3 week 17 Flashcards
whats the diff bw reabsorption and secretion?
- reabsorption: movement of water and solutes from tubular lumen into peritubular capillaries (returned to blood).
- secretion: movement of solute from peritubular capillaries into tubular lumen.
what is the diff bw apical membrane vs basolateral membrane?
- apical: plasma membrane of the epithelial cells facing the tubule lumen, has microvilli.
- basolateral: plasma membrane facing the interstitial fluid.
which parts of the nephron…
a) reabsorb the MOST substances
b) reabsorb ONLY water
c) reabsorb ONLY ions
d) secrete ions
a) proximal tube
b) loop of henle (descending limb)
c) loop of henle (ascending limb)
d) proximal tube, distal tube, collecting duct
describe the role of the proximal tubule in reabsorption (4)
- nonregulated reabsorption
- mass reabsorber (ex 70% sodium/water, 100% glucose)
- brush border = large surface area
- leaky tight junctions = paracellular transport
why is lots of mitochondria found in the proximal tubule?
bc energy is needed for primary active transport
whats the diff bw…
a) active solute reabsorption
b) water reabsorption (passive)
c) passive solute reabsorption via diffusion
a) uses pumps. pumped into peritubular fluid then enters endothelial cells via pores.
b) via aquaporins
c) solutes move from higher to lower concentration. can be through ion channel/transporters (facilitated), EC gradient…
how is glucose reabsorbed?
- actively transported across apical membrane via sodium from tubular fluid to epithelial cell.
- carrier protein for glucose is located on basolateral membrane; high concentration inside the epithelial cell = down concentration gradient into the peritubular fluid via carrier, where it can diffuse into the plasma (facilitated diffusion).
what is transport maximum? renal threshold?
- transport maximum: rate of transport when carriers are saturated.
- renal threshold: the plasma concentration at which the amount of ___ in the filtrate exceeds the transport maximum, and at which ___ appears in the urine.
what is the descending limb of the loop of henle permeable to vs the ascending limb?
- descending: permeable to water, no Na+, K+ or Cl- transport
- ascending: permeable to Na+, Cl-, K+, impermeable to water
describe the role of the distal tubule and collecting ducts in reabsorption
- regulated reabsorption and secretion (see slides 13-14)
- transport is regulated across epithelium
- tight junctions limit paracellular transport
what are the three factors that determine the rate of excretion of a substance?
- filtered load
- secretion rate
- reabsorption rate
- in other words: amount excreted = amount filtered + amount secreted – amount reabsorbed
what is clearance?
- volume of plasma that contains the amount of a substance that has been excreted per unit time
- can be used to estimate GFR
- see slide 18 for how to calculate
describe creatinine
- by-product of muscle metabolism
- freely filtered
- not reabsorbed
- small amount secreted
how is renal plasma estimated?
- via the clearance of para-aminohippuric acid (PAH) (freely filtered, fully secreted, and not reabsorbed)
- amount excreted = amount contained in volume of plasma that entered the kidneys
- normal value = 625 mL/min
how is renal blood flow estimated?
clearance PAH / 1 - hematocrit
how do you determine the fate of solutes in renal tubules?
If Cx > GFR (125mL/min), then substance was secreted
If Cx < GFR (125mL/min), then substance was reabsorbed
how is micturition regulated?
- +volume of bladder = wall expands and stretch receptors activate. at spinal cord…
- -sympa activity = internal urethral sphincter relaxes, so it opens
- +parasympa activity = detrusor muscle contracts, also opens internal urethral sphincter
- -somatic motor activity = external urethral sphincter relaxes, so it opens
- micturition occurs (urination)
reabsorption of solutes increases the ____ of peritubular fluid and plasma.
osmolarity
how is water reabsorbed via osmosis (3)
- through aquaporins
- by paracellular transport (proximal tubule)
- across the lipid portion of plasma membranes
where does water reabsorption take place?
