Comprehensive Study Notes: Blood (Chapter 17) – Structure, Function, and Disorders

Blood: Structure, Composition, and Function

Overview of Blood

Blood is a specialized fluid connective tissue essential for transporting substances throughout the body, maintaining homeostasis, and protecting against disease. It consists of plasma and formed elements, each with distinct roles.

• Plasma: The liquid matrix, making up about 55% of blood volume.

• Formed Elements: Includes erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

Blood Composition and Hematocrit

The composition of blood can be analyzed by centrifugation, separating it into plasma, buffy coat (leukocytes and platelets), and erythrocytes. Hematocrit is the percentage of blood volume occupied by erythrocytes.

• Hematocrit: 47% ± 5% for males; 42% ± 5% for females.

• Buffy coat: ~1% (leukocytes and platelets).

• Plasma: ~55% of blood volume.

Physical Characteristics and Volume

• Color: Scarlet (oxygen-rich) to dark red (oxygen-poor).

• pH: 7.35–7.45 (slightly alkaline).

• Temperature: ~38°C (100.4°F).

• Volume: 5–6 L in males, 4–5 L in females (~8% of body weight).

Functions of Blood

Blood performs three main functions: distribution, regulation, and protection.

• Distribution: Transports oxygen, nutrients, metabolic wastes, and hormones.

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

• Protection: Prevents blood loss (clotting) and infection (immune response).

Blood Plasma

Composition of Plasma

Plasma is about 90% water and contains over 100 dissolved solutes, including proteins, nutrients, electrolytes, gases, and hormones.

• Plasma Proteins: Produced mainly by the liver; include albumin (60%), globulins (36%), and fibrinogen (4%).

• Nutrients: Glucose, amino acids, lipids.

• Electrolytes: Na+, K+, Ca2+, Cl–, HCO3–.

• Other Solutes: Nitrogenous wastes (urea, creatinine), respiratory gases (O2, CO2), hormones.

Formed Elements of Blood

Types and Characteristics

The formed elements include erythrocytes, leukocytes, and platelets. Only leukocytes are complete cells; erythrocytes lack nuclei and organelles, and platelets are cell fragments.

• Erythrocytes (RBCs): Biconcave, anucleate, filled with hemoglobin for gas transport.

• Leukocytes (WBCs): Complete cells involved in immune defense.

• Platelets: Fragments involved in clotting.

Erythrocytes (Red Blood Cells)

Structure and Function

Erythrocytes are specialized for oxygen transport. Their biconcave shape increases surface area for gas exchange, and they lack mitochondria, relying on anaerobic metabolism.

• Hemoglobin: Each RBC contains about 250 million hemoglobin molecules, each capable of binding four O2 molecules.

• Major contributor to blood viscosity.

Hemoglobin Structure and Function

Hemoglobin is a protein composed of four polypeptide chains (two alpha, two beta) each bound to a heme group containing iron. It binds oxygen reversibly and also transports some carbon dioxide.

• Oxyhemoglobin (HbO2): Hemoglobin bound to oxygen (lungs).

• Deoxyhemoglobin (HHb): Hemoglobin after oxygen release (tissues).

• Carbaminohemoglobin (HbCO2): Hemoglobin bound to CO2 (tissues).

Hematopoiesis and Erythropoiesis

Hematopoiesis

Hematopoiesis is the process of blood cell formation, occurring primarily in red bone marrow. All formed elements arise from hematopoietic stem cells (hemocytoblasts).

• Location: Axial skeleton, girdles, proximal epiphyses of humerus and femur.

• Rate: ~100 billion new cells per day.

Erythropoiesis

Erythropoiesis is the production of red blood cells. It involves several stages, including ribosome synthesis, hemoglobin accumulation, and ejection of the nucleus to form reticulocytes, which mature into erythrocytes.

• Regulation: Controlled by erythropoietin (EPO), a hormone released by the kidneys in response to hypoxia (low oxygen).

• Dietary Requirements: Iron, vitamin B12, folic acid, amino acids, lipids, and carbohydrates.

