Chapter 7: Blood – Structure, Function, and Disorders

Blood: Structure, Function, and Disorders

Circulatory System Overview

The circulatory system is essential for transporting substances throughout the body and maintaining homeostasis. It consists of the heart (the pump), blood vessels (the conduits), and blood (the transport medium).

• Transport of Nutrients: Picks up nutrients from the digestive system and distributes them to cells.

• Gas Exchange: Exchanges oxygen and carbon dioxide with the respiratory system.

• Waste Removal: Carries metabolic wastes to the liver and kidneys for removal, and excess water to the urinary system.

• Temperature Regulation: Helps regulate body temperature.

• Immune Defense: Contains specialized cells for immune defense.

The Composition and Functions of Blood

Blood is a specialized connective tissue composed of formed elements and plasma.

• Formed Elements (45%): Specialized cells and cell fragments, including red blood cells (RBCs), white blood cells (WBCs), and platelets.

• Plasma (55%): The liquid component, consisting of water, proteins, hormones, ions, nutrients, and wastes.

• Blood Volume: Adult males: 5–6 liters; Adult females: 4–5 liters.

• Viscosity: Blood is five times more viscous than water.

Table: Main Components of Blood

Component	Percentage	Main Constituents
Plasma	55%	Water, proteins (albumins, globulins, clotting proteins), hormones, ions, nutrients, wastes
Formed Elements	45%	Red blood cells, white blood cells, platelets

Plasma: Water and Dissolved Solutes

Plasma is the liquid portion of blood, making up about 55% of total blood volume.

• Composition: 90% water, 10% dissolved solutes (proteins, hormones, ions, amino acids, carbohydrates, vitamins, metabolic wastes).

• Plasma Proteins:

  • Albumins: Maintain osmotic balance with interstitial fluid.

  • Globulins: Transport substances (alpha, beta, gamma globulins). Beta globulins transport lipids (as lipoproteins: HDL and LDL). Gamma globulins are antibodies.

  • Clotting Proteins: Essential for blood clotting (e.g., fibrinogen).

Red Blood Cells (Erythrocytes): Structure and Function

Red blood cells (RBCs) are specialized for the transport of oxygen and carbon dioxide.

• Structure: Lack a nucleus and most organelles; packed with hemoglobin.

• Hemoglobin: Protein with four polypeptide chains, each with a heme group containing iron for oxygen binding.

• Origin: Produced from stem cells in the bone marrow.

• Life Span: Approximately 120 days.

• Regulation: Erythropoietin (EPO) hormone from the kidneys stimulates RBC production in response to low oxygen.

Hemoglobin and Oxygen Transport

• Oxygen Binding: High oxygen concentration (lungs) promotes binding; low concentration (tissues) promotes release.

• pH Effect: Neutral pH (lungs) strengthens binding; acidic pH (tissues) promotes release.

• Terminology: Oxyhemoglobin (with oxygen, bright red); Deoxyhemoglobin (without oxygen, dark red).

Hematocrit and Hemoglobin Measurement

• Hematocrit: Percentage of blood volume occupied by RBCs (Men: 43–49%, Women: 37–43%).

• Hemoglobin: Amount of hemoglobin in blood (Men: 14–18 gm%, Women: 12–14 gm%).

• Clinical Relevance: Low values may indicate anemia; high values may occur at high elevation or indicate risk due to increased viscosity.

Blood Cell Formation and Life Cycle

• Hematopoietic Stem Cells: Located in bone marrow; produce all blood cells and platelets.

• RBC Maturation: Erythroblasts mature into erythrocytes, losing their nucleus and organelles.

• Destruction: Old RBCs are removed by macrophages in the liver and spleen; iron and amino acids are recycled, heme is converted to bilirubin and excreted.

White Blood Cells (Leukocytes): Defense and Immunity

White blood cells protect the body against infection and regulate inflammation.

• Categories:

  • Granular Leukocytes: Neutrophils (60%, first responders, phagocytosis), Eosinophils (2–4%, defend against parasites, moderate allergies), Basophils (0.5%, release histamine in inflammation/allergy).

  • Agranular Leukocytes: Monocytes (5%, become macrophages), Lymphocytes (30%, immune response; B and T lymphocytes).

• Short Life Span: Most WBCs live only a few days; numbers increase rapidly during infection.

Platelets and Hemostasis (Blood Clotting)

Platelets are cell fragments essential for stopping blood loss (hemostasis).

• Origin: Fragments of megakaryocytes in bone marrow.

• Function: Initiate clotting and participate in vessel repair.

Stages of Hemostasis

1. Vascular Spasm: Constriction of blood vessels to reduce blood flow.

2. Platelet Plug Formation: Platelets stick together to seal the rupture.

3. Coagulation: Formation of a blood clot via a cascade involving at least 12 clotting proteins.

Key Steps in Coagulation

• Platelets release prothrombin activator.

• Prothrombin activator converts prothrombin to thrombin.

• Thrombin converts fibrinogen to fibrin, forming a mesh that traps cells and forms the clot.

Blood Types and Transfusion Compatibility

Blood transfusions require matching of blood types to prevent immune reactions.

• Antigens: Foreign proteins on RBC surfaces (A, B, AB, O types; Rh factor).

• Antibodies: Defensive proteins in plasma that target non-self antigens.

• ABO System:

  • Type A: A antigen, anti-B antibodies; receives A or O.

  • Type B: B antigen, anti-A antibodies; receives B or O.

  • Type AB: A and B antigens, no antibodies; universal recipient.

  • Type O: No antigens, anti-A and anti-B antibodies; universal donor.

• Rh Factor: Rh-positive (antigen present), Rh-negative (antigen absent). Rh-negative individuals can develop antibodies if exposed to Rh-positive blood.

Table: ABO Blood Types and Compatibility

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

Blood Typing and Cross-Matching

• Blood Typing: Identifies ABO and Rh types by adding specific antibodies and observing for agglutination (clumping).

• Cross-Matching: Mixes donor and recipient blood to check for compatibility.

Blood Substitutes

• Reasons for Use: Disease risk, limited supply, storage issues.

• Types: Modified hemoglobin solutions, perfluorocarbons (PFCs) that temporarily carry oxygen.

Blood Disorders

• Infections: Mononucleosis (Epstein-Barr virus), septicemia (bacterial infection of blood).

• Anemia: Reduced oxygen-carrying capacity due to low RBCs or hemoglobin. Types include iron-deficiency, hemorrhagic, pernicious (vitamin B12 deficiency), hemolytic, and anemia due to renal failure.

• Leukemia: Cancer of WBCs; abnormal proliferation of white blood cells.

• Multiple Myeloma: Cancer of plasma cells in bone marrow.

• Thrombocytopenia: Low platelet count, leading to unusual bleeding and bruising.

• Hemophilia: Genetic deficiency of clotting factors, leading to bleeding disorders.

Donating Blood

• Donors are screened for health and risk factors.

• Blood is collected, tested for infectious agents, and can be separated into components for transfusion.

• Donated blood volume is quickly restored by the body.