Blood: Structure, Function, and Disorders – Anatomy & Physiology Study Notes

Blood – Internal Transport System

Overview and Functions

Blood is the life-sustaining transport vehicle of the cardiovascular system, essential for maintaining homeostasis. Its primary functions are categorized as transport, regulation, and protection.

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

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

• Protection: Prevents blood loss and infection through clotting and immune responses.

Transport Functions of Blood

Key Transport Roles

Blood serves as the medium for transporting essential substances throughout the body.

• Monomers from digestion: Amino acids, fatty acids, glucose, etc.

• Hormones: Chemical messengers regulating physiological processes.

• Metabolic waste products: Such as urea and carbon dioxide, transported to excretory organs.

• Gases: Oxygen (O2) and carbon dioxide (CO2).

Regulation Functions of Blood

Homeostatic Roles

Blood helps maintain the internal environment of the body.

• Body temperature: Absorbs and distributes heat throughout tissues.

• pH balance: Maintains normal pH using buffers, especially the alkaline reserve of bicarbonate ions.

• Fluid volume: Ensures adequate fluid volume in the circulatory system.

Protection Functions of Blood

Defense Mechanisms

Blood protects the body from blood loss and infection.

• Preventing blood loss: Plasma proteins and platelets initiate clot formation.

• Preventing infection: Blood carries agents of immunity:

  • Antibodies

  • Complement proteins

  • White blood cells

17.2 Composition of Blood

Blood as a Connective Tissue

Blood is the only fluid tissue in the body, classified as a connective tissue.

• Plasma: Nonliving fluid matrix.

• Formed elements: Living blood cells suspended in plasma:

  • Erythrocytes (RBCs): Red blood cells

  • Leukocytes (WBCs): White blood cells

  • Platelets: Cell fragments involved in clotting

Blood Separation and Hematocrit

When blood is centrifuged, it separates into three layers:

• Erythrocytes: Bottom layer (~45% of whole blood)

• Buffy coat: Middle thin layer (

  • Plasma: Top layer (~55%)

Hematocrit is the percentage of blood volume that is RBCs. Normal values:

• Males: 47% ± 5%

• Females: 42% ± 5%

Physical Characteristics and Volume of Blood

Properties

• Sticky, opaque fluid with metallic odor/taste

• Color varies with oxygen content: scarlet red (high O2), dark red (low O2)

• pH: 7.35–7.45

• Makes up ~8% of body weight

• Average volume: Males 5–6 L, Females 4–5 L

Blood Plasma

Composition and Functions

Blood plasma is a straw-colored, sticky fluid, about 90% water, containing over 100 dissolved solutes.

• Nutrients, gases, hormones, wastes, proteins, inorganic ions

• Plasma proteins: Most abundant solutes, produced mainly by the liver

  • Albumin: 60% of plasma proteins; carrier for molecules, blood buffer, contributes to osmotic pressure

Table: Composition of Plasma

Constituent	Description and Importance
Water	90% of plasma; dissolves and suspends solutes, absorbs heat
Electrolytes	Maintain osmotic balance, pH, and membrane potential
Plasma proteins	Albumin (osmotic pressure), globulins (transport, immunity), fibrinogen (clotting)
Nonprotein nitrogenous substances	By-products of metabolism (urea, uric acid, creatinine)
Nutrients	Glucose, amino acids, fatty acids, vitamins
Respiratory gases	O2 and CO2
Hormones	Steroid and thyroid hormones

Formed Elements

Types and Characteristics

• Red blood cells (RBCs): No nuclei or organelles

• White blood cells (WBCs): Only complete cells

• Platelets: Cell fragments

• Most formed elements survive only a few days in the bloodstream

• Most blood cells originate in bone marrow and do not divide

Blood Cells

Microscopic Appearance

• Erythrocytes: Red, biconcave discs

• Leukocytes: Larger, varied shapes, nucleus present

• Platelets: Small, irregular fragments

17.3 Erythrocytes

Structural Characteristics

• Small diameter (7.5 μm), biconcave disc shape

• Anucleate, no organelles

• Filled with hemoglobin for gas transport

• Contain plasma membrane protein spectrin for flexibility

• Large surface area relative to volume for efficient gas exchange

• No mitochondria; ATP production is anaerobic

Structure of Erythrocytes

• Biconcave disc: 2.5 μm thick, 7.5 μm diameter

• Shape optimizes gas exchange and flexibility

Function of Erythrocytes

• Dedicated to respiratory gas transport

• Hemoglobin binds reversibly with oxygen

• Normal values: Males 13–18 g/100ml; Females 12–16 g/100ml

• Hemoglobin consists of four polypeptide chains (two alpha, two beta), each with a heme group

