RAAS and Renal Physiology

Question 1

Goal of RAAS and Activation

Answer 1

• Main purpose of the Renin-Angiotension-Aldosterone System (RAAS) system is to increase blood pressure
• In the affertent arterioles of the kindey are specialized smooth muscle cells called justaglomerular/granular cells. These cells store prorenin (in inactivated from reinin) and release it when stimulated

Question 2

Juxtaglmerular apparatus

Answer 2

Question 3

Triggers of Renin release

Answer 3

• Low blood pressure/ reduced stretch of afferent arteriole after a reduction in renal perfusion pressure
• Increased renal sympathetic nervous discharge
• Reduced delivery of NaCl to macula densa cells in distal tubule of the nephron - causes macula densa cells to release prostogladin (paracrine hormone) to the juxtaglomerular cells, resulting in renin release.

Question 4

Overview of RAAS

Answer 4

• Decrease BP stimulates renin release.
• Renin moves through blood stream and comes in contact with angiotensinogen (produced by the liver) and cleaves it into angiotension I. As angiotension moves through the capillaries it comes in contact with ACE (mostly in the lungs, but also elsewhere). ACE converts angiotension I into angiotension II.
• Angiontension results in smooth muscle constriction, stimulates kindeys to hold more water, stimulates the pituitary gland to release ADH, and stimulates the adrenal gland to release aldosterone.
• Together these effect result in an increase in blood pressure

Question 5

Angiotension II effects

Answer 5

• Smooth muscles - stimulates vasoconstriction, which increases resistance (Rapid effect)
  
  • ↑P_a - P_v = (SV X HR) X ↑R
• Kidneys - increase Na⁺ reabsorption, which consequently causes can increase in water reabsorption, increasing strove volume, which increases arterial pressure (slow effect)
  
  • ↑P_a - P_v = (↑SV X HR) X R
• Posterior Pituitary Gland - stimulates release of anti-diuretic hormone (ADH)
  
  • Vasoconstriction of smooth muscles
  • Increases water reabsorption at kidneys
• Adrenal Gland - stimulates aldosterone release
  
  • Increases Na⁺ reabsorption at kidneys (results in water reaborption)

Question 6

Sodium vs. Water Reabsorption at the Kidneys

Answer 6

• Aldosterone and angiotension II work on areas of the nephron that are permeable to water. These hormones work by increasing Na⁺ reabsorption, knowing the water following sodium.
• ADH creates channels (aquaporin channels) that allow water to pass through.

Question 7

Aldosterone effects

Answer 7

• Aldosterone is released from the adrenal gland by the zona glmoerulosa (steriod hormone).
• Aldosterone production is stimulated by:
  
  • Angiotensen II
  • Elevated potassium
• Aldosterone works on the
  
  • Late distal convoluted tubule and collecting duct, specifically the principle cells
• Aldosterone effects
  
  • Drives Na/KATP_ase to work harder
  • Adds K⁺ channel on apical surface (urine side) resulting in more K⁺ excretion
  • Adds Na⁺ channels on apical surface resulting in increase Na⁺ and water net reabsorption
  • Net effect: blood looses K⁺ and gains Na⁺ and water resulting in increased strove volume and therefore blood pressure

Question 8

ADH secretion

Answer 8

• ADH is released from nerve cells of supraoptic nucelus into the capillaries of the posterior pituitary - which carry ADH to the rest of the body through the blood stream.
• ADH triggers:
  
  • High blood concentration (osmolarity)
  • Low blood volume -reduction in stretch of SVC, IVC, and right atrium
  • Decrease blood pressure - baroreceptors at aortic arch and carotid arteries signal the brain when pressure is low
  • Angiotension II (RAAS system) - signals brain that pressures are low

Question 9

ADH effects on blood pressure

Answer 9

• ADH targets various organs to increase blood pressure
  
  • Smooth muscle - vasoconstriction (increases resistance)
  • Kidney - increased reabsorption of water (increases stroke volume). Works on collecting duct by causing movement of aquaporins to apical surface allowing water reabsorption.

