Physiology Flashcards
general osmolality of cell fluid?
300
tonicity is dependant on what 2 factors?
osmalarity
ability of a solute to cross the cell membrane
- i.e 2 solutions of the same osmolarity have different ability to cross the membrane due to different structure (i.e urea vs sucrose)
300mM urea is hypotonic to RBCs while 300mM sucrose is isotonic to RBCs, why is this?
cell membrane of red blood cells are very permeable to urea due to urea transporters
as urea moves, it leaves water behind (outside the cell), this creates an osmotic gradient and water moves into the cell
tonicity vs osmolarity?
tonicity = the effect osmolarity has on the cell volume
osmolarity <300?
hypotonic
osmolarit >300?
hypertonic
osmolarity of 300?
isotonic
water makes up what percentage of total body weight in males and females?
males = 60% females = 50% (due to higher amount of fat which contains little water)
total body water consists of what 2 fluid compartments?
intracellular fluid (67% of total body water) extracellular fluid (33% of total body water)
extracellular fluid contains what?
plasma (20%)
interstitial fluid (80%)
lymph and transcellular fluid (negligible)
how is the volume of a fluid compartment measured?
using tracers and obtaining the distribution volume of these tracers
give examples of tracers and how theyre used
ECF = inulin
plasma = labelled albumin
total body water = 3H2O
TBW = ECF + ICF
how do tracers work?
add known quantity of tracer to the body
take a sample of body fluid and measure the concentration of tracer in the sample
distribution volume = quantity of tracer/concentration of tracer in sample
what contributes to water input?
fluid intake
food intake
meatbolism
what contributes to water output?
insensible - skin - lungs sensible loss - sweat - faeces - urine
sensible vs insensible water loss?
sensible = regulatory mechanisms in place in the body to control water loss
greatest water loss from the body?
urine (1.5L per day)
the kidneys compensate for increased water loss by reducing urine output but they cant turn of urine production completely, why is this?
some waste products can only be secreted in solution
effect of kidneys on osmolarity?
kidneys have an effect on ion concentrations
ion compensation of extracellular fluid?
like sea water
- high sodium
- high chloride
- low potassium and bicarbonate
ion compensation of intracellular fluid?
low sodium and chloride
higher potassium and bicarbonate
what separates the plasma and the intracellular fluid (of skeletal muscle) and how does ion composition compare?
interstitial fluid between capillary wall and plasma membrane of skeletal muscle cell
similar ion composition to plasma
osmotic concentrations of ECF and ICF?
the same
300mosmol/L
because changes in solute concentrations lead to immediate changes in water distribution, the regulation of fluid balance and electrolyte balance are tightly intertwined
what is fluid shift?
movement of water between ICF and ECF in response to an osmotic gradient (water moves without salt)
what would happen if the osmotic concentration of ECF increases vs decreases?
increase = ECF becomes hypertonic, volume of ICF decreases and cell shrivels decreased = ECF becomes hypotonic, ICF volume increases, cell swells
how does gain or loss of salt affect fluid homeostasis?
change in fluid osmolarity
Na+ excluded from ICF
osmotic water movements
these 2 factors combine to produce opposite changes in ICF and ECF volume
increase in ECF NaCl = increased volume of ECF, decreased volume of ICF
opposite is true if NaCl is lost from the ECF
how does gain/loss of isotonic fluid affect fluid homeostasis?
no change in fluid osmolarity
change in ECF volume only
the kidneys can only affect the composition and volume of the ECF or the ICF?
ECF
electrolyte balance is important for what 2 reasons?
total electrolyte concentration has a knock on effect on water balance
concentrations of individual electrolytes can affect cell pumps (Na = major ECF component, K = major ICF component, both affect functioning of cells)
importance of sodiumbalance?
> 90% of osmotic concentration of the ECF results from presence of sodium
total sodium in ECF represents balance between input/output)
major determinant of ECF volume (water follows sodium)
importance of K balance?
minor fluctuations in plasma [K] can have a detrimental consequences
K+ plays a role in establishing membrane potential
95% of [K+] is intracellular so small leakages can affect plasma K+ leading to
- muscle weakness (paralysis)
- cardiac irregularities (cardiac arrest)
what contributes to salt input/output and how is this manifested?
intake = fluids and food
output = sweat and faeces, urine
manifested as changes in ECF volume
osmolarity is based on what 2 factors?
molar concentration of the solution
the number of osmotically active particles present
osmolality vs osmolarity?
osmolality = units osmol/kg water
osmolarity = units osmol/L
these are interchangeable for weak salt solutions
10 functions of kidneys?
Water balance
Salt balance
Maintenance of plasma volume
Maintenance of plasma osmolarity
Acid-base balance
Excretion of metabolic waste products (e.g.)
