Renal Physiology and Body Fluid Homeostasis Flashcards
role of the kidney?
matching rate of excretion to intake
why is intake of fluid and electrolytes so variable?
intake is sporadic, can occur as result of social and regulatory stimuli
role of kidney when rate of intake/excretion are extremely different from each other?
can only slow rate of change in body fluids
what processes does the kidney work in conjunction with?
regulating ingestion, regulation of other excretory routes (CO2 by lungs, faeces), regulation of metabolic processes, control of absorption
amount of fluid in the intracellular fluid?
around 25 litres- the largest fluid compartment
further subdivisions of the extracellular fluid?
blood plasma, interstitial fluid, transcellular fluid
what is the blood plasma and what is its volume?
fluid within vasculature, around 3 litres
what is the interstitial fluid and what is its volume?
fluid around cells and outside vasculature, around 13 litres
what is the transcellular fluid and what is its volume?
specialised fluid compartments such as synovial fluid, cerebrospinal fluid. around 1 litre
which fluid compartment in the body is regulated independently from the rest?
transcellular fluid
how does the kidney act on composition of interstitial and intracellular fluid indirectly?
kidney acts directly on composition and volume of plasma fluid which then influences composition of interstitial fluid which then influences composition of intracellular fluid
total volume of extracellular fluid?
around 17 litres
what is osmotic pressure?
pressure required to prevent osmotic water movement
composition of blood? (proportion of plasma vs cells)
55% plasma, 45% cellular components
composition of plasma?
91% water, 7% proteins (albumin, fibrinogen, globulins etc.), 2% electrolytes, nutrients, hormones
cellular components of blood?
leukocytes (WBCs), platelets, erythrocytes
what is seen in centrifuged blood?
liquid plasma layer, leukocyte layer, other cellular components layer
what determines osmotic water movements?
osmolality
what is osmolality?
osmoles per kg of water
what separates the plasma from the interstitial fluid?
capillary membranes
why don’t ions exert osmotic pressure across the capillary membrane?
capillary permeable to ions so freely cross capillary membrane and have similar concentrations in plasma and interstitial fluid
what factor influences the water distribution between blood and interstitium?
colloid osmotic pressure produced by protein concentration
main protein in ECF?
albumin
what does osmotic pressure depend on?
osmolarity x gas constant x absolute temperature
what does it mean for osmolality to be a colligative property?
proportional to the number rather than the type of particle
what force pulls water out of the interstitium into the capillaries?
osmotic pressure as the colloid concentration in the interstitial fluid is negligible
what forces water out of the capillaries?
hydrostatic pressure higher in the capillaries than the interstitium
what is Starling’s theory of capillary water movement?
volume flow is proportional to hydrostatic pressure difference - osmotic pressure difference
why does the capillary pressure drop along the capillary?
primarily due to resistance
net flux at arteriolar end of capillary and venous end of capillary?
outward at arteriolar end, inwards at venous end
causes of oedema?
cardiac failure, septicaemia, lymphatic blockage, protein loss
what provides the majority of ECF osmolality? (compared to ICF)
NaCl
what provides the majority of ICF osmolality?
K+ and membrane impermeant anions
cause of movement between interstitial and intracellular fluid?
osmosis
overarching cause of movement between plasma and interstitial fluid?
Starling forces
how is Na+ excluded from cells?
the Na+/K+-ATPase
how are Cl- and HCO3- excluded from cells?
the membrane potential
how can extracellular ion concentrations change?
by changing amount of solute (salt intake or loss) or by changing volume of solvent (water intake or loss)
effects of increased plasma osmolality on cells?
water moves out of cells into plasma, causes cellular shrinkage, if severe can result in reduced consciousness/fitting as brain function reduced
effects of decreased plasma osmolality on cells?
can cause brain to swell, increases pressure within confines of the skull which can reduce cerebral perfusion as veins become compressed
treatment for decreased plasma osmolality (hyperhydration)?
can give mannitol- unreactive sugar that can cross capillary membrane but not cell membrane so increases interstitial osmolality to draw water from cells by osmosis
how are body water compartment volumes estimated clinically?
if jugular vein overfilled then high enough plasma volume, can measure urine output
what is the dilution technique of measuring body water compartment?
add a known amount of substance A into compartment, measure its concentration, volume = amount/conc. in practice A should be restricted to 1 compartment, evenly distributed, not change V, not be lost (metabolised/excreted), be non-toxic and easily measurable
what substances can be used to measure total body water?
D2O or HTO have fairly rapid excretion but slow compared to volume of distribution
single injection method for measuring plasma volume?
single injection of substance, measure concentration in blood over time- decreases as it is lost so some lost by time it is evenly distributed. can extrapolate back to see what concentration would be if evenly distributed at t=0
how can blood volume be determined from plasma volume?
centrifuge blood to get haematocrit, blood volume = plasma volume/(1-H)
requirements for substances used in single injection method?
slow excretion or metabolism, non-toxic, easily measurable
continuous infusion method for measuring fluid compartment volumes?
infuse substance at constant rate until the plasma concentration becomes constant- then infusion stopped and amount of substance excreted from urine measured to calculate A- as concentration increases excretion increases so will reach steady state when excretion rate= infusion rate which is when can measure concentration in plasma.
requirements for substances used in continuous infusion to measure ECF volume?
fast excretion, single measurable route of excretion, can cross capillary membrane but not cell membrane
substances that can be used in continuous infusion?
inulin, radiolabelled Na+/Cl-, thiosulphate
how can intracellular water be calculated?
by subtraction ECF volume from total body water (estimated with D2O/HTO)
where does blood enter the kidneys? via what?
at the renal hilum via the renal arteries
how much of the resting CO do kidneys receive?
