TEST 1: Fluid & Acid-Base Balance Flashcards
Sodium homeostasis mechanisms
- Renin-Angiotensin Aldosterone System (RASS):
- Antidiuretic Hormone (ADH)
- Atrial Natriuretic Hormone (ANP)
Renin-Angiotensin Aldosterone System as a mechanism for sodium homeostasis
- Renin release: low BP/ blood volume triggers release of renin from the juxtoglomerular cells of the kidney
- Angiotensin II formation: renin converts angiotensin (from liver) into angiotensin I, which is then converted to angiotensin II by the angiotensin-converting enzyme (ACE) in the lungs
- Aldosterone secretion: angiotensin II stimulates adrenal cortex to release aldosterone
- Sodium reabsorption: aldosterone acts on distal tubule and collecting ducts of the kidneys to increase sodium reabsorption and water retention (thus elevating blood pressure and volume)
Anti diuretic hormone as a mechanism for sodium homeostasis
- Released in response to high osmolarity: high plasma osmolarity triggers the release of ADH from the posterior pituitary gland
- Water reabsorption: ADH increases the permeability of collecting ducts in the kidneys to water, which promotes water reabsorption and dilution of the sodium concentration of the blood.
ANP as a mechanism for sodium homeostasis
- Release in response to high blood volume: stretch receptors in the atria of the heart sense increased blood volume and release ANP
- Inhibition of RAAS: ANP inhibits renin, aldosterone, and ADH— promoting the excretion of sodium and water (lowering the BP and pressure)
Potassium homeostasis mechanisms
- Aldosterone
- Insulin
- Acid-Base Balance
- Kidney function
Aldosterone as a mechanism for potassium homeostasis
-Aldosterone promotes excretion of K by the distal tubule and collecting ducts in exchange for sodium reabsorption
-Critical for regulating serum K levels
Insulin as a mechanism for potassium homeostasis
-Insulin facilitates the uptake of K into cells (particularly muscle cells) by activating the sodium-potassium ATP-ase pump
-This lowers extracellular K levels
Acid-Base balance as a mechanism for potassium homeostasis
-Acidosis: hydrogen ions enter cells in exchange for K ions—> hyperkalemia
-Alkalosis: hydrogen ions move out of the cell and K ions enter —> hypokalemia
Kidney function as a mechanism for potassium homeostasis
- Filtration and reabsorption: the kidneys filter K in the glomerulus. Most of the filtered K is reabsorbed in the proximal tubule and loop of Henle. The distule tubule and collecting ducts regulate final excretion of K based on body needs.
- K secretion: K is secreted in exchange for sodium under the influence of aldosterone.
Regulation of NA and K
- Body uses negative feedback mechanisms where deviation from normal levels trigger a response to restore balance.
- Hormones like aldosterone, ADH, and ANP play critical roles in regulation of NA and K
- Adequate dietary intake of NA and K is critical for maintaining balance and the kidneys adjust excretion rates to match intake levels.
Sodium and Potassium homeostasis
-Sodium: primarily regulated by RAAS, ADH, and ANP
-Proper NA balance maintains fluid volume, BP, osmotic balance.
Potassium: mainly controlled by aldosterone, insulin, acid-base balance, and kidneys.
-Proper K balance maintains normal cell function, nerve transmission, and muscle contraction.
3 main fluid compartments
-Intravascular (plasma)
-Interstitial
-Intracellular
Intravascular fluid compartment
(Plasma)
-Includes fluid WITHIN the blood vessels
-Contains a high concentration of proteins (esp albumin) which contribute to oncotic pressure as well as a significant amount of electrolytes
-5% of total body weight
Interstitial Compartment
-The fluid that surrounds the cells IN the tissues, OUTSIDE the blood vessels
-Acts as bridge between the Intravascular compartment and the cells, facilitating nutrient and waste exchange
-Accounts for 15% total body weight
Intracellular compartment
-Fluid WITHIN the cells
-Contains high concentrations of K, phosphate, and proteins
-Accounts from 40% total body weight
Mechanisms of fluid shifts
- Osmosis
- Hydrostatic Pressure
- Oncotic Pressure (colloid osmotic pressure)
Osmosis
-Movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration
-Driven by osmolarity, heavily influenced by electrolytes like NA and K.
Hydrostatic pressure
-Pressure exerted by the fluid within the blood vessels
-Pushes fluid out of the capillaries into the interstitial space.
