BIOL 3542 Lecture Notes - Lecture 8: Cytosol, Ph, Osmotic Concentration
26 Jun 2018
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Human Physiology II
Chapter 20: Integrative Physiology II: Fluid and Electrolyte Balance
Fluid and Electrolyte Homeostasis
kidneys primary route for water loss, ion removal
small amounts of water, ions lost in feces, sweat
lungs lose water, remove hydrogen, bicarbonate (HCO3-) ions by excreting carbon dioxide
drinking only normal way to replace lost water
Salt Appetite: behaviour that leads people, animals to seek, ingest salt
water, sodium, associated with ECF fluid volume, osmolarity
disturbances in potassium balance can cause problems with cardiac, muscle function by
disrupting membrane potential of excitable cells
calcium involved in variety of body processes
hydrogen, bicarbonate balance determines body pH
ECF Osmolarity Affects Cell Volume
water crosses most cells membranes freely
if osmolarity of ECF changes, water moves into/out of cells, changes intracellular volume
cell volume so important, many cells have independent mechanisms for maintaining it
in some cases, changes in cell volume signal initiation of cellular responses
in most cases, inappropriate changes in cell volume impair cell function
maintenance of ECF osmolarity within normal range essential to maintain cell volume
homeostasis
Multiple Systems Integrate Fluid and Electrolyte Balance
process of fluid, electrolyte balance involves respiratory, cardiovascular, renal systems and
behavioural responses
adjustments by lungs, cardiovascular system under neural control, occur rapidly
adjustments by kidneys under endocrine, neuroendocrine control, occur more slowly
change made by one system likely to have consequences that affect others
Water Balance
water most abundant molecule in body
50% body weight of females, 60% of males
2/3 inside cells, 3L in plasma, 11L in interstitial fluid
Daily Water Intake and Excretion are Balanced
only means by which water enters body from external environment is by absorption through
digestive tract
Intravenous Injection: fluid added directly to plasma
Insensible Water Loss: water loss we aren’t consciously aware of that occurs across skin surface,
through exhalation of humidified air
still occurs even though human epidermis has outer layer of keratin to minimize
evaporative water loss
only water loss in urine can be regulated
water loss by diarrhea, excessive sweating can become significant
pathological water loss disrupts homeostasis by:
1. decreasing blood pressure
volume depletion of ECF decreases blood pressure
tissues unable to get adequate oxygen
2. raising osmolarity of body fluids
if fluid lost is hyposmotic to body, solutes left behind raise osmolarity, potentially
disrupting cell function
The Kidneys Conserve Water
once fluid filtered into nephron, it’s part of external environment
kidneys can conserve water, cannot replenish lost water
a major fall in blood volume, pressure shuts down renal function
The Renal Medulla Creates Concentrated Urine
Diuresis: removal of excess water in urine
Diuretics: drugs that promote excretion of urine
urine can be up to 4x as concentrated as blood
to produce dilute urine, kidneys must reabsorb solute without allowing water to follow by
osmosis
apical tubule cell membrane, cell junctions can’t be permeable to water
to concentrate urine, nephron must reabsorb water, without letting solute leave tubule lumen
water reabsorbed by osmosis through aquaporins
renal medulla maintains high osmotic concentration in cells, interstitial fluid, allowing urine
to be concentrated as it flows through collecting duct
renal cortex has interstitial osmolarity of ~300mOsM
reabsorption in proximal tubule isosmotic, filtrate entering loop of Henle has osmolarity
~300mOsM
as nephrons dip into medulla, interstitial osmolarity increases to 1200mOsM where collecting
ducts empty into renal pelvis
fluid in descending limb of loop of Henle lose water to interstitium
tubule fluid at bottom of loop same osmolarity as medulla
in ascending limb, cells in thick portion have apical (facing tubule lumen) surfaces that’re
impermeable to water, but do transport ions out of tubule lumen, decreasing concentration of
tubule fluid
fluid leaving loop of Henle hyposmotic, osmolarity ~100mOsM
loop of Henle primary site where kidney creates hyposmotic fluid
fluid leaves loop of Henle, passes into distal nephron where water permeability of tubule
cells variable under hormonal control
when apical membrane of distal nephron impermeable to water, filtrate remains dilute
small amount of additional solute can be reabsorbed as fluid passes along collecting duct,
making fluid even more dilute (as low as 50mOsM)
when body needs to conserve water, tubule epithelium in distal nephron inserts water pores
into apical membrane, osmosis draws water out of less-concentrated lumen into more
concentrated interstitial fluid (fluid osmolarity can be as high as 1200mOsM)
Vasopressin Controls Water Reabsorption
vasopressin/antidiuretic hormone causes body to retain water
presence of vasopressin causes collecting duct to become permeable to water
water moves by osmosis from tubule to medullary interstitial fluid
in absence of vasopressin, collecting duct impermeable to water
permeability variable depending on how much vasopressin present, allowing body to match
urine concentration closely to its needs
the more vasopressin present, the more water reabsorbed
Vasopressin and Aquaporins
kidney has multiple isoforms of aquaporins
aquaporin-2 regulated by vasopressin, found on apical membrane facing tubule lumen
and in membrane of cytoplasmic storage vesicles
when vasopressin levels low, collecting duct cell stores aquaporin-2 water pores in
cytoplasmic storage vesicles
vasopressin binds to V2 receptors on basolateral side of cell, activating G-protein/cAMP
second messenger system
subsequent phosphorylation of intracellular proteins causes aquaporin-2 vesicles to move
to apical membrane, fuse with it
exocytosis inserts aquaporin-2 water pores into apical membrane
Membrane Recycling: process in which parts of cell membrane are alternately added by
exocytosis, withdrawn by endocytosis
Blood Volume and Osmolarity Activate Osmoreceptors
plasma osmolarity, blood volume, blood pressure control vasopressin secretion
most potent stimulus for vasopressin release is increase in plasma osmolarity
Osmoreceptors: stretch-sensitive neurons that increase firing rate as osmolarity increases
when osmoreceptors shrink, cation channels linked to actin filaments open, depolarizing
cell
primary osmoreceptors for vasopressin release in hypothalamus
when plasma osmolarity below threshold of 280mOsM, osmoreceptors don’t fire,
vasopressin release from pituitary ceases
if plasma osmolarity rises above threshold, osmoreceptors shrink and fire to stimulate
vasopressin release
pituitary receptors for decreased volume stretch-sensitive receptors in atria
blood pressure monitored by same carotid, aortic baroreceptors that initiate cardiovascular
responses
when blood pressure/volume low, receptors signal hypothalamus to secrete vasopressin,
conserve fluid
Document Summary
Chapter 20: integrative physiology ii: fluid and electrolyte balance. Fluid and electrolyte homeostasis kidneys primary route for water loss, ion removal small amounts of water, ions lost in feces, sweat lungs lose water, remove hydrogen, bicarbonate (hco3 drinking only normal way to replace lost water. Salt appetite: behaviour that leads people, animals to seek, ingest salt. Maintenance of ecf osmolarity within normal range essential to maintain cell volume homeostasis. 50% body weight of females, 60% of males. 2/3 inside cells, 3l in plasma, 11l in interstitial fluid. Daily water intake and excretion are balanced only means by which water enters body from external environment is by absorption through digestive tract. Insensible water loss: water loss we aren"t consciously aware of that occurs across skin surface, through exhalation of humidified air. Still occurs even though human epidermis has outer layer of keratin to minimize evaporative water loss only water loss in urine can be regulated.
