Blood: Structure, Function, and Hemostasis

Blood: Structure, Function, and Hemostasis

Overview of Blood

Blood is a specialized connective tissue essential for the transport of gases, nutrients, hormones, and waste products throughout the body. It also plays a critical role in immune defense, hemostasis, and homeostasis.

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

• Cellular elements: Include red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).

Composition of Blood

Blood consists of plasma and cellular elements, each with distinct components and functions.

• Plasma (about 55% of blood volume):

  • Water (92%)

  • Proteins (7%): Albumins, globulins, fibrinogen, transferrin

  • Other solutes (1%): Ions, nutrients, gases, waste products

• Cellular Elements (about 45% of blood volume):

  • Red blood cells (Erythrocytes): Transport oxygen and carbon dioxide

  • White blood cells (Leukocytes): Immune defense

  • Platelets (Thrombocytes): Cell fragments involved in blood clotting

Table: Major Plasma Proteins

Name	Source	Function
Albumins	Liver	Major contributor to colloid osmotic pressure; carrier for various substances
Globulins	Liver and lymphoid tissue	Clotting factors, enzymes, antibodies, carriers for various substances
Fibrinogen	Liver	Forms fibrin threads essential to blood clotting
Transferrin	Liver and other tissues	Iron transport

Note: The liver is the major site of plasma protein synthesis.

Cellular Elements of Blood

The cellular elements of blood are classified into erythrocytes, leukocytes, and platelets, each with specialized functions.

• Red Blood Cells (Erythrocytes): Biconcave, anucleate cells specialized for oxygen and carbon dioxide transport.

• White Blood Cells (Leukocytes):

  • Lymphocytes (Immunocytes): Mediate specific immune responses.

  • Monocytes: Develop into macrophages, which are phagocytic cells.

  • Neutrophils: Phagocytes and granulocytes; first responders to infection.

  • Eosinophils: Granulocytes involved in defense against parasites and allergic reactions.

  • Basophils: Granulocytes; tissue basophils are called mast cells, involved in inflammation and allergic responses.

• Platelets (Thrombocytes): Small, anucleate cell fragments derived from megakaryocytes, essential for blood clotting.

Hematopoiesis

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

• Leukopoiesis: Production of white blood cells.

• Erythropoiesis: Production of red blood cells.

• Thrombopoiesis: Production of platelets.

• Hematocrit: The ratio of red blood cells to plasma, expressed as a percentage.

Table: Cytokines Involved in Hematopoiesis

Name	Sites of Production	Influences Growth or Differentiation of
Erythropoietin (EPO)	Kidney (mainly)	Red blood cells
Thrombopoietin (TPO)	Liver (primarily)	Megakaryocytes (platelet precursors)
Colony-stimulating factors (CSFs)	Endothelium and fibroblasts of bone marrow, leukocytes	All types of blood cells; mobilizes hematopoietic stem cells

• Colony stimulating factors (CSFs) regulate leukopoiesis.

• Thrombopoietin (TPO) regulates platelet production.

• Erythropoietin (EPO) regulates red blood cell production.

The Complete Blood Count (CBC)

The CBC is a common laboratory test that provides information about the cellular components of blood, including red blood cells, white blood cells, and platelets.

Test	Males	Females
Hematocrit (%)	40–54	37–47
Hemoglobin (g/dL)	14–17	12–16
Red blood cell count (106/μL)	4.5–6.5	3.9–5.6
White blood cell count (103/μL)	4.5–11	4.5–11
Platelet count (103/μL)	150–450	150–450

Note: The CBC also includes differential counts of leukocyte types and indices such as mean corpuscular volume (MCV).

Red Blood Cells (Erythrocytes)

Red blood cells are specialized for oxygen transport and have unique structural and functional properties.

• Mature RBCs lack a nucleus, maximizing space for hemoglobin.

• Mean corpuscular volume (MCV): Indicates the average size of RBCs.

• Microcytes: Abnormally small RBCs, often due to iron deficiency.

• Life span: Approximately 120 days.

• Iron metabolism:

  • Dietary iron is absorbed in the small intestine.

  • Transported in blood by transferrin to red bone marrow.

  • Excess iron is stored in the liver as ferritin.

  • Hemoglobin components are recycled.

• Hemoglobin:

  • Composed of four globin chains, each with a heme group containing an iron atom.

  • Essential for oxygen transport.

Example: In sickle cell disease, abnormal hemoglobin causes RBCs to assume a sickle shape, impairing oxygen delivery.

Iron Metabolism and Hemoglobin Synthesis

Iron is a critical component of hemoglobin, and its metabolism is tightly regulated.

• Heme is a porphyrin ring with an iron (Fe) atom at its center.

• Iron is transported by transferrin in the blood and stored as ferritin in the liver.

• Iron is incorporated into new heme groups during erythropoiesis.

Red Blood Cell Breakdown and Bilirubin Metabolism

RBCs are removed from circulation by the spleen after about 120 days. Heme is broken down into bilirubin, which is excreted in bile and urine.

• Anemia: A condition of low RBC count or low hemoglobin, resulting in decreased oxygen transport.

• Types of anemia include hemolytic anemia, sickle cell disease, and iron-deficiency anemia.

• Jaundice: Results from elevated bilirubin levels due to excessive RBC breakdown or liver dysfunction.

Platelets (Thrombocytes)

Platelets are small, anucleate cell fragments derived from megakaryocytes. They play a vital role in hemostasis, immunity, and inflammation.

• Contain mitochondria, smooth endoplasmic reticulum, and granules with cytokines and growth factors.

• Life span: About 10 days.

• Essential for stopping blood loss by forming platelet plugs and contributing to clot formation.

Hemostasis and Coagulation

Hemostasis is the process that prevents blood loss from damaged vessels. It involves three major steps:

1. Vasoconstriction: Reduces blood flow to the injured area.

2. Platelet plug formation: Platelets adhere to exposed collagen and aggregate.

3. Coagulation: A cascade of enzymatic reactions leads to the conversion of fibrinogen to fibrin, forming a stable clot.

• Intrinsic pathway: Initiated by contact activation.

• Extrinsic pathway: Triggered by tissue injury.

• Thrombin converts fibrinogen into insoluble fibrin polymers.

• Fibrinolysis: The breakdown of fibrin by the enzyme plasmin, leading to clot dissolution.

Anticoagulants and Disorders of Hemostasis

Anticoagulants are substances that prevent excessive clotting.

• Thrombus: A blood clot that forms and adheres to an undamaged vessel wall.

• Mechanisms limiting clotting:

  • Inhibition of platelet adhesion

  • Inhibition of the coagulation cascade and fibrin production

• Endogenous anticoagulants: Heparin, antithrombin III, protein C

• Hemophilia: A group of disorders characterized by defective or missing coagulation factors, leading to impaired clotting.

Summary Table: Key Blood Disorders

Disorder	Description
Anemia	Low RBC count or hemoglobin; decreased oxygen transport
Polycythemia vera	Excessive RBC production; increased blood viscosity
Hemolytic anemia	RBC destruction exceeds production (e.g., hereditary spherocytosis, sickle cell disease)
Jaundice	Elevated bilirubin due to excessive RBC breakdown or liver dysfunction
Hemophilia	Deficiency of clotting factors; impaired coagulation

Key Equations

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

Additional info:

• Blood cell development (hematopoiesis) is regulated by cytokines and growth factors, ensuring proper balance and function of all blood cell types.

• Disorders of blood can affect oxygen delivery, immune defense, and hemostasis, with clinical manifestations ranging from anemia to clotting disorders.