Blood: The Internal Transport System

Introduction to Blood

Blood is a vital connective tissue that serves as the internal transport system of the body. It is essential for maintaining homeostasis by transporting substances, regulating body functions, and providing protection against disease and blood loss.

• Functions of Blood:

  • Transport: Delivers oxygen and nutrients to tissues, removes metabolic wastes (such as carbon dioxide and urea), and transports hormones from endocrine glands to target organs.

  • Regulation: Maintains body temperature by absorbing and distributing heat, regulates pH through buffers (such as bicarbonate), and maintains fluid volume in the circulatory system.

  • Protection: Prevents blood loss through clot formation (hemostasis) and protects against infection via antibodies, complement proteins, and white blood cells (leukocytes).

• Components of Blood:

  • Plasma: The liquid matrix, making up about 55% of blood volume, composed mostly of water, proteins, nutrients, hormones, and waste products.

  • Formed Elements: Includes erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (cell fragments involved in clotting).

Composition and Physical Characteristics of Blood

Blood Composition

• Plasma: Straw-colored, sticky fluid (about 90% water) containing dissolved solutes such as nutrients, gases, hormones, electrolytes, and plasma proteins (albumin, globulins, fibrinogen).

• Formed Elements:

  • Erythrocytes (RBCs): Specialized for oxygen and carbon dioxide transport.

  • Leukocytes (WBCs): Involved in immune defense.

  • Platelets: Fragments of megakaryocytes, essential for blood clotting.

When blood is centrifuged, it separates into three layers:

• Plasma (~55%)

• Buffy coat (thin layer of leukocytes and platelets)

• Erythrocytes (hematocrit: Males 47% ± 5%; Females 42% ± 5%)

Physical Properties

• Blood is a sticky, opaque fluid with a metallic taste.

• Color varies with oxygen content: scarlet (oxygen-rich) to dark red (oxygen-poor).

• pH: 7.35–7.45 (slightly alkaline).

• Denser and more viscous than water, due mainly to RBCs.

• Average volume: 5–6 L in males, 4–5 L in females.

Erythrocytes (Red Blood Cells)

Structure and Function

Erythrocytes are small, biconcave, anucleate cells specialized for gas transport.

• Biconcave shape: Increases surface area for gas exchange.

• No nucleus or organelles: Maximizes space for hemoglobin (Hb).

• Flexible membrane: Allows passage through narrow capillaries.

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

Hemoglobin and Gas Transport

• Hemoglobin (Hb): A protein made of four polypeptide chains (two alpha, two beta), each with a heme group containing an iron atom that binds oxygen.

• Oxyhemoglobin: Hb bound to oxygen (in lungs).

• Deoxyhemoglobin: Hb after releasing oxygen (in tissues).

• Carbaminohemoglobin: Hb bound to carbon dioxide (about 20% of CO2 transported this way).

Hematopoiesis: Blood Cell Formation

Overview

Hematopoiesis is the process of blood cell formation, occurring primarily in red bone marrow.

• Hematopoietic stem cells (HSCs): Also called hemocytoblasts, these give rise to all formed elements via specific developmental pathways.

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

• Stages of Erythropoiesis:

  1. Hematopoietic stem cell → Myeloid stem cell → Proerythroblast

  2. Basophilic erythroblast (ribosome synthesis)

  3. Polychromatic erythroblast (hemoglobin accumulation)

  4. Orthochromatic erythroblast (nucleus ejected)

  5. Reticulocyte (enters bloodstream, matures into erythrocyte)

Regulation of Erythropoiesis

• Erythropoietin (EPO): Hormone produced by kidneys (and liver to a lesser extent) in response to hypoxia (low oxygen levels); stimulates RBC production.

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

Fate and Destruction of Erythrocytes

• Lifespan: 100–120 days.

• Destruction: Old RBCs are trapped in the spleen and phagocytized by macrophages.

• Hemoglobin Breakdown:

  • Iron is salvaged and stored as ferritin or hemosiderin.

  • Heme is degraded to bilirubin, which is excreted in bile and converted to stercobilin (gives feces brown color).

  • Globin is broken down into amino acids.

Erythrocyte Disorders

Anemia

Anemia is a condition in which the blood's oxygen-carrying capacity is too low to support normal metabolism.

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

• Causes:

  • Blood loss: Hemorrhagic anemia (acute or chronic).

  • Decreased RBC production: Iron-deficiency anemia, pernicious anemia (vitamin B12 deficiency), renal anemia (lack of EPO), aplastic anemia (bone marrow failure).

  • Increased RBC destruction: Hemolytic anemia (due to abnormal Hb, incompatible transfusions, infections), thalassemia, sickle-cell anemia.

Polycythemia

• Polycythemia vera: Bone marrow cancer causing excessive RBC production; increases blood viscosity.

• Secondary polycythemia: Caused by low oxygen levels (e.g., high altitude) or increased EPO production.

• Blood doping: Artificially increasing RBC count to enhance athletic performance.

Platelets (Thrombocytes)

Structure and Function

• Platelets: Small fragments of megakaryocytes, essential for blood clotting.

• Contain: Chemicals such as serotonin, Ca2+, enzymes, ADP, and platelet-derived growth factor (PDGF).

• Function: Form temporary plugs to seal breaks in blood vessels; kept inactive by nitric oxide (NO) and prostacyclin from endothelial cells.

Platelet Formation (Thrombopoiesis)

• Regulated by thrombopoietin: A hormone produced by the liver and kidneys.

• Development: Megakaryoblast → Megakaryocyte (undergoes mitosis without cytokinesis) → Platelets (fragments enter bloodstream).

• Lifespan: About 10 days.

• Normal count: 150,000–400,000 platelets/μL of blood.

Summary Table: Formed Elements of Blood

Key Equations

• Hematocrit Calculation:

• Oxygen Carrying Capacity:

Example: Sickle-Cell Anemia

Sickle-cell anemia is caused by a single amino acid mutation in the beta chain of hemoglobin, leading to crescent-shaped RBCs that rupture easily and block small blood vessels. This condition is prevalent in individuals of African descent and provides some resistance to malaria.

Additional info:

• These notes are based on Chapter 17 of Marieb and Hoehn's Anatomy & Physiology (11th Edition), covering the structure, function, and disorders of blood.

• Further study should include the mechanisms of hemostasis, blood typing, and transfusion reactions, which are typically covered in subsequent sections.