Blood: Composition, Formed Elements, and Hematopoiesis

Detailed Notes

1.1 Blood as a Connective Tissue

Blood is a specialized connective tissue composed of cells and cell fragments suspended in a nonliving fluid extracellular matrix called plasma. Unlike other connective tissues, blood's extracellular matrix lacks insoluble protein fibers (collagen, elastin) under normal conditions; instead, the fibrous protein fibrinogen remains dissolved in plasma and only polymerizes into visible fibers during the clotting cascade.

The three general components of blood, obtainable by centrifugation of a whole-blood sample, are:

1. Plasma — the straw-colored liquid supernatant, roughly 55% of whole-blood volume.
2. Buffy coat — a thin whitish layer between plasma and red cells, containing leukocytes and platelets, less than 1% of volume.
3. Erythrocytes (red blood cells) — the dense red pellet at the bottom, roughly 45% of volume, a fraction known as the hematocrit.

1.2 Physical Characteristics and Functions of Blood

Physical characteristics of normal adult blood include a temperature of about 38 °C (slightly above core body temperature), a pH range of 7.35–7.45 (slightly alkaline), and a viscosity approximately five times that of water owing to the abundance of formed elements and plasma proteins. Total blood volume averages 5–6 L in an adult male and 4–5 L in an adult female.

Blood performs three broad functions:

• Transport: delivers oxygen, nutrients, hormones, and metabolic heat; carries carbon dioxide and nitrogenous wastes to the lungs and kidneys for elimination.
• Regulation: maintains body temperature (by distributing heat), normal pH (via buffer systems), and fluid balance (through osmotic effects of plasma proteins).
• Protection: prevents blood loss through hemostasis (platelet plug and coagulation) and combats infection through leukocytes and circulating antibodies.

1.3 Plasma Composition

Plasma is approximately 92% water by volume, making it an excellent solvent and suspension medium. The remaining 8% consists of dissolved solutes, which fall into three major groups:

1. Plasma proteins (7% of plasma by weight) — the most abundant solutes by mass. Synthesized primarily in the liver (except gamma globulins, which are produced by plasma cells):
   • Albumin (∼60% of plasma proteins): the major contributor to colloid osmotic pressure (oncotic pressure), which holds water inside capillaries by opposing hydrostatic pressure. Albumin also serves as a nonspecific carrier protein, binding and transporting various hydrophobic molecules including fatty acids, steroid hormones, and many drugs.
   • Globulins (∼36%): divided into alpha and beta globulins (carrier proteins synthesized by the liver, transporting lipids, metal ions, and fat-soluble vitamins) and gamma globulins (immunoglobulins / antibodies produced by plasma cells as part of the adaptive immune response).
   • Fibrinogen (∼4%): the soluble precursor of fibrin, an insoluble protein that forms the meshwork of a blood clot. During coagulation, thrombin cleaves fibrinogen into fibrin monomers that polymerize and cross-link.

1. Other organic solutes (~1%): includes nonprotein nitrogenous substances (urea, uric acid, creatinine, ammonia), nutrients (glucose, amino acids, lipids, vitamins), dissolved gases (O₂, CO₂), and hormones.

1. Electrolytes / inorganic ions (~1%): include Na⁺ (most abundant cation), K⁺, Ca²⁺, Cl⁻ (most abundant anion), HCO₃⁻, and HPO₄²⁻. These ions are critical for maintaining osmotic balance, membrane potentials, and enzyme activity.

Serum is plasma from which the clotting proteins (notably fibrinogen) have been removed — it is the liquid remaining after blood has clotted, whereas plasma is prepared by centrifuging blood that has been treated with an anticoagulant.

1.4 Formed Elements Overview

The formed elements are the cellular and cell-fragment components of blood. All derive from hematopoietic stem cells in red bone marrow and are broadly grouped into three classes:

1.5 Erythrocytes (Red Blood Cells)

1.5.1 Structure

Erythrocytes are the most numerous formed elements. Each mature erythrocyte is a small, biconcave disc — a flattened shape with a central depression on both sides — and is anucleate (lacks a nucleus) as well as devoid of most organelles, including mitochondria and ribosomes. The biconcave shape provides three functional advantages:

• It increases the surface-area-to-volume ratio, enhancing the rate of gas exchange.
• It allows the cell to deform (flex and bend) as it squeezes through narrow capillaries.
• It enables the formation of rouleaux (stacks resembling coin rolls) that streamline flow in larger vessels.

