Structure and Functions of Blood Components and Tissue Fluid

Introduction

Key Concepts

Blood Plasma

Blood plasma is the liquid component of blood, constituting approximately 55% of its volume. It is a pale yellow fluid composed mainly of water (about 90%), electrolytes, nutrients, hormones, waste products, and proteins. Plasma serves as the medium for transporting various substances throughout the body.

Composition of Blood Plasma:

• Water: Acts as a solvent for transporting solutes and maintaining blood volume.
• Proteins: Including albumin, globulins, and fibrinogen, they play roles in osmotic pressure maintenance, immune responses, and blood clotting.
• Electrolytes: Such as sodium, potassium, calcium, and chloride ions, essential for nerve function and muscle contraction.
• Nutrients: Glucose, amino acids, fatty acids, vitamins, and minerals absorbed from the digestive system.
• Waste Products: Urea, bilirubin, and other metabolites removed from tissues.
• Hormones: Chemical messengers transported to target organs.

Functions of Blood Plasma:

• Transportation: Carries nutrients, hormones, and waste products to and from cells.
• Regulation: Maintains pH balance and electrolyte concentrations critical for cellular functions.
• Protection: Transports antibodies and other immune components to defend against pathogens.

Red Blood Cells (Erythrocytes)

Red blood cells are the most abundant cell type in blood, making up about 45% of blood volume. They are characterized by their biconcave shape, which increases surface area for gas exchange and allows flexibility to navigate through capillaries.

Structure of Red Blood Cells:

• Biconcave Shape: Enhances gas diffusion and flexibility.
• Lack of Nucleus: Provides more space for hemoglobin but limits the cell's ability to repair itself.
• Hemoglobin: A protein that binds oxygen and carbon dioxide, facilitating gas transport.

Functions of Red Blood Cells:

• Oxygen Transport: Hemoglobin binds to oxygen in the lungs and releases it in tissues.
• Carbon Dioxide Removal: Carries carbon dioxide from tissues to the lungs for exhalation.

White Blood Cells (Leukocytes)

White blood cells are integral to the immune system, comprising less than 1% of blood volume. They are involved in defending the body against infections and foreign invaders.

Types of White Blood Cells:

• Neutrophils: Engulf and destroy bacteria and fungi.
• Lymphocytes: Include B cells and T cells, which are crucial for adaptive immunity.
• Monocytes: Differentiate into macrophages to phagocytize pathogens and debris.
• Eosinophils: Combat parasitic infections and contribute to allergic responses.
• Basophils: Release histamines during allergic reactions.

Functions of White Blood Cells:

• Immune Response: Detect and neutralize pathogens.
• Inflammation: Initiate healing processes following tissue injury.
• Surveillance: Monitor for abnormal cells, such as cancerous cells.

Platelets (Thrombocytes)

Platelets are small, disk-shaped cell fragments essential for blood clotting, accounting for less than 1% of blood volume.

Structure of Platelets:

• Acellular: Lack a nucleus, derived from megakaryocytes in the bone marrow.
• Granules: Contain proteins vital for clot formation.

Functions of Platelets:

• Clot Formation: Aggregate at injury sites to form a temporary plug.
• Release of Clotting Factors: Secrete substances that facilitate the coagulation cascade.
• Wound Repair: Aid in the healing process by providing growth factors.

Tissue Fluid (Interstitial Fluid)

Tissue fluid, also known as interstitial fluid, surrounds the cells in tissues, accounting for about 15% of blood volume. It acts as an intermediary between blood plasma and cells.

Composition of Tissue Fluid:

• Water: Provides a medium for biochemical reactions.
• Electrolytes: Maintains osmotic balance and pH.
• Nutrients: Delivers glucose, amino acids, and fatty acids to cells.
• Waste Products: Transports carbon dioxide and metabolic wastes back to the blood.

Functions of Tissue Fluid:

• Nutrient Exchange: Facilitates the movement of nutrients from blood to cells.
• Waste Removal: Aids in the elimination of cellular waste products.
• Communication: Transports signaling molecules between cells.
• Protection: Cushions cells and maintains the extracellular environment.