25%
what do the renal arteries divide into?
the interlobar arteries
what do the interlobar arteries give off?
arcuate arteries
where do the interlobar arteries run?
up the renal columns to the junction between the renal cortex and the medulla
where do the arcuate arteries run?
along the corticomedullary border
what do the arcuate arteries give off?
interlobular arteries which supply cortex via afferent arterioles
what do the afferent arterioles in the kidney lead to?
the glomerular capillaries
where does the majority of blood flow from the glomerular capillaries?
through the peritubular capillaries for filtrate reabsorption
where does 1% of blood flow from the glomerular capillaries?
follows the vasa recta
what is the vasa recta?
long capillary loops that descend into the medulla before returning to the cortex
how and why is there a hyperosmotic environment generated and maintained within the medulla?
needed to produce concentrated urine, happens as medulla receives very little renal blood flow compare to cortex
what % of the plasma is filtered from the glomerular capillaries into the renal tubules?
approximately 20%
why is the blood flow through the renal artery and renal vein almost identical?
around 99% of the renal filtrate is reabsorbed into the peritubular capillaries and vasa recta
first step of filtration in kidney?
blood filtered in glomerulus, filtrate containing about 20% of renal plasma flow enters Bowman’s capsule and then the proximal tubule
what is the route taken by the filtrate on leaving Bowman’s capsule?
proximal convoluted tubule, proximal straight tubule, thin descending limb of LoH, thin ascending limb of LoH, thick ascending limb of LoH, distal convoluted tubule, cortical collecting duct, outer medullary collecting duct, inner medullary collecting duct
what occurs in the proximal tubule?
reabsorption of the majority of filtrate and essentially all filtered glucose and amino acids
histological features of the proximal tubule?
large SA, many mitochondria
what occurs in the LoH?
separates reabsorption of solutes and water so fluid leaving LoH is hypo-osmotic to plasma, inner medulla is very hyperosmotic.
what is the distal tubule responsible for?
control of plasma K+ and pH, water reabsorption in concentrating kidney, water impermeable in diluting kidney
what is the collecting duct responsible for?
allows reabsorption of water from the hypo-osmotic tubule into the cortex and then the hyper-osmotic medulla to produce hyper-osmotic urine
where does urine flow from the collecting ducts?
into the minor calyces then the major calyces and then into the renal pelvis and then the ureter
what are the 2 populations of nephrons?
cortical and juxtamedullary
which nephrons have LoHs that extend into the inner medulla?
juxtamedullary nephrons
what are the 3 layers filtrate flows through in Bowman’s capsule?
fenestrated capillary membrane, basal lamina and filtration slits between the foot processes of podocytes
why is filtration in the renal corpuscle considered ultrafiltration?
indicates that separation is occurring at the level of ‘ultra-small’ sizes (molecular sizes)
role of the fenestrated capillary membrane in the filtration barrier?
large pores (around 70nm) that entirely prevent passage of cells, allow passage of largest protein molecules
role of the basement membrane in the filtration barrier?
negatively charged, restricts passage of large solutes
role of the podocytes in the filtration barrier?
have foot processes separated by filtration slits bridged by thin diaphragm with pore approx. 4 by 14 nm. negative charge. most restrictive layer of filtration barrier
why is only very little albumin able to pass into Bowman’s capsule despite it being small enough?
both albumin and the filtration barrier are negatively charged
why is the charge on the filtration barrier irrelevant to small molecules like ions?
charges must be close to interact, small molecules don’t pass sufficiently close to the borders of the filtration barrier
what factors affect the glomerular filtration rate?
difference between glomerular capillary and Bowman’s capsule hydrostatic pressure, difference between colloid osmotic pressure of glomerular capillary and Bowman’s capsule, and the filtration coefficient (product of capillary permeability and SA available for filtration)
how is glomerular filtration rate primarily regulated? what contributes to this factor?
by regulating the glomerular capillary hydrostatic pressure. can be varied by changing resistance of afferent or efferent arterioles
effect of constriction of the afferent arteriole on glomerular capillary hydrostatic pressure? what is the effect of this on GFR?
reduces glomerular capillary hydrostatic pressure, which reduces GFR
when is constriction of the afferent arteriole needed?
when arterial blood pressure is high- would otherwise increase renal plasma flow and pressure in glomerular capillaries increasing GFR
effect of constriction of the efferent arteriole on glomerular capillary hydrostatic pressure? effect of this on GFR?
restricts escape of blood to low pressure venous system, increases glomerular capillary hydrostatic pressure, increases GFR
what mechanism is usually enough to reduce or increase glomerular capillary hydrostatic pressure to control GFR?
afferent arteriole constriction/dilatation
what allows independent control of renal plasma flow and filtration pressure?
dilatation of the efferent arteriole will decrease glomerular capillary hydrostatic pressure while increasing renal plasma flow, dilatation of the afferent arteriole will increase both glomerular capillary hydrostatic pressure and renal plasma flow
what is the evidence for autoregulation of GFR?
over a wide range of ABP, GFR stays relatively constant, outside this range is much more affected
what are the 2 intrinsic mechanisms of regulation of GFR?
the myogenic mechanism and tubuloglomerular feedback
what is the myogenic mechanism of GFR regulation?
afferent arteriole constricts when stretched, relaxes when released from stretch
what is the tubuloglomerular feedback mechanism of GFR regulation?
the macula densa (between ThickAL of LoH and the distal tubule) senses NaCl uptake, raised NaCl uptake suggests more NaCl than normal being delivered to distal areas of nephron, suggests flow rates through nephron are too high for normal levels of reabsorption. in response to raised NaCl delivery macula densa releases ATP, stimulates afferent arteriole constriction which reduces glomerular capillary pressure and renal plasma flow offsetting effects of raised ABP
what are the extrinsic mechanisms controlling GFR?
the renin-angiotensin system and the renal sympathetic nerves
effect of AngII and sympathetic activity on the renal arterioles?
both constrict them
which arteriole does AngII act on primarily at low concentrations?
the efferent arteriole
what cells surround the glomerular capillaries and function to adjust the capillary SA?
mesangial cells- similar to smooth muscle, contraction may reduce capillary SA
why can severe rhabdomyolysis reduce GFR?
skeletal muscle breakdown can release large quantities of myoglobin which can block the filtration pores
what is the hydrostatic pressure in Bowman’s capsule normally?
around 10mmHg
how can urinary tract obstruction impede filtration?
by increasing hydrostatic pressure in Bowman’s capsule so decreasing GFR
what is nephrotic syndrome?
pathologies leading to an increase in glomerular protein permeability and therefore a reduction in the reflection coefficient and reduction in plasma colloid osmotic pressure so reduction in GFR
how does increased RBF affect the glomerular capillary colloid osmotic pressure?
when RBF is high the increase in capillary colloid osmotic pressure over distance is decreased so more filtration can occur at the distal end of the glomerular capillary
how much filtrate is reabsorbed in the proximal tubule, how much of substances such as glucose and and amino acids is reabsorbed in the proximal tubule?
