week 13 Movement of Fluid Between Compartments and Mechanisms of Oedema

Body Water Compartments and Ion Concentrations

• Total body water is divided into:

  • Extracellular fluid (ECF)

  • Intracellular fluid (ICF)

• ECF includes:

  • Interstitial fluid and lymph

  • Intravascular fluid (plasma)

  • Immobile fluid (endothelial glycocalyx = EGC)

  • Urine

  • Intestinal fluid

  • Saliva, sweat, and tears

• Water content varies with age and gender:

  • Males > Females

  • Lean > Obese

  • Young > Old

  • EGC-bound water is approximately 1 liter.

• Intracellular fluid (ICF) is composed of cytosol, which includes water, inorganic ions, and organic substances.

• Ion composition:

  • Extracellular fluid (ECF): High Na+, Cl-, HCO3-, Ca++

  • Intracellular fluid (ICF): High K+, Prot-

• Typical ion concentrations (mmol/l):

  • Na+: ECF = 145, ICF = 12

  • K+: ECF = 4, ICF = 155

  • Ca++: ECF = 4, ICF = 0.0001

  • Cl-: ECF = 120, ICF = 4

  • HCO3-: ECF = 27, ICF = 8

Movement of Water and Ions

• There is constant exchange of water and ions between IVF (intravascular fluid), ISF (interstitial fluid), and ICF.

• Diffusion: Movement of molecules from high to low concentration areas, driven by Brownian motion.

  • Diffusion barriers: fully permeable, semi-permeable, impermeable.

• Diffusion Barriers:

  • Vascular wall (IVF/ISF): Permeable to inorganic ions, glucose, amino acids; impermeable to large protein ions/molecules; fully permeable to water.

  • Cellular membranes (ISF/ICF): Poorly permeable to inorganic ions (controlled permeability); impermeable to most organic ions (except lipid-soluble); fully permeable to water.

Osmotic and Hydrostatic Pressure

• Water distribution is determined by the balance between osmotic and hydrostatic pressures. These pressures regulate the movement of water between different body compartments.

• ICF/ISF: Hydrostatic pressure is similar; osmotic pressure may vary. Because the hydrostatic pressure is similar between ICF and ISF, changes in osmotic pressure primarily drive water movement.

• ISF/Plasma: Both hydrostatic and osmotic pressures vary. Variations in both hydrostatic and osmotic pressures regulate fluid exchange between the ISF and plasma.

• Hydrostatic pressure = Blood Pressure (BP): Hydrostatic pressure is the force exerted by blood against the capillary walls.

• Blood pressure gradient in systemic circulation:-

  • Arterial end: 35 mmHg: Higher pressure at the arterial end facilitates filtration.

  • Venous end: 15 mmHg: Lower pressure at the venous end facilitates reabsorption.

  • Mean pressure: 18-8 mmHg: The average pressure in the capillaries.

Osmotic Pressure

• Osmosis: Water movement from less to more concentrated solution through a water-permeable membrane. This movement aims to equalize solute concentrations on both sides of the membrane.

• Osmotic pressure: Pressure required to prevent water movement across a semi-permeable membrane. It reflects the concentration of non-permeating solutes.

• Conditions for osmotic pressure:-

  • Semi-permeable membrane between two solutions: Allows water to pass but restricts solute movement.

  • Non-permeable solute in one solution: Creates a concentration gradient that drives water movement.

• Osmotic pressure formula: P{osm} = [C{solute}] \, x \, R \, x \, T where:-

  • [C_{solute}] is the concentration of the solute in osmoles per liter: The amount of solute dissolved in a liter of solution.

  • R is the ideal gas constant: A constant that relates the pressure, volume, and temperature of a gas.

  • T is the absolute temperature in degrees Kelvin. Temperature affects the kinetic energy of the molecules.

• Osmotic pressure is denoted by \Pi

• Oncotic pressure: Osmotic pressure produced by large protein molecules. Proteins, such as albumin, contribute significantly to oncotic pressure in the blood.

Osmosis in Action

• Osmosis occurs due to non-diffusible particles (e.g., ions, plasma proteins) on one side of a semi-permeable membrane. These particles create an osmotic gradient, driving water movement.

