Week 5

Question 1

Role of the kidney acid base

Answer 1

Acid base regulation is one of the most important roles
PH is normally maintained using HCO3- buffer:
-the kidney maintains HCO3- concentrations in a normal range to regulate pH
-the kidney can raise or lower HCO3- concentration to adapt to other changes in the body
In the case of renal failure patients become increasingly acidic which causes a whole host of problems

Question 2

Reminders of pH

Answer 2

Definition of pH: pH= -log10[H+]
HCO3- mediated buffering in the circulation:
H+ +HCO3- <—> H2Co3 <—> H2O +CO2
Or more simply:
H+ +HCO3- <—> H2O +CO2

Question 3

Bicarbonate

Answer 3

IUPAC name : hydrogen carbonate
Historical aside: the ‘bi’ comes from the historical observation that Na+ salts have half as much carbonate other carbonates (c.f. NaHCO3 and Na2CO3)

Question 4

Normal values

Answer 4

PH: arterial= 7.4 venous =7.35
HCO3- (mM): Arterial= 24. Venous =25
Pco2 (mmHg) (kPa): Arterial= 40 (5.3). Venous= 46

Note that venous blood attains a similar equilibrium to that in arterial blood but all concentrations are slightly higher because tissues produce CO2 which in turn leads to production of bicarbonate
But this tissue bicarbonate is converted back to CO2 and exhaled in lungs
So peripheral tissue cant be source of net bicarbonate production

Question 5

Henderson-Hasselbalch equation

Answer 5

In general for an acid we have:
HA <—> H+ and A-
From which the H-H equation can be derived:
pH= pK + log10 [base]/[acid]
In particular case of HCO3- this is:
PH= pK +log10 [HCO3-]/[H2CO3] = pK+log10 [HCO3-]/0.03*Pco2
Where Pco2 is partial pressure of CO2 in mmHg
Allows us to predict pH given known bicarbonate and CO2 concentration
The kidney regulates bicarbonate in body and lungs regulate CO2

Question 6

Normal extracellular pH

Answer 6

Substituting normal values we have:
PH= 6.1 + log10 25/0.03*40=7.4
6.1= equilibrium constant
HCO3-= 25mM
PCO2=40mmHg
This equation shows pH/H+ is related to both bicarbonate and CO2 concentration:
H+ concentration proportional concentration of CO2
PH inversely proportional to CO2 conc
H+ conc inversely proportional to bicarbonate conc
PH inversely proportional to [H+]
[H+] a [CO2]/[HCO3-]
PH a [HCO3-]/[H2CO3]

Question 7

H+ as a waste product

Answer 7

Net hydrogen ion production occurs when:
-ATP is hydrolysed
-during anaerobic respiration with the production of lactate
-during the production of ketones (alkanones) (which are high in diabetes mellitus ketones produced by liver as an energy source)
-during ingestion of acids in food
This metabolically produced H+ has to be removed from the body; this occurs by reaction with HCO3- producing CO2 which is exhaled. However this results in the loss of HCO3-
Hence a key role of the kidney is pH regulation is reabsorption of filtered HCO3- and the production of new HCO3- to replace the losses occurring elsewhere in the body

Question 8

HCO3- absorption in the proximal tubule

Answer 8

HCO3- in the filtrate reacts with secreted H+ ions (secreted by the Na+/H+ exchanger, this reaction catalysed by carbonic anhydrase to produce water and CO2
CO2 diffuses through membrane or porons into cell
Within the cells CO2 and H2O in equilibrium producing bicarbonate
Bicarbonate leaves through basolateral membrane through the HCO3-/Na cotransporter this cotransporter uses bicarbonate gradient to drive sodium out the cell
So ultimately bicarbonate reabsorbed by proximal tubule

Question 9

Quantifying proximal reabsorption of filtered HCO3-

Answer 9

If excess bicarbonate present in the filtrate then this is not reabsorbed
This is because HCO3- absorption in proximal tubule is rate limited
Mechanism:
-for a fixed GFR bicarbonate ions are filtered at rate proportional to plasma bicarbonate concentration
-while this bicarbonate concentration stays normal then the proximal tubule can readily reabsorb all bicarbonate
-once the transport maximum rate is reached no more bicarbonate can be reabsorbed
-we are therefore seeing increasing concentration of bicarbonate being excreted in urine
-this is how kidney deals with high plasma bicarbonate concentration
A low bicarbonate concentration involves synthesis of bicarbonate from kidneys