65% of filtrate, nearly 100? of substances like glucose and amino acids
what is clearance?
the rate of excretion expressed as a function of the plasma concentration
what does clearance allow?
comparison of renal handling of different substances even if present in plasma at very different concentrations
filtration rate calculation if a substance is freely filtered?
GFR x plasma concentration
what is clearance for a substance that is freely filtered and neither reabsorbed or secreted? what does this mean?
clearance = GFR. means can use one of these substances to measure GFR
what is the clearance ratio of a substance?
clearance of the substance relative to that of inulin- if >1 implies secretion of the substance and in <1 implies reabsorption or incomplete filtration of the substance
how can creatinine be used to calculate GFR?
creatinine is produced at a relatively constant rate, freely filtered, not reabsorbed and only slowly secreted so only overestimates GFR by a small amount. can calculate creatinine clearance from rate of creatinine excretion in urine measured over 24 hours and plasma creatinine concentration.
how can GFR be estimated from plasma GFR concentration?
if GFR drops less creatinine is excreted, plasma concentration has to rise to increase creatinine excretion again. needs correction for age, sex and body weight as creatinine production is proportional to muscle mass
how can inulin be used to measure GFR?
inulin is freely filtered and not reabsorbed or secreted, doesn’t influence GFR. so can use constant perfusion to calculate arterial plasma concentration and rate of excretion. then can use this to calculate clearance which = GFR
what is PAH?
para-aminohippurate- substane secreted by the cortical peritubular capillaries of kidney and freely filtered that can be used to estimate renal plasma flow
passive mechanisms of transcellular transport?
simple diffusion, facilitated diffusion, solvent drag (carried in water flow)
how can PAH be used to estimate renal plasma flow if plasma concentrations sufficiently low?
if plasma concs. low enough PAH can be almost completely cleared by the kidney, flow = rate of excretion so can measure PAH concentrations in the artery and then renal vein
why does using PAH to estimate renal plasma flow produce an underestimate of around 10%
as PAH is only secreted by the cortical peritubular capillaries and around 10% of blood travels through medullary capillaries instead
methods of secondary active transport?
symport (all substances transported in same direction) and antiport (transport in opposite directions)
active mechanisms of transcellular transport?
primary active transport (directly coupled to hydrolysis of ATP by transport protein), secondary active transport (coupled to electrochemically favourable movement of another substance), endocytosis
active components of reabsorption in the proximal tubule?
sodium and glucose co transport into cells by SGLTs out of PT, then GluT2 transports glucose out coupled to Na+/K+-ATPase so glucose and Na+ transported into blood together. NHE transports Na+ out of PT in exchange for H+. HCO3- actively transported into blood
example of transport maxima seen in clinical practice?
appearance of glucose in urine of a diabetic as maximum reabsorption by glucose transporter cannot return all of the glucose back to the plasma as plasma [glucose] is too high
difference in glucose transport in the early and late proximal tubule?
in early, uses SGLT-2 which transports 1 Na+ to drive glucose transport, in late uses SGLT-2 which uses 2 Na+ to drive glucose transport
why does the Na+ concentration remain constant throughout the proximal tubule even though 70% is being reabsorbed?
water follows the solute as proximal tubule water permeable so reabsorption is isotonic. Starling forces favour water reabsorption as osmotic gradient from ions and also peritubular capillaries have higher colloid osmotic pressure
is Cl- reabsorption greater in the early or plate proximal tubule?
greater in the late than early PT
how does transcellular reabsorption work in the late proximal tubule?
Cl- enters the cell in exchange for various anions. there is more Cl- to reabsorb than anions to secrete so the anions are recycled. anions protonated in the tubule (acidic due to actions of Na+/H+ exchanger). protonated form is uncharged, can therefore diffuse back through cell membrane, in cell dissociates into anion and H+ as cell is more alkaline. anion can then be reused to reabsorb more Cl-, net result is reabsorption of Na+ and Cl-. Cl- then leaves basolateral membrane via K+/Cl- co-transporter (K+ provided by the Na+/K+-ATPase, Na+ leaves via the Na+/K+ ATPase which ultimately provides power for the entire process
simple micropuncture evidence for isotonic reabsorption in the PCT?
sample from early and late PT reveal no change in osmotic pressure, if inulin injected before sampling concentration of inulin found to rise along the PT- must be caused by fluid reabsorption
spilt oil drop experiment for evidence of isotonic reabsorption in the PCT?
mineral oil can be injected into Bowman’s capsule so some enters the PCT- second micropipette used to inject test solution to split the oil drop so that the solution between the oil drops is known. with time oil drops move towards each other indicating reabsorption of fluid between them. can resample test fluid to show is it isosmotic with that injected
organic anions secreted in the PT?
prostaglandins, cAMP and cGMP, bile salts, drugs including penicillin, frusemide, salicylate
organic cations secreted in the PT?
creatinine, adrenaline and noradrenaline, dopamine, drugs including morphine, atropine
what is the K+ distribution between cells and ECF?
less than 2% in ECF, 98% within cells
role of K+ under normal circumstances?
K+ gradient across cell membranes is responsible for the membrane potential, critical for cell functions including volume + pH regulation as well as excitability of nerve and muscle
effect of changes in amount of K+ in extracellular space compared to intracellular space?
same change in amount of K+ in extracellular space will cause much larger changes in membrane potential because there is less in the extracellular space, and the extracellular space is smaller
how can control over extracellular K+ be accomplished in the short term?
by moving K+ between the intracellular and extracellular compartments
how can control over intracellular K+ be accomplished in the long term?
by controlling amount of K+ in the body
insensible K+ losses by the body?
around 10mmol each day in faeces and sweat
K+ intake?
from ingestion- approx. 100mmol per day, intake is sporadic and varies considerably with diet
stresses on K+ homeostasis?
insensible losses (increased by diarrhoea, vomiting, sweating); excess or insufficient intake; dehydration and hyperhydration; cell lysis; acidosis
factors causing intracellular to extracellular K+ shift?
action potentials (in exercising skeletal muscle), dehydration, cell lysis, acidosis
effect of APs on K+ flux?
