Reabsorption and micturition- Renal 2 Flashcards
-Reabsorption
- Secretion
- Countercurrent
multiplication
- Micturition
(urination)
Learning Objectives:
– Describe the routes and mechanisms of tubular
reabsorption and secretion.
– Explain how specific segments of the renal tubule
and collecting duct reabsorb water and solutes.
– Discuss how specific segments of the renal tubule
and collecting duct secrete solutes into the urine.
(considered again).
– Understand the control of the micturition reflex
Tubular reabsorption - is it selective? is it variable?
What is the amount of each substance reabsorbed?
Tubular reabsorption is tremendous, highly selective, and variable.
The amount of each substance reabsorbed = the quantity required to maintain proper internal fluid environment composition and volume
Tubular reabsorption involves:
the transfer of substances from tubular lumen
into peritubular capillaries
Reabsorbed substances must cross how many barrier?
5
Most of tubular reabsorption occurs?
At proximal convoluted tubule
From the tubules to Peritubular capillaries:
Of 125ml filtered per min, how much is reabsorbed?
124mL
How is most water reabsorbed and what % and explain the breakdown of this?
Mostly absorbed by osmosis - 99%
99% H2O – Occurs passively by osmosis
- Obligatory (~80%) - follows solutes that get
reabsorbed in proximal tubule and descending
loop of Henle, passive, antidiuretic hormone
(ADH) independent
- Facultative (~20%) - occurs in the distal and
collecting tubules, regulated by ADH (increase
aquaporin insertion)
Does not require ADH
How much sodium, glucose, amino acids and urea is reabsorbed?
99.5% Na+
100% glucose, amino acids
50% urea - it is the only metabolic waste that is
reabsorbed
What transport does tubular reabsorption involve?
In selective reabsorption what is reabsorbed from filtrate bak into the blood?
Transepithelial transport
Sodium, amino acids, glucose
Passive reabsorption where does this occur and what is it?
No energy is required for the substance’s net movement
Occurs down electrochemical or osmotic gradients
Active reabsorption what is it and how does it occur?
Occurs if any one of the steps in transepithelial transport of a substance requires energy
Movement occurs against electrochemical gradient
See diagram on slide 7
1. Does H20 generally go across the cell membrane?
2. Do aquaporfins use energy - if not what?
3. What substances can move across membrane?
4. What requires active transport?
- H20 generally doesn’t across cell membrane as its polar so doesn’t go across phospholipid tails which are hydrophobic
- Aquaporfins don’t use energy but they allow channels to get H2O across but still passive as energy is not used
- n2+O2 non polar susbances, lipid soluble can across membrane and certain drugs can cross across the membrane
- Ions require active transport
Secondary active transport - how do they generate a gradient?
What mechanisms does this occur by?
- Generate a gradient using energy such as sodium
- Occurs by 2 mechanisms called Antiport - exchange diffusion they move in opposite directions e.g. Cl and Bicarbonate ions in opposite directions
and Symport - cotransport e.g. moving in same direction like glucose, amino acids and sodium
All dependent on carrier proteins - proteins bound by cargo and exposed to other side of the membrane where they can release the proteins
How is Na reasborbed?
1. What is essential for it?
2. How much energy is used for it by the kidneys?
3. What follows Na^+ reasborption?
Concentrated in basolateral membrane of tubular cell
An active Na+/ K+ ATPase pump in basolateral membrane is essential for Na+ reabsorption
* Of total energy spent by kidneys, 80% is used for Na+ transport
* Water follows reabsorbed sodium by osmosis which has a main effect on blood volume and blood pressure
What is sodium ion?
How it it controlled?
Major extracellular cation - typically has a strong difference of concentration across the membrane
Tightly controlled
Basically all sodium is reabsorbed
Most reabsorption tajes place in proximal tubule: 67%
Na/K pump is critical for nervous system
How much Na^+ is reabsorbed in proximal tubule and what is the role here?
67%
Plays role in reabsorbing glucose, amino acids, H2O, Cl- and urea
LOT OF ALL REABSORPTION OCCURS WHERE?
See diagrams on slide 11,12,13
IN PROXIMAL TUBULE
How much Na^+ is reabsorbed in ascending limb of the loop of Henle and what is the role here?
25% - Plays critical role in kidneys’ ability to produce urine of varying concentrations
Glucose
2nd trasnporters - Sglut2: symporter which enables glucsoe to bemoved from lumen into cytosol with1. sodium moecuels down its ocnentartion gardeint and this is repsonieblf ro 80-90% of its reabsorption
sglut 1- requires 2 allows ratio of 4900:1 to be generated in terms fo how how much glucsoe is in cytsoosl relative to the lumen
How much Na^+ is reabsorbed in distal and collecting tubules and what is the role here?
8% - Variable and subject to hormonal control; plays role in regulating ECF volume, BP, K+ secretion
Amino acid uptake is dependent on?