- 70% at proximal tubule
- 15% loop of henle (descending limb)
- 8% distal convoluted tubule and collecting duct
what is needed for the regulated reabsorption of water? (2)
- medullary osmotic gradient
- ADH
T or F: the medullary osmotic gradient decreases as we go down
- FALSE
- increases*
describe how the medullary osmotic gradient is established (7)
- all fluids are at 300 mOsm. active transport of Na+, Cl-, K+ from ascending limb > medullary interstitial fluid = +osmolarity of interstitial fluid and -osmolarity of tubular fluid in ascending limb.
- +osmolarity of medullary interstitial fluid draws water from lumen of the descending limb > interstitial fluid.
- +osmolarity of tubular fluid in descending limb.
- +tubular fluid enters loop of Henle, pushing fluid farther into renal tubules. process of solute transport from ascending limb…
- …followed by water movement from descending limb,
- +osmolarity of tubular fluid in descending limb, followed by +tubular fluid entering loop of Henle, repeats until…
- medullary osmotic gradient is established.
how does urea contribute to the medullary osmotic gradient?
- transport of urea from collecting duct > medullary space is what helps establish gradient.
- contributes approximately 40% of the osmolarity of the gradient.
- moves from high urea to low urea.
how does the vasa recta maintain the medullary osmotic gradient?
- anatomical arrangement of vasa recta capillaries prevents the diffusion of water and solutes from dissipating the medullary osmotic gradient.
- blood picks up salt as it moves down.
- as it comes back up, it picks up water.
- this counter-mechanism means that gradient is maintained.
which nucleus contains cells that secrete ADH?
paraventricular nucleus
how does ADH regulate water reabsorption?
- increases permeability of collecting duct by putting aquaporins on surface of apical membrane.
- makes urine have high osmolarity (1400 mOsm) and low volume.
how does ADH increase water permeability?
- binds to receptor + activates g protein
- activates adenylate cyclase > ATP to cAMP > PKA > phosphorylation of targets, triggers aquaporins to membrane
what stimulates ADH release? (2)
1) increase in plasma osmolarity (osmoreceptors detect, +neurosecretory cell activity, conservation of body water)
2) decrease in blood pressure (baroreceptors detect, +neurosecretory cell activity, conservation of blood volume)
define hypernatremia and hyponatremia
- hypernatremia: high plasma sodium
- hyponatremia: low plasma sodium
where does renal sodium reabsorption take place?
- freely filtered at the glomerulus and actively reabsorbed from the renal tubule
- 60-70% proximal tubule
- 25-30% descending limb (via NaKCl transporter)
- 10% distal tubules and collecting ducts (regulated)
how does sodium reabsorption work at the distal vs proximal tubule?
- distal: sodium and chloride channels + ATPase still moves sodium out of cell and into blood
- proximal: pumps 3 sodiums out for every 2 potassiums in, chloride follows sodium
what hormones regulate the reabsorption of sodium?
- aldosterone (increases sodium reabsorption and potassium secretion)
- atrial natriuretic peptide (increases sodium excretion)
where is aldosterone released from? what is its mechanism of action?
- adrenal glands on kidneys (lipid soluble)
- moves into nucleus, triggers gene transcription
- genes make sodium channels, potassium channels, NaK ATPase
- more sodium and potassium channels on apical side
- more NaK ATPase on basolateral side
- result: reabsorption of sodium into blood stream and potassium exits into urine
what stimulates the release of aldosterone?
- renin-angiotensin-aldosterone system
- high plasma K+ concentrations
how does the renin-angiotensin-aldosterone system work?
- angiotensin II acts as hormone to trigger release of aldosterone
- angiotensinogen circulates in blood, made by liver
- renin cleaves it to produce angiotensin I which is converted into II by ACE
- key thing = renin
what are stimuli for renin release? (3)
- decreased pressure in afferent arteriole
- renal sympathetic nerve activity
- decreases in Na+ and Cl– in distal tubule filtrate (makes macula densa secrete paracrines)
what is the function of atrial natriuretic peptide?
- increases GFR
- decreases Na+ reabsorption by closing Na+ channels in apical membrane
- overall effect: increased Na+ excretion
what stimulates the release of atrial natriuretic peptide?
secreted by atrial cells of the heart in response to distension of atrial wall caused by +plasma volume