Question 10

HCO3-

Answer 10

So proximal tubule reabsorption of HCO3- is tubular maximum limited but this limit depends on H+ in the proximal tubule which in turn relies on the Na/H exchanger
This transport limited process means that almost all of the filtered bicarbonate is reabsorbed under resting conditions but that an excess of HCO3- will not be reabsorbed. This is a method of rapidly correcting HCO3- excess
If the source of CO2 is from the vasa recta rather than the filtrate this acts a mechanism for de novo HCO3- production replacing losses elsewhere in the body. In this case, luminal H+ is buffered by HPO42-

Question 11

HCO3- production in proximal tubule

Answer 11

We have CO2 moving from cortical interstitial space (provided by vasa recta) into the cell
By entering cell reacts with H2O produces bicarbonate ions and H+ (catalysed by CA)
The kidney separates HCO3- and H+produced so we get a net bicarbonate production returning to the body:
-in the kidney excess H+ ions enter into filtrate and leave body
-this separates H+ from HCO3-in a permanent way leading to net production new HCO3-
Normally elsewhere in body for the reaction to be mediated by CA any formed H+ and HCO3- are in equilibrium so no net production of HCO3-
Kidney and gut can separate H+ from HCO3- so get net production of both

Question 12

Distal tubule H+ secretion- another way the kidney regulates pH

Answer 12

In the distal tubule primary active transport is the dominant mechanism for H+ secretion. This is through apical:
-H+ K+ ATPase
-H+ ATPase
This process occurs in the alpha-intercalated cells in the distal tubule- cells specialised for acid secretion

Question 13

Filtrate phosphate

Answer 13

Phosphate is present in the circulation and is freely filtered at glomerulus. Whilst some is reabsorbed in kidney most is excreted
Excess H+ ions need to be buffered in the filtrate to keep free urine H+ concentration low. A key mechanism is through buffering (or binding or sequestration) by hydrogen phosphate
H+ +HPO42-<—> H2PO4-
PH= 6.8+ log10[HPO42-]/[H2PO-]
In plasma we have mainly HPO42- and in urine we have H2PO4-
If plasma pH=7.4
Then [HPO42-]/[H2PO4-]= 10^0.6=4
This shows that in the circulation HPO42- predominates

Question 14

Urinary HPO42-/ H2PO4-

Answer 14

If urinary pH=5 then:
5= 6.8+ log10[HPO42-]/[H2PO4-]
[HPO42-]/[H2PO4-]= 10^-1.8= 0.016
This shows that in acidic urine H2PO4- predominates and that phosphates are buffering the pH
We can see here that along length of tubule system the phosphate is picking up H+ which it can then carry out to urine
So HPO4- is acting as a pH buffer it binds additional H+ and preventing pH from dropping too low
This is important because if all HCO3- is being reabsorbed in proximal tubule urine doesn’t have another pH buffer so relies on phosphate

Question 15

Ammonia secretion

Answer 15

When someone is very acidic it’s possible for kidney to produce ammonium ions that are secreted into urine
The ammonium ion NH4+ is produced in proximal tubule by conversion of glutamine to glutamic acid and alpha-ketoglutarate
NH4+ is in equilibrium with NH3 which being small an uncharged is membrane permeable
NH4+ reforms in the filtrate lumen acting as another reservoir for H+ which helps body regulate pH

Question 16

The extraordinary path of ammonium in the nephron

Answer 16

Glutamine coming from liver can be converted in the epithelial cells in the cortical interstitial space to produce alpha-ketoglutarate as it does so it produced ammonia
Ammonia enters in the lumen of filtrate where it binds to H+ acting as pH buffer ultimately leading to secretion of ammonium
The bicarbonate is recovered from alpha-ketoglutarate and is returned to body via Na/HCO3- cotransporter
This is a way to deal with a situation of heavy acid overload
There is a cycle of ammonium in nephrons:
-ammonium ions are exchanged with sodium in proximal tubule
-so we have ammonium in loop of Henle
-that ammonium acts to replace the K+ in the NKCC2 transporter (Na-K-Cl) leading to reabsorption of ammonium
-the ammonium can be ultimately secreted in collecting ducts travelling via ammonia as it passes through epithelial layer

Question 17

PH along the nephron

Answer 17

By the end of the proximal tubule (primarily by action of Na/H exchanger), the pH has fallen to about 6.9 but by the end of the tubule the pH is highly variable (depending on the body’s acid load) down to about 4.5 k

Question 18

Key acid-base problems

Answer 18

There are 4 main types of acid-base disorders:
- respiratory acidosis
-respiratory alkalosis
-metabolic acidosis
-metabolic alkalosis

Question 19

Respiratory acidosis

Answer 19

Typical cause: hypoventilation
CO2 goes up so H+ increases (i.e pH falls)
To compensate the kidney increases the production of HCO3- returning pH towards normal by shifting equilibrium to right
So someone with respiratory acidosis will be acidic but have high bicarbonate concentration as kidney attempts to compensate by increasing HCO3-