Question 10

Main function of the kidney

Answer 10

• To maintain stable extracellular fluid environment by
  
  • Filtration of circulating blood by glmerulus to form an ultrafillrate of plsam in urinary space
  • Selective reabsorption (urine to blood)
  • Selective secretion (peritubular capillary blood to urine)

Question 11

Glomerular structure

Answer 11

• Movement of ultrafiltration of plasam from glomerulus to bowmans space involves fenestrated capillary endothelium, capillary basement membrane, and visceral epithelial cell layer (podocytes) of Bowmans capsule.
• Mesangial cells fill the area between the capillaries - can contract and alter capillary surface area available for filtration
• Glomerulus filters materials based mainly on size, but also charge (<2nm allowed through; >4nm blocked; anything between is let through based on charge).
  
  • Podocytes and endothelial cells are covered by glycocalyx made of negatively charged glycoproteins and proteoglycans and the basement membrane has heparan sulfate proteoglycans. This negatively charged barrier prevents filtration of large, negatively charged, ions (mostly proteins- albumin).

Question 12

Vectorial Transport

Answer 12

• Net movement of substances from tubular fluid to blood (reabsorption) or vise versa (secretion)
  
  • In order for substances to be able to move across - the cell membrane of the luminal side must have different properties than the cell membrane of the basolateral (peritubular) side. This is called a polarized epithelium
  • Tight junctions limit water and solute moement between cells (paracellular route)
  • Solute transport across the cell membrane can be either passive or active.

Question 13

Passive Transport

Answer 13

• Simple diffusion - moves down electrochemical gradient (electrical + concentration gradient)
• Facilitated diffusion - interaction of molcules with a specific membrane carrier protein that allows passage across a membrane

Question 14

Active transport

Answer 14

• Primary -When ion movement is pumped against its electrochemical gradient requiring ATP
• Secondary - one molecule is trasported against its electrochemical gradient using energy dervied from the downhill movement of another ion (established from primary active transport).

Question 15

Transport of Proximal Tubule

Answer 15

• Na/H exchanger isoform 3 (antiporter) is main route of Na⁺ entry into proximal tuble cell from lumen
• There are also various sodium co-transporters on lumenal membran - this is why the proximal tuble does most of the reabsorption of Na⁺, K⁺, CI^-, and HCO₃^- and almost complete reabsorption of glucose, amino acids, and low-molecular weight proteins. Most of the other filtered solutes are also at least somewhat reabsorbed at the proximal tuble (calcium, phosphate, urea).
  
  • Na⁺ gradient is created by Na/KATP_ase on basolateral surface - creating a downhill Na⁺ concentration gradient allowing for the co-transport of other ions
• Proximal tuble has aquaporin channels on both the apical and basolateral membrane. This allows ~65% of filtered water to be reasorbed here (it is isomotic because the junctions between cells are leaky and unable to maintain transepithelial osmotic gradient)
• Final part of proximal tubule (S2 and S3) also secretes weak acids

Question 16

Structure of Proximal Tubule

Answer 16

• Proximal tubule can be divided into the proximal convoluted tubule (first 2/3) and the proximal straight tubule (last 1/3) Tubule epithelium is divided into 3 segements

1. S1 -initial part of PCT
2. S2 - remainder of PCT and cortical part of PST
3. S3 - medullary part of PST

• Responsbile for bulk reabosprtion of glomerular filtrate
• Epithelia cells have brush border on apical side and membrane folds on basolateral side increasing surface area
• Rich in mitochondria and rely on aerobic metabolism making them suspectible to hypoxia

Question 17

Transport Loop of Henle

Answer 17

• Composed of thin descending limb, thin ascending limb, thick ascending limb and macula densa
• Continues reabsorption of solutes (Na⁺, CI^-, K⁺, Ca²⁺, Mg²⁺)
• Resonsible for kidney’s ability to concentrate of dilute urine (countercurrent multiplication)
• Thick limb produces Tamm-Horsfall protein/uromodulin - most abundant protein in the urine. May have a role in sodium homeostasis, inhibit Ca²⁺ crysalization in tubular fluid and help protect the kidney from inflammation and infection

Question 18

Transport Distal Tubule Nephron

Answer 18

• Has Na/KATP_ase that creates a electrochemical gradient to move Na⁺ across the lumen into the cell. Using this gradient there is a secondary active thiazide sensitive Na-CI co-transportor
• CI^- is passively transported out of the cell (cell to blood)
• Ca²⁺ is reabsorbed in this segment (passively from lumen to cell and actively from cell to blood)

Question 19

Transport Cortical Collecting Duct

Principle Cell

Answer 19

• Principle cell is the major cell of the cortical collecting duct. It is mainly responsible for Na⁺ reabsorption and K⁺ secretion (+ water reabsorption)
  