repolarisation phase causes K+ shift from intracellular to extracellular space
how much of body K+ does skeletal muscle contain?
up to 70%
effect of dehydration on K+ flux?
decrease in plasma osmolality causes cell shrinkage, increases intracellular [K+] which can cause cells to lose K+
effect of cell lysis on K+ flux?
releases K+ into the EC space
effect of acidosis on K+ flux?
movement of H+ into cells displaces K+.
factors causing extracellular to intracellular K+ shift?
hyperhydration, insulin, adrenaline
effect of hyperhydration on K+ flux?
cell swelling produced by drop in plasma osmolality can cause cells to take up more K+
effect of insulin on K+ flux?
activates Na+/K+-ATPase activity so causes K+ to move into cells
effect of adrenaline on K+ flux?
activates Na+/K+-ATPase so causes K+ to move into cells
why is it interesting that insulin and adrenaline enhance movement of K+ into cells?
they are hormones associated with the major physiological stresses on K+ homeostasis (eating and exercise) so act as feed-forward response
effect of extracellular [K+] on excitability of excitable nerves?
low [K+] causes hyperpolarisation so reduces excitability, high [K+] causes depolarisation- brings excitable cells closer to threshold (increases excitability) but can cause inactivation of VGNaCs at high degrees cause reduced excitability
effect of hyperkalaemia on muscle?
can increase risk of cardiac arrythmia by causing increased excitability from mild hyperkalaemia and reduced excitability from extreme hyperkalaemia
effect of hypokalaemia on muscle?
causes muscle weakness by reducing excitability, if severe can cause muscular paralysis (including of diaphragm) and cardiac arrhythmias
why is understanding of K+ homeostasis necessary in a hospital setting?
renal disease, bowel dysfunction, diuretic drugs, IV fluid administration can all disrupt K+ intake and output so hypo- and hyperkalaemia are common in hospital setting
what are the 2 major physiological stresses on K+ homeostasis?
eating and exercising
control of internal balance of K+ (between ECF and ICF)?
Na+/K+-ATPase activity which is increased by insulin, adrenaline and aldosterone
under what physiological stresses is transient hyperkalaemia observed?
after eating in untreated or poorly-treated diabetes, after exercise in patients taking β2-adrenergic receptor blockers
what is required for long term K+ homeostasis?
matching of K+ intake and output by kidneys
journey K+ takes through the kidneys?
freely filtered, unregulated reabsorption of around 67% in PT, further unregulated reabsorption of around 20% in thick AL of LoH. 13% left entering distal segments. smaller degree of unregulated reabsorption in Type A intercalated cells of DT (around 3%) and collecting duct (around 9%). regulated K+ secretion in principal cells of DCT (up to 50% of that filtered) and principal cells of cortical collecting duct (up to 30%). so net secretion of K+ can be between 1-80% of that filtered
3 major factors controlling amount of K+ secreted by principal cells in DCT and CCT?
plasma [K+], aldosterone, tubular flow rate
how does plasma [K+] influence K+ secretion in the DCT and CCT?
high plasma [K+] increases interstitial [K+] so enhances K+ transport into principal cells, enhances K+ gradient across luminal membrane
how does aldosterone influence K+ secretion in the DCT and CCT?
aldosterone is released by adrenal cortex in response to increased plasma [K+], causes increased activity of all K+ secretion components- the SK, ENaC channels and basal Na+/K+-ATPase, by stimulating protein synthesis. aldosterone also enhances Na+ reabsorption in the principal cells of the DT- opposed by effect of reduced tubular flow rate. changes in aldosterone levels alone significantly influence K+ excretion
effect of aldosterone on Na+/K+-ATPase density in long term?
increases it- suggests it stimulates its synthesis
how does tubular flow rate influence K+ secretion in the DCT and CCT?
since K+ secretion across luminal membrane is essentially passive it slows if K+ is allowed to build up in tubule. high tubular flow rates remove secreted K+ allowing further secretion- relevant in hypo/hypervolaemia. reduced tubular flow rate in hypovolaemia opposes effect of aldosterone enhancing Na+ reabsorption and therefore K+ secretion
how does ADH both reduce and increase K+ secretion so the effects cancel out?
ADH promotes water reabsorption so reduces flow through DCT and CCT, which means K+ secretion is reduced, however ADH also stimulates luminal K+ conductance in principal cells so enhances secretion
what is the importance of pH homestasis?
charges of many proteins are pH dependent so functions of proteins sensitive to pH, and intake and intrinsic production of acids and alkalis varies significantly under normal physiological conditions
why does CO2 lower blood pH?
it rapidly hydrates to HCO3- and H+ in presence of carbonic anhydrase
why does CO2 production not produce severe stress on acid-base balance?
healthy lungs are able to excrete as much CO2 as is produced
examples of pH buffers in plasma?
static buffers: inorganic phosphate and plasma proteins, haemoglobin also acts as a buffer, dynamic buffer: HCO3- system
which buffer provides more buffering capacity than all other extracellular buffers combined?
HCO3-
what is needed for the HCO3- buffer system to be sustainable?
HCO3- levels to be maintained by renal production of bicarbonate
why do we assume that buffering an acid by HCO3- in blood only reduces [HCO3-] and doesn’t influence PCO2?
the amount of CO2 produced by the buffer system is negligible compared to the amount produced by respiration
what is the respiratory control of plasma pH?
PCO2 levels detected in medulla, or low pH detected by peripheral chemoreceptors -> direct feedback control of PCO2 by changes in alveolar ventilation rate
what does net production of HCO3- by the kidneys require?
reabsorption of filtered HCO3-, production of HCO3- and H+ in proximal tubule allows secretion of H+ and return of HCO3- to plasma, buffering of tubular H+ allows further secretion
how is HCO3- reabsorbed in the kidneys?
reabsorbed in all segments by: secretion of H+ acidifying tubule, driving reaction to produce more CO2 and H+, CO2 diffuses into cell, cell more alkaline due to H+ secretion so reaction in cell driven back to left yielding HCO3- and H+, HCO3- transported out of cell across basolateral membrane, H+ secreted restarting the cycle. catalysed by carbonic anhydrase
why does pH in urine need to be buffered?