Because mature erythrocytes lack mitochondria, they generate ATP exclusively through anaerobic glycolysis, which ensures they never consume the oxygen they are transporting.

1.5.2 Hemoglobin

The cytoplasm of an erythrocyte is packed almost entirely with hemoglobin (Hb), a conjugated protein responsible for its red color. Each hemoglobin molecule is a tetramer composed of four polypeptide globin chains (two alpha and two beta chains in adult HbA), each enclosing a heme group. Each heme group contains one iron (Fe²⁺) ion that can reversibly bind one molecule of O₂, so a single hemoglobin molecule can carry up to four O₂ molecules.

Hemoglobin also transports roughly 20–23% of the CO₂ produced by tissues. CO₂ binds directly to the amino groups of the globin chains (not to the heme iron), forming carbaminohemoglobin. The remainder of CO₂ is transported in plasma as bicarbonate ions (HCO₃⁻).

Oxyhemoglobin (HbO₂) is the bright-red form carrying oxygen; deoxyhemoglobin (HHb) is the darker, purplish-red form after releasing oxygen. Arterial blood is bright red; venous blood is darker red—not blue. Veins appear blue through skin due to optical properties of light scattering through tissue, not their actual color.

1.5.3 Erythropoiesis

Erythropoiesis, the production of new erythrocytes, occurs in the red bone marrow of the axial skeleton, pectoral and pelvic girdles, and proximal epiphyses of the humerus and femur in adults. The process takes roughly 3–5 days from stem cell to reticulocyte.

The primary hormonal regulator is erythropoietin (EPO), a glycoprotein hormone produced mainly by the kidneys (∼90%, with ∼10% from the liver). When renal interstitial fibroblasts sense tissue hypoxia (low O₂), they increase EPO secretion. EPO stimulates the proliferation and differentiation of committed erythroid progenitor cells in the bone marrow. The key steps include:

1. Decreased O₂ delivery to kidneys (hypoxia) — triggered by anemia, high altitude, lung disease, or blood loss.
2. Kidney releases EPO into the bloodstream.
3. EPO stimulates proerythroblasts in red bone marrow, accelerating their division and maturation.
4. Within days, reticulocytes (immature RBCs that still contain some ribosomal remnants) are released into the circulation, where they mature into fully functional erythrocytes within 1–2 days.
5. RBC count rises, O₂-carrying capacity of blood increases, relieving the original hypoxic stimulus (negative feedback).

Dietary requirements for normal erythropoiesis include iron (for heme), vitamin B₁₂ and folic acid (for DNA synthesis during rapid cell division), copper, and vitamin C (which enhances iron absorption).

1.5.4 Erythrocyte Destruction and Iron Recycling

The average erythrocyte lifespan is ∼120 days. As they age, RBC membranes become less flexible and more fragile. Worn-out RBCs are phagocytosed primarily by macrophages in the spleen (the "erythrocyte graveyard"), as well as in the liver and bone marrow. The breakdown process includes:

• Globin chains are broken down into amino acids, which re-enter the general amino acid pool.
• Heme is split into iron (Fe²⁺) and biliverdin by the enzyme heme oxygenase.
  • Iron is bound to transferrin (a plasma transport protein) and carried to the bone marrow for reuse in new hemoglobin, or stored in the liver bound to ferritin and hemosiderin.
  • Biliverdin is rapidly reduced to bilirubin (a yellow pigment), which is released into the blood, bound to albumin, and transported to the liver. In the liver, bilirubin is conjugated and secreted into bile; it ultimately reaches the intestines and is converted to stercobilin (giving feces its brown color) or reabsorbed and excreted by the kidneys as urobilin (giving urine its yellow color).

If bilirubin accumulates in the bloodstream (e.g., due to liver dysfunction or excessive RBC breakdown), it deposits in tissues, producing a yellow discoloration called jaundice.