There is what?
largely dependent on Na movement down concentration gradient
different carrier proteins some involve symporters with sodium some have hydrogen ions with it
Plasma concentration of K^+ outside and inside cell - what can affect this?
Plasma concentration of K^+ are generally lower outside of cell compared to inside the cell but even a small disturbance in this really affects functions in nerves, etc by affecting resting membrane potential of cells
Important for controlling pH via hydrogen ions
Where does K^+ secretion occur and what does it rely upon?
Cortical collecting tubule and relies upon active transport of K^+ across basolateral membrane (ROMK-renal outer medullary potassium channels) and passive exit across apical membrane into tubular fluid
Achieved by principal cells - mainly for Na^+ but has other roles with K^+
Criteria for diagnosing acute kidney injury (acute renal failure):
- Rise in creatinine >26 μmol/L in 48hrs
- Rise in creatinine >1.5*baseline within 7 days
- Urine output <0.5mL/kg/h for >6 consecutive hours.
What is osmosis?
Osmosis is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane in the direction that tends to equalise the solute concentrations on the two sides.
What is another important thing reabsorbed by tubular secretion?
What is it?
How is it removed?
Reabsorption?
Plasma levels?
Creatine
Creatinine is a breakdown product of creatine phosphate in muscle, produced at a steady rate by the body.
Creatinine is removed from the blood mainly by glomerular filtration, but also by proximal tubular secretion.
Tubular reabsorption of creatinine is minimal. Creatinine plasma level is 70 - 150 μmol/L
Acute Kidney Injury
Risk factors:10
See diagram 18
- Chronic kidney disease
- History of urinary symptoms
- Diabetes
- Drugs especially newly started
- Poor fluid intake/increased losses
- Chronic liver disease
- Over the age of 75
- Cardiac failure
- Peripheral vascular disease
- Sepsis
Common causes of acute kidney injury?
Ischaemia, sepsis, nephrotoxins although protsaic disease causes up to 25% in some studies and has the best prognosis
What does controlling ECF osmolarity prevent?
Changes in ICF volume
Above 300mOsm?
300 mOsm?
Lower than 300 mOsm?
Hypertonic: increased solute concentration outside cell
Isotonic: equal and ideal
Hypotonic: Increased solute concentration inside cell causing cells to burst/lyse
Depending on the body’s state of hydrate the kidney’s secrete?
Urine of varying concentrations
Urine volume ans osmotic concentration are regulated by?
Controlling water reabsorption
Too much H20 in ECF?
Water deficit in ECF?
Hypotonic ECF
Hypertonic ECF
Examples of Micturition reflex problems:
- Sphincter muscles lose tone: – leading to incontinence
- Control of micturition can be lost due to:
– a stroke
– Alzheimer’s disease
– CNS problems affecting cerebral cortex or
hypothalamus - In males, urinary retention may develop if enlarged prostate gland compresses the urethra and restricts urine flow
Mechanism of altered urine production depends on what 2 mechanisms?
Countercurrent multiplier
Countercurrent exchange
Loop of Henle helps control?
Amount of H20 reabsorbed
Countercurrent multiplier?
A countercurrent multiplication loop is a system where fluid flows in a loop so that the entrance and exit are at similar low concentration of a dissolved substance but at the far end of the loop there is a high concentration of that substance.
Nephron
Countercurrent exchange?
A countercurrent exchange system is a system in which two fluids of different properties flow parallel and against each other in semipermeable tubes, which allow compensation for these
differences
Vasa recta
Ascending and descending loops of Henle are different how?
Ascending loop of henle: ions ACTIVELY pumped into the interstitial space.
Creates a really salty environment
Descending loop of henle is permeable to water
water will flow into the high concentration of salt in the interstitial space
What does countercurrent multiplier lead to?
A large, vertical osmotic gradient is established in the interstitial fluid of the medulla (from 100 to 1200-1400 mOsm).
=> The medullary vertical osmotic gradient is established by countercurrent multiplication
There is countercurrent flow made by?
Close proximity of 2 limbs
COUNTERCUREENT MULTIPLICATION
fluid is moving trough loop at all time and this is how vertical is made
Ascending: trasnporters pump ions out so we have reduction in amount of solutes and increase in concentaryion of solutes in interstial fluids so it flows out then there’s a
Higher solute concentraion in descending limb
moves to bottom of limb
=> pumping ions out of ascending limb and dragging more water out of descending limb
Repeat cycles 4-6: get a salty extracellular environment and you get this gradient
Flow is very important to drive reasborption of ions and water an cmaintin this concetration gradient
=> Generates vertical osmotic gradient
Countercurrent exchange?
What happens as the vasa recata descends?
What happens as vessel ascends?
Maintains this vertical osmotic gradient
H20 diffuses and solutes in
H20 flows back in and solutes out and this maintains the medullary osmotic gradient
What does the Vasa Recta do?
