of granulocytic precursor cells in the bone marrow. B. Chronic myeloid

7. A common type of clotting disorder resulting in a decrease in the platelet count is called: D. Thrombocytopenia