H+ is secreted by kidney to buffer blood pH, this will lower urine pH, without buffer would either need to produce very large urine volume or have very acidic urine.
what buffer is used to buffer urine pH?
inorganic phosphate as the body can afford to lose it
why do the kidneys use ammoniagenesis and what is it?
needed as inorganic phosphate alone can’t buffer all of the H+ that needs to be excreted, so ammoniagenesis used to excrete H+ ions as NH4+ (buffered by NH3)- uses glutamine from waste amino acids in the liver, HCO3- produced and returned to circulation
how is HCO3- produced in the proximal tubule?
cells relatively alkaline, NH4+ dissociates to NH3 and H+, NH3 diffuses into acidic tubular lumen, forms NH4+ which can’t diffuse across membrane so is trapped in tubule. HCO3- transported across basolateral membrane
path taken by NH4+ in the kidney?
reabsorbed in the thick ascending limb substituting for K+ on the Na+-K+-2Cl- co-transporter, builds up in medullary interstitium. NH3 form diffuses through collecting duct cells, is protonated in tubule forming NH4+ which traps it so its excreted in urine
how is the balance of NH4+ in the medulla returning to the bloodstream or being trapped in the tubular lumen of the collecting duct controlled?
by plasma pH: at high pH the activity of the luminal proton pumps is low so ammonium trapping is poor, at low pH a high proportion of NH4+ is trapped and excreted.
control of renal H+ and HCO3- handling?
H+ secretion (needed for HCO3- reabsorption) is enhanced by low pH (increases expression and activity of transporters). increased pCO2 entering tubular cells lowers their pH enhancing H+ secretion + HCO3- reabsorption. influenced by hormones: cortisol, PTH, angiotensin II, aldosterone
influence of cortisol on renal H+ and HCO3- handling?
released in response to low pH, increases transcription of the Na+/H+ exchanger and Na+/3HCO3- co-transporter in PT, so increases H+ secretion and HCO3- reabsorption
influence of PTH on renal H+ and HCO3- handling?
in prolonged acidosis promotes acid secretion in thick AL + DT, also reduces inorganic phosphate reabsorption in PCT thereby increasing buffering of tubular fluid.
influence of angiotensin II on renal H+ and HCO3- handling?
stimulates Na+/H+ exchange in proximal tubule enhancing Na+ reabsorption and H+ secretion so HCO3- reabsorption
influence of aldosterone on renal H+ and HCO3- handling?
stimulates K+/H+-ATPase in type A intercalated cells enhancing H+ secretion (so HCO3- reabsorption) and K+ reabsorption
what are the 4 possible stresses on acid-base regulation?
respiratory acidosis, respiratory alkalosis, metabolic acidosis, metabolic alkalosis
what are the 8 possible acid base disorders?
compensated and non-compensated respiratory acidosis, respiratory alkalosis, metabolic acidosis, metabolic alkalosis
signs of non-compensated respiratory acidosis?
high PCO2 (above 40mmHg) and normal HCO3- (around 24mmol), low pH (below 7.4)
signs of metabolically compensated respiratory acidosis?
high PCO2 (above 40mmHg), high HCO3- (above 24mmol), normal pH (7.4)
signs of non-compensated respiratory alkalosis?
low PCO2 (below 40mmHg). normal HCO3- (around 24mmol), high pH (above 7.4)
signs of metabolically compensated respiratory alkalosis?
low PCO2 (below 40mmHg).low HCO3- (below 24mmol), normal pH (7.4)
signs of non-compensated metabolic acidosis?
low HCO3- (below 24mmol), normal PCO2 (around 40mmHg), low pH (below 7.4)
signs of respiratory compensated metabolic acidosis?
low HCO3- (below 24mmol), low PCO2 (below 40mmHg), normal pH (7.4)
signs of non-compensated metabolic alkalosis?
high HCO3- (above 24mmol), normal PCO2 (around 40mmHg), high pH (above 7.4)
signs of respiratory compensated metabolic alkalosis?
high HCO3- (above 24mmol), high PCO2 (above 40mmHg), normal pH (7.4)
what is a Davenport diagram?
graphical method of diagnosing acid-base disorders
what is plasma osmolality normal range?
380-303mOsm/kg
why is plasma osmolality regulated by the kidney?
plasma osmolality influences the distribution of water between the extracellular and intracellular spaces and therefore influences cell volumes
how is plasma osmolality controlled by the kidney?
by regulating urine osmolality- more concentrated urine = more dilute plasma and vice versa. urine osmolality is regulated by controlling amount of water in urine using ADH
what releases ADH?
posterior pituitary
osmoregulatory and volume regulation priorities of the body?
avoiding hypotension, maintaining ECF osmolality, maintaining ECF volume
why won’t changing the amount of NaCl in the body change plasma osmolality even though NaCl is the major determinant of ECF osmolality?
most NaCl reabsorption is isotonic, ADH and thirst adjust water excretion to maintain plasma osmolality
how many excess solutes are usually excreted in urine per day?
total of around 450-1500mOsm per day dissolved in water
why is variation of urine osmolality important?
allows independent control of solute excretion and water excretion, so independent control of ECF volume and osmolality
why is it not straightforward for kidneys to produce urine that is different to plasma osmolality?
no active water pump, water can only flow due to an osmotic or pressure gradient, there is a maximum transcellular osmotic gradient as ion transport becomes less energetically favourable and back leakage increases as gradient builds up
how does kidney produce urine with osmolality different to plasma osmolality?
pumps ions across cells that are impermeable to water thereby creating osmotic gradient for later water transport, employ countercurrent multiplication
summary of production of hypo-osmotic urine (dilute)?
transport of ions and water separated in LoH. as fluid travels through LoH ions transported into medullary interstitium while water largely retained. tubular fluid becomes dilute. in absence of ADH the remaining segments of nephron are impermeable to water and re-absorb further solutes so tubular fluid becomes increasingly hypo-osmotic
summary of production of hyper-osmotic (concentrated) urine?
ions pumped out of LoH into medullary interstitium, water largely retained, so tubular fluid becomes dilute. in presence of ADH DT and CCT are permeable to water, so water drawn out of them into the isosmotic cortex. in MCT fluid is therefore isosmotic to plasma, but renal medulla is hyperosmotic to plasma (due to actions of LoH) so water is reabsorbed from MCT into medulla, hyperosmotic urine is produced
evidence for medullary hypertonicity to plasma?
obtained from microcryoscopy- rapid freezing and sectioning of the kidney allows estimation of solute concentrations by measuring melting point of different regions
how is the medullary interstitium made hypertonic?
