Chapter 17: Blood – Structure, Function, and Disorders

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Blood Composition and Overview

Main Components of Blood

Blood is a specialized fluid connective tissue composed of plasma and formed elements. The formed elements include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets. Each component plays a distinct role in maintaining homeostasis and supporting physiological functions.

Plasma: The liquid matrix, primarily water, containing proteins, nutrients, electrolytes, and waste products.
Erythrocytes (RBCs): Responsible for oxygen transport.
Leukocytes (WBCs): Key players in immune defense.
Platelets: Essential for blood clotting (coagulation).

Microscopic view of blood cells: erythrocytes, leukocyte, and platelets Diagram of blood vessel showing plasma, red blood cells, white blood cells, and platelets

Hematocrit and Blood Volume

The hematocrit is the percentage of blood volume occupied by erythrocytes. Typical values are 47% ± 5% for males and 42% ± 5% for females. Leukocytes and platelets make up about 1%, with plasma constituting the remainder. Blood is a sticky, opaque fluid with a pH of 7.35–7.45 and accounts for approximately 8% of body weight.

Blood sample centrifugation showing plasma, buffy coat, and erythrocytes

Functions of Blood

Distribution

Blood transports oxygen and nutrients to cells, removes metabolic wastes, and delivers hormones from endocrine organs to target tissues.

Regulation

Body temperature: Blood absorbs and distributes heat.
pH balance: Maintained by buffers in plasma.
Fluid volume: Blood proteins help maintain osmotic pressure.

Protection

Against blood loss: Platelets and plasma proteins initiate clot formation.
Against infection: Antibodies, complement proteins, and WBCs defend against pathogens.

Blood Plasma

Composition

Plasma is 90% water and contains over 100 dissolved solutes, including plasma proteins (albumin, globulins, fibrinogen), metabolic by-products, nutrients, electrolytes, respiratory gases, and hormones.

Plasma bags showing yellow color of plasma

Formed Elements

Cellular Components

Only WBCs are complete cells; RBCs lack nuclei and organelles, and platelets are cell fragments. Most formed elements originate in bone marrow and have limited lifespans.

Blood smear showing platelets, erythrocytes, monocytes, neutrophils, and lymphocytes

Erythrocytes (Red Blood Cells)

Structure and Function

Erythrocytes are biconcave, anucleate discs filled with hemoglobin. Their shape increases surface area for gas exchange and allows flexibility. RBCs are the major contributors to blood viscosity.

Diagram of erythrocyte showing biconcave shape

Hemoglobin and Gas Transport

Hemoglobin (Hb) binds reversibly with oxygen. Each molecule consists of four globin chains (two alpha, two beta) and four heme groups, each containing an iron atom that binds one O2 molecule. Each RBC contains about 250 million Hb molecules.

O2 loading: Forms oxyhemoglobin (ruby red).
O2 unloading: Forms deoxyhemoglobin (dark red).
CO2 loading: Forms carbaminohemoglobin (carries 20% of CO2).

Hemoglobin structure showing globin chains and heme groups Flasks showing color difference between oxyhemoglobin and deoxyhemoglobin

Hematopoiesis

Blood Cell Formation

Hematopoiesis occurs in red bone marrow and produces all formed elements from hematopoietic stem cells (hemocytoblasts). Hormones and growth factors direct differentiation.

Sinusoid capillary diagram showing red bone marrow Erythropoiesis developmental pathway from stem cell to erythrocyte

Erythropoiesis Regulation

RBC production is regulated by erythropoietin (EPO), released by the kidneys in response to hypoxia. Adequate nutrients, iron, vitamin B12, and folic acid are required for erythropoiesis.

Homeostasis diagram showing regulation of erythropoiesis Reticulocyte blood smear

Fate and Destruction of Erythrocytes

Lifecycle and Breakdown

RBCs live 100–120 days. Old cells are engulfed by macrophages in the spleen, liver, and bone marrow. Hemoglobin is broken down: iron is salvaged, heme is degraded to bilirubin, and globin is metabolized to amino acids.

Diagram of erythrocyte breakdown and recycling

Erythrocyte Disorders

Anemia

Anemia is a condition of abnormally low O2-carrying capacity, resulting in fatigue, paleness, shortness of breath, and chills. Causes include insufficient erythrocytes, low hemoglobin content, and abnormal hemoglobin.

Pale appearance as a symptom of anemia Iron-rich foods for anemia prevention Normal vs. sickle-shaped erythrocytes in sickle-cell anemia

Polycythemia

Polycythemia is an excess of RBCs, increasing blood viscosity. It may result from bone marrow cancer, high altitude, increased EPO production, or blood doping.

Leukocytes (White Blood Cells)

Types and Functions

Leukocytes make up less than 1% of blood volume and are crucial for immune defense. They can leave capillaries via diapedesis and move through tissues by amoeboid motion and chemotaxis.

Blood smear showing leukocyte types

Granulocytes

Neutrophils: Most numerous, phagocytic, "bacteria slayers."
Eosinophils: Digest parasitic worms, modulate immune response.
Basophils: Release histamine, functionally similar to mast cells.

Neutrophil blood smear Eosinophil blood smear Basophil blood smear

Agranulocytes

Lymphocytes: Crucial to immunity; T cells attack infected cells, B cells produce antibodies.
Monocytes: Largest WBCs, differentiate into macrophages, phagocytic.

Lymphocyte blood smear Monocyte blood smear

Leukocyte Disorders

Leukopenia and Leukemia

Leukopenia is a low WBC count, often drug-induced. Leukemia is a cancerous condition involving abnormal WBC clones, leading to impaired immune function and increased risk of infection.

Platelets and Hemostasis

Platelet Structure and Function

Platelets are fragments of megakaryocytes, regulated by thrombopoietin. They form temporary plugs to seal vessel breaks and contain granules with clotting factors.

Platelet blood smear

Hemostasis

Hemostasis is the process of stopping bleeding, involving three steps: vascular spasm, platelet plug formation, and coagulation.

Vascular spasm: Vasoconstriction reduces blood loss.
Platelet plug formation: Platelets adhere to collagen, release chemicals to recruit more platelets.
Coagulation: Clotting factors transform blood from liquid to gel, reinforcing the plug with fibrin threads.

Platelet plug formation diagram Coagulation diagram showing fibrin mesh Microscopic view of blood clot with fibrin mesh

Coagulation Cascade

The coagulation cascade involves three phases:

Formation of prothrombin activator (intrinsic and extrinsic pathways).
Conversion of prothrombin to thrombin.
Thrombin catalyzes the joining of fibrinogen to form a fibrin mesh.

Phase	Key Event
1	Prothrombin activator formation
2	Prothrombin → Thrombin
3	Fibrinogen → Fibrin mesh

Blood Groups and Transfusions

ABO and Rh Blood Groups

Blood groups are determined by antigens (agglutinogens) on RBC membranes. The ABO system includes types A, B, AB, and O. The Rh system is based on the presence of Rh factors, especially the D antigen.

Transfusion reactions: Occur if mismatched blood is infused, leading to agglutination and hemolysis.
Hemolytic disease of the newborn: Rh– mother exposed to Rh+ blood may produce anti-Rh antibodies, affecting subsequent Rh+ pregnancies.

Blood transfusion bags

Summary Table: Blood Components

Component	Main Function	Key Features
Plasma	Transport, regulation	90% water, proteins, solutes
Erythrocytes	Oxygen transport	Biconcave, hemoglobin-rich
Leukocytes	Immune defense	Granulocytes & agranulocytes
Platelets	Clotting	Cell fragments, clotting factors

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