Blood: Composition, Function, and Disorders

Chapter 7: Blood

7.1 Composition of Blood

Blood is a vital connective tissue responsible for transporting substances throughout the body, defending against pathogens, and maintaining homeostasis. It consists of two main components: formed elements and plasma.

• Formed Elements (45%)

  • Red Blood Cells (RBCs, Erythrocytes): Transport oxygen to tissues and remove carbon dioxide.

  • White Blood Cells (WBCs, Leukocytes): Defend the body against infection and disease.

  • Platelets (Thrombocytes): Aid in blood clotting to prevent blood loss.

• Plasma (55%)

  • Water: Primary constituent (about 90% of plasma); acts as a solvent and medium for transport.

  • Electrolytes (Ions): Includes sodium, potassium, chloride, bicarbonate, calcium, hydrogen, and magnesium. These ions regulate cell function, volume, and electrical charge.

  • Proteins:

    • Albumins: Maintain blood volume and osmotic balance; transport molecules and drugs.

    • Globulins: Serve as antibodies (immunoglobulins) and transport substances. Beta globulins form lipoproteins (e.g., LDL, HDL).

    • Clotting Proteins: Essential for blood coagulation.

  • Hormones: Chemical messengers (e.g., insulin, growth hormone, testosterone, estrogen) that regulate body functions.

  • Gases: Oxygen (O2) for metabolism; carbon dioxide (CO2) as a waste product.

  • Nutrients and Wastes: Glucose, amino acids, urea, and heat are transported throughout the body.

Additional info: Plasma proteins are primarily synthesized in the liver. Albumins constitute about two-thirds of plasma proteins and are crucial for maintaining osmotic pressure.

Red Blood Cells (Erythrocytes)

• Most abundant formed element; mature RBCs lack a nucleus and organelles.

• Generate ATP anaerobically, so they do not consume the oxygen they transport.

• Hemoglobin: Oxygen-binding protein composed of four polypeptide chains, each with a heme group containing an iron atom.

• Oxygen binds reversibly to iron in hemoglobin; each hemoglobin can carry up to four O2 molecules (oxyhemoglobin).

• Deoxyhemoglobin (without oxygen) is dark purple; venous blood appears dark red.

• Hemoglobin also transports about 25% of CO2.

Hematocrit: The percentage of blood volume occupied by RBCs; indicates oxygen-carrying capacity. Low hematocrit suggests anemia; high hematocrit may indicate polycythemia (excess RBCs).

Origin and Lifespan of Blood Cells

• All blood cells and platelets originate from hematopoietic stem cells in red bone marrow.

• RBCs develop from erythroblasts, lose their nucleus and organelles, and mature in about one week.

• RBC lifespan: ~120 days; removed by macrophages via phagocytosis.

• Hemoglobin breakdown: Polypeptide chains are recycled; iron is reused; heme is converted to bilirubin (excreted in bile).

• Failure to excrete bilirubin can cause jaundice.

Regulation of RBC Production

• Controlled by a negative feedback loop involving the hormone erythropoietin (EPO), produced by the kidneys in response to low oxygen levels.

• EPO stimulates red bone marrow to increase RBC production.

• Blood doping: Artificially increasing RBC count to enhance oxygen delivery; banned due to increased blood viscosity and cardiovascular risk.

White Blood Cells (Leukocytes)

• Make up about 1% of blood volume; have nuclei but lack hemoglobin.

• Short lifespan (hours to days), except for monocytes (months) and lymphocytes (days to years).

• Removed by the liver and spleen after death.

• Produce colony-stimulating factors to regulate WBC production and release.

• Can exit blood vessels and migrate into tissues (diapedesis).

Types of White Blood Cells

• Granular Leukocytes

  • Neutrophils (60%): Phagocytize bacteria and fungi; first responders to infection.

  • Eosinophils (2-4%): Defend against parasites; moderate allergic reactions.

  • Basophils (0.5%): Release histamine during inflammation; promote blood flow and tissue repair.

• Agranular Leukocytes

  • Monocytes (5%): Migrate into tissues, become macrophages, and phagocytize pathogens and debris; active in chronic infections.

