Anatomy & Physiology: Blood (Chapter 18) Study Notes

Blood: Structure, Function, and Components

Overview of Blood in the Circulatory System

Blood is a specialized connective tissue that plays a vital role in the circulatory system, which is divided into the cardiovascular and lymphatic systems. It serves as the main transport medium for essential substances throughout the body.

• Cardiovascular system: Composed of the heart and blood vessels, responsible for circulating blood.

• Lymphatic system: Returns excess tissue fluid to the bloodstream and is involved in immune responses.

• Functions of blood:

  • Transports nutrients, signaling molecules (e.g., hormones), respiratory gases (O2, CO2), and waste products.

  • Distributes hormones from endocrine glands.

  • Conveys immune cells to sites of infection or injury.

  • Regulates body temperature by distributing heat.

Composition of Blood

Blood Volume and Components

Blood consists of both cellular and liquid components, making it a unique connective tissue.

• Average blood volume:

  • Males: 5–6 liters

  • Females: 4–5 liters

• Formed elements (blood cells): Living components, including erythrocytes (RBCs), leukocytes (WBCs), and platelets.

• Plasma: The non-living, liquid matrix of blood.

Hematocrit and Buffy Coat

Blood can be separated into its components using centrifugation, which is important for clinical diagnostics.

• Hematocrit: The percentage of blood volume occupied by red blood cells (RBCs).

  • Males: 47% ± 5%

  • Females: 42% ± 5%

• Buffy coat: Thin layer between plasma and RBCs, containing leukocytes and platelets (<1% of blood volume).

Table: Major Components of Whole Blood

Component	Percentage of Whole Blood	Description
Plasma	~55%	Liquid matrix, least dense
Buffy coat	<1%	Leukocytes and platelets
Erythrocytes	~45%	Red blood cells, most dense

Blood Plasma

Composition and Functions

Plasma is a straw-colored, sticky fluid that makes up about 90% water and contains over 100 dissolved solutes.

• Ions (electrolytes): Sodium (Na+), chloride (Cl-), etc.

• Nutrients: Glucose, amino acids, lipids

• Wastes: Carbon dioxide (CO2), urea, ammonia

• Proteins: Produced mainly by the liver

  • Albumin (60%): Maintains osmotic pressure, prevents water loss from blood vessels.

  • Globulins (36%): Includes antibodies and transport proteins for lipids, iron, and copper.

  • Fibrinogen (4%): Essential for blood clotting.

Formed Elements of Blood

Erythrocytes (Red Blood Cells)

Erythrocytes are the most numerous blood cells and are specialized for oxygen transport.

• Structure: Biconcave discs (~7.5 μm diameter), lack nuclei and organelles, filled with hemoglobin (Hb).

• Hemoglobin: Each molecule has four polypeptide chains, each with an iron-containing heme group that binds O2.

  • Each Hb can carry 4 O2 molecules.

  • Each RBC contains ~250 million Hb molecules.

• Function: Transport O2 from lungs to tissues; CO2 from tissues to lungs.

• Adaptations: Biconcave shape increases surface area; flexible membrane (spectrin protein); lack mitochondria (ATP produced anaerobically).

Leukocytes (White Blood Cells)

Leukocytes protect the body from infectious microorganisms and function mainly outside the bloodstream in connective tissues.

• Normal count: 5,000–11,000 per mm3 (less than 1% of blood volume).

• Diapedesis: Process by which WBCs leave capillaries to enter tissues.

• Leukocytosis: Elevated WBC count, usually in response to infection.

Classification of Leukocytes

• Granulocytes: Contain visible cytoplasmic granules; lobed nuclei; all are phagocytic to some degree.

  • Neutrophils (50–70%): Most numerous; multi-lobed nucleus; first responders to infection; phagocytize bacteria; release enzymes and germ-killing substances.

  • Eosinophils (1–4%): Large red granules; combat parasitic worms; modulate allergic responses by degrading histamine.

  • Basophils (0.5–1%): Least common; secrete histamine (vasodilation, inflammation); similar to mast cells.

• Agranulocytes: Lack visible granules; nuclei are spherical or kidney-shaped.

  • Lymphocytes (20–45%): Key cells of the immune system; act against specific antigens; found mostly in lymphoid tissues.

    • T cells: Attack infected or tumor cells directly.

    • B cells: Produce antibodies; differentiate into plasma cells.

  • Monocytes (3–8%): Largest WBCs; kidney-shaped nucleus; become macrophages in tissues; phagocytize pathogens and activate lymphocytes.

Table: Relative Percentages of Leukocyte Types

Leukocyte Type	Relative Percentage	Main Function
Neutrophils	50–70%	Phagocytosis of bacteria
Lymphocytes	20–45%	Immune response (T and B cells)
Monocytes	3–8%	Phagocytosis; become macrophages
Eosinophils	1–4%	Combat parasites; modulate allergies
Basophils	0.5–1%	Release histamine; inflammation

Platelets (Thrombocytes)

Platelets are small, disc-shaped cell fragments essential for blood clotting.

• Origin: Fragments of megakaryocytes in bone marrow.

• Function: Form temporary platelet plugs to seal breaks in blood vessels; participate in clotting cascade.

• Lifespan: ~10 days; formation regulated by thrombopoietin.

Hematopoiesis: Blood Cell Formation

Sites and Process

Hematopoiesis is the process by which new blood cells are formed, primarily in the red bone marrow.

• Red bone marrow: Actively produces blood cells; found in axial skeleton, girdles, and proximal epiphyses of humerus and femur in adults.

• Yellow bone marrow: Contains fat; dormant but can produce blood cells in emergencies.

• Blood stem cell (hematopoietic stem cell): Gives rise to all blood cell types through differentiation.

Blood Disorders

Disorders of Erythrocytes

• Polycythemia: Excess RBCs; increases blood viscosity; can be due to bone marrow cancer (polycythemia vera) or secondary causes (e.g., high altitude, increased EPO).

• Anemia: Reduced O2-carrying capacity; causes include blood loss, decreased RBC production, or increased RBC destruction.

  • Iron-deficiency anemia: Due to low iron; RBCs are small (microcytic) and pale.

  • Pernicious anemia: Lack of intrinsic factor or vitamin B12; RBCs are large (macrocytic).

  • Hemolytic anemias: Premature RBC destruction; causes include transfusion reactions, infections, or genetic defects (e.g., sickle cell disease).

  • Sickle cell disease: Inherited disorder; abnormal hemoglobin causes RBCs to sickle under low O2 conditions.

Disorders of Leukocytes and Platelets

• Leukemia: Cancer of WBCs; classified as lymphoblastic or myeloblastic.

• Thrombocytopenia: Low platelet count; leads to abnormal bleeding; can be drug-induced or due to impaired liver function.

• Clotting factor deficiencies: Impaired liver function (e.g., hepatitis, cirrhosis) can reduce clotting factor production, leading to bleeding disorders.

Key Terms and Definitions

• Hematocrit: Percentage of blood volume occupied by RBCs.

• Hematopoiesis: Formation of blood cells.

• Blood stem cell: Multipotent cell in bone marrow that gives rise to all blood cell types.

• Diapedesis: Movement of WBCs out of capillaries into tissues.

• Thrombopoietin: Hormone regulating platelet production.

Additional info:

• Mnemonic for leukocyte order by abundance: "Never Let Monkeys Eat Bananas" (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils).

• Blood doping is used by athletes to increase RBC count and improve oxygen delivery, but it increases blood viscosity and risk of complications.