1.6 Leukocytes (White Blood Cells)

Leukocytes are the only formed elements that are complete cells, containing nuclei and organelles. Unlike erythrocytes, they operate primarily in the interstitial spaces of tissues, using the bloodstream primarily as a highway for transport. Leukocytes can leave capillaries by squeezing between endothelial cells—a process called diapedesis (also known as extravasation)—and migrate toward sites of infection or injury following chemical gradients in a process termed chemotaxis.

Leukocytes are classified into two major groups based on the presence or absence of cytoplasmic granules visible with standard staining:

1.6.1 Granular Leukocytes (Granulocytes)

All granulocytes have multilobed nuclei and obvious cytoplasmic granules.

• Neutrophils (50–70% of circulating WBCs): The most abundant leukocyte. Their nucleus has 3–5 lobes connected by thin chromatin threads (hence the alternate name polymorphonuclear leukocytes or "polys"). Neutrophil granules contain hydrolytic enzymes and antimicrobial proteins (including defensins and lysozyme). They are the first responders to bacterial infection, performing phagocytosis and releasing a respiratory burst (generating reactive oxygen species to kill engulfed pathogens). They are also key players in the inflammatory response. Lifespan: hours to a few days. An increase in neutrophil count is called neutrophilia and typically signals acute bacterial infection or tissue necrosis.

• Eosinophils (2–4% of circulating WBCs): Distinguished by a bilobed nucleus and large, red-orange cytoplasmic granules that stain with eosin (an acidic dye). Their granules contain enzymes that are particularly effective against parasitic worms (helminths) and participate in modulating allergic and inflammatory responses by releasing histaminase and other mediators that degrade histamine. Eosinophil numbers rise in parasitic infections and allergic conditions — a condition called eosinophilia.

• Basophils (0.5–1% of circulating WBCs): The rarest leukocyte, with a bilobed or S-shaped nucleus obscured by large, dark purple-black granules that stain with basic dyes. Basophil granules contain histamine (a vasodilator that increases capillary permeability) and heparin (an anticoagulant). When basophils degranulate, they initiate and amplify the inflammatory response. They are functionally similar to mast cells found in connective tissues, though the two cell types arise from different precursor lineages.

1.6.2 Agranular Leukocytes (Agranulocytes)

Agranulocytes lack obvious cytoplasmic granules under light microscopy (they do contain some granules, but they are much smaller and less visible).

• Lymphocytes (25–33% of circulating WBCs): The second-most numerous leukocyte. These are relatively small cells (somewhat larger than RBCs) with a large, round, dark-staining nucleus that occupies most of the cell volume and a thin rim of pale-blue cytoplasm. Lymphocytes are the cornerstone of adaptive immunity and exist in three major functional subtypes, which are indistinguishable by light microscopy alone:
  • T lymphocytes (T cells) — mature in the thymus; responsible for cell-mediated immunity. They include helper T cells (CD4⁺; activate other immune cells), cytotoxic T cells (CD8⁺; directly kill virus-infected cells and tumor cells), and regulatory T cells (suppress immune responses).
  • B lymphocytes (B cells) — mature in bone marrow; responsible for humoral immunity. When activated by an antigen, B cells differentiate into plasma cells that secrete large quantities of antibodies (immunoglobulins).
  • Natural killer (NK) cells — part of innate immunity; recognize and kill virus-infected cells and tumor cells without prior sensitization.

• Monocytes (3–8% of circulating WBCs): The largest leukocytes, with a large, kidney-shaped or horseshoe-shaped nucleus and abundant gray-blue cytoplasm. Monocytes circulate in the blood for only 1–2 days before migrating into tissues, where they differentiate into macrophages — large, long-lived phagocytic cells. Tissue macrophages are the cleanup and defense specialists, performing phagocytosis of pathogens, cellular debris, and worn-out cells. They also act as antigen-presenting cells (APCs), displaying fragments of engulfed pathogens to T lymphocytes to trigger adaptive immune responses. Tissue-resident macrophages have tissue-specific names: Kupffer cells in the liver, alveolar macrophages (dust cells) in the lungs, microglia in the central nervous system, and osteoclasts in bone.