Excretion of exogenous foreign compounds (e.g.)
Secretion of renin (control of arterial blood pressure)
Secretion of erythropoietin (EPO; RBC production)
Conversion of vitamin D into active form (Calcitriol: Ca2+ absorption in GI tract)
primary function of the kidney?
regulate volume, composition and osmolarity of the body fluids
kidneys role = controlled excretion
how much of the cardiac output do the kidneys receive?
25%
3 functional mechanisms of the nephron?
filtration
reabsorption
secretion
what lines tubular component of nephron?
single layer thick epithelium
blood supply to nephron?
renal artery > afferent arteriole > glomerular capillary > efferent arteriole > network of capillaries which surround nephron tubules (known as the peritubular capillaries) > drains back into the venule > renal vein
functions of peritubular capillaries?
supply tubular cells
receive any substances which are being reabsorbed
secrete substances from blood into the tubules to be excreted
path of intertubular fluid?
bowmans capsule via glomerular filtration
then enters proximal tubule
drains into loop of henle (descending then ascending limb)
then drains into distal tubule
empties into collecting duct
drains into renal pelvis
single collecting duct per nephron, true or false?
false
single collecting duct for several nephrons
what is the juguloglomerular apparatus?
gap between afferent and efferent arterioles where part of distal tubule passes through
tubular fluid vs urine?
tubular fluid travels through tubules and collecting duct
becomes urine once passes into the renal pelvis where no more modification will take place
2 types of nephron? what are the differences between them?
juxtamedullary - longer loop of henle - don't have network of capillaries, instead have a single capillary known as vasa recta - produce more concentrated urine cortical - shorter loop of henle - have peritubular capillaries
what nephron type is more common?
cortical (80%)
juxtamedullary only make up 20%
cortex vs medullary appearance?
cortex = granular medulla = striated (nephrons and collecting ducts)
outer vs inner layer of bowmans capsule?
outer = single layer of epithelium inner = podocytes
what lines the capillaries of the glomerulus?
single layer of endothelium
thick basement membrane
components of the glomerular membrane?
endothelium
basement membrane
podocytes
- acts as a filter
production and secretion of renin?
granular cells in juxtaglomerular apparatus
macula densa cells?
group of salt sensitive cells which regulate amount of salt in tubular fluid passing through the juxtaglomerular apparatus
what is urine?
modified filtrate of blood
the kidneys incorporate what 3 mechanisms in the production of urine?
filtration system
rich blood supply
mechanisms for urine modification (reabsorption and secretion)
path of tubular fluid through tubules?
20% of plasma in glomerular fluid is filtered out to form the initial tubular fluid
then passes through segments of nephron
tubular secretion and absorption from peritubular capillary takes place as it passes through tubules
then excreted as urine
rate of excretion is equal to what?
rate of filtration + rate of secretion - rate of reabsorption
glomerular filtration + secretion = reabsorption + excretion
how is rate of filtration of substance X calculated?
rate of filtration = mass of X filtered into bowmans capsule per unit time
rate of filtration of X = [X] plasma X GFR (constant)
how is rate of excretion of X calculated?
rate of excretion = mass of X excreted per unit time
rate of excretion of X = [X]urine x Vu
(Vu) = urine production rate
how is rate of reabsorption of X calculated?
if rate of filtration > rate of excretion, net reabsorption of that substance has occurred
rate of reabsorption of X = rate of filtration of X - rate of excretion of X
how is rate of secretion of a substance calculated?
if rate of filtration < rate of excretion, net secretion of the substance has occurred
rate of secretion of X = rate of excretion of X - rate of filtration of X
what are the 3 barriers to glomerular filtration (glomerular membrane)?
glomerular capillary epithelium (barrier to RBCs) basement membrane (basal lamina) (plasma protein barrier) slit processes of podocytes (plasma protein barrier) (glomerular epithelium)
net filtration pressure comprises what 4 forces?
pushing out
- glomerular capillary blood pressure (55mm Hg)
- bowmans capsule oncotic pressure (0mm Hg)
pushing in
- bowmans capsule hydrostatic pressure (15mg)
- capillary oncotic pressure (30mm Hg)
net filtration pressure = (55+0) - (15+30) = 10m Hg
what is glomerular filtration rate?
rate at which protein-free plasma is filtered from the glomeruli into the bowman’s capsule per unit time
GFR = Kf X net filtration pressure
- Kf = filtration coefficient (how porous glomerular membrane is)
what is normal GFR?
