Blood: Structure, Function, and Hemostasis – Study Guide

Blood: Structure, Function, and Hemostasis

Introduction

Blood is a vital connective tissue that plays essential roles in transport, regulation, and protection within the human body. Understanding its composition, the functions of its components, and the mechanisms of hemostasis is fundamental in Anatomy & Physiology.

Blood Components and Plasma

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

• Interstitial Fluid: Fluid found between tissue cells, derived from plasma but with a different composition due to selective permeability of capillary walls.

• Cytosol: The intracellular fluid within cells, distinct from plasma and interstitial fluid.

• Barriers: The capillary wall separates plasma from interstitial fluid, while the cell membrane separates cytosol from interstitial fluid.

Plasma Constituents

• Major plasma proteins: Albumin (maintains osmotic pressure), globulins (immune function, transport), and fibrinogen (clotting).

• Other components: Nutrients (glucose, amino acids), waste products (urea, creatinine), gases (O2, CO2), hormones, and electrolytes (Na+, K+, Ca2+).

Formed Elements of Blood

• Erythrocytes (Red Blood Cells): Specialized for oxygen and carbon dioxide transport. Lack nuclei and most organelles in mature form.

• Leukocytes (White Blood Cells): Involved in immune defense. Several types exist, each with specific functions (see below).

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

Plasma Proteins: Types and Functions

• Albumin: Maintains blood osmotic pressure and transports substances.

• Globulins: Include antibodies (immunoglobulins) and transport proteins.

• Fibrinogen: Precursor to fibrin, essential for blood clotting.

Hematopoiesis

• Definition: The process of blood cell formation, primarily occurring in red bone marrow.

• Stem Cells: Hematopoietic stem cells differentiate into all blood cell types.

• Sites: In adults, mainly in the axial skeleton, pelvic girdle, and proximal epiphyses of femur and humerus.

Erythrocytes: Structure and Function

• Structure: Biconcave discs, lack nuclei and mitochondria, filled with hemoglobin.

• Function: Transport oxygen (via hemoglobin) and carbon dioxide.

• Hemoglobin: Protein with four subunits, each containing a heme group that binds O2. Also carries some CO2.

• Globin and Heme: Globin is the protein part; heme contains iron and binds oxygen.

Hemoglobin and Gas Transport

• Oxygen Transport: Each hemoglobin molecule can bind up to four O2 molecules.

• Carbon Dioxide Transport: Some CO2 binds to globin; most is transported as bicarbonate ions in plasma.

Erythropoiesis Regulation

• Stimulus: Low blood oxygen (hypoxia) stimulates erythropoietin (EPO) release from the kidneys.

• Effect: EPO promotes red blood cell production in bone marrow.

Destruction and Recycling of Erythrocytes

• Phagocytosis: Old erythrocytes are removed by macrophages in the liver, spleen, and bone marrow.

• Recycling: Iron is salvaged and reused; heme is converted to bilirubin and excreted in bile.

Leukocytes: Types and Functions

• Granulocytes: Neutrophils (phagocytosis), eosinophils (combat parasites, allergies), basophils (release histamine).

• Agranulocytes: Lymphocytes (B cells, T cells, NK cells – immune response), monocytes (differentiate into macrophages).

• Leukopoiesis: Formation of white blood cells, regulated by cytokines and growth factors.

Platelets and Hemostasis

• Platelets: Fragments of megakaryocytes, essential for clot formation.

• Hemostasis: The process that stops bleeding, involving three main steps:

  1. Vascular Spasm: Vasoconstriction reduces blood flow.

  2. Platelet Plug Formation: Platelets adhere to exposed collagen and aggregate.

  3. Coagulation: Fibrin mesh forms, stabilizing the clot.

• Clot Retraction and Fibrinolysis: Clot contracts and is eventually dissolved by plasmin.

Blood Types and Antigens

• ABO System: Determined by presence of A and/or B antigens on erythrocytes.

• Rh System: Presence (+) or absence (–) of Rh antigen (D antigen).

• Antibodies: Plasma proteins that react with foreign antigens.

• Transfusion Reactions: Occur if incompatible blood is transfused.

Summary Table: Blood Cell Types and Functions

Cell Type	Main Function	Relative Abundance
Erythrocytes	Transport O2 and CO2	~99% of formed elements
Neutrophils	Phagocytosis of bacteria	Most abundant WBC
Lymphocytes	Immune response (B, T, NK cells)	Second most abundant WBC
Monocytes	Phagocytosis; become macrophages	3rd most abundant WBC
Eosinophils	Combat parasites, allergies	Rare
Basophils	Release histamine, inflammation	Rarest WBC
Platelets	Clot formation	Numerous, but less than RBCs

Hemostasis vs. Coagulation

• Hemostasis: The overall process of stopping bleeding, including vascular spasm, platelet plug formation, and coagulation.

• Coagulation: The specific step in hemostasis where fibrin forms a stable blood clot.

Key Equations

• Oxygen Carrying Capacity of Hemoglobin:

$\text{O}_2\ \text{carried} = \text{Number of Hb molecules} \times 4\ \text{(O}_2\ \text{per Hb)}$

• Bicarbonate Buffer Equation (CO2 transport):

$\text{CO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{CO}_3 \leftrightarrow \text{H}^+ + \text{HCO}_3^-$

Additional info:

• Some details, such as the precise percentages of plasma proteins or the full list of cytokines involved in leukopoiesis, are inferred from standard Anatomy & Physiology knowledge.