Week 10 - water Flashcards
Water homeostasis - function
- Acts as a universal solvent - Transports dissolved solutes
- Distributes body heat
- Cushions & protect organs - tissues
- Lubricates organs & tissues as they move
Water homeostasis - factors
- Water intake
- Kidney function (GFR)
- Digestive functions
- Physical activity → reduces blood flow to kidneys & digestive organs → reduce water output
- Medication
Intracellular compartment
- Known as intracellular fluid (ICF)
- Composed of the fluid found within cells (cytosol)
- ICF accounts for about 60% of the body’s fluid
Extracellular compartment
- Known as extracellular fluid (ECF)
- Composed of the fluid found between cells (interstitial fluid) a variety of other body fluids found outside of cells ( blood plasma, CSF, digestive secretions, synovial fluid)
- ECF accounts for about 40% of the body fluids
Body fluid composition - non electrolytes
- Do not dissociate in water (bc of covalent bonds)
- No charged particles created
Body fluid composition - Electrolytes
- In water electrolytes dissociate into ions, which are charged particles → conduct an electrical current
- Most abundant solutes in body fluids
- Involved with most chemical & physical reactions
- Greater osmotic power than non-electrolytes
- Ability to cause fluid shifts between fluid compartments
Hydrostatic pressure gradient
Force that fluid exerts on cells; tends to push water away from higher hydrostatic pressure to one with lower hydrostatic pressure
Osmotic pressure gradient
- Force of water movement generated by concentration of solutes in a solution
- Movement of water is achieved via the process of osmosis
- A solutions OP is determined by no. of solute particles present in solution (Osmolality)
High hydrostatic pressure
Dominates at arteriolar end of most blood vessels; pushes water out of vessel into surrounding interstitial fluid
high osmotic pressure
Dominates at venular end of most blood vessels; pulls most of water lost to interstitial fluid at arteriolar end back into vessel by osmosis
Electrolyte homeostasis
- Electrolytes obtained from diet = those lost from a variety of route from body
- Several mechanisms (mostly hormonal) maintain electrolyte balance
- Ion concentration is dependent not only on no. of ions in a body fluid, like blood, but also on amount of water in body fluid
- Fluid balance → critical factor that determines electrolyte balance
Obligatory water loss
Fluid lost through urine. Produced daily, irrespective of fluid intake
Sensible water loss
Amount of water lost in feces daily is a noticeable (sensed)
Insensible water loss
an unnoticed (not sensed) amount of daily water loss through sweat and respiration
Volume depletion (hypovolemia)
proportionate amounts of water and sodium are lost without replacement
- total body water decreases, osmolarity normal
- hemorrhage, severe burns, chronic vomiting or diarrhea
Dehydration
More water than sodium is eliminated
- total body water decreases, osmolarity rises
- Lack of drinking water, diabetes, profuse sweating, diuretics, exposure to extreme temperatures
- Affects all fluid compartments
Factors regulating water intake - Neural
- Governed by hypothalamic thirst centre. Thirst mechanism driving force for water intake
- Hypothalamic osmoreceptors detect ECF osmolarity; activated by an increase in plasma osmolarity of 1-2%
Factors regulating water intake - Hormonal
- Renin Angiotensin Aldosterine System (RAAS)
- Complex system that maintains systemic blood pressure primarily via changes to blood volume and urine output
Hypothalamic Thirst centre
- Stimulated by changes in 1-2% inc in ECF osmolality or a 5-10% dec in plasma volume.
- Osmoreceptors in the hypothalamus detect changes in blood osmolarity → Dry mouth, sensation of thirst, dec blood vol or pressure.
- Negative feedback mechanisms → relief of dry mouth, activation of stomach and intestinal stretch receptors
Hypotonic Hydration
- Cellular over-hydration, or water intoxication
- Occurs with renal insufficiency or rapic excess water ingestion
- Leads to net osmosis into tissue cells, swelling of cells, severe metabolic disturbances
- NR: Hypothalamic → thirst centre is inhibited
- HR: through Atrial Naturetic Peptide (ANP) control can increase urine loss. Inhibition of RAAS
Sodium
- Accounts for 90-95% of osmolarity of ECF, msot important solute determining total body water & distribution of water along FC
- Sodium concentration between ECF & ICF compartments are primarily maintained through the Na+/K+ pump.
