Renal Physiology 2 Flashcards
Describe filtration
Filtration is nonselective process
- Both useful & waste materials are filtered freely from plasma
- However tubular transport mechanisms are different in different parts of the tubule
-So what is excreted depends on the transport mechanisms
Our daily urine output is about 1.5 L 99% of what is filtered is absorbed
Most substances are reabsorbed in thebpr9ximal segments of the nephron. About 3% of Reabsorption occurs in the distal & collecting tubules. These segments are important in maintaining the fluid & electrolyte balance, & plasma osmolarity
Summarize Na+ Reabsorption
Percentage of filtered load of sodium being reabsorbed in different parts of tubules
Reabsorption in DCT and CDis variable depending on body’s need for sodium because this Reabsorption regulated by aldosterone
- 67% in proximal convoluted tubule
- 25% in thick ascending limb
- 5% of distal convoluted tubule
- 3% in collecting duct
- less than 1% excreted
Describe early proximal tubule for Na+ Reabsorption
In early proximal tubule Na+ absorption is linked to nutrient transport
The Na+/K+ -ATPase droves Na+ Reabsorption all along the renal tubule on the basolateral side
- Na+/H+ anti port allows H+ secretion fir HCO3- Reabsorption
- Carbonic anhydrase enzyme is present lumen & in cell
Summarize late proximal tubule for Na+ Reabsorption
Na+/H+ anti porter coupled to Cl- Reabsorption and formate and secretion
- 67% of the Na+ is absorbed in the proximal tubule
- Water follows sodium
- Na+ & H2O Reabsorption are linked
- Hence the tubular fluid in the proximal tubule is isosmotic
Describe glucose transport
- Na+ moving down its electrochemical gradient using the SGLT protein pulls glucose into the cell against its concentration gradient
- Glucose diffuses out the basolateral side of the cell using the GLUT protein
- Na+ is pumped out by Na+-K+-ATPase
What is transport maximum(Tm)?
Is transport rate at saturation
What is renal threshold?
Plasma concentration at which saturation occurs
Explain glucose correlations
(A) Filtration of glucose is roportional to the plasma.
(B) Reabsorption of glucose is proportional to plasma concentration until the transport maximum (Tm) is reached where it plateaus
(C) glucose excretion is zero until the renal threshold is reached
Describe Glomerular balance
Neural, hormonal, and other mechanisms ensure that a constant fraction of sodium is reabsorbed in the PCT regardless of changes in GFR; I.e. when GFR increases or decreases, Na+ Reabsorption also increases or decreases, respectively. Overall, this serves as primary regulatory influence on PCT function
Ensures that constant fraction of filtered Na+ (67%) is reabsorbed via filtration/Reabsorption coupling
Explain glomerutubular balance mechanism
If the GFR increased by 1% then FF (GFR/RPF) would also increase (provided RPF is constant)
This increases protein or oncotic pressure in the Glomerular capillaries & peritubular capillaries which favors Reabsorption of Na+ & H2O
Describe a thin ascending & descending loop of henle
The thin descending limb has low permeability to ions and urea, while being highly permeable to water
The thin ascending limb is impermeable to water; but is permeable to ions allowing for some sodium Reabsorption by passive diffusion. Salt moves out of the tubule and into the interstitium due to osmotic pressure created by countercurrent system
Explain thick ascending loop of henle (TLOH)- Na+ & Ca2+
- Na+ Cl+, K+ or Na2ClK transporter on the luminal side which promotes uptake of all these ions
- Na+ & Cl- are reabsorbed mostly via pumps or transporters on the basolateral side
- Luminal membrane contains a K+ channel, allowing K+ to diffuse back into lumen
- This creates a positive luminal potential which promotes Ca2+ & Mg2+ Reabsorption via a paracellular pathway
- Thick ascending loop is impermeable to water this us known as the diluting segment
25% of Na+ is reabsorbed in TLOH
Explain loop of henle physiology
- Important fir setting up medullary interstitial gradient
- It can achieve interstitial gradients up to 1200-1400 mOsm deep in the medulla
Loop of henle can only set up a gradient of 600 mOsm
Requires urea to achieve 1200 mOsm or more
-Establishment of gradient has to do with properties of Loop of Henle and low blood flow to the interstitium. Medulla only receives only 5% of the total blood flow to the kidney.