4 stage process: active NaCl reabsorption and countercurrent multiplication operating in both the concentrating and diluting kidney, urea cycling and passive NaCl reabsorption in concentrating kidney
active NaCl reabsorption to make medullary interstitium hypertonic?
in thick ascending limb of LoH, ions (Na+, K+, Cl-) actively pumped out of tubule, which is water impermeable so tubular fluid becomes hypotonic, medullary interstitium becomes hypertonic
countercurrent multiplication to make medullary interstitium hypertonic?
combination of 2 gradients: transmural gradient and gradient of increasing osmolarity toward LoH apex. the thick AL makes the medulla hypertonic by active pumping, water is drawn out of the LoH descending limb, so fluid entering deepest segment of ascending limb is hypertonic so thick AL can make medulla more hypertonic.
urea cycling in concentrating kidney to make medullary interstitium hypertonic?
increases renal medulla concentration of urea. urea movements driven by osmotic gradients set up by LoH. about 50% of filtered urea is passively reabsorbed in the PT down gradient created by water reabsorption. inner medullary collecting duct has relatively high urea permeability which is increased by ADH. large amount of urea reaching the IMCD is reabsorbed into medullary interstitium, as large gradient created by water reabsorption in urea impermeable DCT and CCD. urea from medullary interstitium secreted into thin limbs of LoH
how much of the maximum osmolarity of fully concentrated urine does urea account for?
half
passive NaCl reabsorption in concentrating kidney to make medullary interstitium hypertonic?
thin AL of LoH reabsorbs NaCl and thereby contributes to high medullary [NaCl] and countercurrent multiplication. unlike thick AL, doesn’t actively pump out NaCl, reabsorbs it passively due to gradients from urea cycling. water pulled from thin AL by hypertonicity of medullary interstitium until osmolality of tubular fluid at tip of LoH is same as in medullary interstitium. medullary tonicity approx. 50% due to urea, 50% due to NaCl, in thin ascending limb is more due to NaCl- so NaCl is reabsorbed down concentration gradient. only occurs due to urea cycling caused by ADH
why does ADH affect tonicity in the LoH?
through its effect on urea cycling
the medullary blood supply
vasa recta use countercurrent exchange mechanism. capillaries branch from efferent arterioles of juxtamedullary nephrons, descend into deepest region of medulla then loop and ascend straight back to cortex to open into veins- as capillaries descend into increasingly hypertonic interstitium water moves out and NaCl + urea move in so as blood ascends it is very hypertonic with respect in interstitium so water is regained and NaCl and urea move back out- so overall relatively little fluid lost and solute taken up, maintaining medullary hypertonicity
why is removal of substances by the vasa recta important?
vasa recta is only route for water, urea and NaCl building up in the medulla to leave via
normal human extracellular osmolality?
range of 282-290mOsmolal
why is osmoregulation necessary?
changes to ECF osmotic pressure will cause water to move into or out of cells causing them to swell or shrink which can disrupt cell function, dehydration can result in circulatory collapse due to low blood volume, low osmotic pressure can cause brain to swell in skull and increased pressure on brainstem can cause coma and death
what are the 2 major osmoregulatory mechanisms?
regulation of water by ADH and regulation of water intake by thirst
what determines ADH secretion in osmoregulation?
osmoreceptors in the organum vasculosum laminae terminalis (OVLT) in the hypothalamus which detect changes in osmotic pressure of the ECF
what is ADH?
a cyclic nonapeptide
molecular weight of ADH?
1084
major and secondary physiological actions of ADH?
increases renal water reabsorption by increasing the water permeability of all parts of the collecting duct system. also increases urea permeabilities of the inner medullary collecting duct and inner medullary thin descending limb, and is a vasoconstrictor
which has higher affinity for ADH, V1 receptors or V2 receptors?
V2 receptors
what are the V2 effects of ADH?
osmoregulatory (increasing water permeability and urea permeability)
what are the V1 effects of ADH?
vasoconstrictor
when does the vasoconstrictor effect of ADH occur?
only at plasma [ADH] well above the normal osmoregulatory range
effect of drinking and water absorption on ADH release?
inhibit ADH release by reflexes from the gut and liver
how is ADH release controlled?
osmoreceptors in OVLT of hypothalamus, signals from stretch receptors in circulatory system, inhibited by cardiopulmonary receptors in atria and great veins and arterial baroreceptors in carotid sinus and aortic arch when blood volume is large- when this inhibition is reduced when blood volume falls large rise in ADH release
why is there a rapid diuretic response to drinking water?
drinking causes inhibition of ADH secretion by nervous reflex from the throat and gut
what is the response to drinking water?
rapid diuretic response as drinking causes inhibition of ADH secretion, as water absorbed plasma osmolality falls- detected by hypothalamic osmoreceptors which reduced stimulation of ADH release maintaining the diuresis. osmoreceptors in liver also maintain diuresis as osmolality of hepatic portal blood falls due to water arriving from gut
what is the response to drinking isotonic saline?
produces small initial diuresis via nervous inhibition of ADH release, since no change in osmolality response is short lived
what cells synthesise ADH?
neurosecretory cells known as magnocellular neurones
where are most of the cell bodies of magnocellular neurones (which synthesise ADH)?
most are in the supraoptic nucleus (SON) of the hypothalamus, some are in the paraventricular nucleus
where is ADH transported after production?
transported down axons of the magnocellular neurons that produce it for storage in vesicles in the nerve endings that lie in the posterior pituitary gland (neurohypophysis)
how is ADH secreted?
arrival at APs at the magnocellular neuron terminals initiates secretion by calcium-dependent exocytosis, ADH enters capillaries in posterior pituitary
what does amount of ADH released depend on?
frequency of APs arriving from the supraoptic nucleus which depends on activities in the various inputs to the hypothalamus that synapse with the magnocellular neurons
experiments of Verney?
