Chapter 17: Blood – Structure, Function, and Disorders

Blood: Structure, Function, and Disorders

17.1 The Functions of Blood: Transport, Regulation, and Protection

Blood is a specialized connective tissue that serves as the internal transport system of the body. Its functions are categorized into three main areas: transport, regulation, and protection.

• Transport Functions:

  • Delivers oxygen (O2) and nutrients to body cells.

  • Transports metabolic wastes (e.g., CO2, urea) to lungs and kidneys for elimination.

  • Carries hormones from endocrine organs to target tissues.

• Regulatory Functions:

  • Maintains body temperature by absorbing and distributing heat.

  • Maintains normal pH using buffers (e.g., bicarbonate ions provide an "alkaline reserve").

  • Maintains adequate fluid volume in the circulatory system.

• Protective Functions:

  • Prevents blood loss via clot formation (plasma proteins and platelets initiate clotting).

  • Prevents infection by transporting agents of immunity:

    • Antibodies

    • Complement proteins

    • White blood cells (WBCs)

17.2 Blood Consists of Plasma and Formed Elements

Blood is composed of cellular (formed elements) and liquid (plasma) components.

• Formed Elements:

  • Erythrocytes (red blood cells, RBCs)

  • Leukocytes (white blood cells, WBCs)

  • Platelets (cell fragments)

• Formed elements are suspended in plasma (the fluid extracellular matrix).

• When blood is centrifuged, it separates into three layers:

  • RBCs (bottom; ~45% of blood, called hematocrit)

  • Buffy coat (middle; <1%, contains WBCs and platelets)

  • Plasma (top; ~55%)

Physical Characteristics:

• Viscous, opaque fluid with metallic taste; ~5x more viscous than water (due to RBCs).

• Color varies with O2 content: scarlet (O2-rich), dark red (O2-poor).

• pH: 7.35–7.45; volume: 5–6 L (males), 4–5 L (females); ~8% of body weight.

Blood Plasma

Plasma is a straw-colored, sticky fluid making up about 55% of blood volume and is 90% water. It contains over 100 dissolved solutes, including nutrients, gases, hormones, wastes, proteins, and inorganic ions.

• Plasma Proteins (~8% of plasma by weight):

  • Most produced by the liver (except antibodies and hormones).

  • Albumin (60% of plasma proteins):

    • Acts as a blood buffer (regulates pH).

    • Carrier for certain molecules.

    • Major contributor to plasma osmotic pressure (keeps water in blood).

Table: Main Components of Plasma

Constituent	Description and Importance
Water	90% of plasma volume; dissolves and suspends solutes, absorbs heat
Electrolytes	Maintain osmotic balance, pH, and membrane potential
Plasma proteins	Osmotic balance, transport, immunity, clotting
Nutrients	Energy sources and building blocks for cells
Respiratory gases	O2 and CO2 transport
Hormones	Regulation of physiological processes

Formed Elements

• Erythrocytes and platelets are not true cells (RBCs lack nuclei and most organelles; platelets are cell fragments).

• Most formed elements survive only a few days in blood.

• Most blood cells do not divide; they are replaced by stem cells in red bone marrow.

17.3 Erythrocytes: Oxygen and Carbon Dioxide Transport

Structural Characteristics of Erythrocytes

• Small-diameter cells (~7.5 μm), biconcave disc shape, lack nucleus and most organelles.

• Filled with hemoglobin (Hb) for gas transport.

• Flexible cytoskeleton allows passage through narrow capillaries.

• 97% of cell volume is hemoglobin (excluding water).

• No mitochondria; ATP produced anaerobically (does not consume transported O2).

Functions of Erythrocytes

• Dedicated to respiratory gas transport.

• Hemoglobin binds reversibly with O2.

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

  • Each Hb molecule: 4 polypeptide chains (2 alpha, 2 beta) + 4 heme groups (each with 1 iron atom).

  • Each iron binds 1 O2; each RBC contains ~250 million Hb molecules.

• O2 loading in lungs: forms oxyhemoglobin (bright red).

• O2 unloading in tissues: forms deoxyhemoglobin (dark red).

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

Production of Erythrocytes (Erythropoiesis)

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

• Hematopoietic stem cells (hemocytoblasts) give rise to all formed elements.

• Stages of erythropoiesis:

  1. Hemocytoblast → myeloid stem cell → proerythroblast (committed cell)

  2. Proerythroblast → basophilic erythroblast (ribosome synthesis)

  3. Basophilic erythroblast → polychromatic erythroblast (hemoglobin synthesis)

  4. Polychromatic erythroblast → orthochromatic erythroblast (nucleus ejected)

  5. Reticulocyte (enters bloodstream, matures to erythrocyte)

Regulation and Requirements of Erythropoiesis

• Number of RBCs is kept constant by balancing production and destruction (>2 million RBCs produced per second).

• Erythropoietin (EPO) is the hormone that stimulates erythropoiesis.

  • Produced by kidneys in response to hypoxia (low O2).

  • Testosterone increases EPO production (higher RBC count in males).

• Dietary requirements:

  • Amino acids, lipids, carbohydrates

  • Vitamin B12 and folic acid (for DNA synthesis in rapidly dividing cells)

  • Iron (essential for hemoglobin synthesis; 65% in Hb, rest in liver, spleen, bone marrow)

Fate and Destruction of Erythrocytes

• Life span: 100–120 days (anucleate, cannot synthesize new proteins).

• Old RBCs become fragile; hemoglobin degenerates.

• Macrophages in spleen engulf and break down dying RBCs:

  • Heme, iron, and globin are separated.

  • Iron stored as ferritin or hemosiderin for reuse.

  • Heme degraded to bilirubin (yellow pigment); excreted in bile, converted to stercobilin (brown pigment in feces).

  • Globin broken down to amino acids.

Erythrocyte Disorders

• Classified as anemia (too few RBCs or low Hb) or polycythemia (too many RBCs).

• Anemia:

  • Blood's O2-carrying capacity is too low to support normal metabolism.

  • Symptoms: fatigue, pallor, shortness of breath, chills.

  • Causes:

    • Blood loss (hemorrhagic anemia: acute or chronic)

    • Not enough RBCs produced (iron-deficiency anemia, pernicious anemia, lack of B12)

    • Too many RBCs destroyed (hemolytic anemia, sickle-cell anemia, thalassemia)

• Polycythemia:

  • Abnormal excess of RBCs; increases blood viscosity, causing sluggish blood flow.

  • Can be caused by bone marrow cancer (polycythemia vera) or secondary to increased EPO (e.g., high altitude).

Table: Types of Anemia (Summary)

Type	Cause	Key Features
Hemorrhagic	Blood loss	Acute (trauma) or chronic (ulcer); treat underlying cause
Iron-deficiency	Low iron intake/absorption	Microcytic, hypochromic RBCs; treat with iron supplements
Pernicious	Lack of intrinsic factor (B12 absorption)	Macrocytic RBCs; treat with B12 injections
Hemolytic	RBC destruction	Genetic (sickle-cell, thalassemia) or acquired (infections, transfusion reactions)

Example: Sickle-cell anemia is caused by a genetic mutation in the beta-globin gene, resulting in abnormal hemoglobin (HbS) that causes RBCs to become crescent-shaped under low oxygen conditions, leading to blockages in small vessels and increased hemolysis.

Additional info: This summary covers the first half of Chapter 17, focusing on blood composition, erythrocyte structure and function, erythropoiesis, and erythrocyte disorders. For a complete understanding, students should also study leukocytes, platelets, hemostasis, and blood typing, which are typically covered in the remainder of the chapter.