Fluid and Electrolyte Balance: Fluid Dynamics
Fluid and Electrolytes: Part 1 - Fluid Section
Introduction to Fluid Compartments
Concept Importance: Fluid and electrolytes are fundamental to how cells and the body function.
The human body contains three primary fluid types:
Intracellular Fluid (ICF): Fluid located inside the cells.
Extracellular Fluid (ECF): Fluid found outside the cells, encompassing various compartments.
Interstitial Fluid (ISF): "In-between" fluid that acts as cushioning for cells, not residing in major compartment
What happens when fluid goes into the the isf? causes edema, if there is too much pressure or fluid
Other Extracellular Fluids: Plasma, lymph, and transcellular fluids (not explicitly detailed in this transcript but part of ECF).
Composition: Intracellular fluid primarily consists of water.
Body Water Content: The body is predominantly water, with variations based on age:
Babies: Up to
Middle-aged adults: Approximately
Elderly: Down to
Cell Membrane and Fluid Exchange
Semi-permeable Membrane: Cells are enclosed by a membrane that facilitates fluid and electrolyte exchange while restricting larger particles to maintain cellular balance.
Fluid Shifts:
Swelling/Edema: Occurs when water diffuses into the intracellular compartment.
Cellular Dehydration: Occurs when cells lose fluid, causing them to shrink.
Third Spacing: When fluid exchange is compromised, excess cellular fluid can accumulate in the interstitial spaces (outside normal pockets), making it unavailable for metabolic processes.
ECF Function: Consists of electrolytes, nutrients, and glucose, which are exchanged with ICF to nourish cells.
Waste Excretion: Cells metabolize products and excrete waste into the ECF for removal from the body.
Fluid Pressures
Importance of Pressure: Pressure is crucial for maintaining blood flow, circulation, and tissue oxygenation.
Hydrostatic Pressure:
Mechanism: Generated by the heart's pumping action, representing the force of fluid pressure in the bloodstream.
Action: Pushes water from capillaries into the interstitial fluid and then into the ECF compartment.
Pushing water and pushing blood through the vessels.
Osmotic Pressure:
Mechanism: Pressure exerted by solids (solutes) in a solution.
Action: Pulls water into the bloodstream from the ICF and ISF compartments, shifting fluid from intracellular and interstitial spaces into the bloodstream.
Decreased Osmotic Pressure: If osmotic pressure drops, fluid moves out of the bloodstream into the interstitial and intracellular spaces.
Pulling water, the driving force of osmosis that pulls water back into the bloodstream
Too much solute=pull water
Too much pressure= push fluid into isf
Albumin keeps fluid from leaking out
Maintaining Fluid in Vessels: Both hydrostatic pressure (from the heart) and osmotic pressure (from solutes) are vital for keeping fluid within the vascular system. A decrease in either can lead to fluid leakage out of the vascular system.
Oncotic Pressure (Colloid Osmotic Pressure):
Mechanism: A specific type of osmotic pressure primarily due to albumin (a blood protein) in the blood.
Action: Helps retain fluid within its compartment, preventing leakage into the interstitial space.
Hypoalbuminemia: Low albumin levels can lead to edema because insufficient protein allows fluid to leak into the interstitial spaces.
Sucking water in
Osmolarity vs. Osmolality
Definition: Both refer to how concentrated a solution is with fluid in relation to electrolytes and protein.
Primary Difference: How they are measured.
Osmolality: Measured in milliosmoles per kilogram (solute/kg). (Think "L" for kilogram, as in "kg" containing an "L" from the word liter, though the analogy given was for "columns" in measurement).
Osmolarity: Measured in milliosmoles per liter (). (Think "R" for liter, or "volume").
Clinical Relevance:
Osmolarity is more commonly reported in clinical settings (e.g., blood and urine are liquids, making liters a more practical unit).
Urine osmolarity and plasma osmolarity can be checked to determine hydration status.
Osmolality is sometimes used to calculate osmolarity.
Solution Concentration Terms (relative to a referent solution, often plasma):
Hyperosmotic: Osmolarity is greater than the referent solution (often plasma) (more concentrated). (More solutes than plasma)
Hypoosmotic: Osmolarity is less than the referent solution (less concentrated). (Less solutes than plasma)
Isosmotic: Osmolarity is identical to the referent solution.
Starling's Law of Capillary Forces
Explanation: Describes fluid movement at the capillary beds.
Major Opposing Forces:
Hydrostatic Pressure: Pushes fluid out of capillaries.
Osmotic Pressure (Oncotic Pressure): Pulls fluid into capillaries (exerted by electrolytes and proteins).
Counterbalance: These two pressures oppose each other to maintain fluid balance.
Alterations Leading to Edema:
Increased blood volume= increased hydrostatic pressure. pushes fluid out into isf which causes edema
Lower albumin= reduced osmotic pressure.
