Comprehensive Study Guide: Composition and Characteristics of Blood

Overview of Blood as a Connective Tissue

• Definition and Classification: Blood is a connective tissue, which, by definition, is composed of cellular elements and an extracellular matrix.

• Cellular Elements (Formed Elements):     * Red Blood Cells (RBCs): Known clinically as erythrocytes.     * White Blood Cells (WBCs): Known clinically as leukocytes.     * Cell Fragments: Known clinically as platelets or thrombocytes.

• Extracellular Matrix (Plasma): Blood is unique among connective tissues because its extracellular matrix, plasma, is fluid. It is composed mostly of water and serves to perpetually suspend the formed elements, allowing them to circulate through the cardiovascular system.

Principal Functions of Blood

• Primary Function: The fundamental role of blood is to deliver oxygen ($O_2$) and nutrients to body cells and to remove waste products from them.

• Transportation Mechanisms:     * Nutrients: Absorbed in the digestive tract, most travel via the bloodstream to the liver to be processed before being released back into circulation for delivery to cells.     * Oxygen: Diffuses from the air in the lungs into the blood, where it is pumped by the heart to the rest of the body.     * Hormones: Endocrine glands release hormones directly into the bloodstream, which carries them to distant target cells.     * Waste Removal: Blood picks up cellular wastes and byproducts for transport to specific organs for excretion.         * Carbon Dioxide ($CO_2$): Transported to the lungs for exhalation.         * Other Wastes: Transported to the kidneys and liver for excretion in the form of urine or bile.

• Defense Functions:     * External Threats: WBCs protect the body from invading pathogens, such as disease-causing bacteria that enter through wounds.     * Internal Threats: WBCs seek out and destroy internal hazards, including cells with mutated DNA (potential cancer cells) or cells infected with viruses.     * Hemostasis (Blood Loss Prevention): When blood vessels are damaged, platelets and specific plasma proteins interact to block the ruptured area, preventing excessive blood loss.

• Maintenance of Homeostasis:     * Temperature Regulation: Regulated via a negative-feedback loop. During exercise or high heat, blood is transported from the core to the cooler body periphery (skin) to dissipate heat. In cold conditions, blood is diverted away from the skin to the core to maintain warmth, though extreme cases can result in frostbite.     * Chemical Balance: Proteins and other compounds within the blood act as buffers to regulate the pH of body tissues.     * Water Content: Blood helps regulate the volume of water within body cells.

Clinical Composition and the Hematocrit

• Blood Analysis: Common tests include measuring lipids or glucose in the plasma, or checking the quantities and types of formed elements.

• Hematocrit: A test measuring the percentage of erythrocytes in a blood sample.     * Procedure: A blood sample is spun in a specialized centrifuge. This causes heavier elements to separate from the lighter-weight liquid plasma.     * Centrifugation Layers:         * Erythrocytes: Settled at the very bottom because they are the heaviest elements.         * Buffy Coat: A pale, thin layer above the erythrocytes containing leukocytes (WBCs) and thrombocytes (platelets). It normally constitutes less than $1\%$ of the sample.         * Plasma: A pale, straw-colored fluid that floats at the top.

• Packed Cell Volume (PCV): Another term for the volume of erythrocytes after centrifugation.

• Hematocrit Values:     * Normal Sample Average: Approximately $45\%$ erythrocytes.     * Standard Range: Generally between $36\%-50\%$, though this varies by gender.     * Female Values: Range is $37$ to $47$, with a mean value of $41$.     * Male Values: Range is $42$ to $52$, with a mean value of $47$.

• Plasma Percentages (Mean):     * Females: Approximately $59\%$ ($100 - 41$).     * Males: Approximately $53\%$ ($100 - 47$).

Physical Characteristics of Blood

• Color:     * Bright Red: Blood that has just taken up oxygen in the lungs.     * Dusky Red: Blood that has released oxygen in the tissues.     * This color variance is due to hemoglobin, a pigment that changes color based on oxygen saturation levels.

• Viscosity: A measure of a fluid's thickness or resistance to flow.     * Blood is approximately five times ($5 \times$) more viscous than water.     * Viscosity is influenced by plasma proteins and formed elements and significantly impacts blood pressure and flow.

• Temperature:     * The normal temperature of blood is approximately $38\,^{\circ}C$ ($100.4\,^{\circ}F$).     * This is slightly higher than the internal body temperature of $37\,^{\circ}C$ ($98.6\,^{\circ}F$).     * The extra heat is produced by friction and resistance as blood flows through vessels, particularly as vessels age and lose elasticity.     * Daily variations of $0.5\,^{\circ}C$ are considered normal.

• pH Levels:     * The average pH of blood is $7.4$.     * The healthy range is $7.35$ to $7.45$, making blood slightly basic (alkaline) compared to pure water ($pH = 7.0$).

• Volume and Weight:     * Blood constitutes approximately $8\%$ of adult body weight.     * Adult Males: Average $5-6\,L$ of blood.     * Adult Females: Average $4-5\,L$ of blood.

Components and Proteins of Blood Plasma

• General Composition: Plasma is approximately $92\%$ water. Most of the remaining portion consists of suspended or dissolved proteins.

• Plasma Proteins ($7\%$ of plasma volume): Total non-water volume is nearly all proteins, including specific plasma proteins and smaller amounts of regulatory proteins (enzymes and hormones).     * Albumin:         * The most abundant plasma protein ($\approx 54\%$ of total plasma protein).         * Clinical levels: $3.5-5.0\,g/dL$ of blood.         * Manufactured by the liver.         * Functions: Serves as a transport vehicle for hydrophobic fatty acids and steroid hormones. It is the primary contributor to blood osmotic pressure, holding water inside vessels and drawing it from tissues to maintain blood volume and pressure.     * Globulins:         * The second most common plasma proteins ($\approx 38\%$ of total plasma protein).         * Clinical levels: $1.0-1.5\,g/dL$ of blood.         * Alpha and Beta Globulins: Produced by the liver; transport iron, lipids, and fat-soluble vitamins ($A, D, E, K$) to cells and contribute to osmotic pressure.         * Gamma Globulins: Known as antibodies or immunoglobulins. Unlike other proteins, these are produced by specialized leukocytes called plasma cells and are essential for immunity.     * Fibrinogen:         * Third major group ($\approx 7\%$ of total plasma protein).         * Clinical levels: $0.2-0.45\,g/dL$ of blood.         * Produced by the liver and essential for the blood clotting process.

• Other Plasma Solutes ($\approx 1\%$ of plasma volume):     * Electrolytes: Sodium, potassium, and calcium ions.     * Dissolved Gases: Oxygen ($O_2$), carbon dioxide ($CO_2$), and nitrogen ($N_2$).     * Organic Nutrients: Vitamins, lipids, glucose, and amino acids.     * Metabolic Wastes: Various breakdown products from cellular metabolism.