Blood: Structure, Function, and Hemostasis – Study Notes for Anatomy & Physiology

Blood: Structure and Components

Overview of Blood

Blood is a connective tissue that circulates through the body, delivering essential substances and removing waste. It consists of plasma and formed elements, each with distinct roles in maintaining homeostasis.

• Plasma: The liquid extracellular matrix of blood, making up about 55% of total blood volume. It is 90% water and contains proteins, nutrients, hormones, and waste products.

• Formed Elements: The cellular components of blood, including erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (thrombocytes).

• Blood Volume: The average adult has about 5 liters of blood, which is approximately 8% of total body weight.

Layers of Blood After Centrifugation

When blood is centrifuged, it separates into three distinct layers:

• Top Layer: Plasma, constituting about 55% of total blood volume.

• Middle Layer: Buffy coat, containing leukocytes and platelets, less than 1% of total blood volume.

• Bottom Layer: Erythrocytes, making up about 45% of total blood volume. The percentage of erythrocytes is called the hematocrit.

Plasma Proteins

Plasma proteins form a colloid and account for about 9% of plasma volume. Major plasma proteins include:

• Albumin: Maintains osmotic pressure and transports substances.

• Globulins: Involved in immune responses and transport.

• Fibrinogen: Essential for blood clotting.

Functions of Blood

Overview of Blood Functions

Blood performs several critical functions necessary for life:

• Exchanging gases: Transports oxygen from lungs to tissues and carbon dioxide from tissues to lungs.

• Distributing solutes: Delivers nutrients, hormones, and waste products throughout the body.

• Performing immune functions: Contains cells and proteins that defend against pathogens.

• Maintaining body temperature: Distributes heat generated by metabolism.

• Sealing damaged vessels by forming blood clots: Prevents excessive blood loss after injury.

• Preserving acid-base homeostasis: Buffers blood pH to maintain a stable internal environment.

• Stabilizing blood pressure: Maintains fluid volume and pressure within vessels.

Erythrocytes and Oxygen Transport

Structure and Function of Erythrocytes

Erythrocytes (red blood cells) are specialized for oxygen transport. Their biconcave shape increases surface area for gas exchange and allows flexibility in capillaries.

• Mature RBCs: Lack organelles and nuclei, maximizing space for hemoglobin.

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

• Oxygen Binding: Each hemoglobin molecule can bind up to four oxygen molecules. Oxygen binding forms oxyhemoglobin ($HbO_2$); when oxygen is released, it forms deoxyhemoglobin.

• Carbon Dioxide Transport: Hemoglobin also binds carbon dioxide ($CO_2$), accounting for about 23% of $CO_2$ transport in blood.

Life Span and Erythropoiesis

RBCs have a lifespan of about 120 days. Damaged or old RBCs are removed by the spleen and liver. Erythropoiesis is the process of RBC formation from hematopoietic stem cells (HSCs) in the bone marrow.

• Stages of Erythropoiesis: HSCs differentiate into erythrocyte colony-forming units (CFUs), then into proerythroblasts, erythroblasts, reticulocytes, and finally mature erythrocytes.

• Regulation: Erythropoietin (EPO), produced by the kidneys, stimulates erythropoiesis in response to low oxygen levels.

Destruction of Erythrocytes

Old erythrocytes become less flexible and are trapped in the spleen, where they are phagocytosed and broken down. Hemoglobin is recycled or converted to bilirubin for excretion.

Anemia

Anemia is a condition characterized by decreased oxygen-carrying capacity of the blood. Causes include:

• Decreased hemoglobin: Often due to iron deficiency.

• Decreased hematocrit: Blood loss or reduced RBC production.

• Abnormal hemoglobin: Genetic disorders such as sickle cell disease.

Leukocytes and Immune Function

Types of Leukocytes

Leukocytes (white blood cells) are divided into two main categories:

• Granulocytes: Contain visible cytoplasmic granules. Types include:

  • Neutrophils: Most common, multi-lobed nucleus, phagocytize bacteria, stain with both acidic and basic dyes.

  • Eosinophils: Bilobed nucleus, combat parasitic infections, stain red with acidic dyes.

  • Basophils: Least common, S-shaped nucleus, release histamine, stain dark purple with basic dyes.

• Agranulocytes: Lack visible granules. Types include:

  • Lymphocytes: Large nucleus, include B cells (produce antibodies) and T cells (destroy abnormal cells).

  • Monocytes: Largest WBCs, U-shaped nucleus, differentiate into macrophages that phagocytose pathogens and debris.

Leukocyte Formation (Leukopoiesis)

Leukopoiesis is the process by which HSCs in the bone marrow differentiate into various leukocyte types. Myeloid and lymphoid cell lines produce granulocytes, monocytes, and lymphocytes.

Platelets and Hemostasis

Platelet Structure and Function

Platelets (thrombocytes) are small cell fragments involved in blood clotting. They contain granules with clotting factors and enzymes.

• Function: Form plugs to seal damaged blood vessels and initiate clotting cascades.

• Formation: Platelets are derived from megakaryocytes in the bone marrow, which fragment into thousands of platelets.

• Lifespan: Platelets survive for about 7–10 days before being removed by the liver and spleen.

Hemostasis

Hemostasis is the process that stops bleeding after vessel injury. It involves three main steps:

1. Vascular Spasm: Immediate vasoconstriction to reduce blood flow.

2. Platelet Plug Formation: Platelets adhere to exposed collagen and aggregate to form a temporary plug.

3. Coagulation: A cascade of reactions leads to the conversion of fibrinogen to fibrin, stabilizing the clot.

Coagulation Pathways

Coagulation involves two pathways that converge on a common pathway:

• Intrinsic Pathway: Initiated by damage inside the vessel; involves clotting factors XII, XI, IX, VIII.

• Extrinsic Pathway: Initiated by tissue factor outside the vessel; involves factor VII.

• Common Pathway: Both pathways activate factor X, leading to the formation of fibrin.

Key Equation:

$\text{Prothrombin} \xrightarrow{\text{Factor Xa}} \text{Thrombin}$ $\text{Fibrinogen} \xrightarrow{\text{Thrombin}} \text{Fibrin}$

Blood Typing and Transfusion

ABO and Rh Blood Groups

Blood typing is based on the presence of antigens (A, B, Rh) on erythrocyte membranes. The ABO system includes four main blood types: A, B, AB, and O. The Rh factor determines positive or negative status.

• Antibodies: Individuals produce antibodies against antigens not present on their own RBCs.

• Transfusion Compatibility: Matching donor and recipient blood types is essential to prevent transfusion reactions.

Blood Transfusion Reactions

Transfusion reactions occur when antibodies attack transfused RBCs with incompatible antigens, leading to agglutination and hemolysis.

Summary Table: Blood Cell Types and Functions

Cell Type	Main Function	Key Features
Erythrocytes (RBCs)	Oxygen and carbon dioxide transport	Biconcave, no nucleus, contains hemoglobin
Leukocytes (WBCs)	Immune defense	Granulocytes (neutrophils, eosinophils, basophils); Agranulocytes (lymphocytes, monocytes)
Platelets	Blood clotting	Cell fragments, contain clotting factors

Additional info: Academic context and definitions have been expanded for clarity and completeness. All equations are provided in LaTeX format as required.