Blood: Structure, Function, and Disorders

Blood: Structure, Function, and Disorders

Overview of 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.

• Normal blood volume: 4–6 liters (7–9% of body weight)

• pH: Slightly alkaline, ranging from 7.35 to 7.45

• Functions: Transport of gases, nutrients, hormones, and waste; regulation of pH and temperature; protection against pathogens and blood loss

Composition of Blood

Blood is composed of plasma (the liquid portion) and formed elements (cells and cell fragments).

• Plasma: 55% of blood volume; contains water, proteins (albumins, globulins, fibrinogen), nutrients, electrolytes, gases, and waste products

• Formed elements: 45% of blood volume; includes erythrocytes (RBCs), leukocytes (WBCs), and platelets (thrombocytes)

Blood Plasma

Plasma is the liquid matrix of blood, primarily composed of water and dissolved substances.

• Major plasma proteins:

  • Albumins: Maintain osmotic pressure and water balance

  • Globulins: Include antibodies for immune defense

  • Fibrinogen and Prothrombin: Essential for blood clotting

• Serum: Plasma without clotting factors; contains antibodies

Formed Elements of Blood

The formed elements are the cellular components of blood, each with specialized functions.

• Red Blood Cells (Erythrocytes): Transport oxygen and carbon dioxide

• White Blood Cells (Leukocytes): Defend against infection and disease

• Platelets (Thrombocytes): Involved in blood clotting

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

Hematopoiesis

Hematopoiesis is the process of blood cell formation, occurring primarily in red bone marrow (myeloid tissue) and lymphoid tissue.

• Red bone marrow: Produces all blood cells except some lymphocytes and monocytes

• Lymphoid tissue: Produces lymphocytes and monocytes; found in lymph nodes, thymus, and spleen

Red Blood Cells (Erythrocytes)

RBCs are biconcave, anucleate cells specialized for gas transport. Their structure maximizes surface area for gas exchange and flexibility for passage through capillaries.

• Hemoglobin (Hb): Iron-containing protein that binds oxygen and carbon dioxide

• Normal RBC count: 4.2–6.2 million/mm3

• Lifespan: ~120 days

Hemoglobin Structure

Hemoglobin consists of four polypeptide chains (two alpha, two beta) each with an iron-containing heme group that binds oxygen.

Blood Cell Counts and Clinical Tests

Blood cell counts and related tests are essential for diagnosing and monitoring health conditions.

• Complete Blood Cell Count (CBC): Measures RBCs, WBCs, platelets, hemoglobin, hematocrit, and differential WBC count

• Hematocrit (Hct): Percentage of blood volume occupied by RBCs

Blood Types and Transfusion Compatibility

Blood types are determined by the presence or absence of specific antigens on RBCs. The ABO and Rh systems are the most clinically significant.

• ABO System: Types A, B, AB, O based on A and B antigens

• Rh System: Rh-positive (antigen present) or Rh-negative (antigen absent)

• Universal donor: Type O-

• Universal recipient: Type AB+

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%

Red Blood Cell Disorders

Disorders of RBCs can affect oxygen transport and overall health.

• Anemia: Reduced oxygen-carrying capacity due to low RBC count, abnormal RBCs, or low/defective hemoglobin

• Polycythemia: Excessive RBC production, often due to bone marrow cancer

Types of Anemia

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

Iron Deficiency Anemia

Characterized by microcytic (small), hypochromic (pale) RBCs due to inadequate hemoglobin synthesis.

Sickle Cell Anemia

A genetic disorder resulting in abnormal hemoglobin (HbS), causing RBCs to assume a sickle shape under low oxygen conditions. This leads to hemolytic anemia, blood stasis, and painful crises.

Hemolytic Disease of the Newborn (Erythroblastosis Fetalis)

Occurs due to ABO or Rh incompatibility between mother and fetus, leading to maternal antibodies attacking fetal RBCs. Prevention is possible with RhoGAM administration to Rh-negative mothers.

White Blood Cells (Leukocytes)

WBCs are crucial for immune defense and are classified as granulocytes or agranulocytes based on the presence of cytoplasmic granules.

• Granulocytes: Neutrophils, eosinophils, basophils

• Agranulocytes: Lymphocytes (B and T cells), monocytes

• Normal WBC count: 5,000–10,000/mm3

Phagocytosis

Neutrophils and monocytes are phagocytes, engulfing and destroying pathogens and debris.

White Blood Cell Disorders

• Leukopenia: Abnormally low WBC count; may occur in immune disorders

• Leukocytosis: Abnormally high WBC count; common in infections and leukemia

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

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

Platelets and Blood Clotting

Platelets (thrombocytes) are cell fragments essential for hemostasis (stopping bleeding) by forming platelet plugs and releasing clotting factors.

• Clotting mechanism:

  1. Damaged tissue releases clotting factors

  2. Prothrombin activator and calcium convert prothrombin to thrombin

  3. Thrombin converts fibrinogen to fibrin, forming a clot

• Disorders: Hemophilia (factor VIII deficiency), thrombocytopenia (low platelet count), vitamin K deficiency

Summary Table: Blood Cell Types and Functions

Cell Type	Main Function
Erythrocyte	Oxygen and carbon dioxide transport
Neutrophil	Phagocytosis of bacteria
Eosinophil	Defense against parasites, allergic reactions
Basophil	Release histamine and heparin (inflammation)
Lymphocyte (B)	Antibody production
Lymphocyte (T)	Cell-mediated immunity
Monocyte	Phagocytosis, become macrophages
Platelet	Blood clotting

Key Equations

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

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

Clinical Applications

• Cardiac blood tests: Enzyme levels (CK, LDH, SGOT, troponins) can indicate myocardial infarction

• Blood doping: Artificially increasing RBC count to enhance athletic performance is dangerous and unethical

Review Questions

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

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

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

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

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

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

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