Chapter 18: Blood – Structure, Function, and Formation

Blood: Structure, Function, and Formation

Overview of Blood and the Circulatory System

Blood is a specialized connective tissue that serves as the transport medium for nutrients, gases, hormones, and waste products throughout the body. It is a vital component of the circulatory system, which is subdivided into the cardiovascular system (blood, heart, blood vessels) and the lymphatic system (vessels returning tissue fluid to blood).

• Cardiovascular system: Includes blood, heart, and blood vessels.

• Lymphatic system: Returns tissue fluid to the blood.

• Blood functions: Transports nutrients, signaling molecules, hormones, respiratory gases, and waste products.

• Blood circulation: Powered by the heart, blood leaves the heart in arteries, branches into capillaries, and returns via veins.

• Regulation: Blood helps regulate body temperature and conveys immune system cells.

Composition of Blood

Blood consists of cellular and liquid components. The cellular portion is called the formed elements, and the liquid portion is plasma.

• Blood volume: ~8% of body mass (Males: 5–6 L, Females: 4–5 L).

• Formed elements: Erythrocytes (RBCs), leukocytes (WBCs), and platelets.

• Plasma: Straw-colored, sticky fluid, ~90% water, contains ions, nutrients, wastes, and proteins.

Hematocrit and Buffy Coat

Hematocrit is the percentage of blood volume occupied by red blood cells. The buffy coat is the thin layer between plasma and RBCs, containing leukocytes and platelets.

• Hematocrit: Males: 47% ± 5%, Females: 42% ± 5%.

• Buffy coat: Contains leukocytes and platelets, part of immune and injury response systems.

Blood Plasma: Components and Functions

Blood plasma is the liquid matrix of blood, containing water, ions, nutrients, wastes, and proteins.

• Ions: Na+, Cl-, etc.

• Nutrients: Simple sugars, amino acids, lipids.

• Wastes: Carbon dioxide, urea, ammonia.

• Proteins:

  • Albumin: Maintains osmotic pressure, prevents water loss from blood vessels.

  • Globulins: Include antibodies and transport proteins for lipids, iron, copper.

  • Fibrinogen: Involved in blood clotting reactions.

• Serum: Plasma without clotting factors, produced after coagulation.

Formed Elements of Blood

The formed elements include erythrocytes, leukocytes, and platelets. Most cannot divide and have specialized functions.

• Erythrocytes: Lack nuclei and organelles; specialized for oxygen transport.

• Leukocytes: Several types; function in immune defense.

• Platelets: Cell fragments involved in clotting.

Blood Smears and Staining

Blood cells are visualized using blood smears and specific stains:

• Acidic dye (eosin): Stains pink.

• Basic dye (methylene blue): Stains blue and purple.

Erythrocytes (Red Blood Cells)

Structure and Function

Erythrocytes are small, biconcave cells specialized for oxygen transport. Their shape increases surface area for gas exchange and flexibility for capillary passage.

• Diameter: ~7.5 μm.

• Most numerous formed element: Females: 4.3–5.2 million/mm3; Males: 5.1–5.8 million/mm3.

• No organelles or nuclei.

• Hemoglobin (Hb): Oxygen-carrying protein; each molecule has four chains, each with an iron atom binding O2.

• Color: Oxidation of iron gives blood its red color.

• Gas exchange: Pick up O2 at lungs, release O2 at tissues; also transport CO2.

• Lack mitochondria: Do not consume O2 they transport.

Leukocytes (White Blood Cells)

General Features

Leukocytes are less numerous than erythrocytes but are essential for immune defense. They function mainly outside the bloodstream in connective tissues.

• Diapedesis: Movement of leukocytes through capillary walls to infection sites.

• Originate in bone marrow.

Classification of Leukocytes

Leukocytes are classified into five types, grouped by the presence of cytoplasmic granules:

• Granulocytes: Neutrophils, eosinophils, basophils.

• Agranulocytes: Lymphocytes, monocytes.

Relative abundance mnemonic: Never Let Monkeys Eat Bananas (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils).

Granulocytes

• Neutrophils: Most abundant (~60%). Multilobed nucleus, granules with digestive enzymes. First line of defense; phagocytize bacteria and release enzymes. Pus consists of dead neutrophils and debris.

