Blood: Physical Characteristics and Functions

Overview of Blood

Blood is a specialized connective tissue essential for transport, regulation, and protection within the human body. It maintains homeostasis by delivering oxygen and nutrients, removing wastes, and supporting immune defense.

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

• pH Range: Strictly regulated between 7.35 and 7.45, keeping blood slightly alkaline.

• Total Volume: Average adult volume is 5 liters (4–5 L in females, 5–6 L in males), representing roughly 8% of body weight.

• Viscosity: Blood is denser and five times more viscous than water due to its cellular elements and plasma proteins.

Functions of Whole Blood

Blood performs three primary functions:

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

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

• Protection: Ensures hemostasis (clotting) and prevents infection.

Components of Whole Blood

Blood Composition and Separation

Whole blood consists of plasma and formed elements. Centrifugation separates blood into three layers by density:

• Plasma: Straw-colored fluid, 55% of blood.

• Buffy Coat: Leukocytes and platelets (<1%).

• Packed Erythrocytes: Bottom layer of RBCs (~45%).

Blood Plasma Structure

Plasma is a complex, non-living extracellular matrix that carries nutrients, wastes, gases, and vital proteins.

• Water (92%): Primary solvent for dissolving and transporting substances.

• Plasma Proteins (7%): Specialized proteins synthesized mostly by the liver.

• Other Solutes (1%): Electrolytes (Na+, K+, Cl-), nutrients (glucose, lipids), gases (CO2, O2), and metabolic wastes (urea).

Functional Plasma Proteins

Plasma proteins maintain osmotic pressure and vascular function.

• Albumin (60%): Regulates osmotic pressure; transports lipids and hormones.

• Globulins (36%): Alpha/beta transport lipids/ions; gamma provide immunity.

• Fibrinogen (4%): Soluble precursor converted to insoluble fibrin for clotting.

Hematocrit and Clinical Measurement

Understanding Hematocrit

Hematocrit (Hct) measures the percentage of erythrocyte volume in whole blood.

• Normal Ranges: Females: 37%–48%; Males: 42%–54% (testosterone elevates male RBC production).

• Clinical Note: Dehydration reduces plasma volume, artificially elevating Hct despite stable RBC counts.

Hemopoiesis: Blood Cell Formation

Hemopoiesis in Red Bone Marrow

Formed elements are produced via hemopoiesis in red bone marrow.

• Hemocytoblast: Every formed element originates from a single multipotent stem cell.

• Myeloid Stem Cells: Give rise to erythrocytes, megakaryocytes (platelets), granulocytes, and monocytes.

• Lymphoid Stem Cells: Give rise exclusively to lymphocytes (T cells, B cells, NK cells).

• Location: Red bone marrow in adults is concentrated in the axial skeleton, pelvic girdles, and proximal humerus and femur.

Erythrocyte Structure and Function

Erythrocyte Adaptations

Mature erythrocytes are specialized for gas transport.

• Biconcave Geometry: 7.5 µm disc maximizes surface area for gas exchange.

• Anucleate State: No nucleus/organelles; maximizes hemoglobin volume.

• Spectrin Flexibility: Cytoskeletal spectrin allows capillary deformation.

Structure of Hemoglobin

Hemoglobin (Hb) is the primary oxygen-carrying molecule in erythrocytes.

• Globin Protein: Four polypeptide chains (two alpha, two beta).

• Heme Groups: Four pigment molecules bound to globin.

• Iron Core: Each heme has one Fe2+ iron atom that reversibly binds one O2 molecule.

• Total Capacity: One erythrocyte carries up to 1 billion oxygen molecules.

Oxygen Dynamics: Oxyhemoglobin vs Deoxyhemoglobin

Hemoglobin reversibly binds oxygen in the lungs and releases it to tissues.

• Oxyhemoglobin (Hb-O2): In lung capillaries, iron binds O2; blood is ruby red.

• Deoxyhemoglobin (HHb): In systemic tissues, O2 dissociates; venous blood is maroon.

Calculate Oxygen Transport Capacity

The oxygen transport capacity of blood can be calculated using hemoglobin concentration and binding capacity.

• Formula:

• Total Capacity (5L):

• Iron Deficiency: Low iron reduces heme synthesis, lowering hemoglobin and decreasing blood oxygenation.

Erythropoiesis: Formation and Regulation of RBCs

Stages of Erythropoiesis

Stem cell to mature erythrocyte differentiation takes 3–5 days in red bone marrow.

1. Hemocytoblast: Multipotent stem cell commits to myeloid line.

2. Proerythroblast: Committed cell begins rapid division.

3. Erythroblast: Synthesizes hemoglobin; nucleus ejected.

4. Reticulocyte: Enters blood; matures within 48 hours.

Hormonal Regulation of Erythropoiesis

Erythropoiesis is regulated by erythropoietin (EPO) via a negative feedback loop.

• Hypoxia: Kidney cells detect low oxygen from altitude, blood loss, or anemia.

• EPO Release: Kidneys secrete erythropoietin (EPO) into circulation.

• Marrow Stimulation: EPO accelerates proerythroblast maturation in red bone marrow.

• Homeostasis: Increased RBC count raises oxygen, inhibiting further EPO release.

Nutritional Requirements for RBCs

Building millions of new red blood cells every second requires specific dietary nutrients:

• Iron: Essential for heme synthesis. Stored in cells as ferritin or hemosiderin; transported in blood by transferrin.

• Vitamin B12 & Folic Acid: Required for DNA synthesis during rapid cell division of erythroblasts.

• Amino Acids: Required for synthesizing globin protein chains.

Erythrocyte Lifecycle and Recycling

RBC Recycling

Anucleate RBCs cannot synthesize proteins; they age and become fragile after 120 days.

• Phagocytosis: Macrophages engulf aged RBCs.

• Globin & Iron Reuse: Globin yields amino acids. Iron is salvaged and returned to marrow.

• Heme Breakdown: Heme degrades to bilirubin; liver excretes it via bile.

Clinical Conditions: Anemia and Polycythemia

Disorders of Erythrocyte Count and Function

Disruptions in erythrocyte count or hemoglobin impair blood function:

• Anemia: Abnormally low oxygen-carrying capacity. Caused by insufficient RBCs (hemorrhage), low hemoglobin (iron deficiency), or abnormal Hb (sickle-cell anemia).

• Polycythemia: Abnormal excess of erythrocytes that dangerously increases blood viscosity, slowing blood flow and straining the heart.

Summary of Blood Physiology

From plasma protein balance to erythrocyte recycling, blood maintains precise homeostasis. Next lesson: Leukocytes, Platelets, and Hemostasis.