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