• Eosinophils: 1–4% of WBCs. Bilobed nucleus, red granules. Enzymes active in allergic reactions and parasitic infections; degrade histamines and attach to parasites.

• Basophils: Rarest (~0.5%). Purplish-black granules. Contain histamines and inflammatory mediators; function in inflammation and parasitic infection, similar to mast cells.

Agranulocytes

• Lymphocytes: 20–45% of WBCs. Large nucleus, thin rim of cytoplasm. Most important immune cells; found in lymphoid tissues. Two main classes:

  • T cells: Attack foreign cells directly.

  • B cells: Produce antibodies that bind antigens for destruction by macrophages.

• Monocytes: 4–8% of WBCs. Largest leukocytes, kidney-shaped nucleus. Transform into macrophages in tissues; phagocytize foreign cells and debris.

Thrombocytes (Platelets)

Structure and Role in Clotting

Platelets are disc-shaped cell fragments derived from megakaryocytes. They are essential for blood clotting and repair of vessel walls.

• Plug small tears in vessel walls to limit bleeding.

• Initiate blood clotting: Sequence of reactions generates a network of fibrin strands among platelets.

Table: Summary of Formed Elements of the Blood

Cell Type	Description	Number per mm3	Life Span	Function
Erythrocytes	Biconcave, no nucleus	4–6 million	100–120 days	Transport oxygen and carbon dioxide
Neutrophils	Multilobed nucleus, pale granules	3000–7000	6 hours–few days	Phagocytize bacteria
Eosinophils	Bilobed nucleus, red granules	100–400	5 days	End allergic reactions, destroy parasites
Basophils	Bilobed nucleus, purplish granules	20–50	Few hours–few days	Release histamine, mediate inflammation
Lymphocytes	Large nucleus, thin rim of cytoplasm	1500–3000	Hours–years	Mount immune response
Monocytes	Kidney-shaped nucleus	100–700	Months	Phagocytosis, develop into macrophages
Platelets	Cell fragments	150,000–500,000	5–10 days	Seal small tears, blood clotting

Blood Cell Formation (Hematopoiesis)

Process and Sites

Hematopoiesis is the process by which blood cells are formed, beginning in the embryo and continuing throughout life. All blood cells originate in the bone marrow.

• ~100 billion new blood cells formed daily.

• Bone marrow: Primary site of hematopoiesis.

Bone Marrow Types

• Yellow marrow: Contains fat cells, located in medullary cavity of long bones; dormant, produces blood cells only in emergencies.

• Red marrow: Contains immature erythrocytes, actively generates new blood cells; found in spongy bone of axial skeleton, pectoral/hip girdles, proximal epiphyses of humerus and femur.

Bone Marrow Structure and Function

• Reticular connective tissue: Contains fat cells and developing blood cells.

• Blood sinusoids: Wide capillaries where mature blood cells enter the bloodstream.

• Macrophages: Capture antigens in blood for 'blood-cleaning' function.

• Mesenchymal stem cells: Can differentiate into fat cells, osteoblasts, chondrocytes, fibroblasts, and muscle cells.

Cell Lines in Blood Cell Formation

All blood cells originate from a hemopoietic blood stem cell (pluripotent hematopoietic stem cell), which produces two main lines of progenitor cells:

• Lymphoid stem cells: Give rise to lymphocytes.

• Myeloid stem cells: Give rise to all other leukocytes and erythrocytes.

Formation of Specific Blood Cells

• Erythrocytes: Committed cells are proerythroblasts; enter bloodstream as reticulocytes, which mature in 1–2 days and accumulate iron for hemoglobin production.

• Non-lymphocyte leukocytes: Myeloblasts differentiate into granular leukocytes; monoblasts become monocytes/macrophages; megakaryoblasts become megakaryocytes, which break apart into platelets.

Key Equations and Definitions

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

• Hemoglobin binding: Each hemoglobin molecule can bind up to four oxygen molecules.

Additional info:

• Blood disorders, such as anemia, polycythemia, sickle cell disease, leukemia, and thrombocytopenia, affect the formed elements and can have significant clinical consequences.

• Blood cell formation is tightly regulated to maintain homeostasis and respond to physiological needs.