Hematology: Structure and Function of Blood

Blood: Structure and Function

Overview of Blood

• Blood is a specialized fluid connective tissue formed in the bone marrow.

• It consists of two main components:

  • Plasma: The non-living liquid matrix.

  • Formed elements: The living cells suspended in plasma, including erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

• Whole blood = plasma + formed elements.

• Serum = plasma minus clotting proteins.

Physical Characteristics of Blood

• Color varies with oxygen content: scarlet (oxygen-rich) to dark red (oxygen-poor).

• Viscosity: 4–5.5 times more viscous than water due to formed elements.

• pH: Slightly alkaline, 7.35–7.45.

• Temperature: 38°C (100.4°F), slightly higher than body temperature.

• Volume: 5–6 L in males, 4–5 L in females (about 8% of body weight).

Functions of Blood

• Distribution:

  • Transports nutrients, oxygen, wastes, and hormones.

• Regulation:

  • Maintains body temperature, pH, fluid volume, and electrolyte balance.

• Protection:

  • Prevents blood loss (clotting) and infection (immune cells and proteins).

• Homeostasis: Maintains stable internal conditions for cells.

Blood Composition

Blood Plasma

• Accounts for ~55% of blood volume; ~91% water.

• Contains dissolved solutes: proteins, nutrients, electrolytes, gases, and wastes.

• Major plasma proteins:

  • Albumin (60%): Maintains osmotic pressure; transports substances.

  • Globulins (35%): Transport proteins and antibodies (immunoglobulins).

  • Clotting proteins: Fibrinogen, prothrombin (produced in liver).

  • Other proteins: Enzymes, hormones, immune proteins.

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

• Electrolytes: Ions such as Ca2+, Na+, K+.

• Respiratory gases: Dissolved O2 and CO2.

• Wastes: Urea, uric acid, lactic acid.

Formed Elements

• Make up ~45% of blood volume.

• Include red blood cells (RBCs), white blood cells (WBCs), and platelets.

• Hematopoiesis: Formation of blood cells (in red bone marrow after birth).

Red Blood Cells (Erythrocytes)

Structure and Function

• Biconcave disks (~7.5 μm diameter) for increased surface area and flexibility.

• Lack nuclei and most organelles at maturity; cannot divide or synthesize proteins.

• Lifespan: ~120 days.

• Contain hemoglobin for O2 and CO2 transport.

Hemoglobin (Hb)

• Protein with four polypeptide chains, each with a heme group (contains iron).

• Each Hb molecule binds up to four O2 molecules.

• O2 binding forms oxyhemoglobin (HbO2); release forms deoxyhemoglobin.

• ~20% of CO2 binds to globin portion (carbaminohemoglobin).

RBC Production (Erythropoiesis)

• Occurs in red bone marrow; stimulated by erythropoietin (from kidneys) in response to low O2.

• Requires vitamin B12, iron, and folic acid.

• Negative feedback regulates production.

Destruction of RBCs

• Old RBCs are destroyed by the reticuloendothelial system (mainly spleen and liver).

• Hemoglobin is broken down:

  • Iron is recycled (stored as ferritin/hemosiderin in liver).

  • Heme is converted to biliverdin, then bilirubin (excreted in bile).

  • Globin is broken down to amino acids.

Hematocrit

• Percentage of blood volume occupied by RBCs.

• Normal: Males 40–54%, Females 37–47%.

• Increased by dehydration or erythropoietin; decreased by bleeding or RBC production problems.

Anemia

• Low RBC count or reduced Hb content; impairs O2 delivery.

• Symptoms: Weakness, lethargy, confusion.

• Sickle cell anemia: Genetic disorder causing abnormal Hb and sickle-shaped RBCs; confers malaria resistance.

• Pernicious anemia: Due to vitamin B12 deficiency or absorption issues.

White Blood Cells (Leukocytes)

General Features

• Defend against pathogens; only formed elements with nuclei and organelles.

• Formed in red bone marrow; can migrate out of bloodstream (diapedesis).

• Capable of amoeboid movement and positive chemotaxis.

• Classified as granulocytes or agranulocytes.

Granulocytes

• Neutrophils (65%): Phagocytic, first responders to infection, multi-lobed nucleus.

• Eosinophils (3%): Bilobed nucleus, combat parasites, involved in allergic responses.

• Basophils (1%): Release histamine (inflammation) and heparin (anticoagulant).

Agranulocytes

• Lymphocytes (30%):

  • B cells: Humoral immunity (antibody production).