125ml/min (across both kidneys)
what is the major determinant of GFR?
glomerular capillary fluid (blood) pressure
how do glomerular capillaries differ to normal capillaries?
they have bigger gaps/pores
results in leaky endothelial layer
pressure remains constant along the length of the capillary between afferent/efferent capillaries (due to reduction in diameter from afferent to efferent)
describe basement membrane
made of collagen and glycoproteins
acellular
the basement membrane has a net negative charge, what is the impact of this?
repells negatively charged plasma proteins
prevents excretion of important plasma proteins so they are retained
what prevents excretion of RBC through the capillary endothelium?
they are too big to fir through the pores
is fluid filtration active or passive?
passive
what drives process of glomerular filtration?
net filtration pressure (result of hydrostatic and oncotic pressures)
largest force involved in net filtration pressure?
glomerular blood capillary pressure (hydrostatic) = 55mm Hg
effect of plasma proteins on net filtration pressure?
plasma proteins only found in capillaries (not in bowmans capsule)
plasma proteins try to drag fluid into capillary to dilute concentration
creates capillary oncotic pressure
- no bowmans capsule oncotic pressure as no plasma proteins present in bowmans capsule
balance of hydrostatic and oncotic pressures is known as what?
starling forces
how does GFR affect urine production?
high GFR = kidneys filtering lots = more urine produced
low GFR = kidneys filtering less = less urine produced
extrinsic vs intrinsic regulation of GFR?
extrinsic = sympathetic control via baroreceptor reflex intrinsic = autoregulation = myogenic mechanism, tubuloglomerular feedback mechanism
how does renal blood flow affect GFR?
increased arterial BP increases blood flow to glomerulus = increased glomerular capillary blood pressure = increased net filtration pressure = increased GFR
opposite is also true
describe extrinsic (sympathetic) control of GFR
fall in blood volume (e.g haemorrhage) > reduced MABP > detected by aortic and carotid sinus baroreceptors > increased sympathetic activity > generalised arteriolar vasoconstriction > constriction of afferent arterioles > reduced capillary BP > reduced GFR > reduced urine volume > help compensate for reduced blood volume
progressive increase in MABP causes a sharp increase in GFR, true or false?
false
increase only causes slight increase in GFR due to autoregulation which prevents short term changes in systemic arterial pressure affecting GFR
means GFR remains fairly constant over a wide range of high/low MABPs
myogenic mechanism of autoregulation (intrinsic control of GFR)?
if vascular smooth muscle is stretched (i.e arterial pressure is increased), it contracts thus constricting the arteriole
tubuloglomerular feedback mechanism of autoregulation (intrinsic control of GFR)?
involved juxtaglomerular apparatus
if GFR rises, more NaCl flows through tubule which is detected by macula densa cells
these cells release vasoactive chemicals causing smooth muscle to contract leading to constriction of afferent arterioles
give 4 examples of pathology which can affect GFR
kidney stone - increases bowmans capsule hydrostatic pressure = reduced GFR
diarrhoea - increased capillary oncotic pressure (dehydration causes relative increase in plasma protein concentration) = reduced GFR
severe burns - reduced capillary oncotic pressure (loss of plasma proteins via site of burns) = increased GFR
change in surface area available for filtration - reduced Kf = reduced GFR
what is plasma clearance?
measure of how effectively the kidneys can clean the blood of a substance
equals volume of plasma completely cleared of a particular substance per minute (each substance has its own specific plasma clearance value)
how is plasma clearance calculated?
always in ml/min
rate of excretion of X/plasma concentration of X
- rate of excretion of X = [X]urine x volume of urine
features of inulin clearance, how can this be used?
freely filtered at glomerulus neither absorbed or secreted not metabolised by kidney not toxic easily measured in urine and blood therefore inulin clearance = GFR - creatinine also used in same way
how is inulin clearance (GFR) calculated?
[inulin]urine x Volume of urine / [inulin]plasma
can glucose be used to estimate GFR?
glucose
- filtered and completely reabsorbed (clearance = 0)
urea
- partly reabsorbed, only 50% of urea is cleared (clearance GFR)
clearance of what 2 substances can be taken as GFR?
inulin
creatinine
what does clearance value tell you about the substance?
clearance < GFR = substance is reabsorbed
clearance = GFR = substance is neither reabsorbed nor secreted
clearance > GFR = substance is secreted into tubule
how can renal plasma flow be calculated?
para-hippuric acid (PAH)
PAH is freely filtered at glomerulus, secreted into tubule, not reabsorbed and completely cleared from the plasma
therefore all the PAH in the plasma that escapes filtration is secreted from the peritubular capillaries
any substance used as a clearance marker must meet what criteria?
non-toxic
inert (not metabolised)
easy to measure
GFR marker vs RPF marker?
GFR = filtered freely, not secreted or reabsorbed RPF = filtered and completely secreted
what is filtration fraction and how is it calculated?
fraction of plasma flowing through the glomeruli that is filtered into the tubules
filtration fraction = GFR/RPF