Proteins in Blood

Blood proteins play diverse roles in maintaining physiological balance and facilitating various bodily functions.

Types of Blood Proteins:

• Albumin: Maintains osmotic pressure and transports hormones and drugs.
• Globulins: Involved in immune responses and transport of lipids.
• Fibrinogen: Essential for blood clot formation.

Functions of Blood Proteins:

• Osmotic Balance: Albumin prevents excessive fluid loss from blood vessels.
• Immune Defense: Globulins (antibodies) identify and neutralize pathogens.
• Clotting: Fibrinogen is converted to fibrin to form blood clots.

Hormones Transported in Blood

Blood serves as the transport medium for hormones, which are crucial for regulating physiological processes.

Examples of Hormones:

• Insulin: Regulates blood glucose levels.
• Thyroxine: Controls metabolic rate.
• Adrenaline: Prepares the body for "fight or flight" responses.

Functions of Hormones in Blood:

• Regulation: Control various bodily functions like growth, metabolism, and reproduction.
• Coordination: Ensure different organs and systems work harmoniously.

Gas Transport in Blood

Blood is responsible for the transport of respiratory gases—oxygen (O₂) and carbon dioxide (CO₂).

Oxygen Transport:

• Oxygen binds to hemoglobin in red blood cells, forming oxyhemoglobin.
• Approximately 98.5% of oxygen is transported bound to hemoglobin.
• The remaining 1.5% is dissolved directly in plasma.

Carbon Dioxide Transport:

• CO₂ is primarily transported as bicarbonate ions ($$HCO_3^-$$).
• About 70% is in the form of bicarbonate, 23% bound to hemoglobin, and 7% dissolved in plasma.
• The conversion of CO₂ to bicarbonate in plasma involves the enzyme carbonic anhydrase:

$$CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^- + H^+$$

Importance of Gas Transport:

• Oxygen Delivery: Essential for cellular respiration and energy production.
• CO₂ Removal: Prevents acidification of blood and maintains pH balance.

Electrolyte Balance in Blood and Tissue Fluid

Electrolytes are charged ions crucial for various physiological functions, including nerve impulse transmission, muscle contraction, and maintaining fluid balance.

Key Electrolytes:

• Sodium ($$Na^+$$): Regulates water balance and nerve function.
• Potassium ($$K^+$$): Essential for heart and muscle function.
• Calcium ($$Ca^{2+}$$): Important for bone structure, blood clotting, and muscle contraction.
• Chloride ($$Cl^-$$): Helps maintain osmotic pressure and acid-base balance.

Functions of Electrolytes:

• Nerve Function: Electrolytes facilitate the transmission of electrical impulses along neurons.
• Muscle Contraction: Ionic gradients allow muscle fibers to contract and relax.
• Fluid Balance: Sodium and chloride ions regulate the movement of water between compartments.
• pH Regulation: Bicarbonate ions buffer blood pH, preventing drastic fluctuations.

Transport of Nutrients and Waste Products

Efficient transport of nutrients and waste products is vital for cell survival and overall organism health.

Nutrient Transport:

• Glucose: Carried by blood to provide energy for cellular activities.
• Amino Acids: Building blocks for protein synthesis delivered to cells.
• Fatty Acids: Transported bound to albumin for energy storage and membrane synthesis.

Waste Transport:

• Urea: Excreted by kidneys after being transported in blood from liver metabolism.
• Carbon Dioxide: Removed from tissues to be expelled via the lungs.
• Bilirubin: Waste product from hemoglobin breakdown, processed by the liver.

Hormonal Regulation and Transport

Blood transports hormones, which are critical for regulating physiological processes and maintaining homeostasis.

Mechanism of Hormone Transport:

• Binding to Carriers: Steroid hormones are often bound to carrier proteins for solubility and transport.
• Targeting: Hormones travel through the bloodstream to reach specific receptor sites on target cells.

