Chapter 11: Blood – Structure, Function, and Clinical Relevance

Blood: Structure and Function

Functions of Blood

Blood is a specialized liquid connective tissue essential for maintaining homeostasis in the body. It performs several vital functions:

• Transportation: Carries dissolved gases (O2, CO2), nutrients, hormones, and metabolic wastes throughout the body.

• Regulation: Maintains pH, ion composition, and osmotic balance of interstitial fluids.

• Protection: Restricts fluid loss at injury sites via clot formation and defends against toxins and pathogens.

• Temperature Stabilization: Distributes heat to regulate body temperature.

Physical and Chemical Characteristics of Blood

Composition of Blood

Blood consists of cells (formed elements) suspended in a liquid matrix called plasma. Key characteristics include:

• Volume: Adult males: 5–6 L; females: 4–5 L

• Temperature: 38°C (slightly above body temperature)

• Viscosity: Five times more viscous than water due to plasma proteins and formed elements

• pH: Slightly alkaline, ranging from 7.35–7.45

Blood Collection and Analysis

• Venipuncture: Most common method; blood is drawn from veins (median cubital vein).

• Peripheral Capillaries: Used for small samples (fingertip, earlobe).

• Arterial Puncture: Used to assess gas exchange efficiency in the lungs.

Components of Whole Blood

Main Components

• Blood Plasma: Liquid extracellular matrix containing dissolved substances.

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

Hematopoiesis is the process of blood cell formation from pluripotent stem cells, primarily occurring in red bone marrow.

Formed Elements: Quantities and Types

• Red Blood Cells (Erythrocytes): 4.8–5.4 million/μL

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

• Granular Leukocytes:

  • Neutrophils: 60–70%

  • Eosinophils: 2–4%

  • Basophils: 0.5–1%

• Agranular Leukocytes:

  • T and B lymphocytes, NK cells: 20–25%

  • Monocytes: 3–8%

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

Blood Separation by Centrifugation

When blood is centrifuged, its components separate by density:

• Plasma: ~55%

• Red Blood Cells: ~45%

• Buffy Coat (WBCs & Platelets): ~1%

Plasma and Plasma Proteins

Plasma Composition

Plasma makes up about 55% of whole blood and is the main component of extracellular fluid (ECF). It contains:

• Plasma Proteins:

  • Albumins (58%): Maintain osmotic pressure and pH

  • Globulins (38%): Transport proteins and antibodies

  • Fibrinogen (4%): Forms fibrin for blood clotting

• Hormones

• Nutrients: Glucose, fatty acids, amino acids, vitamins

• Waste Products: Urea, uric acid, creatinine, bilirubin, ammonium

• Respiratory Gases: O2 and CO2

• Electrolytes: Sodium, potassium, calcium, magnesium, chloride, hydrogen phosphate, bicarbonate, sulfate

Plasma minus clotting proteins (like fibrinogen) is called serum. About 90% of plasma proteins are synthesized by the liver; liver disorders can alter blood composition and function.

Table: Major Plasma Proteins and Their Functions

Protein	Function
Albumins	Osmotic pressure, pH regulation
Globulins	Transport proteins, antibodies
Fibrinogen	Blood clotting (forms fibrin)

Hematopoiesis and Blood Cell Development

Hematopoiesis

Hematopoiesis is the formation of blood cells from stem cells (hemocytoblasts). In the fetus, it occurs in the liver, thymus, spleen, lymph nodes, and red bone marrow. After birth, it is mainly in red bone marrow, with some WBCs produced in lymphatic tissues.

Stages in RBC Maturation

• Embryonic cells become pluripotent stem cells (hemocytoblasts).

• Hemocytoblasts produce myeloid stem cells.

• Erythroblasts (immature RBCs) synthesize hemoglobin.

• Loss of nucleus forms reticulocytes, which enter the bloodstream and mature into RBCs.

Erythrocytes (Red Blood Cells)

Structure and Function

• Account for 99.9% of formed elements.

• Contain hemoglobin for oxygen and carbon dioxide transport.

• Measured as RBC count (cells/μL) and hematocrit (% of whole blood).

• Unique biconcave shape increases surface area for gas exchange and flexibility for capillary passage.

• During maturation, organelles are lost; RBCs cannot divide and rely on anaerobic metabolism.

• Life span: ~120 days; ~1% replaced daily.

Anatomy of Red Blood Cells

• Diameter: 6.45–8.14 μm

• Thickness: 1.9–2.6 μm

Production of Erythrocytes (Erythropoiesis)

• Triggered by decreased blood O2 (hypoxia), stimulating kidneys to release erythropoietin (EPO).

• EPO acts on myeloid stem cells in red bone marrow, increasing cell division and maturation rate.

• Clinical application: EPO infusions aid recovery from blood loss and chemotherapy-induced anemia.

Equation: $\text{EPO} \xrightarrow{\text{stimulates}} \text{RBC production}$

Hemoglobin: Structure and Function

Hemoglobin

• Main component of mature erythrocytes.

• Composed of four globin proteins (2 alpha, 2 beta), each attached to a heme group containing iron.

• O2 binds to iron; low iron leads to anemia.

• Oxyhemoglobin: Hemoglobin with O2

• Carboxyhemoglobin: Hemoglobin with CO2

Hemoglobin Function

• O2-heme bond is weak, allowing easy binding and release.

• High plasma O2: Hemoglobin gains O2 (lung capillaries).

• Low plasma O2 and high CO2: Hemoglobin releases O2 (systemic capillaries).

Anemia

• Reduction in blood's oxygen-carrying capacity.

• Indicated by low hematocrit and hemoglobin levels.

• Causes: Iron or B12 deficiency, internal bleeding.

• Symptoms: Fatigue, weakness, lack of energy, poor concentration.

Hemoglobin Recycling

Process Overview

• Most RBC components recycled in liver and spleen.

• Macrophages engulf RBCs and remove hemoglobin.

• Only ~10% of RBCs rupture (hemolyze) in bloodstream.

Steps in Hemoglobin Recycling

1. Globular proteins broken into amino acids.

2. Heme stripped of iron, converted to biliverdin.

3. Biliverdin converted to bilirubin (orange-yellow pigment).

4. Liver absorbs bilirubin; becomes part of bile.

5. If not excreted, bilirubin causes jaundice.

6. Iron released into blood, binds to transferrin, stored by liver.

Leukocytes (White Blood Cells)

Types and Functions

• Larger than RBCs; involved in immune responses.

• Contain nucleus and organelles; lack hemoglobin.

• Granulocytes: Neutrophils, eosinophils, basophils (cytoplasmic granules, lobed nuclei).

• Agranulocytes: Lymphocytes, monocytes (lack visible granules).

• General quantities (most to least): Neutrophils, lymphocytes, monocytes, eosinophils, basophils.

Table: Major Leukocyte Types and Functions

Type	Function
Neutrophils	Phagocytosis of bacteria; first responders
Eosinophils	Combat parasites; modulate allergic responses
Basophils	Release histamine; involved in inflammation/allergy
Lymphocytes	Specific immune responses (T cells, B cells, NK cells)
Monocytes	Phagocytosis; become macrophages

Additional info:

• Further details on platelets, blood loss mechanisms, blood typing, and clinical disorders are covered in subsequent slides and notes.