Question 20

Respiratory alkalosis

Answer 20

Typical causes: hyperventilation, high altitude
At high altitude we need to breathe more as there is a low PO2 and as a result we increase excretion of CO2 at lungs
[CO2] goes down equilibrium shifts right H+ goes down pH rises
To compensate kidney decreases production or recovery of HCO3- returning pH towards normal

Question 21

Metabolic acidosis

Answer 21

Typical cause: renal failure, lactic acidosis, ketoacidosis (due to DM), poisoning (eg aspirin)
H+ goes up (i.e pH falls) so HCO3- decreases or HCO3- goes down so H+ increases
To compensate the CNS increases ventilation rate decreasing CO2 returning pH towards normal

Question 22

Metabolic alkalosis

Answer 22

Typical causes: vomiting, contraction alkalosis (occurs in low volume states)
H+ goes down (i.e pH rises), so HCO3- increases or HCO3- increases so H+ decreases (pH rises)
To compensate the CNS decreases ventilation rate increasing CO2 returning pH towards normal

Question 23

Anion gap

Answer 23

There are many different causes of metabolic acidosis
The differential diagnosis is assisted by measurement of anion gap
So that we don’t explode the sum of positive charges and negative charges in our bodies has to be equal
However if we look at the difference between the commonly measured cations and anions we find something disturbing - concentration of cations exceeds anions
This is because there’s more unmeasured anions than cations, theres also negative charges on circulating proteins which we do not measure
The anion gap is usually measured as:
[Na+]-[Cl-]-[HCO3-]
This gap is exacerbated by divalent cations (Ca2+, Mg2+) and is only partially explained by other well known anions (eg HPO4-)

Question 24

Increased anion gap

Answer 24

The reference range for the normal anion gap varies between laboratories but is typically 3-11 mmol.l-1
An increase in the anion gap suggests that there is an high concentration of anions that are not being counted.
Causes of such an acidosis includes:
-lactate (produced during anaerobic metabolism)
-ketones (in diabetes or alcohol toxicity)
-sulfates, phosphates, urate, hippurate (accumulating during renal failure)
-aspirin overdose (complicated by direct increase in central respiratory drive, which can also cause alkalosis)
So several causes of metabolic acidosis change the anion gap

Question 25

Changing volume

Answer 25

The body cant easily measure the total extracellular water content
What is measured and so regulated is the effective circulating volume
Usually, actual extracellular volume and the effective circulating volume change together: eg haemorrhage, sweating
In the case of haemorrhage, lose blood initially from vascular supply causes hydrostatic pressure changes which will shunt blood from interstitial space to intravascular space
However in heart failure the heart is not appropriately circulating blood this is detected and so water is retained

Question 26

Juxtaglomerular apparatus

Answer 26

JGA is the area of the kidney most important for regulation of blood pressure
JGA is the complex of later distal tubule in association with renal afferent arteriole (both of the same nephron)
There are granular cells in the afferent arteriole
Thickening of the wall of the early distal tubule (or distal TAL) is the macula densa
The aim of the renin-angiotensin-aldosterone system is to increase the effective circulating volume

Question 27

The renin-angiotensin system

Answer 27

With low circulating volume you will have low amount sodium being filtered in glomerulus low GFR
This means by the time the filtrate reaches distal tubule there will be low amount of sodium in the filtrate
This low sodium is detected by macula densa cells within wall of distal tubule
The macula densa cells are in close contact with granule cells they signal to the granule (juxtaglomerular) cells causing them to release an enzyme called renin into the circulation
Renin acts on angiotensinogen (liver and adiposites) converting it to angiotensin I
Angiotensin I converted to angiotensin II by angiotensin converting enzyme ACE found in lungs
Angiotensin II acts on the efferent arteriole to cause vasoconstriction increasing GFR directly, proximal tubule to increase Na+ Reabsorption, adrenal cortex releasing aldosterone which increase Na+ reabsorption by distal tubule and collecting duct
This process leads to an increase in body sodium and hence a retention of volume as if the kidney can retain more sodium it retains more volume
Therefore maintains GFR and total circulating volume

Question 28

Aldosterone

Answer 28

There are two main physiological trigger for aldosterone release:
-angiotensin II
-hyperkalaemia
So aldosterone is involved in volume and potassium regulation
These 2 signalling pathways converge at the collecting duct where they increase activity of Na reabsorption and K secretion in the kidney
The convergence of these signalling pathways means that, were aldosterone alone the only regulator of ATII action on the kidney, K+ and volume regulation could not be independently regulated. This leads to the implication that ATII must have other renal actions