Functions of Hormonal Transport:

• Regulation of Metabolism: Hormones like insulin and glucagon manage blood glucose levels.
• Growth and Development: Hormones such as growth hormone control growth processes.
• Stress Response: Adrenaline and cortisol prepare the body to respond to stressors.

Advanced Concepts

Osmotic Pressure and Fluid Balance

Osmotic pressure is a critical factor in maintaining fluid balance between blood plasma and tissue fluid. It is primarily regulated by the concentration of solutes, particularly proteins like albumin.

Colloid Osmotic Pressure:

• Also known as oncotic pressure, it is the osmotic pressure exerted by plasma proteins.
• Prevents excessive water from leaving the blood vessels into the interstitial space.
• Maintained by albumin concentration:

  $$\pi = iCRT$$ where $$\pi$$ is osmotic pressure, $$i$$ is the ionization constant, $$C$$ is concentration, $$R$$ is the gas constant, and $$T$$ is temperature.

Hydrostatic Pressure:

• The pressure exerted by blood against the vessel walls.
• Promotes the movement of water and solutes out of capillaries into tissue fluid.
• Dependent on cardiac output and vascular resistance.

Balance Between Hydrostatic and Colloid Osmotic Pressure:

• Fluid movement is determined by the balance of hydrostatic pressure pushing fluid out and oncotic pressure pulling fluid in.
• Disruptions can lead to edema (excess fluid in tissues) or dehydration of cells.

Transport Across the Capillary Wall

The capillary wall is selectively permeable, allowing specific substances to pass between blood and tissues.

Mechanisms of Transport:

• Diffusion: Movement of small, non-polar molecules like oxygen and carbon dioxide directly through the endothelial cells.
• Filtration: Bulk flow of plasma through openings between endothelial cells, driven by pressure gradients.
• Transcytosis: Vesicular transport of larger molecules like proteins across endothelial cells.
• Facilitated Diffusion and Active Transport: Specific transporters assist in moving ions and glucose against concentration gradients.

Factors Affecting Transport Efficiency:

• Capillary Permeability: Varies between different tissues based on endothelial cell structure.
• Surface Area: Increased surface area enhances the capacity for substance exchange.
• Distance: Short diffusion distances in well-vascularized tissues facilitate rapid exchange.

Acid-Base Balance in Blood

Maintaining acid-base balance is essential for optimal enzymatic and metabolic functions. Blood pH is tightly regulated within the narrow range of 7.35 to 7.45.

Buffer Systems:

• Bicarbonate Buffer: The primary buffer in blood, involving the equilibrium between bicarbonate ions and carbonic acid:

  $$HCO_3^- + H^+ \leftrightarrow H_2CO_3 \leftrightarrow CO_2 + H_2O$$

• Protein Buffers: Albumin and hemoglobin can bind or release hydrogen ions.
• Phosphate Buffer: Plays a minor role in extracellular fluid buffering.

Respiratory Regulation:

• The respiratory system adjusts blood pH by altering the rate and depth of breathing to expel or retain CO₂.
• Increased respiration removes CO₂, reducing acidity.

Renal Regulation:

• The kidneys manage acid-base balance by excreting or reabsorbing bicarbonate and hydrogen ions.
• They also generate new bicarbonate ions to replenish those lost.

Blood Typing and Immunology

Blood typing is crucial for safe blood transfusions and understanding immune responses related to blood antigens.

Blood Group Systems:

• ABO System: Categorizes blood based on the presence of A and B antigens on red blood cells.
• Rh System: Determines the presence (+) or absence (-) of the Rh antigen.

Compatibility and Transfusions:

• Receiving incompatible blood can trigger immune reactions, leading to hemolysis.
• Understanding blood types prevents transfusion-related complications.

Immunological Functions:

• White blood cells recognize and respond to foreign antigens.
• Antibodies produced by B lymphocytes specifically target antigens for neutralization or destruction.

