Chapter 17: Blood – Structure, Function, and Disorders

Chapter 17: Blood

Introduction

Blood is a vital connective tissue that plays essential roles in transport, defense, and homeostasis. This chapter covers the composition of blood, the function and classification of white blood cells, the process of hemostasis (clotting), and common blood disorders.

Blood Groups and Hemolytic Disease of the Newborn (HDN)

Rh Factor and Pregnancy

The Rh factor is an antigen present on red blood cells (RBCs). If an Rh-negative mother carries an Rh-positive fetus, her immune system may produce anti-Rh antibodies after exposure to fetal blood, especially during delivery. In subsequent pregnancies, these antibodies can cross the placenta and attack fetal RBCs, causing hemolytic disease of the newborn (HDN).

• HDN Symptoms: Fetal anemia (low RBC count), jaundice (high bilirubin from destroyed RBCs).

• ABO incompatibility: Maternal anti-A or anti-B antibodies do not cross the placenta, so ABO incompatibility is less dangerous than Rh incompatibility.

Prevention: RhoGAM

• RhoGAM is an injection containing anti-Rh antibodies.

• Given to Rh-negative mothers during and after pregnancy if the fetus/father is Rh-positive.

• RhoGAM destroys fetal RBCs in maternal circulation before the mother’s immune system can react, preventing the formation of maternal anti-Rh antibodies.

• FDA approved RhoGAM in 1968; it has nearly eliminated HDN-related mortality in developed countries.

• Saves over 10,000 babies annually in the US.

White Blood Cells (Leukocytes)

Classification of White Blood Cells

White blood cells (WBCs) are crucial for immune defense. They are classified based on the presence or absence of cytoplasmic granules:

Class	Cell Type	Main Function
Granular Leukocytes	Neutrophils	General phagocytes; first responders to infection
Granular Leukocytes	Eosinophils	Phagocytize allergens and parasites; reduce inflammation
Granular Leukocytes	Basophils	Release histamine to increase inflammation
Agranular Leukocytes	Monocytes	Become macrophages; phagocytize pathogens and debris
Agranular Leukocytes	Lymphocytes	Defend against specific pathogens and toxins (adaptive immunity)

White Blood Cell Functions

• Neutrophils: Most abundant; general phagocytes for bacteria and debris.

• Eosinophils: Combat multicellular parasites and modulate allergic responses.

• Basophils: Release histamine and heparin; promote inflammation.

• Monocytes: Differentiate into macrophages in tissues; engulf pathogens and dead cells.

• Lymphocytes: Include B cells (produce antibodies), T cells (cell-mediated immunity), and NK cells (destroy abnormal cells).

WBC Movement and Response

• WBCs migrate through connective tissues, using the bloodstream for rapid transport.

• They exit blood vessels by diapedesis (squeezing between endothelial cells).

• WBCs are attracted to sites of infection or injury by chemotaxis (movement toward chemical signals).

• Phagocytic WBCs (neutrophils, eosinophils, monocytes) engulf pathogens or debris (phagocytosis).

WBC Differential Count

• Measures the percentage of each type of WBC in a blood smear.

• Changes in WBC proportions can indicate infection, inflammation, or disease.

• Example: Elevated eosinophils suggest allergies or parasitic infection; increased lymphocytes may indicate viral infection.

Hemostasis: The Clotting Response

Overview

Hemostasis is the process that stops bleeding and repairs damaged blood vessels. It involves three overlapping phases:

1. Vascular Phase: Vasoconstriction reduces blood flow; endothelial cells release chemicals to promote repair and make surfaces sticky for platelets.

2. Platelet Phase: Platelets adhere to damaged endothelium and each other, forming a platelet plug. They release chemicals (ADP, Ca2+, platelet factors) to recruit more platelets and promote clotting.

3. Coagulation Phase: A cascade of clotting factors converts fibrinogen to fibrin, forming a stable mesh that traps blood cells and seals the vessel.

Key Steps and Factors

• ADP: Stimulates platelet aggregation and secretion.

• Calcium ions (Ca2+): Essential for platelet aggregation and activation of clotting factors.

• Clotting factors: Most are proteins produced by the liver; some require vitamin K for synthesis.

• Thrombin: Enzyme that converts fibrinogen to fibrin.

Coagulation Cascade (Simplified):

• Factor X activates thrombin.

• Thrombin converts fibrinogen to fibrin.

• Fibrin forms a mesh that stabilizes the clot.

• Positive feedback: Thrombin stimulates more clotting factor release.

Equation:

$\text{Fibrinogen} \xrightarrow{\text{Thrombin}} \text{Fibrin}$

Clot Retraction and Healing

• Platelets contract, pulling the edges of the wound together (clot retraction).

• Clot is gradually dissolved by enzymes as tissue repair completes.

Blood Disorders

Nutritional Blood Disorders

• Iron deficiency anemia: Low dietary iron reduces hemoglobin synthesis; RBCs are microcytic (small) and hypochromic (pale).

• Pernicious anemia: Vitamin B12 deficiency (or lack of intrinsic factor for absorption) impairs RBC production; RBCs are macrocytic (large) and may be misshapen.

• Clotting issues: Require adequate Ca2+ and vitamin K for synthesis of clotting factors.

Congenital Blood Disorders

• Sickle cell disease (SCD): Inherited disorder causing abnormal hemoglobin (HbS); RBCs become sickle-shaped, fragile, and can block capillaries.

• Sickle cell trait: Heterozygous carriers have some resistance to malaria, as infected RBCs sickle and are destroyed by macrophages.

• Hemophilia: X-linked disorder with reduced production of clotting factors; leads to excessive bleeding.

Blood Infections

• Malaria: Caused by Plasmodium parasites, transmitted by mosquitoes. Parasites infect liver and RBCs, causing cycles of fever and chills as RBCs rupture.

• Sepsis (Septicemia): Pathogens multiply in blood, spreading infection throughout the body.

• Bacteremia: Bacteria in the blood.

• Viremia: Viruses in the blood.

Malaria Vaccine: Approved in 2015/2021, reduces infections by 75%, requires boosters, and is being distributed in Africa.

Blood Cell Cancers

• Leukemias: Cancers of blood-forming tissues (red bone marrow), leading to abnormal proliferation of WBCs.

• Lymphoid leukemia: Affects lymphocytes.

• Myeloid leukemia: Affects neutrophils, eosinophils, basophils, or monocytes.

Venipuncture

• Common method for drawing blood, usually from the median cubital vein in the antecubital region.

• Veins are preferred due to thin walls and low pressure, making them easy to access and heal.