Fluid Movement:
Filtration: Fluid exits capillaries when capillary hydrostatic pressure is greater than osmotic pressure (fluid "leaks" out).
No Net Movement: Occurs when hydrostatic pressure equals osmotic pressure (e.g., hydrostatic vs. colloidal osmotic pressure).
Reabsorption: Fluid moves back into the vessel when hydrostatic pressure is less than colloidal osmotic pressure.
Clinical Significance: Essential for understanding hydration status and maintaining fluid in the vascular space.
Renal System and Fluid Balance
Primary Regulator: The renal system (kidneys) is crucial for maintaining fluid status.
Kidney Function:
Filters waste products.
Regulates sodium and water retention or excretion as needed.
Holds onto fluid when necessary and removes excess fluid when not needed.
Tonicity and Intravenous (IV) Fluids
Tonicity: A measure of a solution's concentration of solids compared to the bloodstream.
Goal of IV Fluids: Most often, IV fluids aim to mimic the natural composition of blood.
Isotonic Fluids:
Characteristics: Have the same tonicity as blood (similar physiologic constituents).
Effect: No net fluid shift between compartments or change in cell size.
Examples:
Normal Saline ( Sodium Chloride): Main type used in adults; expands volume, dilutes medication, keeps veins open.
Lactated Ringer's (LR): Crystalloid, closest solution to blood products; used for significant volume resuscitation (e.g., surgeries, trauma, labor & delivery) when anticipating blood loss.
D5W (Dextrose in Water): Isotonic outside the body, but becomes hypotonic inside the body as glucose is metabolized. Used for sodium and volume replacement; must be given slowly.
Hypertonic Fluids:
Characteristics: Higher particle concentration, less water than blood.
Effect: Pulls water from the intracellular fluid into the extracellular fluid (shrinks cells).
Use Cases: Rare, primarily for severe cellular swelling, such as:
Cerebral edema (e.g., trauma, strokes, increased intracranial pressure (ICP))
Diabetic ketoacidosis (DKA) when the goal is to shift fluid out of swollen cells.
Examples:
Normal Saline (caution: do not give to trauma or head injury patients as it can worsen cerebral edema if not carefully managed or for specific indications).
Mannitol (an osmotic diuretic, works similarly to saline to decrease brain swelling).
D5 half normal saline or D5 normal saline (caution with head injury patients).
Hypotonic Fluids:
Characteristics: Fewer particles, more water than blood.
Effect: Promotes fluid shift from the extracellular fluid into the intracellular fluid (swells cells).
Use Cases: Severe dehydration (to rehydrate cells).
Example: ½ normal saline (1/2 NS) 0.45% NACL as treatment for dehydration
Regulation of Fluid Balance
Osmoreceptors (Hypothalamus):
Stimulation: Activated by increased plasma concentration (elevated serum osmolarity).
Action: Initiate the thirst mechanism, signaling the body needs more water.
Antidiuretic Hormone (ADH) / Vasopressin:
Release: From the posterior pituitary gland.
Stimulation: Triggered by increased plasma osmolarity (osmoreceptors) and decreased blood volume/pressure.
Action: Stimulates kidney nephrons to reabsorb more water, decreasing serum osmolarity and increasing circulating blood volume.
Renin-Angiotensin-Aldosterone System (RAAS):
Activators: Hypotension (decreased blood pressure), hypokalemia, dehydration, low cardiac output, reduced renal perfusion.
Pathway:
Renin Release: Kidneys release renin in response to activators.
Angiotensinogen Conversion: Renin converts angiotensinogen (from the liver) to Angiotensin I.
Angiotensin I to Angiotensin II: Angiotensin-Converting Enzyme (ACE) in the lungs converts Angiotensin I to Angiotensin II.
Angiotensin II Actions:
Potent Vasoconstrictor: Constricts blood vessels, increasing blood pressure.
Aldosterone Release: Activates the adrenal cortex to release aldosterone.
Aldosterone Actions:
Increases sodium and water reabsorption by the kidneys.
Increases potassium excretion by the kidneys.
Overall Effect: Increases blood pressure and blood volume through vasoconstriction and sodium/water retention in the ECF.
Importance: Critical for maintaining blood pressure and fluid homeostasis. (ACE inhibitors are medications that target this system).
Natriuresis (Reverse of RAAS):
Mechanism: Excretion of large amounts of sodium and water when the body has too much volume (inverse process to RAAS).
Natriuretic Peptides: Three peptides promote natriuresis:
Atrial Natriuretic Peptide (ANP): Released from the heart atria when stretched due to excess volume.
B-type Natriuretic Peptide (BNP):
Clinical Significance: Most important for clinical application.
Source: Primarily from heart ventricles (despite "B" sometimes referring to "brain").
Trigger: Excessive volume stretching the heart ventricles.