Blood Clotting Cascade

The blood clotting process involves a series of enzymatic reactions known as the coagulation cascade, essential for preventing excessive bleeding.

Stages of Clot Formation:

1. Vascular Spasm: Immediate constriction of blood vessels to reduce blood flow.
2. Platelet Plug Formation: Platelets adhere to the injury site and aggregate to form a temporary seal.
3. Coagulation Cascade: Activation of clotting factors leads to the transformation of fibrinogen into fibrin strands:

$$Fibrinogen \xrightarrow{Thrombin} Fibrin$$

Fibrin forms a stable mesh that, along with platelets, constitutes the blood clot.

Regulation of Clotting:

• Antithrombin: Inhibits thrombin and other clotting factors to prevent excessive clotting.
• Fibrinolysis: Plasmin breaks down fibrin, dissolving clots once the vessel is healed.
• Vitamin K: Essential for synthesizing certain clotting factors.

Clinical Relevance:

• Hemophilia: A genetic disorder characterized by deficient clotting factors, leading to excessive bleeding.
• Deep Vein Thrombosis (DVT): Formation of clots in deep veins, which can be life-threatening if they dislodge.

Blood Volume Regulation

Maintaining appropriate blood volume is crucial for ensuring adequate tissue perfusion and preventing circulatory collapse.

Mechanisms of Regulation:

• Thirst Mechanism: Triggered by increased plasma osmolarity or decreased blood volume, prompting water intake.
• Antidiuretic Hormone (ADH): Stimulated by the posterior pituitary in response to high plasma osmolarity, increasing water reabsorption in the kidneys.
• Renin-Angiotensin-Aldosterone System (RAAS): Activated by low blood pressure, leading to vasoconstriction and increased sodium and water reabsorption.

Clinical Implications:

• Dehydration: Reduced blood volume due to excessive fluid loss can impair organ function.
• Hypertension: Excessive blood volume increases blood pressure, contributing to cardiovascular diseases.

Interrelationship Between Blood and Lymphatic Systems

The blood and lymphatic systems collaborate to maintain fluid balance and immune function.

Fluid Exchange:

• Fluid exits capillaries into interstitial spaces and is collected by the lymphatic system.
• The lymphatic system returns excess interstitial fluid to the bloodstream, preventing edema.

Immune Surveillance:

• The lymphatic system transports white blood cells to sites of infection.
• Lymph nodes filter pathogens, facilitating immune responses.

Fat Absorption:

• Lacteals in the small intestine absorb dietary fats, transporting them via the lymphatic system before entering the bloodstream.

Regulation of Blood pH

Blood pH is tightly regulated to ensure optimal enzyme function and metabolic processes. Deviations can disrupt cellular activities and lead to severe health issues.

Buffer Systems in Detail:

• Bicarbonate Buffer System:

  $$HCO_3^- + H^+ \leftrightarrow H_2CO_3 \leftrightarrow CO_2 + H_2O$$

  Buffering capacity is influenced by respiration and renal excretion.
• Hemoglobin Buffer System:

  Hemoglobin can bind to free hydrogen ions, aiding in buffering excess acidity.

Respiratory Compensation:

• In response to acidosis, respiration increases to expel CO₂, shifting the equilibrium to reduce H⁺ concentration.
• In alkalosis, respiration slows to retain CO₂, increasing H⁺ concentration.

Renal Compensation:

• The kidneys excrete or retain bicarbonate and hydrogen ions to adjust blood pH.
• In metabolic acidosis, kidneys increase H⁺ excretion and bicarbonate reabsorption.
• In metabolic alkalosis, kidneys retain H⁺ and excrete bicarbonate.

Blood Resistance and Blood Flow

Blood resistance within the circulatory system affects blood flow, influenced by vessel diameter, blood viscosity, and vessel length.

Poiseuille’s Law:

$$\text{Flow Rate} (Q) = \frac{\Delta P \pi r^4}{8 \eta l}$$

• ΔP: Difference in pressure between two points.
• r: Radius of the blood vessel.
• η: Blood viscosity.
• l: Length of the vessel.