series of experiments on dogs that showed that osmoreceptors were present in the head, localised them to hypothalamus and showed that they regulated release of an ADH from the posterior pituitary. induced diuresis by giving water through stomach tube- found that injecting small volumes of hypertonic solutions into carotid artery caused antidiuresis, as did injecting extract from posterior pituitaries intravenously. after removal of pituitary gland hypertonic injections into carotid had no effect, injecting posterior pituitary extract still did. when hypertonic solutions injected intravenously instead of into carotid, didn’t cause antidiuresis. injecting urea into carotid had no effect.
explanation of Verney’s results? look at rohan notes
proved osmoreceptors detect change in osmotic pressure as changing osmolality with hypertonic injections caused antidiuresis- also proved osmoreceptors were in carotid. also proved they caused response in pituitary as without pituitary no response
how do osmoreceptors work?
shrink or swell with changes in extracellular osmotic pressure altering activity of stretch-inactivated ion channels which influence MP and so AP frequency
why no antidiuretic response to intracarotid hypertonic urea?
urea crosses cell membranes on urea transporters so causes no change in cell volume as no associated water movement
mechanism of action of ADH?
acts on the principal cells of the collecting duct which contain vesicles that have the water channel aquaporin 2 (AQP2) present in their membranes. ADH binds to V2 receptors on basolateral membranes of principal cells- GPCR coupled to adenylyl cyclase which is activated resulting in formation of intracellular second messenger cAMP- activates PKA< phosphorylates AQP2, triggers fusion of AQP2-containing vesicles with luminal plasma membrane thus inserting more AQP2 into luminal membrane increasing its water permeability
effect of ADH on concentrating ability of kidney?
AQP3 and 4 in basolateral membrane means always highly water permeable. rate limiting step in water reabsorption is at luminal membrane. vesicles and the AQP2 they contain are constantly removed from luminal membrane by endocytosis. density of AQP2 in luminal membrane reflects balance between exocytotic insertion of AQP2 and its removal by endocytosis- when ADH levels fall water permeability of luminal membrane decreases as endocytotic removal of AQP2 exceeds exocytotic insertion, and vice versa. ADH increases urea permeability in IMCD by stimulating activation of urea transporters in luminal membrane and inserting additional transporters into IM thin descending limb
constraint on water conservation?
need to produce at least some to dissolve waster products in- usually around 500ml/day
why can’t mammals survive drinking seawater?
excess salts can’t be excreted in urine
what stimulates thirst?
high osmotic pressure, reduced ECF volume, dry throat- all integrated in thirst centre in hypothalamus
how does high plasma osmotic pressure cause thirst?
osmoreceptors in OVLT respond to high osmotic pressure (different to those controlling ADH secretion) by signaling thirst to hypothalamus
how does reduced ECF volume stimulate thirst?
reduced inhibition of thirst centre in hypothalamus by arterial and cardiopulmonary baroreceptors in circulation + rise in Ang II which stimulates thirst
what is diabetes insipidus?
production of large volume of dilute, insipid urine. caused by failure of ADH production or secretion or failure of kidneys to respond to ADH. the polyuria gives rise to polydipsia
what determines volume of ECF? why?
its Na+ content- since Na+ is excluded from the cells
how is Na+ content of the ECF controlled?
by varying loss of Na+ in urine + some regulation of Na+ intake in response to severe volume depletion (sodium appetite)
why must ECF volume be regulated in long term?
influences blood volume and so ABP
principle of increasing Na+ in blood?
reduce filtration by reducing GFR, increase reabsorption. water follows Na+ so this will increase BP (and ECFV)
physical factors influencing sodium excretion?
arterial blood pressure, colloid osmotic pressure
how does ABP influence sodium excretion?
when ECFV increases so does ABP, this increases GFR causing increased Na+ loss in urine- pressure natriuresis. increased glomerular capillary hydrostatic pressure increases net filtration pressure + so GFR. increased cortical peritubular capillary hydrostatic pressure reduces fluid movement into these capillaries raising RIHP which reduces fluid reabsorption from PT
how does colloid osmotic pressure influence sodium excretion?
colloid osmotic pressure= conc of protein- changes affect GFR and reabsorption of fluid by cortical peritubular capillaries. addition of NaCl and water to ECF reduces COP, so GFR rises, reabsorption of fluid from PT falls, more sodium excreted
nervous and hormonal factors influencing sodium excretion
renal nerves, renin, angiotensin I, angiotensin II, aldosterone, atrial natriuretic peptide
effect of renal nerves of sodium excretion?
increased activity in renal sympathetic nerves directly stimulates Na+ reabsorption by activating NHE3 for Na+/H+ exchange, also constricts renal arterioles (afferent more than efferent) so reduces GFR, so less Na+ filtered. also increases secretion of renin which increases amount of Ang II and aldosterone
what modulates renal sympathetic nerve activity?
inputs to CNS from cardiopulmonary receptors and arterial baroreceptors- less input= less inhibition of sympathetic outflow
what is the renin angiotensin system?
renin= proteolytic enzyme secreted by modified smooth muscle cells in wall of afferent arteriole. catalyses production of Ang I from angiotensinogen. Ang I further cleaved to Ang II by ACE. Ang II reduces Na+ excretion
stimuli to renin secretion?
fall in pressure in afferent arteriole, renal sympathetic nerve outflow, change in composition/flow rate at macula densa (if fall in NaCl reabsorption, renin release increases)
effects of angiotensin II?
stimulates Na+ reabsorption in proximal tubule by stimulating NHE3. stimulates aldosterone synthesis by AT2 receptors. stimulates thirst, stimulates sodium appetite, causes aldosterone release, stimulates vasoconstriction, reduces RIHP by constricting efferent more than afferent to increase COP (does cause increased GFR but GFR falls since decreased RBF outweighs increased FF)
where is most sensitive to Ang II?
proximal tubules and efferent arterioles
what is the only factor that stimulates Na+ reabsorption at the distal parts of the renal tubule?
aldosterone
what secretes aldosterone?
zona glomerulosa
effects of aldosterone?
mainly acts on CCD. promotes Na+ reabsorption, K+ secretion into renal tubules (so K+ excretion in urine), promotes H+ secretion
mechanism of action of aldosterone?
stimulates new protein synthesis in principal cells. resulting aldosterone-induced proteins include channels for Na+ and K+ and more Na+/K+-ATPase in long term. increases activity and density of the ENaC, density of SK channels (responsible for K+ secretion), and in long term density of Na+/K+-ATPase in adluminal membrane. so increases rate and capacity of Na+ reabsorption. also increases H+ secretion
stimuli to aldosterone secretion?