  • Na⁺ enters the cell from the lumen via epitheliam sodium channels (ENaC) and exits into the blood from basolateral Na/KATP_ase -this creates a lumen-negative transepithelial potential different (allows passive excretion of K⁺ into the urine)
  • K⁺ enters principle cell from blood by basolateral Na/KATP_ase and leaves by K⁺ transport pathways on BOTH membrans - but depolarization of apical membrane by Na⁺ favour secretion of K⁺ into the lumen (via ROMK channels)

Question 20

Transport Cortical Collecting Duct

Intercalated Cells

Answer 20

• Type α - responsible for H⁺ secretion into urine
• Type β -responsible for HCO₃^- secretion into urine

Question 21

Transport Medullary Collecting Duct

Answer 21

• There is a gradual transition in the epithelium where there a fewer and fewer intercalated cells and principle cells are modified to reabsorbed Na^+, lack apical K⁺ channels (no longer secrete K⁺)

Question 22

Glomerulotubular balance

Answer 22

• The proportion of filtered Na⁺ excreted in the urine is normally <1% . Because of this, it is important that there are compensatory changes in Na⁺ reabsorption so that an increase in GFR does not lead to a significant increase in Na⁺ excretion
• Glomerulotubular balance - the amount of Na⁺ reabsorption in a given segment depends on the amount of Na⁺ delivered to that segement
• Perfect glomerulotubular balance would mean that reabsorption and excretion of Na⁺ woud change in exacatly the same proportion as a change in GFR (not perfect in real life)

Question 23

Where does glomerulotubular balance occur?

Answer 23

• Proximal tubule, thick limb of Henle, and distal tubule
  
  • This is why diuretics working on proximal tubule are pretty ineffect

Question 24

Countercurrent Multiplication Physiology

Answer 24

• The nephron can be divided into 3 parts - the cortex, the inner medulla, and the outer medulla. As you go deeper down (towards the medulla) the solution becomes more concentrated due to ion reabsorption.
• At the loop of Henle, the
  
  • Descending limb reabsorbs H₂O and is impearmeable to ions
  • Ascending limb reabsorbs ions (Na⁺, CI^-, K⁺) and is impearmeable to water
    
    • Na⁺ is reabosrbed passively in the thin ascending limb (mechanism not understood) and actively at thick ascending limb. Thick ascending limb also has a pump-leak system: basolateral Na/KATP_ase maintains electrochemical gradient driving passive Na⁺ entry from the lumen (urine) throught the Na-2CI-K cotransporter asn to a lesser extent, the Na-H exchanger. K⁺ exits through the basolateral ion channels, but also re-enters the lumen through apical membrane channels. This K⁺ recycling is needed to operate the Na-2CI-K channel since K⁺ is the limiting factor from the transporter. K⁺ recycling also creates a lumen + transepithelial potential difference allowing Na⁺ (and other cation) reabsorption through paracellular pathway.

Question 25

Countercurrent Multiplication -The Basics

Answer 25

• When we actively reabsorb ions in the ascending limb and make the medulla more concentrated, by not reabsorbing water, that drives water to be reabosrbed passively in the descending limb

Question 26

Urea reabsorption

Answer 26

• The collecting ducts have the ability to reabsorb urea in the medulla in order to maintain osmolarity (which will in turn increase water reabsorption in the descending limb of the loop of Henle
  
  • Inner medullary collecting duct expresses ureas transporters allowing passive reabsorption of urea. This process is under the control of ADH

Question 27

Main function of Loop of Henle

Answer 27

• Generation and maintenance of interstitial osmotic gradient that increases from renal cortex (~290mOsm/kg) to tip fo medulla (~1200mOsm/kg)
  
  • Osmotic equilibrium occurs in descending limb
  • NaCI reabsorption occurs in the water-impermeable ascending limb
  • Hypotonic fluid is delivered to the distal tubule and in the absence of ADH it reamins hypotonic as it moves through the distal tubule and collecting duct, despite large osmotic gradient favoring water reabsorption, resulting in large volume of dilute urine.
  • If ADH is secreted, water is reabsorbed down osmotic gradeint so that tubular fluid is isotonic in cortical collecting duct and hypertonic in medullar collecting duct, resultin in small volume of concentrated urine (remember that ADH causes expressionof aquaporin channels in collecting duct).

