BY453: Overview of Blood and Its Functions

BY453 Connective Tissue: Blood - Overview

  • Date of Lecture: 19th October 2015

  • Instructor: Dr. Peter Bush

  • Course: Special Topics in Human Physiology

Introduction to Blood

  • Definition of Blood:

    • Blood is defined as the fluid that circulates in the heart, arteries, capillaries, and veins of vertebrate animals. It functions primarily to carry nourishment and oxygen to all parts of the body while also removing waste products.

Characteristics of Blood

  • Classification:

    • Blood is classified as a connective tissue.

  • Components:

    • Contains an extracellular matrix known as plasma.

    • Includes formed elements which consist of living cells.

  • Volume:

    • Approximately 5 to 6 litres in circulation.

Functions of Blood

  • Major Functions Include:

    • Transport:

    • Gases (e.g., O2, CO2)

    • Nutrients (e.g., glucose)

    • Waste products

    • Processed molecules (e.g., Vitamin D)

    • Regulatory molecules (hormones and enzymes)

    • Regulation:

    • pH (normal range 7.4)

    • Osmosis (via bicarbonate and other ions)

    • Body temperature (regulation from core to skin)

    • Protection:

    • Against foreign substances through innate (phagocytes) and adaptive (antibodies) defenses.

    • Repair:

    • Clot formation (initial step in tissue repair).

Composition of Blood

  • Components:

    • Living Cells (Formed Elements):

    • Erythrocytes (Red Blood Cells)

    • Leukocytes (White Blood Cells)

    • Platelets (Cell Fragments)

    • Non-living Matrix (Plasma):

    • A straw-colored, fluid portion of blood where formed elements are suspended.

    • Differentiated from serum as it retains fibrinogen.

    • Contains approximately 90% water alongside a complex mixture of solutes including nutrients, salts (metal ions like Na⁺ and K⁺), gases, hormones, proteins, and waste products.

Plasma Proteins

  • Production:

    • All plasma proteins are produced by the liver except antibodies (g-globulins), which are produced by B-lymphocyte cells.

  • Functions of Plasma Proteins Include:

    • Oncotic Pressure:

    • Measures approximately 25 mmHg, crucial for interstitial fluid reabsorption.

    • Transport Functions:

    • Function in carrying gases, nutrients, hormones, and waste products.

    • Viscosity:

    • Plasma viscosity is about 2.5 times greater than water, assisting in maintaining arterial pressure.

    • pH Buffering:

    • Contributes to about 1/6th of total blood buffering capacity with amino groups acting as bases and carboxylic groups as acids depending on pH levels.

    • Antibody Function:

    • Protects against pathogens.

Formed Elements of Blood

  • Definition:

    • Formed elements refer to the cellular components of blood, which are non-fluid elements consisting of blood cells.

  • Types of Formed Elements Include:

    • Erythrocytes (Red Blood Cells - RBCs):

    • Count: 4-6 million (approximately 1000 times more than leukocytes).

    • Function: Oxygen transport primarily due to hemoglobin (Hb).

    • Structure: Biconcave disks with no membrane-bound organelles; lifespan of about 120 days with 1% replaced daily from red bone marrow.

    • Leukocytes (White Blood Cells - WBCs):

    • Function: Essential for immune defense against bacteria, fungi, and viruses; complete cells capable of diapedesis (movement through blood vessel walls).

    • Platelets (Thrombocytes):

    • Size: 2-3 µm in diameter, anucleate, derived from megakaryocytes; crucial for blood clotting.

Oxygen Transport

  • Dissolved Oxygen:

    • Molecular oxygen (O₂) has low solubility in water, at approximately 0.3%.

  • Evolutionary Adaptations:

    • Increased demand for O₂ in more active animals led to complex respiratory systems, circulatory apparatus, and the development of oxygen-transporting pigments like hemoglobin.

Hemoglobin Structure & Function

  • Structure:

    • Hemoglobin is characterized by a heterogeneous quaternary structure comprising four different protein chains (protomers), with each chain containing approximately 130 amino acids and a central hem group.

    • The hem group consists of four pyrrole rings with ferrous iron (Fe²⁺) at the center, structured as 2 alpha and 2 beta chains (a₂b₂).

  • Oxygen Binding:

    • Each hem group can bind with one O₂ molecule, allowing a total of four O₂ molecules per hemoglobin molecule (forming oxyhemoglobin, HbO₂).

    • Binding of O₂ promotes a conformational change in hemoglobin, enhancing its affinity for additional oxygen, a phenomenon known as cooperativity.

O2-Hb Dissociation Curve

  • Representation:

    • Shows the relationship between the percent O₂ saturation of hemoglobin and the volume of O₂ unloaded to tissues.

  • Factors Influencing the Curve:

    • Higher metabolic demand shifts the curve to the right, promoting O₂ release to tissues, while lower demand shifts it left, favoring O₂ binding.

    • Changes in pH or temperature can also affect affinity, where increases in pH or decreases in temperature shift it left, and vice versa for right shifts.

Types of Leukocytes

  • Classification:

    • Leukocytes can be divided into granulocytes (70% of WBCs) characterized by granules in the cytoplasm, and agranulocytes (30% of WBCs) that do not have these granules.

  • Granulocyte Types:

    • Neutrophils

    • Eosinophils

    • Basophils

  • Agranulocyte Types:

    • Lymphocytes

    • Monocytes (which serve as precursors for phagocytes).

Platelets

  • Origin:

    • Derived from ruptured multinucleate cells known as megakaryocytes.

  • Structure:

    • Small, disk-shaped (approximately 2-3 µm diameter) and anucleate, essential for the clotting process.