Oncotic pressure (Colloid osmotic pressure)
-Exerted by plasma proteins (albumin) within the blood vessels
-Pulls fluid into the capillaries from the interstitial space
Patterns of fluid shifts between Intravascular and interstitial compartments
- Capillary hydrostatic pressure: drives fluid out of capillaries into the interstitial space (balanced by—>
- Interstitial hydrostatic pressure: opposes movement of fluid out of the capillaries
- Capillary oncotic pressure: pulls fluid back into the capillaries from the interstitial space
- Interstitial oncotic pressure: pulls fluid out of the capillaries into the interstitial space
Starlings Law of Capillary Forces
-The net movement of fluid is determined by the balance of pressures
-Fluid that moves out of capillaries is usually reabsorbed but any excess is drained by lymphatic system
Pattern of fluid shift between interstitial and intracellular compartments
-Osmotic gradient: drives water movement in and out of the cells.
-If interstitial fluid becomes hypertonic (high solute concentration) water will move out of cells into interstitial space
-If interstitial fluid becomes hypotonic (low solute concentration) water will move into the cells.
Pathological fluid shifts
- Edema
- Dehydration
- Third spacing
Edema
-Accumulation of fluid in the interstitial space
Causes:
-Increased capillary hydrostatic pressure (CHF, venous obstruction)
-Decreased capillary oncotic pressure (low albumin d/t liver disease)
-Increased capillary permeability (inflammation, trauma)
-Lymphatic obstruction
(Lymphedema)
Dehydration
-A deficiency of fluid in the interstitial AND intracellular compartments.
-Typically affects interstitial space first (mild dehydration)
-If it Affects intracellular fluid volume it impairs cell function (severe dehydration)
Third-Spacing
-Fluid shifts from Intravascular compartment to potential spaces (ie peritoneal, pleural)
-Often occurs when severe burns, pancreatitis, or sepsis
Intravascular V. Interstitial V. Intracellular fluid
-Intravascular: fluid within the blood cells (high in protein and electrolytes)
-Interstitial: fluid surrounding cells (acts as intermediary between blood vessels and cells)
-Intracellular: fluid inside cells (rich in potassium and protein)
Principles of fluid balance
- Homeostasis
- Fluid compartments:
-ICF: 40% of body weight, primarily within the cells
-ECF: 20% of body weight, consists of Intravascular fluid (plasma) and interstitial fluid - Osmotic pressure: force that drives water across a cell membrane from
An area of low concentration to high until equilibrium is reached. - Hydrostatic pressure: pressure exerted by a fluid due to its weight and the force of the hearts pumping
- Oncotic pressure: osmotic pressure exerted by large molecules (esp albumin) in the blood. Helps to retain fluid in Intravascular space by attracting water.
Mechanisms regulating fluid balance
1.Renal system
2.Endocrine system
3. Thirst Mechanism
4. Fluid intake and output
How the renal system regulates fluid balance
- Kidneys filter blood, reabsorb needed substances, excrete waste and excess fluids
- GFR: determines the rate at which blood is filters to form urine
- NA reabsorption/ electrolyte balance: key to regulating fluid volume due to osmotic effect of NA
- ADH: increases water reabsorption in the kidneys (reducing urine output and increasing intravascular volume)
How the endocrine system regulates fluid balance
- Renin-Angiontensin-Aldosterone system (RAAS):
Renin: converts angiotensin to angiotensin I
Angiotensin II: causes vasoconstriction and stimulates aldosterone release, increasing NA and water reabsorption
Aldosterone: promotes sodium reabsorption and potassium excretion in the kidneys, increasing fluid volume - ANP: released by atrial cells in response to high blood pressure/ volume, inhibits RAAS, and promotes NA and water excretion
How the thirst mechanism regulates fluid balance
-Regulated by hypothalamus in response to increased plasma osmolarity or decreased blood volume
-Stimulated the desire to drink fluids thereby increasing fluid intake
How fluid intake and output affects regulating fluid balance
- Intake: drinking fluids and eating food
- Output: urine, feces, sweat, insensible loss through respiration
-daily balance is usually regulated to match intake with output
Factors influencing fluid balance
- Electrolyte concentrations
- Plasma proteins
- Blood pressure and volume
- Capillary permeability
- Pathophysiologic conditions
How electrolyte contributions influence fluid balance
-Affect osmotic gradients and fluid distribution
-NA is primary ECF cation that influences fluid volume and distribution