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 (RBCs), white blood cells (WBCs), 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 RBCs, WBCs, and platelets, comprising about 45% of blood volume.

• Normal Blood Volumes: Whole blood volume averages 4–6 L in adults, accounting for 7–9% of total body weight.

Blood pH and Donation

• Blood pH: Normally ranges from 7.35 to 7.45, making blood slightly alkaline. A decrease toward neutral is termed acidosis.

• Blood Donation: Approximately 14 million units are donated annually. Plasma expanders can temporarily maintain blood volume after hemorrhage, but donated blood can only be stored for up to 6 weeks.

Blood Plasma

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

• 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 include three main types:

• Red Blood Cells (Erythrocytes)

• White Blood Cells (Leukocytes)

  • Granular Leukocytes: Neutrophils, eosinophils, basophils

  • Agranular Leukocytes: Lymphocytes, monocytes

• Platelets (Thrombocytes)

Numbers of Formed Elements

• RBCs: 4.2–6.2 million/mm3

• WBCs: 5,000–10,000/mm3

• Platelets: 150,000–400,000/mm3

Hematopoiesis

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

Classes of Blood Cells and Their 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

Red Blood Cells (Erythrocytes)

RBCs are biconcave, anucleate cells specialized for gas transport. Their shape increases surface area for gas exchange and allows flexibility to pass through capillaries. The absence of a nucleus provides more space for hemoglobin (Hb), the oxygen-carrying protein.

Hemoglobin and Gas Transport

• Oxyhemoglobin: Hemoglobin bound to oxygen

• Carbaminohemoglobin: Hemoglobin bound to carbon dioxide

• RBCs also help maintain acid-base balance by converting CO2 to bicarbonate

Hematocrit and Blood Testing

The hematocrit (Hct) is the percentage of blood volume occupied by RBCs. It is measured using a centrifuge, which separates blood into plasma, buffy coat (WBCs and platelets), and RBCs.

Blood Types and Transfusion Compatibility

Blood types are determined by the presence or absence of specific antigens on RBC membranes. The two main systems are ABO and Rh.

• 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 develop after exposure

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%

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

• Low RBC count or abnormal hemoglobin reduces oxygen-carrying capacity

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

Types of Anemia

Anemia	Folate Content	Hemoglobin	Hematocrit	Iron Content	RBC Size	Vitamin B12 Content
Aplastic	Normal	Low	Low	Normal	Low	Normal
Pernicious	Low to normal	Low	Low	Normal	High	Low
Hemorrhagic	Normal	Low	Low	High	Normal	Normal
Iron deficiency	Normal	Low	Low	Low	Low	Normal
Hemolytic	Normal	Low	Low	Normal to high	Low	Normal

Iron Deficiency Anemia

• Caused by insufficient iron for hemoglobin synthesis

• RBCs are microcytic and hypochromic

• Treatment: Oral iron supplements

Sickle Cell Anemia

• Genetic disorder causing abnormal hemoglobin (HbS)

• RBCs become sickle-shaped under low oxygen, leading to hemolysis and vascular blockage

White Blood Cells (Leukocytes)

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

Leukocyte Disorders

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

• Leukocytosis: High WBC count, common in infections and leukemia

• Leukemia: Cancer of WBCs, classified as acute or chronic, lymphocytic or myeloid

Platelets and Blood Clotting

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

• Clotting Mechanism:

  • Damaged tissue releases 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

Clotting Disorders

• Hemophilia: X-linked disorder causing deficiency of clotting factor VIII; leads to excessive bleeding

• Thrombocytopenia: Low platelet count, causing bleeding and purpura

• Vitamin K Deficiency: Impairs synthesis of clotting factors

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
Basophil	Release histamine and heparin
Lymphocyte	Immune response (B and T cells)
Monocyte	Phagocytosis; become macrophages
Platelet	Blood clotting

Key Equations

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

• Oxygen Transport: $\mathrm{Hb} + \mathrm{O}_2 \rightarrow \text{Oxyhemoglobin}$

• Carbon Dioxide Transport: $\mathrm{Hb} + \mathrm{CO}_2 \rightarrow \text{Carbaminohemoglobin}$

Clinical Applications

• Complete Blood Cell Count (CBC): Measures RBC, WBC, platelet counts, hemoglobin, hematocrit, and more to assess health status.

• Blood Typing: Essential for safe transfusions and organ transplantation.

• Cardiac Blood Tests: Enzyme levels (e.g., troponins) help diagnose myocardial infarction.

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