Preserve the vertical osmotic gradient by countercurrent exchange
Urine transport, strorage and elimination takes place in the?
urinary tract
Micturition ?
spinal reflex based on strecth receptors in lining of mucosa in the bladder
it can be overriden voluntarily
Pelvic nerve - sensory afferents: stretch recepetors in mucosa
Hypogastric nerve -> Sympathetic From L2:
NA, B3 receptor- Inhibit detrusor
Contracts int. sphincter
Pelvic nerve-> parasympathetic From S2-4: ACh, M3 receptor - Activates detrusor & opening of int. sphincter
Pudendal nerve -> Motor, voluntary control Activates ext sphincter
- Primary Active Transport occurs by?
- Which side faces interstitial space and which towards the membrane?
- Which way does it drive Na and K?
- Explain symporters?
- Primary active transport: Na/K pump
- Side facing into interstitial fluid the basolateral side facing out towards the membrane
- Can drive sodium against its concentration out into interstitial fluid and drive potassium from interstitial fluid out and into cytosol by using Primary active transport
- Symporters carry 2 things in same direction: Na down its concentration and simultaneously like glucose, amino acids can move up its concentration gradient
Secondary active transport occurs via 2 mechanisms:
1. Antiport which is exchange diffusion
2. Symport which is co transport
Explain them and give examples?
- In antiport, driving ions and transport molecules move in the opposite direction. Most of the ions are exchanged in this way. For example, coupled movement of chloride and bicarbonate ions across the membrane is initiated by this mechanism.
- In symport, the solute and driving ions move towards the same direction. For example, sugars such as glucose and amino acids are transported across the cell membrane by this mechanism.
Tubular secretion involves the transfer of what from where to where?
What transport is used?
Transfer of substances from peritubular capillaries into the tubular lumen
Involves transepithelial transport
Tubular secretion - kidney tubules can selectively do what?
Kidney tubules can selectively add some substances to the substances already filltered
Most important ions in secretory systems?
K^+, H^+ ions and organic ions
Where is K^+ secreted?
What must concentration be to maintain normal membrane excitability in muscles and nerves?
Secreted only in the distal and collecting tubules under control of aldosterone (lecture 3).
Keeps plasma K+ concentration at 3.5 - 5 mmol/L to
maintain normal membrane excitability in muscles and nerves
Where is H^+ ion secreted and what is it important in regulating?
Secreted in proximal, distal, and collecting tubules
Important in regulating acid-base balance (lecture 4).
Organic ions accomplish what and where are they secreted?
Accomplish more efficient elimination of foreign
organic compounds from the body
Secreted in the proximal tubule
Cortical nephrons which are 85% of all nephrons mainly perform what functions?
Excretory and regulatory functions
Juxtamedullary nephrons which are 15% of all nephrons main functions is to?
Concentrate and dilute urine
Medullary vertical osmotic gradient is established by?
Countercurrent multiplication
Countercurrent flow is made by?
Close proximity of 2 limbs
Ascending limb of loop of Henle actively transports?
What is it impermeable to and what does this mean?
Where does the interstitial fluid face in countercurrent multiplication?
The ascending limb actively transports NaCl out of the tubular lumen into the surrounding interstitial fluid.
It is impermeable to water. Therefore, water does not follow the salt by osmosis.
This interstitial fluid faces against the flow of fluid
(countercurrent) in the descending limb, attracting the water by osmosis for reabsorption
Descending Limb of loop is highly permeable to?
Water but impermeable to sodium for reabsorption
As the Vasa recta descends?
As it ascends?
What does this do?
H20 diffuses out and solutes in
As vessels ascend H20 flows back in and solutes out
The maintains the medullary osmotic gradient
If we were so overhydrated we had no ADH what happens?
Decreased osmolarity of extracellular fluids
Then decreased ADH release from posterior pituitary
Decreased number of aquaporins (H20 channels) in collecting duct
Then decreased H20 reabsorption from collecting duct
Leading to a large volume of dilute urine being produced and released
If we were so dehydrates we had maximal ADH what happens?
Increased osmolarity of extracellular fluids, increased ADH release from posterior pituitary and increased number of aquaporins (H20 channels) in collecting duct and increased H20 reabsorption from collecting duct and thus a small volume of concentrated urine made and released
Normally urine is?
Is a clear, sterile solution
Yellow colour (due to pigment urobilin)
Generated in kidneys from urobilinogens
Urinary tract composed of?
Ureters, urinary bladder and urethra
Urinary bladder -
1. Structure?
2. Function?
3. Full bladder has?
4. Emptying =?
- Hollow, muscular organ
- Functions as temporary reservoir urine storage
- Full bladder can contain 1 liter of urine
- Emptying = mictruition
Micturition - urination takes place via what kind of reflex?
How can it be overriden?
Spinal reflex
Can be overriden voluntarily