increased plasma ang II, increased plasma [K+], decreased plasma [Na+]
ACTH and aldosterone?
aldosterone is synthesised from a glucocorticoid precursor so ACTH is permissive factor is aldosterone synthesis
Addison’s disease?
aldosterone and glucocorticoids deficient- complete absence of aldosterone= natriuresis -> reduced ECF and plasma volume -> hypotension and circulatory collapse and failure to regulate extracellular [K+]
aldosterone excess?
increased ECF volume, hypertension, potassium depletion and metabolic alkalosis
natriuretic factors and Na+ excretion?
atrial natriuretic peptide precursor granules in atrial myocytes- ANP released when atrial stretch increased if ECF increased. ANP inhibits Na+ reabsorption in medullary and CCD, increases intracellular cGMP to activate PKG to phosphorylate ENaC reducing Na+ entry across luminal membrane + to inhibit the Na+/K+-ATPase. inhibits Na+ reabsorption in PT by increasing dopamine so increasing cAMP, increasing PKA, causing phosphorylation and inhibition of NHE3 and Na+/K+-ATPase. inhibits renin secretion. dilates mesangial cells increasing GFR, inhibits aldosterone secretion, vasodilates afferent + efferent arterioles (afferent more) increasing GFR. inhibits ADH secretion so increases water loss
response to ingestion of 1 litre fresh water?
water diuresis. purely osmotic response- insignificant change in ECF volume. excess water excreted within an hour
response to ingestion of 1 litre isotonic saline?
no change in osmotic pressure. NaCl remains in ECF, so water will too- so volume change more significant, slow excretion of excess volume
response to ingestion of 1 litre seawater?
likely vomiting- to remove fluid form stomach- as has very high osmotic pressure- if remains in gut would draw water into gut from ECF causing osmotic pressure rise in body fluid and fall in ECF volume- then when gut contents absorbed still high osmotic pressure but high ECF volume that can’t be excreted
demonstration of dominance of osmoregulation over volume regulation?
keep dogs without water overnight- develop high osmotic pressure and low ECF volume so experience osmotic and hypovolaemic thirst. when released in morning measure volume drunk. if osmotic pressure restored to normal by intracarotid water infusion without significantly altering ECF volume drank 30% of control volume. if ECF volume increased to 6% above normal but osmotic pressure kept high still drank 70% of control volume
functions of Ca2+?
structural (bones and teeth), second messenger, stability of excitable cell membranes
most important reason for acute ECF calcium homeostasis?
stability of excitable cell membranes
effect of hypocalcaemia?
reduced threshold for APs in excitable cells, spontaneous activity- motor nerves particularly susceptible- causing spontaneous activity so tetany in skeletal muscle. moderate cases cause Trousseau’s sign and Chvostek’s sign. can cause prolonged QT interval, death from asphyxiation. could be due to PTH insensitivity, deficiency, 1,25-DHCC insufficiency (also cause abnormal bone demineralisation)
effect of hypercalcaemia?
raises threshold for APs in excitable cells so results in sluggish CNS function, muscle weakness, arrhythmias. calcium phosphates may precipitate causing kidney stones. most common cause is hyperparathyroidism (primary or secondary) leading to PTH excess - chronic kidney disease
calcium distribution in body?
1kg in bone as hydroxyapatite- 99% of calcium. 1g of this lines surface of canals filled with bone fluid and is available for exchange with the ECF. ECF contains 1g of Ca2+. total Ca2+ concentration in plasma is 2.5mM. about 10g Ca2+ inside cells
when does positive calcium balance occur (intake exceeds loss)
growing children, pregnancy, bone healing
when does negative calcium balance occur (loss exceeds intake)
old age, prolonged bed rest, prolonged weightlessness during space travel
3 regulatory steps promoting balanced concentration of Ca2+ in ECF?
conc in ECF balanced between intake of calcium ingested in diet and secretion of Ca2+ from ECF to enhance uptake of calcium ions by gut. bone undergoes continuous remodeling- constant breakdown and deposition of calcium in formation. kidneys can increase output of Ca2+ from body in urine
endocrine control of Ca2+ homeostasis?
parathyroid hormone, 1,25-DHCC, calcitonin
parathyroid hormone secretion?
by chief cells of parathyroid glands- regulated by plasma [Ca2+] (if increases this reduces PTH secretion)
role of parathyroid hormone?
raises ECF Ca2+, lowers ECF phosphate
how does increased [Ca2+] reduce PTH secretion?
binds to surface receptor with relatively low Ca2+ affinity - GPCR- results in production of IP3 which releases intracellular Ca2+ and this along with DAG formed activates PKC which inhibits PTH synthesis and secretion
what does PTH act on?
directly on bone and kidney, indirectly on gut through 1,25-DHCC
PTH actions on bone?
stimulates osteocytes to take up Ca2+ from bone fluid and transfer to bone-lining cells which secrete it into interstitial and so ECF. also stimulate osteoblasts to release cytokines to stimulate osteoclasts - slows mineralisation of bone
PTH actions on kidney?
decreases Ca2+ reabsorption in proximal tubule and TAL (secondary effect to reduced Na+ reabsorption). this is outweighed by stimulating Ca2+ reabsorption in DCT and collecting duct. stimulates synthesis of 1,25-DHCC in kidney
effect of 1,25-DHCC on bone?
acts synergistically with PTH to promote bone dissolution- needed for normal bone mineralisation
effect of 1,25-DHCC on kidney?
increases Ca2+ reabsorption and phosphate reabsorption by increasing expression of Ca2+ transporters in DCT and CD and expression of phosphate transporters in PST
effect of 1,25-DHCC on gut?
increases absorption of Ca2+ and phosphate in SI by increasing expression of transporter proteins
what secretes calcitonin?
parafollicular/C cells of thyroid gland
main action of calcitonin?
inhibits osteoclast activity favouring osteoblastic activity and so bone deposition- so prevents hypercalcaemia
what causes CT secretion?
rise in ECF [Ca2+] - Ca2+ binds to low affinity Ca2+ GPCR activating PLC, produces IP3 releasing intracellular calcium to stimulate CT secretion. also gastrin stimulates CT release (feed-forward). also potentially stimulated by the sex steroids
