Blood: Structure, Function, and Clinical Relevance

Blood: Structure, Function, and Clinical Relevance

Overview of Blood Functions

Blood is a specialized connective tissue essential for transporting substances, regulating physiological parameters, and protecting the body from disease and blood loss.

• Transport: Delivers oxygen, nutrients, hormones, and removes metabolic wastes.

• Regulation: Maintains body temperature, pH balance, and fluid volume.

• Protection: Prevents blood loss via clotting and combats infection through immune cells.

Blood Components

Blood consists of plasma (liquid matrix) and formed elements (cells and cell fragments). Centrifugation separates these components for analysis.

• Plasma: 55% of blood; contains water, proteins, nutrients, hormones, and waste products.

• Buffy Coat: <1% of blood; contains leukocytes (white blood cells) and platelets.

• Erythrocytes: 45% of blood; red blood cells responsible for gas transport.

Physical Characteristics of Blood

Blood is slightly alkaline (pH 7.35–7.45), more viscous than water, and its color varies with oxygenation.

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

• Hematocrit: Percentage of blood volume occupied by erythrocytes; normal values differ by gender.

Plasma

Composition and Function

Plasma is the clear, straw-colored liquid portion of blood, serving as the medium for transporting nutrients, hormones, and waste products.

Constituent	Description and Importance
Water	90% of plasma volume; dissolving and suspending medium for solutes of blood; absorbs heat
Electrolytes	Most abundant solutes; help maintain plasma osmotic pressure and normal blood pH
Plasma proteins	8% of plasma; includes albumin (osmotic pressure), globulins (transport, antibodies), and fibrinogen (clotting)
Nonprotein nitrogenous substances	By-products of metabolism (urea, uric acid, creatinine)
Nutrients	Glucose, amino acids, fatty acids, vitamins, etc.
Respiratory gases	Oxygen and carbon dioxide
Hormones	Steroid and thyroid hormones carried by plasma proteins

Transport of Lipids in Plasma

Lipids are insoluble in water and are transported in plasma bound to soluble plasma proteins, forming lipoproteins.

Formed Elements

Overview

Formed elements include erythrocytes, leukocytes, and platelets. Most are short-lived and originate from stem cells in the bone marrow.

Erythrocytes (Red Blood Cells, RBCs)

Erythrocytes are specialized for oxygen and carbon dioxide transport. They are biconcave, anucleate cells packed with hemoglobin.

• Shape: Biconcave disc increases surface area for gas exchange and flexibility for capillary passage.

• Hemoglobin: Each RBC contains ~250 million hemoglobin molecules, each capable of binding four O2 molecules.

• Oxyhemoglobin: Hemoglobin bound to oxygen (bright red).

• Deoxyhemoglobin: Hemoglobin after oxygen release (dark red).

• Carbaminohemoglobin: Hemoglobin bound to CO2.

Regulation of Erythropoiesis

Red blood cell production is regulated by erythropoietin (EPO), primarily produced by the kidneys in response to hypoxia (low oxygen levels).

• Essential nutrients: Iron, copper, zinc, folate, and vitamin B12 are required for erythropoiesis.

• Disorders: Polycythemia (excess RBCs), anemia (deficiency in RBCs or hemoglobin).

RBC Life Cycle and Breakdown

RBCs have a lifespan of about 120 days. Old or damaged RBCs are phagocytized in the liver, spleen, and bone marrow. Components are recycled or excreted.

• Globin: Broken down into amino acids.

• Iron: Stored as ferritin or hemosiderin, transported by transferrin.

• Heme: Converted to biliverdin, then bilirubin, and excreted in bile.

RBC Disorders

• Anemia: Reduced oxygen-carrying capacity due to blood loss, decreased production, or increased destruction of RBCs.

• Polycythemia: Excess RBCs increase blood viscosity.

• Hemolytic, sickle cell, and thalassemia: Genetic or acquired disorders affecting RBC structure or function.

Leukocytes (White Blood Cells, WBCs)

Types and Functions

Leukocytes are the only complete cells among the formed elements and are crucial for immune defense. They are classified as granulocytes or agranulocytes based on the presence of cytoplasmic granules.

• Granulocytes: Neutrophils, eosinophils, basophils

• Agranulocytes: Lymphocytes, monocytes

• Neutrophils: Phagocytize bacteria; most abundant WBC.

• Eosinophils: Combat parasitic infections and modulate allergic responses.

• Basophils: Release histamine and heparin; involved in inflammation and allergic reactions.

• Lymphocytes: B cells (antibody production), T cells (cell-mediated immunity), and natural killer cells.

• Monocytes: Differentiate into macrophages for phagocytosis of pathogens and debris.

Leukopoiesis and Diapedesis

Leukocytes originate from hematopoietic stem cells in the bone marrow. They can exit the bloodstream via diapedesis to reach sites of infection or injury.

Platelets (Thrombocytes)

Structure and Function

Platelets are cell fragments derived from megakaryocytes. They play a critical role in hemostasis by forming plugs at sites of vessel injury and releasing factors that promote clotting and tissue repair.

• Normal count: 150,000–450,000/µL

• Lifespan: ~10 days

Hemostasis

Steps of Hemostasis

Hemostasis is the process of stopping blood loss from damaged vessels. It involves a series of tightly regulated steps:

1. Vascular Spasm: Vasoconstriction reduces blood flow.

2. Platelet Plug Formation: Platelets adhere to exposed collagen and aggregate.

3. Coagulation: Cascade of clotting factors leads to fibrin mesh formation.

4. Clot Retraction and Fibrinolysis: Clot contracts and is eventually dissolved.

Clotting Disorders

• Hemophilia: Genetic deficiency of clotting factors (e.g., factor VIII in Hemophilia A).

• Thrombocytopenia: Low platelet count, leading to excessive bleeding and petechiae.

• Thrombosis/Embolism: Excessive clot formation or dislodged clots causing vessel blockage.

Blood Groups and Transfusion

ABO Blood Group System

The ABO system classifies blood based on the presence of A and/or B antigens on erythrocytes. Mixing incompatible blood types causes agglutination (clumping) and potentially dangerous transfusion reactions.

Rh Blood Group System

The Rh system is based on the presence (Rh+) or absence (Rh−) of the D antigen. Rh incompatibility can cause hemolytic disease of the newborn if an Rh− mother carries an Rh+ fetus.

Summary Table: ABO and Rh Blood Types

Blood Type	Antigens on RBC	Antibodies in Plasma	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	A, B, AB, O	AB
O	None	Anti-A, Anti-B	O	A, B, AB, O

Additional info: Rh+ can receive Rh− blood once, but subsequent exposures may cause reactions due to antibody formation.