Chapter 17: Blood – Comprehensive Study Notes

Red Blood Cell Production (Erythropoiesis)

Location and Key Terms

Red blood cell (RBC) production, or erythropoiesis, is a tightly regulated process essential for oxygen transport. It occurs primarily in the red bone marrow of adults.

• Hematopoiesis: Formation of all blood cells.

• Erythropoiesis: Formation of erythrocytes (RBCs) only.

• Starting cell: Multipotent hematopoietic stem cell (hemocytoblast).

Stem Cell Pathways and RBC Lineage

• Hemocytoblasts divide into:

  • Myeloid stem cells

  • Lymphoid stem cells

• RBC lineage: Myeloid stem cell → Proerythroblast → Erythroblast → Reticulocyte → Erythrocyte

Stages of RBC Development

1. Commitment stage: First recognizable RBC precursor is the proerythroblast.

2. Proerythroblast → Erythroblasts: Actively produce hemoglobin.

3. Nucleus is ejected: Cell becomes biconcave.

4. Reticulocytes: Enter bloodstream and mature within 1–2 days.

Regulation of Erythropoiesis

Materials Required for RBC Production

• Iron

• Amino acids

• Vitamins (especially vitamin B12, folic acid)

Vitamin B12

• Required for DNA synthesis.

• Absorption requires intrinsic factor from the stomach.

• B12 deficiency results in pernicious anemia.

• Cause: Inability to absorb or obtain vitamin B12.

Hormonal Control

• Hormone: Erythropoietin (EPO)

• Released by kidneys (and liver to a lesser extent).

• Triggered by low oxygen levels (hypoxia).

• Low oxygen → kidneys release EPO → bone marrow increases RBC production.

White Blood Cell Production

Stem Cell Lines

• Myeloid stem cells

• Lymphoid stem cells

Myeloid Stem Cells Produce

• Neutrophils

• Eosinophils

• Basophils

• Monocytes (become macrophages in tissues)

Lymphoid Stem Cells Produce

• T cells

• B cells

• Natural Killer (NK) cells

Development occurs in bone marrow and lymphoid tissues.

Colony Stimulating Factors (CSFs)

• Chemical signals that stimulate specific WBC production.

Shared Characteristics of White Blood Cells (WBCs)

• All circulating WBCs can:

  • Leave the bloodstream (diapedesis):

    • Margination: Adherence to vessel wall.

    • Emigration: Movement through vessel wall into tissues.

  • Respond to chemical signals (chemotaxis).

  • Move through tissues using amoeboid movement.

  • Destroy threats via phagocytosis.

White Blood Cell Overview

• Have a nucleus

• Contain organelles

• Larger than RBCs

Main Roles

• Defense against pathogens

• Removal of toxins, wastes, and abnormal cells

WBC Counts

• Differential count measures number and types of WBCs.

• Leukopenia: Low WBC count.

• Leukocytosis: High WBC count.

• Leukemia: Uncontrolled production of abnormal WBCs.

Types of White Blood Cells

Type	Percent of WBCs	Main Function
Neutrophils	50–70%	First responders during infection; phagocytize bacteria; die after engulfing 10–20 bacteria; accumulation forms pus.
Eosinophils	2–4%	Target cells tagged by antibodies; defense against parasites; release cytotoxic substances.
Basophils	<1%	Involved in inflammation and allergic responses; release histamine and heparin; increase vessel permeability and blood flow.
Monocytes	3–8%	Largest WBC; circulate for ~24 hours; become macrophages in tissues; release signals for inflammation and tissue repair.
Lymphocytes	20–40%	Central to adaptive immunity; T cells (cell-mediated immunity), B cells (produce antibodies), NK cells (destroy infected or abnormal cells).

Platelets

• Platelets are cell fragments involved in hemostasis (blood clotting).

• Lifespan: 9–12 days.

• Spleen removes old platelets and stores approximately 1/3 of platelets.

Clinical Terms

• Thrombocytopenia: Low platelet count.

• Thrombocytosis: High platelet count.

Platelet Production

• Formed from megakaryocytes in bone marrow.

• Regulated by thrombopoietin.

• During inflammation, interleukin-6 increases platelet production.

Blood Typing

• Blood type is determined by antigens (agglutinogens) on RBC surfaces.

• Major antigens: A, B, Rh (positive or negative).

• Antibodies bind foreign antigens.

• Agglutination results in clumping of RBCs.

• Why agglutination is dangerous: Clumped RBCs can block blood vessels and cause hemolysis, leading to organ damage.

Blood Physiology – Erythropoietin (EPO)

• EPO is released when blood oxygen is low (hypoxia).

• Triggers include:

  • Anemia

  • Reduced kidney blood flow

  • Lung oxygen limitation

  • Damage to respiratory surfaces

• EPO effects:

  • Increases RBC precursor cell division

  • Speeds up RBC maturation

  • Increases hemoglobin concentration

• Clinical connection: Artificial EPO use = blood doping (can increase risk of clotting).

RBC Turnover and Hemoglobin Breakdown

• Old or damaged RBCs removed by:

  • Macrophages in liver

  • Spleen

  • Bone marrow

• Hemoglobin breakdown:

  • Globin → amino acids

  • Heme → iron + biliverdin

  • Iron transported by transferrin and recycled in bone marrow

• If hemoglobin is not phagocytized:

  • Hemoglobinuria = hemoglobin in urine

  • Hematuria = intact RBCs in urine

• Heme pigment pathway:

  • Heme without iron → biliverdin (green)

  • Converted to bilirubin (yellow)

  • Processed by the liver and excreted in bile

  • Converted in intestines to urobilin and stercobilin pigments

  • Some pigments excreted by kidneys

Hemostasis

• Hemostasis means stopping bleeding.

• Phases:

  1. Vascular phase

  2. Platelet phase

  3. Coagulation phase

• Fibrin forms a mesh that stabilizes the clot.

• Clot retraction pulls wound edges together.

• Control mechanisms:

  • Antithrombin

  • Vitamin K

  • Protein C

  • Anticoagulant drugs

• Clot removal: Fibrinolysis via plasmin.

Blood Disorders

Hemolytic Disease of the Newborn

• Occurs when Rh-negative mother carries Rh-positive fetus.

• Mother becomes sensitized after first exposure.

• In later pregnancies, anti-Rh antibodies cross placenta.

• Result: Destruction of fetal RBCs.

Leukocyte Disorders

• Leukemia: Cancer of WBCs; can be acute or chronic; myeloid or lymphoid origin.

• Mononucleosis: Caused by Epstein-Barr virus; results in excess lymphocytes.

Anemia

• Definition: Reduced oxygen delivery to tissues.

• Types:

  • Iron-deficiency anemia: Low iron

  • Renal anemia: Decreased EPO production

  • Aplastic anemia: Bone marrow damage or suppression

  • Sickle cell anemia: Abnormal hemoglobin disorder

Hemostatic Disorders

• Thrombus: Clot that remains in place.

• Embolus: Traveling clot fragment.

• Embolism: Blockage caused by embolus.

• Hemophilia: Inherited clotting factor deficiency.

Key Equations and Concepts

• Oxygen Carrying Capacity:

• $\text{O}_2\ \text{carrying capacity} = \text{Hemoglobin concentration} \times 1.34\ \text{mL O}_2/\text{g Hb}$

• Hematocrit:

• $\text{Hematocrit} = \frac{\text{Volume of RBCs}}{\text{Total blood volume}} \times 100\%$

Additional info: Equations and some clinical context were added for completeness.