Implications of Poiseuille’s Law:

• Even small changes in vessel radius significantly impact blood flow.
• Vasoconstriction increases resistance and decreases flow, whereas vasodilation decreases resistance and increases flow.

Factors Affecting Blood Viscosity:

• Hematocrit: Higher red blood cell concentration increases viscosity.
• Temperature: Increased temperature can decrease viscosity.
• Plasma Protein Levels: Elevated proteins can increase viscosity.

Regulation of Blood Flow:

• Autoregulation: Tissues can control their own blood flow based on metabolic needs.
• Neural Control: Sympathetic and parasympathetic nervous systems modulate vessel tone.
• Hormonal Control: Hormones like adrenaline induce vasoconstriction or vasodilation.

Thermoregulation and Blood Flow

Blood plays a crucial role in regulating body temperature by distributing heat generated by metabolic processes and dissipating excess heat.

Heat Distribution:

• Blood transports heat from core organs to the skin surface, where it can be lost to the environment.
• Vasodilation increases blood flow to the skin, enhancing heat loss through radiation and convection.

Heat Conservation:

• Vasoconstriction reduces blood flow to the skin, minimizing heat loss during cold conditions.

Evaporative Cooling:

• Sweating releases heat through the evaporation of water from the skin surface.
• Blood flow to sweat glands is increased to provide the necessary fluids for evaporation.

Clinical Relevance:

• Hyperthermia: Excessive heat can lead to heat stroke, requiring measures to enhance heat loss.
• Hypothermia: Excessive cold exposure can cause vital organs to fail if heat conservation mechanisms are overwhelmed.

Regulation of Blood Pressure

Blood pressure is the force exerted by circulating blood on the walls of blood vessels, essential for maintaining adequate blood flow to organs.

Components of Blood Pressure:

• Systolic Pressure: The pressure when the heart contracts during systole.
• Diastolic Pressure: The pressure when the heart relaxes during diastole.

Regulatory Mechanisms:

• Baroreceptor Reflex: Stretch-sensitive receptors detect changes in blood pressure and signal adjustments in heart rate and vessel diameter.
• Renin-Angiotensin-Aldosterone System (RAAS): Regulates blood volume and systemic vascular resistance.
• Antidiuretic Hormone (ADH): Controls water reabsorption in kidneys, affecting blood volume and pressure.

Factors Influencing Blood Pressure:

• Cardiac Output: The volume of blood the heart pumps per minute.
• Peripheral Resistance: The resistance of arteries to blood flow.
• Blood Volume: Total amount of blood within the circulatory system.
• Vascular Elasticity: Flexibility of blood vessel walls affects pressure dynamics.

Clinical Implications:

• Hypertension: Chronic high blood pressure increases the risk of heart disease and stroke.
• Hypotension: Abnormally low blood pressure can lead to inadequate tissue perfusion.

Transport of Lipids in Blood

Lipids, including fatty acids and cholesterol, are transported in blood plasma in forms that allow their solubility and prevent aggregation.

Forms of Lipid Transport:

• Chylomicrons: Transport dietary triglycerides from the intestines to tissues.
• Very Low-Density Lipoproteins (VLDL): Transport endogenous triglycerides from the liver to peripheral tissues.
• Low-Density Lipoproteins (LDL): Carry cholesterol to cells, often associated with atherosclerosis.
• High-Density Lipoproteins (HDL): Remove excess cholesterol from tissues and transport it back to the liver for excretion.

Functions of Lipid Transport:

• Energy Supply: Deliver fatty acids to muscles and other tissues for energy production.
• Membrane Synthesis: Provide components for cell membrane structures.
• Hormone Production: Supply cholesterol for steroid hormone synthesis.

Clinical Relevance:

• Atherosclerosis: Accumulation of LDL cholesterol in arterial walls leads to plaque formation.
• Heart Disease: Imbalances in lipoprotein levels contribute to cardiovascular risk.

Comparison Table

Summary and Key Takeaways