The proximal tubule and the loop of Henle Flashcards
where does most reabsorption occur?
Proximal tubule
how often is plasma filtered?
65 times per day
what do the kidneys reabsorb?
99% of fluid 99% of salt 100% of glucose 100% of amino acids 50% of urea 0% creatinine
Is reabsorption specific or non specific?
specific - unlike filtration which is non-specific
what is glomerular filtrate?
modified filtrate of blood- contains ions and solutes at plasma concentration but lacks RBCs and large plasma proteins
how does the osmolarity change between the fluid reabsorbed in the proximal tubule and the filtrate?
iso-osmotic - no change in osmolarity
what is reabsorbed in the proximal tubule?
Sugars Amino acids Phosphate Sulphate Lactate
what is secreted in the proximal tubule?
H+ Hippurates Neurotransmitters Bile pigments Uric acid Drugs Toxins
what are the two membranes of the wall of the nephron?
apical membrane - faces tubular fluid and lumen
Basolateral membrane - faces interstitial fluid
what is paracellular reabsorption?
some substances are reabsorbed between adjacent epithelial cells through “leaky” tight junctions
what is essential for Na+ reabsorption?
An energy-dependent Na+-K+ ATPase transport mechanism at the basolateral membrane
what is iso osmotic fluid reabsorption across “leaky” proximal tubule epithelium due to?
(1) Standing Osmotic Gradient
(2) Oncotic Pressure Gradient
what causes oncotic drag of peritubular plasma?
After losing 20% plasma , the plasma protein concentration is higher in peritubular capillaries and so helps drag fluid from lateral space into the blood
what causes paracellular reabsorption of negative chloride ions?
the net movement of positive charge from the movement of Na+ from tubular fluid into blood through the transcellular route sets up an electrical gradient and so negative ions move
why does the osmolarity not change?
water follows reabsorbed salt and so salt and water are absorbed in equal proportions
how much glucose is reabsorbed in the proximal tubule?
100%
what happens to rate of filtration if you increase plasma concentration?
rate of filtration increases
why does reabsorption plateau?
Membrane transporter proteins are fully saturated with glucose
what is the function of the loop of henle?
- Generates a cortico-medullary solute concentration gradient
- This enables the formation of hypertonic urine
what is countercurrent flow?
opposing flow in two limbs
what do the loop and vasa recta establish together?
hyper-osmotic medullary interstitial fluid
what does the ascending limb reabsorb and what is its permeability to water?
- Along the entire length of the ascending limb Na+ & Cl- are being reabsorbed
- relatively impermeable to water
what does the descending limb reabsorb and what is its permeability to water?
-does not reabsorb NaCl -highly permeable to water
what does the selective permeabilities of the ascending and descending limbs enable?
an osmotic gradient to be established in the medulla
what is the triple co-transporter?
transporter that allows the movement of Na+, K+ and Cl- from the filtrate to the interstitial fluid
how is K+ recycled and what does this mean?
- Potassium is moved across both basolateral and luminal membranes so there is not net movement of potassium
- recycling means that NaCl is absorbed into the interstitial fluid
what blocks the triple co-transporter?
loop diuretics
why does water not cross the membrane?
the tight junctions are to small to fit water though and so water cannot follow the salt
what happens when the triple co-transporter pumps solute from the ascending limb?
- Solute removed from lumen of ascending limb (water cannot follow)
- Tubular fluid is diluted and osmolality of interstitial fluid is raised
- Interstitial solute cannot enter the descending limb
- Water leaves the descending limb by osmosis
- Fluid in the descending limb is concentrated
what is the difference ins osmotic state from leaving proximal tube and entering distal tubule?
- its iso osmotic leaving the proximal tubule
- its hypo osmotic entering the distal tubule
in a steady state, what is the osmolarities of the cortex and medulla?
- it remains at 300 in the cortex but progressively increases in osmolarity in the medulla
what other contributes to half the medullary osmolarity?
urea cycle
What is the purpose of countercurrent multiplication?
To concentrate the medullary interstitial fluid
why is concentrating the medullary interstitial fluid essential?
To enable the kidney to produce urine of different volume and concentration according to the amounts of circulating antidiuretic hormone (ADH = vasopressin)
what runs alongside the long loop of Henle of juxtamedullary nephrons?
vasa recta
what does the loop of henle and vasa recta form?
countercurrent system
how is the problem of essential blood flow through the medulla that washes away NaCl and urea minimised?
1) Vasa recta capillaries follow hairpin loops
2) Vasa recta capillaries freely permeable to NaCl and water
3) Blood flow to vasa recta is low (few juxtamedullary nephrons)
what ensures that the solute is not washed away?
Passive exchange across the endothelium preserves medullary gradient - blood equilibrates at each layer.