  • Lymphocytes (30%): Found in blood and lymphatic organs; include B cells (produce antibodies) and T cells (destroy infected or abnormal cells).

Platelets (Thrombocytes)

• Less than 1% of blood volume; derived from megakaryocytes in bone marrow.

• Fragments of cytoplasm and membrane; not true cells.

• Essential for blood clotting (hemostasis).

7.2 Hemostasis

Hemostasis is the process that stops blood loss after vessel injury. It involves three main stages:

1. Vascular Spasm: Constriction of blood vessels to reduce blood flow; lasts up to 30 minutes.

2. Formation of Platelet Plug: Platelets adhere to the damaged site, swell, and stick together to form a temporary plug.

3. Blood Clotting (Coagulation):

   • Damaged vessels and platelets release prothrombin activator, which (with calcium ions) converts prothrombin to thrombin.

   • Thrombin converts soluble fibrinogen into insoluble fibrin threads, forming a stable clot.

   • Platelets contract to pull vessel edges together.

Hemophilia: Genetic disorder causing deficiency in clotting factors, leading to excessive bleeding.

Key Equations:

• $\text{Prothrombin} \xrightarrow{\text{Prothrombin Activator + Ca^{2+}}} \text{Thrombin}$

• $\text{Fibrinogen} \xrightarrow{\text{Thrombin}} \text{Fibrin (insoluble threads)}$

7.3 Blood Transfusions

Blood transfusions require compatibility between donor and recipient blood types to prevent immune reactions.

• Antigens: Foreign cell surface proteins that trigger immune responses.

• Antibodies: Plasma proteins (gamma globulins) produced by lymphocytes; bind specifically to antigens.

• Antibody-antigen reactions can cause agglutination (clumping) and block blood vessels, leading to transfusion reactions.

ABO Blood Types

Blood Type	Antigens on RBCs	Antibodies in Plasma	Can Receive From	Can Donate To
A	A	Anti-B	A, O	A, AB
B	B	Anti-A	B, O	B, AB
AB	A, B	None	A, B, AB, O	AB
O	None	Anti-A, Anti-B	O	A, B, AB, O

AB+ is the universal recipient; O- is the universal donor.

Rh Blood Typing

• Based on the presence (Rh+) or absence (Rh-) of the Rh antigen on RBCs.

• 85% of Americans are Rh+; 15% are Rh-.

• Rh- individuals can develop antibodies against Rh antigen after exposure (e.g., pregnancy or transfusion).

• Hemolytic Disease of the Newborn: Occurs when an Rh- mother carries an Rh+ fetus; maternal antibodies attack fetal RBCs, especially in subsequent pregnancies.

Additional info: ABO incompatibility between mother and fetus is less severe due to lower antigen density on fetal RBCs.

7.4 Blood Substitutes

Blood substitutes are developed to address shortages, disease transmission risks, and storage limitations of human blood.

• Hemoglobin-Based Oxygen Carriers (HBOCs): Modified hemoglobin from animals or genetically engineered bacteria, dissolved in solution.

• Perfluorocarbons (PFCs): Synthetic, colorless liquids containing carbon and fluorine; must be emulsified for use. They do not transmit diseases but are rapidly removed from the body via the lungs.

7.5 Blood Disorders

• Mononucleosis: Viral infection of lymphocytes; causes fatigue and swollen lymph nodes.

• Septicemia: Bacterial infection of the blood (blood poisoning); can be life-threatening.

• Anemia: Reduced oxygen-carrying capacity due to:

  • Iron deficiency (impaired hemoglobin synthesis)

  • Hemorrhagic anemia (blood loss)

  • Pernicious anemia (vitamin B12 deficiency)

  • Hemolytic anemia (premature RBC destruction, e.g., sickle cell disease)

  • Renal failure (insufficient erythropoietin production)

• Leukemia: Excessive production of abnormal WBCs; impairs normal blood function.

• Multiple Myeloma: Overproduction of plasma cells; disrupts bone marrow function.

• Thrombocytopenia: Low platelet count; increases bleeding risk.