Renal Physiology - Tubular Function Flashcards
What are the two types of nephrons?
cortical nephrons and juxtamedullary nephrons
Cortical nephrons?
shorter nephron loop (loop of Henle)
Glomerulus further from corticomedullary junction
Efferent arteriole supplies peritubular capillaries
Juxtamedullary nephrons?
long nephron loop deep into medulla
glomerulus close to corticomedullary junction
efferent arteriole supplies vasa recta
what is the ratio between the two types of nephrons?
in humans it is 85:15 (cortical:juxtamedullary)
changes in species depending on water conservation required
what gradient is the loop of henle responsible for?
responsible for a hyperosmotic gradient within the medulla
desert animals - type of nephron?
- desert animals have far more juxtamedullary nephrons relative to cortical nephrons and a greater medullary area relative to cortex
longer loop of henle results in? and why?
results in greater water retention due to the correlation of the longer the loop the higher the gradient
what does a hyperosmotic gradient result in?
results in water retention i.e. the water is not excreted into the collecting duct
what is the function of the tubular system?
it is to transform a large volume of glomerular filtrate to a small but variable volume of urine with a composition that stabilises the composition of the extracellular fluid
tubular reabsorption?
water/substances from tubular lumen to peritubular capillary
tubular secretion?
from peritubular capillaries to lumen
what is the tubular system composed of?
proximal convoluted tubule
loop of Henle
Distal convoluted tubule
collecting duct
what happens to the substances in the tubular system?
they can be reabsorbed from the tubular system back into the blood or can be secreted into the tubular fluid
where is the majority of the water/solutes reabsorbed from the tubular system?
into the proximal convoluted tubule
what does the loop of henle reabsorb more of?
reabsorbs more Na+ than water
loop of henle variability?
variable length in different species - longer loops in animals where water conservation is more important
what does the collecting duct reabsorb more of?
more water than solutes
reabsorption into collecting duct regulated by?
regulated by antidiuretic hormone
(ADH/vasopressin) for water and aldosterone for Na+
99% of glomerular filtrate is reabsorbed by?
by the renal tubular system
how are cells of the tubular system adapted to their function? - PCT cells
they have microvilli - increase SA + mitochondria to provide energy for transporter proteins
what three ways do PCT cells reabsorb substances in?
primary active transport e.g. Na+/K+ ATPase pump
Secondary active transport following electrochemical gradient e.g. Na+ into cell
Passive diffusion e.g. solutes following absorption of water
what do peritubular capillaries transport?
they transport reabsorbed substances
primary active transport for the reabsorption into the PCT - what happens?
ATP required
Na+/K+ ATPase (basal side only) - pumps sodium out of cell (into blood) and exchanges w/ potassium
Pumps Na+ out of the cell exchanging w/ K+
Secondary active transport for reabsorption into PCT- describe how it happens?
high intracellular [K+] means K+ diffuses out
Diffusion of K+ results in negative membrane potential
Na+ diffuses into the cell from apical side due to negative membrane potential
carrier proteins facilitate amino acids and glucose transport
passive diffusion for reabsorption into PCT- describe how it happens?
usually requires specific channels to facilitate
water reabsorbed via osmosis following ions and solutes
other substances e.g. lipid soluble compounds reabsorbed readily across cell membranes
50-60% filtered urea also reabsorbed - one reason why the kidneys must filter so much
Describe what is happening with the counter-current multiplier involving the ascending thick limb of the loop of Henle:
impermeable to water (lacks aquaporins)
basolateral surface of epithelial cells has Na+/K+ ATPase
Na+ pumped into interstitium
K+ follows from tubule, through the cell, to the interstitium down conc gradient
Cl- also follows down electrochemical gradient
what environment does the counter current multiplier create?
creates a hypertonic environment in the interstitium of the renal medulla - necessary or concentrating urine
Descending thin limb of loop of henle - permeability?