1.6.3 Leukocyte Formation (Leukopoiesis)

Leukopoiesis is stimulated by colony-stimulating factors (CSFs) and interleukins, which are glycoprotein cytokines released by various cells including macrophages, lymphocytes, and bone marrow stromal cells. Each CSF promotes the production of a specific leukocyte lineage or group of lineages from hematopoietic stem cells. For example, granulocyte colony-stimulating factor (G-CSF) stimulates neutrophil production, while granulocyte-macrophage CSF (GM-CSF) stimulates both granulocyte and monocyte production.

1.7 Platelets (Thrombocytes)

Platelets are not cells but small, anucleate, membrane-enclosed cytoplasmic fragments pinched off from giant bone marrow cells called megakaryocytes. Each megakaryocyte can produce thousands of platelets by extending long cytoplasmic processes (proplatelets) that fragment into the circulation. Platelets are discoid when inactive, roughly 2–4 μm in diameter, and have a lifespan of about 10 days. Aged platelets are removed by macrophages in the spleen and liver.

The cytoplasm of a platelet contains:

• Granules packed with clotting factors, serotonin, ADP, platelet-derived growth factor (PDGF), and calcium ions.
• A marginal bundle of actin and myosin filaments that enables shape change and contraction during clot retraction.
• An open canalicular system of membrane channels that facilitates granule release.

Platelet function centers on hemostasis — the stopping of bleeding. When a blood vessel is injured, platelets adhere to exposed collagen in the damaged vessel wall (via von Willebrand factor), become activated, change shape, and release the contents of their granules. This release attracts more platelets, which aggregate to form a temporary platelet plug. The platelet plug is later reinforced by fibrin strands during the coagulation cascade. Platelets also secrete PDGF, which stimulates the repair and regrowth of the damaged vessel wall.

A deficiency in platelet number is called thrombocytopenia (increased bleeding risk); an excess is called thrombocytosis (increased clotting risk). The hormone thrombopoietin (TPO), produced primarily by the liver, is the main regulator of platelet production.

1.8 Hematopoiesis: The Formation of Blood Cells

1.8.1 Overview and Location

Hematopoiesis (also called hemopoiesis) is the continuous process by which all formed elements of blood are produced and replaced. In the developing embryo and fetus, hematopoiesis occurs sequentially in the yolk sac, liver, spleen, and thymus before shifting definitively to the bone marrow. After birth and throughout adult life, all blood cell production is confined to red bone marrow (myeloid tissue), which resides in the trabecular cavities of spongy bone in the axial skeleton (skull, vertebrae, ribs, sternum, pelvis) and the proximal ends of the humerus and femur. Yellow bone marrow, which fills the shafts of long bones, is largely composed of adipocytes and is not hematopoietic under normal conditions, though it can convert back to red marrow in cases of extreme, prolonged hematopoietic demand.

1.8.2 Hematopoietic Stem Cells

All blood cells arise from a common population of hematopoietic stem cells (HSCs), also called hemocytoblasts. HSCs are multipotent — they have the capacity to differentiate into any type of blood cell but not into non-hematopoietic tissues. HSCs are rare (∼1 in every 10,000 bone marrow cells) and reside in specialized niches within the red bone marrow where stromal cells, extracellular matrix, and local growth factors regulate their self-renewal and commitment to differentiation.

The first branch point in hematopoiesis divides stem cells into two major progenitor lines:

1. Myeloid stem cells (myeloid lineage) — give rise to all formed elements except lymphocytes. Their descendants include:
   • Proerythroblasts → erythrocytes (via erythropoiesis, stimulated by EPO).
   • Myeloblasts → granulocytes (neutrophils, eosinophils, basophils).
   • Monoblasts → monocytes → macrophages.
   • Megakaryoblasts → megakaryocytes → platelets (via thrombopoiesis, stimulated by TPO).

1. Lymphoid stem cells (lymphoid lineage) — give rise to:
   • T lymphoblasts → T lymphocytes (mature in thymus).
   • B lymphoblasts → B lymphocytes (mature in bone marrow).
   • NK cell precursors → natural killer cells.

Commitment to a particular lineage is driven by exposure to specific hematopoietic growth factors, including EPO, TPO, and the various colony-stimulating factors and interleukins. Once a stem cell commits to a progenitor cell line, the process is generally irreversible.