Kidney Tubular Function Flashcards
How much filtrate is reabsorbed by the kidneys?
99%
What part of the nephron is responsible for re-absorption of filtrate?
Renal tubule
Describe the role of reabsorbtion by the renal tubule.
Reabsorbs and conserves molecules that the body wants to keep (that are important to us). Allows the conc. of molecules to be kept within a narrow range Allows fine-tuning of urine composition
What is reabsorbed by the kidney?
-Water and nutrients (glucose, AA’s) -Waste products (urea, creatinine) -Molecules needed for cellular processes (Na+, Cl-, Ca2+, K+, bicarbonate)
What waste product do we reabsorb more of?
Urea (creatinine is freely filtered but very little reabsorbed)
What makes up the renal tubule?
-Proximal convoluted tube -Loop of Henle -Distal convoluted tube -Collecting duct
What is the function of the proximal convoluted tube?
Major site of re-absorption and some secretion
What is the function of the loop of henle?
Counter-current multiplier to primarily reabsorb sodium and water
What is the function of the distal convoluting tube?
Re-absorption and secretion (fine-tune urine)
What is the function of the collecting duct?
Collect urine
What is the meaning of reabsorption in kidney tubules?
the movement of molecules from the renal filtrate in the tubule lumen through the tubule wall and into the blood
What is the meaning of secretion in tubular kidney function?
The active movement of molecules from the blood and into the tubule lumen to form part of the urine
Where in the kidneys is Na+ reabsorbed?
-65% in proximal convoluted tubule -25% in ascending loop of Henle -8% in distal convoluted tubule
What percent of Na+ is reabsorbed by the kidneys?
-98%
How much of the energy used by the kidney is used to reabsorb sodium?
80% of total energy used by kidney
What is the major site of absorption?
The proximal convoluted tubule
How much filtrate is reabsorbed in the proximal convoluted tubule?
up to 2/3rds
Describe the filtrate that enters the proximal convoluted tubule.
It is an early form of urine containing both waste products and nutrients that our body wants to retain
How is the structure of the proximal convoluted tubule designed to enhance its function?
-It is convoluted (curved) to maximise surface area in the small space it has -The epithelial cells of the tubule have microvilli to maximise SA -The proximal convoluted tubule is closely associated with peritubular capillaries allowing reabsorption
Why is blood in the peritubular capillaries low in molecules and nutrients?
Because they have been passed into the tubular lumen
Describe the conc. gradient in the PCT.
There is a high conc. of molecules and nutrients in the tubular lumen and a low conc. in the blood creating a concentration gradient
By what process are molecules and nutrients reabsorbed into the blood?
Passive diffusion (is facilitated)
Describe the movement of molecules from the tubular lumen to the peritubular capillaries.
Move from a high con. in the tubular lumen through the epithelial cell wall, through the interstitial space and into the peritubular capillary
What is required to facilitate the passive reabsorption in the PCT?
Pores or carrier proteins to allow movement through the phospholipid bilayer
How is H2O transported from the tubular lumen?
Through aquaporins (pores that allow the free movement of water molecules down a conc. gradient)
How are glucose and amino acids transported through the tubular lumen membrane?
Via Na+ co-transporters (get a Na+ plus glucose transporter and a Na+ plus amino acid transporter)
What is required to maintain the movement of amino acids and glucose?
A sodium concentration gradient
What percent of the renal filtrate can be reabsorbed by the PCT without any energy expenditure?
up to 50%
What other process (not passive diffusion) occurs to maximise the amount of filtrate reabsorbed by the PCT?
Active transportation of Na+ via sodium potassium pump in the basolateral membrane
Together passive diffusion and active sodium transport can reabsorb up to what percent of renal filtrate?
65% (2/3rds)
What is the difference between the basolateral membrane and the apical membrane?
Apical membrane = membrane between the tubular lumen and the tubular cell wall
Bilateral membrane = membrane between the tubular cell wall and the interstitial space
What facilitates the active transport of sodium?
Na+/K+ pump on basolateral membrane
Describe the movement of molecules via the Na+/K+ pump.
3 Na+ moved against conc gradient into interstitial space 2 K+ moved against conc gradient from interstitial space into tubule wall
What does the Na+/K+ pump need?
1 ATP per 3Na+/2K+
How does the movement of Na+ molecules by active transport facilitate the movement of other molecules?
-Water follows Na+ due to principles of osmosis -Glucose and AA co-transported with Na+ -Chloride and other -ve ions follow Na+
How does co-transportation of glucose and AA lead to their reabsorbtion?
-Glucose and AA moved against conc. gradient when co-transported with sodium from tubule lumen to tubule wall -Causes high conc. of nutrients in the tubule wall so they will move down conc. gradient through interstitial space and into blood
What type of transport is the co-transportation of glucose and AA with Na+?
Secondary active transport (moved against conc. gradient but doesn’t require energy)
What percent of nutrients can be reabsorbed by secondary active transport (co-transportation) ?
Up to 100%
Describe the conc. gradients of Na+ in the process of active transport for reabsorbtion in the proximal convoluted tubule.
-high conc. gradient in tubule lumen -low conc. in tubule wall -high in interstitial space -low in pertitubular capillary
Describe the osmotic/electrical gradient in the PCT during active transport of Na+.
