Blood: Structure, Function, and Disorders

Blood Composition and Volume

Overview of Blood Components

Blood is a specialized connective tissue composed of a liquid matrix called plasma and various cellular elements known as formed elements. It plays a critical role in transport, regulation, and protection within the human body.

• 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 to 6 liters in adults, accounting for 7% to 9% of total body weight.

Blood pH and Donation

Blood is slightly alkaline, with a normal pH range of 7.35 to 7.45. Deviations toward neutrality result in acidosis, a potentially dangerous condition. Blood donations are essential for medical care, but stored blood has a limited shelf life of about 6 weeks.

Blood Plasma

Composition and Function

Plasma is the liquid fraction of blood after removal of formed elements. It consists mainly of water (91%) and dissolved substances such as nutrients, salts, gases, and plasma 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 without clotting factors; contains antibodies.

Formed Elements of Blood

Types and Formation

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

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

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

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

Blood cells are produced in the red bone marrow (myeloid tissue) and lymphoid tissues (lymph nodes, thymus, spleen).

Hematopoiesis

Hematopoiesis is the process of blood cell formation. Myeloid tissue forms all blood cells except some lymphocytes and monocytes, which are produced in lymphoid tissue.

Red Blood Cells (Erythrocytes)

Structure and Function

RBCs are biconcave discs lacking nuclei and most organelles, maximizing space for hemoglobin (Hb). Their flexible membrane allows passage through capillaries. The average lifespan is about 120 days.

• Function: Transport oxygen (O2) and carbon dioxide (CO2), and help maintain acid-base balance.

• Hemoglobin: The red pigment responsible for O2 and CO2 binding.

Hemoglobin and Gas Transport

• Oxyhemoglobin: Hb bound to O2

• Carbaminohemoglobin: Hb bound to CO2

• CO2 is also converted to bicarbonate by RBCs.

Hematocrit and Blood Testing

The hematocrit (Hct) is the percentage of blood volume occupied by RBCs. It is measured as part of the complete blood count (CBC), a key diagnostic tool.

Red Blood Cell Abnormalities

Classification by Size and Hemoglobin Content

• Size:

  • Normocytes: Normal size (8–9 μm)

  • Microcytic: Small

  • Macrocytic: Large

• Hemoglobin Content:

  • Normochromic: Normal Hb

  • Hypochromic: Low Hb

  • Hyperchromic: High Hb

Blood Types and Transfusion

ABO and Rh Systems

Blood types are determined by the presence or absence of specific antigens on RBC membranes. The ABO system includes types A, B, AB, and O, while the Rh system classifies blood as Rh-positive or Rh-negative.

• 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 incompatibility can cause erythroblastosis fetalis in newborns if an Rh-negative mother carries an Rh-positive fetus.

Red Blood Cell Disorders

Polycythemia

Polycythemia is an abnormal increase in RBC count, often due to cancerous transformation of red bone marrow. It leads to increased blood viscosity, slow flow, and risk of clotting or hemorrhage.

Anemia

Anemia is characterized by a decrease in the oxygen-carrying capacity of blood due to low RBC count, abnormal RBCs, or low/defective hemoglobin. Symptoms include fatigue, pallor, weakness, and compensatory increases in heart and respiratory rates.

• Hemorrhagic Anemia: Due to acute or chronic blood loss.

• Aplastic Anemia: Bone marrow failure, often from toxins or radiation.

• Pernicious Anemia: Vitamin B12 deficiency, often with CNS symptoms.

• Folate Deficiency Anemia: Common in malnutrition and alcoholism.

• Iron Deficiency Anemia: Microcytic, hypochromic RBCs; most common worldwide.

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

Leukocytes (White Blood Cells)

Types and Functions

WBCs are classified as granulocytes (with granules) or agranulocytes (without granules). They play essential roles in immune defense, inflammation, and phagocytosis.

• Granulocytes:

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

  • Eosinophils: Defend against parasites, involved in allergies.

  • Basophils: Release histamine and heparin, involved in inflammation.

• Agranulocytes:

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

  • Monocytes: Largest WBCs, become macrophages in tissues.

White Blood Cell Disorders

• Leukopenia: Low WBC count, often due to immune disorders or bone marrow failure.

• Leukocytosis: 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, causing bone lesions and anemia.

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

Platelets and Blood Clotting

Role in Hemostasis

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

• Clotting involves conversion of prothrombin to thrombin, which then converts fibrinogen to fibrin, forming a mesh that traps blood cells.

• Clotting can be altered by drugs (e.g., heparin, Coumadin), vitamin K deficiency, or genetic disorders (e.g., hemophilia).

Clotting Disorders

• Thrombus: Stationary clot

• Embolus: Circulating clot

• Hemophilia: X-linked disorder, deficiency of factor VIII

• Thrombocytopenia: Low platelet count, risk of bleeding

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

Tables

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

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%

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	Low	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	High	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