impermeable to NaCl (lacks necessary ATPase pumps) but water can diffuse following the concentration gradient
What is the vasa recta?
the vascular component of the nephron, the vasa recta runs counter-current to the tubular loop of Henle
capillaries of the vasa recta permeability?
these are permeable to both water and NaCl
What is the descending vasa recta close to?
it is close to ascending loop and loses water increasing its osmolarity toward the hairpin
What is the ascending vasa recta close to?
close to descending loop and absorbs water decreasing its osmolarity and increasing flow
what happens to the urea in the vasa recta?
it is excreted from the capillaries into interstitium and then into tubular system
blood plasma is isosmotic w/?
with medullary interstitial fluid (tissue fluid) when entering the medulla
as vessels travel deeper into the medulla, what happens to the medullary fluid?
the hypertonicity of these medullary fluid (due to NaCl reabsorption from thick loop of Henle) water is drawn from vessels
osmolarity of vasa recta?
the osmolarity (moles of solutes per kg) increases toward the hairpin (matching that of the intersitium)
as the vessels of the vasa recta ascend - what do they do?
they draw much more water in by osmosis
overall effect for the vasa recta?
overall effect is a net increase in flow through the vasa recta (absorbed from the tubular system), while maintaining the hyperosmotic gradient
glucose - tubular handling of important substances:
same for amino acids, ketones, lactic acid, water soluble vitamins (pathological)
As easily filtered as water
completely reabsorbed in the PCT
Secondary active trasnport - co-transported with Na+
Only excreted in the urine when plasma load exceeds resorptive capacity called Renal Threshold (usually several times higher than normal filtered load)
protein - tubular handling of important substances:
glomerular filtrate an effective barrier for most proteins
very small peptides and small amounts of albumin can’t be filtered
reabsorbed by endocytosis
ions (phosphate and sulphate) - tubular handling of important substances:
filtration close to renal threshold
normally only small amounts excreted - increases with increased dietary intake
reabsorbed by secondary active transport
ions: sodium - tubular handling of important substances:
primarily regulated by kidney
driving force for secondary active transport
reabsorption of Na+ coupled to K+ and H+ secretion
what is ADH synthesised by?
by hypothalamic neurons in the supraoptic and paraventricular nuclei and released from the posterior pituitary
when is ADH released?
released in response to an increase in osmolarity of extracellular fluid
what does ADH do for the kidneys?
it increases water reabsorption in the DCT and collecting duct (greater effect in collecting duct)
ADH binds to G-protein coupled receptors
Activate adenylate cyclase which converts ATP to cAMP
AQP2 within vesicles is inserted into apical membrane via secondary messengers
increased water reabsorption helps restore extracellular fluid osmolarity, blood volume and arterial pressure
where is aldosterone produced and secreted from?
from the zona glomerulosa of the adrenal gland
why is aldosterone produced?
produced in response to activation of the renin-angiotensin-aldosterone system
what is aldosterone stimulated by?
by systemic hypotension or high K+ intake
where does aldosterone cross?
crosses basolateral membrane and binds to mineralocorticoid nuclear receptor
what does aldosterone activate?
activates transcription and synthesis of:
New Na+ channels
New K+ channels
New Na+/K+ pumps
what does aldosterone act to do?
acts to increase reabsorption of Na+ and secretion of K+
what nephrons do birds have?
mixture of nephrons -
mammalian-type cortical and juxtamedullary nephrons
reptile-like cortical nephrons that lack a loop of henle
as a waste product, instead of urea what do birds produce?
uric acid - no urea
what happens due to the absence of urea and fewer loops of henle for birds?
they have a much lower medullary osmolarity
what is most of the Na+ and Cl- excretion in birds due to?
it is via the salt glands which are paired glands emptying into the nasal cavity instead of the kidneys
what are avian kidneys (and reptillian) supplied by - blood vessel?
portal vein in addition to the arterial supply
where is uric acid secreted into?
into the tubular system and precipitates out so urine can hold more solutes and conserve water