Also requires Antidiuretic hormone(ADH)
Why this gradient?
So kidneys can excrete dilute or concentrated urine
Normal plasma osmolarity is 290 mOsm (calculations we use 300
Urine can vary from 30-1200mOsm
Explain osmolarity changes in the nephron
- Isotonic fluid leaving the proximal convoluted tubule becomes progressively more concentrated in the descending limb
- Removal of solute in the thick ascending limb creates hyposmotic fluid
- Hormones control distal nephron permeability to water and solutes.
- Urine osmolarity depends on the Reabsorption in the collecting duct
Explain countercurrent multiplication & exchange
The kidneys have a unique ability to set up medullary interstitial gradient up to 1200 mOsm.
Multiplier mechanism
- The thin descending limb is permeable to H2O but not solutes
- Thin ascending limb is permeable to solutes but not H2O
- Thick ascending loop of henle is impermeable to H2O & pumps Na & Cl into the interstitium
Vasa rectus mechanism
- provides O2 & nutrients to the medullary interstitium
- the blood vessels are permeable to both solutes & H2O
- The arrangement of vessels allows for passive movement of solutes & H2O
- The low blood flow & passive movement maintains the gradient by preventing the- washout
Summarize countercurrent multiplication in loop of henle
-Filtrate entering the descending limb becomes progressively more concentrated as it loses water. Osmotic gradient 300 mOsm —> osmotic gradient 1200 mOsm
Blood in vasa recta removes water leaving the loop of henle
The ascending limb pumps out Na+, K+, and Cl- and filtratebecomes hyposmotic
Describe urea
Urea is a waste product of protein metabolism
- 50% absorbed in proximal tubule - As water gets absorbed it’s concentrated of urea in tubular fluid rises
ADH increases both water & urea permeability DCT & MCT
-Urea diffuses into the interstitium contributing to increase medullary gradient
Some of it recycled into LOH & some is excreted into urine
LOH can only set gradients up to 600 mOsm requires urea to achieve gradients up to 1200 mOsm
Describe Na+ & Ca2+ reabsorption in the early distal tubule
- Is also known as diluting segment
- is impermeable to H2O
- Both Na+& Cl- are reabsorbed from the luminal side. Na+ is extruded by Na+ K+ ATPase pump & Cl- diffuses through a Cl- channel on to basolateral side
- 5%-Na+ Reabsorption occurs here
Ca2+ enters the cell passively the luminal side
Once it enters it binds calbindin (Ca2+ binding protein), maintains low calcium gradient in the cell
Then calcium is actively extruded into the peritubular fluid via Ca2+-ATPase—> requires PTH
Describe late distal tubule and collecting duct
There are 2 types of cells:
a-intercalated: K+ Reabsorption; H+ secretion (coupled with HCO3- Reabsorption)
- B-intercalated: coupled with HCO3^- secretion )
Principal: K+ secretion (cou0led with Na+ Reabsorption ) this is under hormonal control of aldosterone
-3% Na+ Reabsorption occurs here. Thus segment is responsible for fine tuning Na+ levels to ensure Na+ balance
Describe kidney and potassium balance
Most of the total body potassium is located in ICF (98%)
A small shift of K+ into or out of cells can produce a large change in ECF potassium concentration
Potassium balance:
Urinary excretion of K+ must match with dietary intake potassium balance is done by combination of filtration, Reabsorption and secretion by the kidneys
- In PCT and LOH, potassium is only reabsorbed
- DCT and collecting duct are responsible for K+ balance:
Reabsorption or secretion occurs here depending on the need for potassium balance
Secretion of K+ in DCT and CD is variable depending on:
- Dietary potassium
- Aldosterone
- Acid-base STRs- alkalosis favors secretion
Maximum fraction of tubular load is reabsorbed in PCT
Summarize mechanism of potassium secretion in DCT and collecting duct
- Secretion occurs in the P cells of DCT and CD
- It’s secretion is linked to Na+ absorption
- Is under hormonal control. Does require aldosterone
Summarize Mechanism of potassium Reabsorptionin DCT and Collecting duct
Occurs only at low K+ diet
Occurs in alpha intercalated cells
-At luminal membrane: H+-K+-ATPase moves potassium into & H+ ion out of the cell
In basolateral membrane: potassium diffuses ou5 of cell by potassium channels