Fate and Destruction of Erythrocytes

RBCs have a lifespan of 100–120 days. Old RBCs are removed by macrophages in the spleen, liver, and bone marrow. Hemoglobin is broken down: iron is recycled, heme is converted to bilirubin (excreted in bile), and globin is metabolized to amino acids.

Erythrocyte Disorders

Anemia

Anemia is a condition where blood has a reduced oxygen-carrying capacity. It is a symptom, not a disease, and can result from blood loss, decreased RBC production, or increased RBC destruction.

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

• Types: Hemorrhagic, hemolytic, aplastic, iron-deficiency, pernicious, thalassemias, sickle-cell anemia.

Polycythemia

Polycythemia is an abnormal excess of erythrocytes, increasing blood viscosity. Causes include bone marrow cancer (polycythemia vera), high altitude, increased EPO production, or blood doping.

Leukocytes (White Blood Cells)

Types and Functions

Leukocytes are crucial for immune defense. They can leave blood vessels (diapedesis) and move toward infection sites (chemotaxis). Leukocytosis is an increase in WBC count, often due to infection.

• Granulocytes: Neutrophils, eosinophils, basophils (contain granules, lobed nuclei).

• Agranulocytes: Lymphocytes, monocytes (lack visible granules).

Granulocytes

• Neutrophils: Most abundant, phagocytic, "bacteria slayers."

• Eosinophils: Attack parasitic worms, modulate allergies.

• Basophils: Release histamine, involved in inflammation.

Agranulocytes

• Lymphocytes: Crucial for immunity; T cells attack infected/tumor cells, B cells produce antibodies.

• Monocytes: Largest WBCs; become macrophages in tissues, phagocytize pathogens and debris.

Platelets and Hemostasis

Platelets

Platelets are cell fragments derived from megakaryocytes, essential for blood clotting. They form a temporary plug at sites of vessel injury and release chemicals that promote clotting.

Hemostasis

Hemostasis is the process of stopping bleeding, involving three steps:

1. Vascular Spasm: Vasoconstriction reduces blood loss.

2. Platelet Plug Formation: Platelets adhere to exposed collagen, release chemicals, and aggregate.

3. Coagulation: Clotting cascade forms a fibrin mesh, stabilizing the plug.

Coagulation Cascade

The coagulation process involves a series of reactions leading to the conversion of fibrinogen to fibrin, forming a stable clot. It includes intrinsic and extrinsic pathways converging on the activation of factor X, leading to thrombin formation and fibrin mesh development.

Blood Groups and Transfusions

ABO and Rh Blood Groups

Blood groups are determined by the presence or absence of specific antigens (agglutinogens) on RBC membranes. The ABO system includes types A, B, AB, and O. The Rh system is based on the presence (Rh+) or absence (Rh–) of the D antigen.

• Transfusion Reactions: Occur if mismatched blood is transfused, leading to agglutination and hemolysis.

• Hemolytic Disease of the Newborn: Can occur if an Rh– mother carries an Rh+ fetus.

Blood Disorders

Bleeding and Clotting Disorders

• Thromboembolic Disorders: Formation of undesirable clots (thrombus, embolus).

• Bleeding Disorders: Thrombocytopenia (low platelets), impaired liver function, hemophilias (clotting factor deficiencies).

Summary Table: Main Components of Blood

Component	Percentage of Blood	Main Function
Plasma	~55%	Transport of nutrients, wastes, hormones; maintains osmotic balance
Erythrocytes	~45%	Oxygen and carbon dioxide transport
Leukocytes	<1%	Immune defense
Platelets	<1%	Clotting (hemostasis)

Key Equations

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

• Oxygen Carrying Capacity: $\text{O}_2 \text{ capacity} = \text{Hb concentration} \times 1.34 \text{ mL O}_2/\text{g Hb}$

Additional info: This guide expands on the original notes with definitions, examples, and academic context to ensure a comprehensive understanding of blood structure, function, and disorders for college-level anatomy and physiology students.