  • T cells: Cell-mediated immunity (helper, cytotoxic, suppressor functions).

  • Natural Killer (NK) cells: Immune surveillance, destroy abnormal cells.

• Monocytes: Differentiate into macrophages; phagocytic, antigen-presenting cells.

Platelets (Thrombocytes)

• Small, anucleate cell fragments derived from megakaryocytes.

• Essential for blood clotting (hemostasis); lifespan ~10 days.

• Functions:

  • Release clotting chemicals.

  • Form temporary platelet plugs.

  • Secrete serotonin (vasoconstriction).

Hemostasis (Prevention of Blood Loss)

Phases of Hemostasis

• Vascular phase: Vasoconstriction and vessel spasm after injury.

• Platelet phase: Platelet adhesion, aggregation, and plug formation.

• Coagulation phase: Formation of fibrin clot via coagulation cascade.

Coagulation Pathways

• Extrinsic pathway: Triggered by tissue damage; involves tissue factor (factor III).

• Intrinsic pathway: Triggered by blood contact with collagen; involves factor XII.

• Common pathway: Both pathways activate factor X, leading to conversion of prothrombin to thrombin, and fibrinogen to fibrin.

Key Equations:

• Prothrombinase formation: $\text{Prothrombinase} + \text{Prothrombin} \xrightarrow{Ca^{2+}} \text{Thrombin}$

• Fibrin formation: $\text{Thrombin} + \text{Fibrinogen} \xrightarrow{Ca^{2+}} \text{Fibrin}$

Clot Retraction and Fibrinolysis

• Clot retraction: Platelets contract to reduce clot size (30–60 min).

• Fibrinolysis: Plasminogen is activated to plasmin, which digests fibrin and dissolves the clot.

Factors Affecting Coagulation

• Promoted by rough endothelium, blood pooling, thromboplastin.

• Inhibited by aspirin, vitamin K deficiency, liver disease, low calcium, anticoagulant drugs, low platelet count, hemophilia.

Blood Cell Counts

Cell Type	Normal Range (Male)	Normal Range (Female)	Clinical Terms
RBC	4.5–6.0 × 106/mm3	4.2–5.5 × 106/mm3	Polycythemia (>6.0 × 106), Anemia (<4.0 × 106)
WBC	5.0 × 103 – 1.1 × 104/mm3		Leukocytosis (>1.1 × 104), Leukopenia (<5.0 × 103)
Platelets	1.5 × 105 – 3.6 × 105/mm3		Thrombocytopenia (<1.5 × 105)

Blood Types and Transfusion

ABO Blood Group System

• Agglutinogens: Antigens on RBC membrane (A, B).

• Agglutinins: Antibodies in plasma (anti-A, anti-B).

• Blood type determined by presence/absence of A and B agglutinogens.

• Genotypes: A, B, AB, O (gene on chromosome 9).

Blood Type	Agglutinogen(s)	Agglutinin(s)	Can Receive From	Can Donate To
A	A	Anti-B	A, O	A, AB
B	B	Anti-A	B, O	B, AB
AB	A, B	None	All types	AB
O	None	Anti-A, Anti-B	O	All types

Type AB: Universal recipient Type O: Universal donor

Rh Blood Group

• Rh factor (D antigen) on RBC membrane; gene on chromosome 1.

• Rh+ if any Rh antigen present; Rh– if absent.

• Anti-Rh antibodies form only after exposure (e.g., transfusion, pregnancy).

Erythroblastosis Fetalis (Hemolytic Disease of the Newborn)

• Occurs when Rh– mother carries Rh+ fetus.

• First pregnancy usually unaffected; subsequent pregnancies at risk if mother is sensitized.

• Maternal anti-Rh antibodies cross placenta, destroy fetal RBCs, causing anemia, jaundice, or more severe effects.

Summary Table: Major Blood Properties

Property	Value/Range
Plasma % of blood	55%
Formed elements %	45%
Blood pH	7.35–7.45
Osmolarity	280–300 mOsm/L
Mean salinity	0.85%
Hemoglobin (male)	13–18 g/100 mL
Hemoglobin (female)	12–16 g/100 mL

Example: Blood Transfusion Compatibility

• A person with type A blood (anti-B antibodies) must not receive type B or AB blood.

• Type AB individuals can receive any blood type (universal recipient).

• Type O individuals can donate to any blood type (universal donor).

Additional info: This guide expands on the original notes with definitions, clinical context, and summary tables for exam preparation.