Role: Attempts to promote natriuresis. In heart failure, if compensatory mechanisms fail, BNP continues to be released in increasing amounts.
Diagnostic Use: Direct measure of the severity of heart failure exacerbation; higher levels indicate more significant heart failure.
C-type Natriuretic Peptide (CNP): Found in endothelial cells of arteries and ventricles. Less understood; thought to increase kidney filtration rate; not typically measured clinically.
Edema
Definition: Accumulation of excess fluid in the extracellular compartment of the interstitial fluid.
Primary Causes:
Increased Capillary Filtration/Hydrostatic Pressure: Forces fluid from capillaries into ISF (e.g., in heart failure due to increased ECF volume or general pressure).
Decreased Capillary Osmotic Pressure: Allows fluid to move to the interstitial space (e.g., hypoalbuminemia from liver failure or protein malnutrition).
Increased Capillary Permeability: Alters capillary wall integrity, allowing proteins to leak into ISF, which increases interstitial osmotic pressure (e.g., due to histamine and inflammatory response).
Obstructive Lymph Flow (Lymphedema): Fluid in the interstitial space cannot return to systemic circulation.
Sodium Retention: Due to illness or consumption of salty fluids. Where sodium goes, water follows.
Pitting Edema: Occurs when accumulated fluid in peripheral interstitial spaces exceeds tissue absorption capacity; pressure leaves an indentation.
Sequestered Fluids (Third Spacing)
Definition: Fluid accumulation in body cavities normally free of fluid (e.g., pericardial, peritoneal, pleural spaces).
Fluid Name: An "effusion" when present in these areas.
Types of Effusions:
Transudative Effusions: Occur due to increased hydrostatic pressure or low plasma osmotic pressure (e.g., in cirrhosis, increased pressure in one space).
Exudative Effusions: Occur due to inflammation and increased capillary permeability.
Clinical Intervention: Fluid can often be removed via needle aspiration (e.g., for pleural effusion).
Fluid Volume Imbalances
Hypervolemia (Fluid Volume Excess/Overload)
Cause: Excess fluid volume.
Associated Conditions: Kidney failure (impaired fluid maintenance, eventually leading to activation of RAAS and compensatory failure), heart failure (RAAS activation from low perfusion, high hydrostatic forces leading to edema).
Clinical Manifestations:
Neurological: Changes in LOC (confusion, headaches, seizures) often due to decreased sodium concentration.
Respiratory: Pulmonary congestion, potential pleural effusion.
Cardiovascular: Bounding pulse, increased blood pressure, increased jugular vein distention (JVD), S3 heart sound, tachycardia.
Other: Anorexia, nausea, dependent pitting edema (esp. in lower extremities).
Hypovolemia (Fluid Volume Deficit/Dehydration)
Definition: Diminished water volume in the body, low ECF volume.
Mechanism: Low ECF concentration pulls water from tissues via osmosis, causing cellular dehydration/shrinking.
Body's Response (Compensatory Mechanisms):
Osmoreceptors stimulate thirst.
ADH released to retain water.
Circulatory system constricts (due to Angiotensin II).
Heart rate increases to maintain perfusion.
RAAS system activated due to decreased circulating volume.
Causes: Reduced fluid intake, reduced ADH, kidneys unresponsive to ADH, burns, fever, excessive perspiration, osmotic diuresis (e.g., with elevated blood glucose).
Most Common Cause: Diarrhea.
Clinical Manifestations:
Dehydration involves both ICF and ECF volume depletion.
Tachycardia (increased HR).
Low blood pressure (hypotension).
Inadequate circulating blood volume leading to inadequate perfusion.
Dry skin, thirst, sticky or dry mucous membranes.
Weight loss (related to fluid loss).
Concentrated urine (except in diabetes insipidus, which causes dilute urine).
Tenting skin (poor skin turgor).
Weak pulse.
Postural hypotension (dizziness upon standing).
Confusion.
Remember: Sodium and water are "best friends"; water follows sodium. Replenishing sodium helps maintain fluid volume during dehydration.
Assessment of Fluid Status
Daily Weights: One of the most accurate ways to determine fluid volume status. Weight changes often directly correlate to fluid shifts rather than fat.
Intake and Output (I&O): Accurate documentation of all fluid intake and output, reported in milliliters (). Essential for patients at risk of imbalance (e.g., dehydration, trauma).
Vital Signs:
Heart rate (tachycardia can indicate dehydration).
Blood pressure (decreased BP, hypotension).
Orthostatic hypotension (dizziness/BP drop upon standing).
Physical Assessment:
Mucous membranes (dryness).
Urine output (concentration, volume).
Edema (presence and type, e.g., pitting edema).
Conclusion (Fluid Section)
Key Takeaways: Understand fluid location (vascular vs. extravascular), their consequences, and gain an adequate understanding of the RAAS system for future topics.