High in tubule lumen and low in peritubular capillary
How many -ve ions are reabsorbed in the PCT?
up to 2/3rds
What happens to the filtrate that remains after re-absorption in the the PCT?
It moves into the loop of Henle which employs counter-current multiplication
What are the 2 types of nephrons you get? Describe them.
Corticle nephrons - most numerous, short loops of Henle that only extend to the medulla
juxtamedullary nephrons - long loops of Henle that extend deep into the medulla (specialised for urine concentration)
What can the walls of the loop of henle be divided into? What is this based on?
-Thin descending limb -Thin ascending limb -Thick ascending limb
Based on different structural and permeability properties
What can the subdivisions of the Loop of Henle walls be termed?
Loop rules
Describe the permeability properties of the thin descending and thin ascending limb.
They are permeable to water but no Na+ re-absorption occurs here
Describe the permeability of the thick ascending limb of the loop of Henle.
It is not permeable to water due to the absence of aquaporin channels It is permeable to Na+ (it is the site of ACTIVE Na+ re-absorption)
What does the thick ascending Loop of Henle pump into the interstitial space and what does this result in?
-Actively pumps Na+, Cl- and K+ into the interstitium -The interstitium becomes “salty” ( no water able to leave the thick ascending limb to dilute) which creates an osmotic gradient -This facilitates water reabsorption from the thin limbs
What percent of total sodium re-absorption occurs in the loop of Henle?
25% - specifically in the ascending loop of H
Describe the process of Na+ re-absorption in the thick ascending limb of the Loop of Henle.
-Have Na+, Cl- (x2) and K+ pump instead of the glucose and AA co-transporters -This pump works as secondary active transport -Still have the sodium/potassium pump on the basolateral membrane which moves Na+ against its conc gradient. This causes accumulation of Na+ in the interstitial space -leads to passive transport of na+ from interstitial space into the blood
What are the Na+, Cl- (x2) and K+ pumps a target of?
Diuretic drugs which inhibit the action of the pump - prevents the reabsorption of sodium and hence water (helps control blood pressure)
The difference in osmotic gradient between the thin descending limb and the salty interstitial space facilitates the movement of water. What kind of transport is this?
Passive (osmosis)
What is the difference in osmotic pressure of the filtrate when it moves form the PCT in the cortex to the LofH in the medulla?
in cortex it is isotonic with the intersitium (same osmotic pressure) In medulla it is hypotonic with the interstitium (lower osmotic pressure [so will move into interstitium])
Why is the process in the Loop of Henle called the Counter-current Multiplication?
Counter-current = Because the filtrate moves in opposite directions through loop of henle (down descending limb and up ascending limb) Multiplication = Because the deeper you get into the medulla, the osmotic gradient is multiplied
What is the ratio of water to sodium at the top of loop of H vs the bottom?
Top = high water to Na+ ratio Bottom = low water to Na+ ratio
Why is there a higher salt concentration as you get further down the medulla?
To maintain water reabsorption
What nephrons participate in counter current multiplication?
Only juxtamedullary nephrons
What part of the peritubular capillaries reabsorbs molecules form the interstitial space around the loop of H?
Vasa recta
Why is the vasa recta a special portion of the peritubular capillaries?
It employs a counter-current exchange system (arterial blood descends into medulla and venous blood ascends out) which maintains the osmotic gradient
Describe the flow of blood in the vasa recta?
Slow and sluggish
What kind of transport is the exchange of water and salt with the vasa recta?
the exchange of water and salt with the vasa recta?
Passive - free movement
What term describes the concentration of the filtrate entering the distal convoluted tubule relative to the interstitial space here?
Hypotonic (water wants to move into the interstitial space)
How much sodium can be reabsorbed in the PCT?
Up to 8%
What is the difference between the reabsorption of water and salts in the PCT and the pervious sections of the kidney?
Reabsorption can be regulated here dependent on whether an individual is more hydrated or dehydrated (this keeps the concs of stuff within the narrow limits required)
How is the reabsorption controlled in the distal convoluted tubule?
Through hormones
What hormones control reabsorption in the PCT? What do they do?
Anti-diuretic hormone (ADH) = increases water reabsorption
aldosterone = increases Na+ reabsorption
Atrial natriuretic hormone (ANH) = promotes water and Na+ secretion (blocks the action of ADH and aldosterone)
The filtrate in the PCT is hypotonic. Why does it not move into the interstitial space in the absence of ADH?
Because there are no aquaporin channels present
When is ADH released, where is it released from and how does it work?
-when the body recognises you are becoming dehydrated (during exercise etc.) -Pituitary gland -Inserts AQP channels to allow water reabsorption in PCT and collecting ducts
What kind of urine is produced as a result of the actions of ADH?
A smaller volume of concentrated urine
Describe the actions of aldosterone and when it is released.
-Released due to fluid loss -Causes upregulation of Na+/K+ pump -leads to reabsorption of Na+ and indirectly secreted K+ into urine (has an effect on BP)
What kind of urine is produced as a result of the actions of aldosterone?
Concentrated urine
When, how and where is atrial natriuretic hormone released?
-released when there is an increase in fluid volume -increase in fluid volume causes an increase in blood volume and therefore BP. Smooth muscle cells in the heart detect this increased BP and release ANH
Describe the action of atrial natriuretic hormone and what kind of urine is produced as a result.
-blocks the action of aldosterone and ADH -large volume of dilute urine produced