Question 28

Countercurrent Exchange

Answer 28

• The U-shaped arrangement of hte capillaries ensures that solute entery and water loss in the descending vasa recta are offset by solte loss and water entery in the ascending vasa recta. This process is entirely passive.
• Ensures that capillaries don’t dissipate the medullary osmotic gradient because of equilibrium of hypertonic intersitium with isotonic capillary blood

Question 29

Potassium reabsorption at the Kidneys

Answer 29

• ~90% of filtered K⁺ is reabsorbed
  
  • 65% in proximal convoluted tubule
  • 25% in thick ascending limb of Loop of Henle

Question 30

Potassium Reabosorption Proximal Tubule Cell

Answer 30

• Active Na⁺ reabsorption drives net fluid reabsorption across proximal tubule. This drives K⁺ reabsorption through solvent drag
• Most K^{+ is} reabsorped by paracellular diffusion
  
  • As fluid goes down proximal tubule we see a lumenal voltage shift from slightly negative to slightly positive, favoring K⁺ diffusion through paracellular pathway

Question 31

Potassium Reabsorption thick ascending limb

Answer 31

• K⁺ reabsorption occurs by paracellular and transcellular mechanisms.
• ROMK channels provide pathway for K⁺ recycling back into the lumen to ensure there is enough K⁺ to supply Na2CIK channel
• ROMK provides + lumen voltage allowing K⁺ reabsorption through paracellular pathways

Question 32

Potassium Secretion at the Kidneys

Answer 32

• Renal K⁺ secretion occurs in the late distal tuble and cortical collecting duct

Question 33

Potassium Secretion Distal Tubule

Answer 33

• Na⁺ uptake occurs through thiazide sensitive Na-Cl cotransporters, energized by basolateral Na/K_^ATPase (creates favorable Na⁺ concentration gradient). Na-CI cotransporter is heavily present in eary DCT but not in the last DCT.
• Late DCT sees start of alodterone sensitive distal nephron. See presence of epithelial Na⁺ channel (ENaC) (transport Na⁺ into the cell from the urine) and a K/CI cotransporter (transports K⁺ and CI^- out of the cell into the urine). Conditions that cause low lumenal CI^- will increase K⁺ secretion
• ROMK is expressed thought DCT

Question 34

Potassium secretion principle cell

Answer 34

• Na/K_^ATPase creates a high K⁺ concentration, making a favourable diffusion gradient of K⁺ from the cell into the lumen (via ROMK)
• Na/K_^ATPase also makes a gradient making it favourable for Na⁺ to move into the cell from the lumen (via ENaC)

Question 35

Potassium and α-intercalated cell

Answer 35

• Reabsorption of HCO₃^- is dependent on apical H⁺ secretion by α-intercalated cell
• 2 transporters secret H⁺, both use ATP. One only secrets H⁺ and the other is an antiporter with K⁺ (K⁺ reabsorption).
  
  • Activity of H/K_^ATPase increases when there is K⁺ depletion, resulting in increased K⁺ reabsorption

Question 36

Factors that effect K⁺ reabsorption and secretion

Answer 36

1. Aldosterone - leads to K⁺ secretion through various mechanism (distal nephron)
   
   • Increases intracellular K⁺ ​cocentration by stimulating Na/K_^ATPase on basolateral membrane
   • Stimulates Na⁺ reabsorption across luminal membrane, increasing electronegativity of the lumen, making an electrochemical gradient that favors K⁺ ​secretion (opens ENaC on luminal membrane)
   • Affects luminal membrane to increase K⁺ ​permeability (increases number of open K channels)
2. Rate of distal delivery of Na⁺ and water
   
   • Increases distal Na⁺ delivery and stimulates distal Na⁺ absorption - makes lumen potential more negative increasing K⁺ ​secretion
3. Acid-base balance
   
   • Metabolic alkalosis increases K⁺ ​excretion
   • Metabolic acidosis decreases K⁺ ​excretion
4. ADH - stimulates aquaporin 2 receptors, which in turn activates ENaC in cortial collecting and principle cells, causing K⁺ ​secretion
5. Angiotension II -inhibits apical ROMK channels in principle cells of collecting tubule and collecting duct, reducing K⁺ ​secretion
6. Tissue Kallikrein - increases ENaC activity, increasing K⁺ ​secretion
7. Anions - Delivery of anions to distal nephron creates a lumen negative voltage gradient that favoura K⁺ ​secretion