• Capillary walls are freely permeable to water and small inorganic ions, glucose, and amino acids, but not to large protein molecules. This differential permeability is crucial for fluid exchange between blood and tissues.

• Concentrations of inorganic ions are similar in plasma and ECF, but protein content differs significantly (approximately 25mmol in plasma and near zero in ECF). This difference leads to oncotic pressure.

• Oncotic pressure is the osmotic pressure caused by large organic ions (proteins). It helps retain fluid within the blood vessels.

• Oncotic pressure remains constant along the capillary length. This constant pressure ensures continuous fluid reabsorption into the capillaries.

• Endothelial glycocalyx (EGC) is a hydrated gel-like layer made of glycoproteins/glycolipids with

Starling’s Forces

• Oncotic Pressure: Draws fluid into the capillary due to the presence of proteins (especially albumin). It opposes hydrostatic pressure.

• Hydrostatic Pressure: Pushes fluid out of the capillary into the interstitial space. It is highest at the arterial end and lowest at the venous end.

• Net Fluid Movement: Determined by the balance between hydrostatic and oncotic pressures. At the arterial end, hydrostatic pressure is typically higher, leading to net filtration (fluid moving out). At the venous end, oncotic pressure is typically higher, leading to net absorption (fluid moving in).

Lymphatic Drainage

• Role of Lymphatics: Lymphatic vessels collect excess fluid, proteins, and other substances from the interstitial space and return them to the bloodstream.

• Mechanism: Lymphatic capillaries are highly permeable and can absorb large molecules and fluid that cannot be directly reabsorbed into blood capillaries.

• Importance: Prevents the accumulation of fluid in the interstitial space, which would lead to edema.

Mechanisms of Edema

• Venous Obstruction:-

  • Mechanism: Increased venous pressure impairs reabsorption of fluid from the interstitial space into the capillaries.

  • Example: Deep vein thrombosis (DVT) or heart failure.

• Lymphatic Obstruction:-

  • Mechanism: Blockage of lymphatic vessels prevents the removal of excess fluid and proteins from the interstitial space.

  • Example: Lymphedema following surgical removal of lymph nodes or filariasis.

• Lowered Oncotic Pressure:-

  • Mechanism: Reduced protein concentration in the blood (e.g., hypoalbuminemia) decreases the oncotic pressure, leading to less fluid reabsorption into the capillaries.

  • Example: Liver disease (reduced albumin synthesis), kidney disease (proteinuria), malnutrition.

• Increased Capillary Permeability:-

  • Mechanism: Damage to capillary walls increases their permeability, allowing more fluid and proteins to leak into the interstitial space.

  • Example: Inflammation, burns, allergic reactions.

Consequences of Pulmonary Edema

• Interstitial Edema:-

  • Effect on Gas Exchange: Increased fluid in the interstitial space impairs diffusion of oxygen and carbon dioxide between the alveoli and blood capillaries.

  • Symptoms: Shortness of breath, rapid breathing, and decreased oxygen saturation.

• Alveolar Edema:-

  • Effect on Gas Exchange: Fluid accumulation in the alveoli reduces the surface area available for gas exchange and diffusion, leading to severe hypoxemia and hypercapnia.

  • Symptoms: Severe shortness of breath, frothy sputum (red or pink), and respiratory failure.

Cardiovascular Reflexes Triggered by Decreases in O2 and Increases in CO2

• Chemoreceptors:-

  • Peripheral Chemoreceptors: Located in the carotid and aortic bodies; detect changes in arterial PO2, PCO2, and pH.

  • Central Chemoreceptors: Located in the medulla; primarily detect changes in PCO_2 and pH in the cerebrospinal fluid.

• Reflex Responses:-

  • Increased Ventilation: Stimulation of chemoreceptors leads to increased respiratory rate and tidal volume to eliminate CO2 and increase O2 uptake.

  • Increased Heart Rate and Blood Pressure: Sympathetic nervous system activation increases heart rate and vasoconstriction to improve oxygen delivery to tissues.

  • Vasoconstriction: Peripheral vasoconstriction increases blood flow to essential