→ Exchanges intracellular Na+ for extracellular K+
→ Creates gradient for cotransport of other solutes (glucose, potassium and calcium)
→ Generates body heat
Sodium - Functions
- Controls ECF volume and water distribution
- Changes in Na+ levels affect plasma volume, BP and ECF & IF vol
- Principal ion responsible for resting membrane potentials
- Depolarisation important for nerve and muscle function
- NaHCO3 has major role in buffering pH of ECF
Sodium - Regulation
Critical for both Na+ ion balance and fluid balance & water reabsorption in kidneys
- NR: no known receptors but linked to BP and BV
- HR: Aldosterone, ADH and ANP regulate sodium loss in urine
Sodium regulation - Aldosterone
“Salt retaining hormone”
- Primary role in adjustment of sodium excretion
- Hypernatremia/hypokalemia inhibits release from adrenal cortex
Sodium regulation - Antidiuretic hormone (ADH)
- Modifies water excretion independently of sodium excretion → can change sodium concentration
- Inc blood Na+ levels stimulate ADH release from the pituitary gland
- Kidneys reabsorb more water ( without retaining more Na+)
- ADH can stimulate thirst
Sodium regulation - ANP (atrial natriuretic peptide)
- Inhibits sodium and water reabsorption and secretion of renin and ADH
- From stretched atria
- Kidneys excrete more Na+ and H20 → decreasing BP/volume
Sodium imbalance- Hypernatremia
- ECF (= plasma sodium) > 145 mEq/L
- Result of dehydration
- Causes water retention, hypertension nad edema
Sodium imbalance - Hyponatremia
- ECF (=plasma) sodium < 136 mEq/L
- Result of excess body water, quickly corrected by ecretion of excess water; → if uncorrected produces symptoms of hypotonic hydration
Potassium - functions
- Determines intracellular osmolarity and cell volume
- Membrane potentials and action potentials (with sodium)
- Na+ - K+ pump
- Cotransport
- Thermogenesis
- Essential co-factor for protein synthesis
Potassium regulation
Kidneys determine the potassium conc. in ECF via hormonal regulation
- Aldosterone regulates ion pumps along distal portion of nephron and collecting system → facilitate potassium secretion by exchanging potassium for sodium ions
- Aldosterone secretion inc when
→ plasma conc of K+ is high
→ECF sodium levels are low
Potassium Imbalances - Hyperkalemia
Plasma K+ greater than 4.5 mEq/l
- Dangerous, potentially fatal electrolyte imbalance
- Makes resting membrane potential of excitable cells more positive rendering critical cells INCAPABLE of functioning normally
- Inactivates voltage-gates Na+ channels, nerve and muscle cells become less excitable
- Severe cardiac arrhythmias
Potassium Imbalances - Hypokalemia
Plasma K+ less than 3.9 mEq/l
- From sweating, chronic vomiting or diarrhea
- Commonly caused by DIURETICS → excess K+ loss in urine
- Makes resting membrane potential more negative, leaving excitable cells HYPERPOLARISED and less responsive to stimuli
Chloride - function
- Stomach acid: required in formation of HCl
- Chloride shift: CO2 loading and unloading in RBCs
- pH: Major role in regulating pH
Chlorine - regulation
Primary homeostasis achieved as an effect of Na+ homeostasis as it passively follows Na+
Chemical buffer system
Consists of a weak acid and its CONJUGATE WEAK BASE; function to resist large sings in pH
Physiological buffer systems
Allow maintenance of a slightly alkaline pH by ensuring an adequate no. of base ions in boy fluids and by providing a mechanism for elimination of H+ ions
- Respiratory mechanisms: act within 1-3 mi by removing CO2
- Renal mechanisms: most potent but require hours to days to effect pH changes removes hydrogen and inc. reabsorption of bicarbonate.
Bicarbonate buffer system
- Involves carbonic acid (H2CO3) and bicarbonate(HCO3)
- Major extracellular buffer system
- If strong acid added:
→ HCO3 ties up H+ and forms H2CO3
→ pH decreases only slightly
→ HCO3 - conc closely regulated by kidneys - If strong base added
→ Causes H2Co3 to dissociate and donate H+ to form HCO3-
→ pH rises only slightly
Phosphate buffer system
- Action nearly identical to bicarbonate buffer
- Involves dihydrogen phosphate (H2PO4-) and monohydrogen phosphate (HPO42-
- Effective buffer in URINE AND INTRACELLULAR FLUID, where phosphate conc. is high
-If strong acid added:
→HPO42- ties up H+ and forms H2PO4-
→pH decreases only slightly - If strong base added:
→ It causes H2PO4- to dissociate and donate H+ to form HPO42-
→ pH rises only slightly
Protein buffer system
- Intracellular proteins are most plentiful and powerful buffers; plasma
proteins (extracellular) also important - Protein molecules are can function as both weak acid and weak base
- When pH rises (more alkaline), organic acid or carboxyl (COOH)
groups release H+ - When pH falls (more acidic), NH2 (amide anion) groups bind H+ to
make NH3 (Ammonia) - Protein buffer system:
→ Hemoglobin buffer system
→ Amino acid buffer system
→ Plasma protein buffer system
pH imbalance- Respiratory
Fast, limited compensation
→ Hypercapnia (inc CO2) stimulates pulmonary ventilation
→ Hypocapnia reduces it
pH imbalance - renal
- slow, powerful compensation
→ Effective for imbalances of a few days or longer
→ Acidosis causes in H+ secretion
→ Alkalosis causes bicarbonate secretion
Metabolic acidosis
- High H+ levels stimulate respiratory centres
- Rate and depth of breathing elevated
- Nlood pH is below 7.35 and HCO3- level is low
- As CO2 eliminated by RS, PCO2 falls below normal
Metabolic alkalosis
- Slow, shallow breathing
- Allows CO2 accumulation in blood
- Blood pH is over 7.45 and HCO3- level is high
- As CO2 is retained, PCO2 rises to normal levels
Alkalosis compensation
- Kidney secrete HCO3-
- Recalim h+ to acidify blood
Acidosis compensation
- Kidneys secrete H+
- Reclaim HCO3- to increase the alkalinity of the blood
- Rate of H+ secretion changes with extracellular fluid CO2 levels
- Inc CO2 in peritubular capillary blood causes an inc race of H+ secretion
- System responds to both rising and falling H+ conc/