• Each heme contains iron atom that binds one O2

Structure of Hemoglobin

• Globin: protein composed of four chains

• Heme: iron-containing pigment

• Each Hb molecule can transport four O2

• Each RBC contains ~250 million Hb molecules

Oxygen and Carbon Dioxide Transport

• O2 loading in lungs: produces oxyhemoglobin (ruby red)

• O2 unloading in tissues: produces deoxyhemoglobin (dark red)

• CO2 loading in tissues: 20% of CO2 binds to Hb, producing carbaminohemoglobin

Production of Erythrocytes

Hematopoiesis and Erythropoiesis

• Hematopoiesis: Formation of all blood cells in red bone marrow

• Hematopoietic stem cells (hemocytoblasts): Give rise to all formed elements

• Hormones and growth factors direct differentiation

Stages of Erythropoiesis

• Process takes about 15 days

• Transformation sequence:

  1. Hematopoietic stem cell → myeloid stem cell

  2. Myeloid stem cell → proerythroblast

  3. Proerythroblast divides, becomes basophilic erythroblast

  4. Basophilic erythroblast synthesizes ribosomes

  5. Polychromatic erythroblast: synthesizes hemoglobin

  6. Orthochromatic erythroblast: ejects organelles, nucleus degrades

  7. Reticulocyte: still contains ribosomes

  8. Mature erythrocyte: ribosomes degrade, cell becomes mature RBC

• Reticulocyte count indicates rate of RBC formation

Regulation and Requirements of Erythropoiesis

Hormonal Controls

• Erythropoietin (EPO): Hormone stimulating RBC formation

• Produced by kidneys (and some by liver) in response to hypoxia

• Low O2 levels trigger EPO release via hypoxia-inducible factor (HIF)

• Testosterone increases EPO production

Dietary Requirements

• Amino acids, lipids, carbohydrates

• Iron: 65% found in hemoglobin; stored as ferritin and hemosiderin; transported by transferrin

• Vitamin B12 and folic acid: required for DNA synthesis in rapidly dividing cells

Fate and Destruction of Erythrocytes

Life Span and Breakdown

• Life span: 100–120 days

• RBCs are anucleate; cannot synthesize new proteins or divide

• Old RBCs become fragile; hemoglobin degenerates

• Macrophages in spleen engulf and break down dying RBCs

Breakdown Products

• Heme, iron, and globin are separated

• Iron stored as ferritin/hemosiderin or recycled

• Heme degraded to yellow pigment bilirubin

• Liver secretes bilirubin in bile; in intestines, bilirubin is converted to urobilinogen and then to stercobilin (excreted in feces)

• Globin is metabolized to amino acids

Erythrocyte Disorders

Anemia

Anemia is defined as abnormally low O2-carrying capacity of blood, insufficient to support normal metabolism. Symptoms include fatigue, pallor, dyspnea, and chills. Causes are grouped as:

• Blood loss: Hemorrhagic anemia (rapid or chronic blood loss)

• Not enough RBCs produced:

  • Iron-deficiency anemia: Microcytes, small pale RBCs; treated with iron supplements

  • Pernicious anemia: Autoimmune, lack of intrinsic factor for B12 absorption; treated with B12 injections

  • Renal anemia: Lack of EPO, often due to kidney disease; treated with synthetic EPO

  • Aplastic anemia: Destruction/inhibition of bone marrow; treated with transfusions or stem cell transplants

• Too many RBCs destroyed:

  • Hemolytic anemias: Premature lysis of RBCs due to transfusions, infections, or genetic disorders

  • Thalassemias: Faulty globin chain, common in Mediterranean ancestry; severity varies

  • Sickle-cell anemia: Mutated hemoglobin S; RBCs become crescent-shaped under low O2, rupture easily, block vessels, cause pain

Polycythemia

Polycythemia is an abnormal excess of RBCs, increasing blood viscosity and risk of clotting.

Sickle-Cell Anemia: Special Notes

• Prevalent in individuals of African descent; confers some resistance to malaria

• Treatment includes transfusions, hydroxyurea, stem cell transplants, gene therapy, and nitric oxide for vasodilation

Example: Sickle-Cell Anemia

Normal erythrocytes are round and flexible, while sickle cells are crescent-shaped and rigid, leading to blockages in small blood vessels and reduced oxygen delivery.