Question 29

Inhibiting the renin angiotensin system

Answer 29

This system can be clinically inhibited in 4 different locations in order to control blood pressure (indicating the importance of this system for blood pressure control):
-ACE inhibitors (captopril, enalopril)
-AT1 receptor antagonists (candesartan, irbesartan), used for treatment of hypertension
-aldosterone receptor antagonists (spironolactone), used in heart failure
-renin inhibition (aliskiren)
All of these lower circulating volume hence lower blood pressure

Question 30

Angiotensin II receptors

Answer 30

The main receptor in the periphery for the effect of AT II is the AT1 receptor
It’s mainly coupled though Gq so it is linked to an increase in IP3/DAG signalling and increased Ca2+ release from intracellular stores in for example smooth muscle cells (vasoconstriction) and the granule cells of the juxtaglomerular apparatus
The inhibitors of AT1 receptors, the ‘sartans’ dont have the side effect of cough that the ACE inhibitors can have; so while more expensive they’re commonly prescribed for hypertension.
ACE inhibitors as well as inhibiting ACE also inhibit bradykinin metabolism so people taking ACE inhibitors will get increase in bradykinin (inflammatory mediator) which will trigger dry cough

Question 31

Angiotensin II

Answer 31

4 key actions to increase circulating volume;
-increase Na/H exchange in the proximal tubule and hence proximal Na+ and water Reabsorption
-increase in aldosterone release from the adrenal cortex which increases distal Na+ absorption
-cause ADH release
-causes thirst- replenish volume
NB: efferent arteriole vasoconstriction helps to maintains the GFR

Question 32

Haemorrhage

Answer 32

This leads to decreased vascular volume, decreased venous pressure, decreased cardiac filling (startling’s law), decreased cardiac output, decreased circulating volume (with decreased tissue and organ perfusion)
To decreased blood pressure to increased sympathetic activity to increased renin release (action of transmitter on granule cells)
With the decrease in blood pressure this is sensed by the afferent arteriole causing a fall in wall tension and causes release of renin

Question 33

Effects of activating sympathetic innervation of afferent arteriole

Answer 33

Three actions of sympathetic activation to the afferent arteriole:
-vasoconstriction upstream of the granule cells causes a further fall in the pressure sensed by these cells and hence amplifies the fall in wall pressure generated by a fall in blood pressure- stimulate renin release
-direct stimulation of renin release from granule cells
-afferent arteriole vasoconstriction drops glomerular hydrostatic pressure to the glomerulus and hence lowers GFR, retain water increase volume

Question 34

Sympathetic effector transmission in the afferent arteriole

Answer 34

The key recognised sympathetic transmitter is noradrenaline
On the vascular smooth muscle cells, vasoconstriction is caused by an action of alpha1-adrenoceptors. These are Gq coupled
On the granule cells (for the regulation of renin release) the main receptors are beta1-adrenoceptors as elsewhere these are Gs coupled

Question 35

A third stimulus to renin release

Answer 35

With a fall in the blood volume the venous pressure falls, as the venous system is the main source of capacitance in the circulation. I.e the site where most blood volume is found
This fall in venous pressure causes a fall in the pressure in the vasa recta and hence an increase in the uptake of fluid from the renal interstitial space
this means a greater loss of fluid from filtrate particularly in the descending limb of the loop of Henle
This decrease Na+ delivery to the distal tubule which acts as further stimulus to renin release

Question 36

ADH release following haemorrhage

Answer 36

Decreased cardiac filling activation of the baroreceptor reflex, and the central actions of ATII all cause an increase in the release of ADH following haemorrhage
This release leads to an increase in water reabsorption and hence the maintenance of circulating volume
This effect however will lower Osmolality because this mechanism does not retain Na+. So the acute response to haemorrhage will involved hyponatraemia
This means volume regulation disturbs osmoregulation
The body however accepts decreased osmolality in order to maintain low volume

Question 37

What affects ADH release

Answer 37

ADH release will be increased by:
-increased osmolality
-stress
-decreased volume
-nicotine
ADH release will be decreased by:
-decreased osmolality
-increased volume
-alcohol
This mechanism by which excessive alcohol consumption suppresses ADH release leading to diuresis

Question 38

Atrial natriuretic peptide ANP

Answer 38

28 amino acid peptide with a 17 amino acid ring released from atria
Increased venous return, leads to increased atrial filling increased ANP release
ANP travels to the kidney acts on ANPa,b receptors (NPR1-2) activating the intrinsic guanylyl cyclase activity increasing cGMP
Theres a similar peptide produced in kidney called urodilatin
Some sites and mechanisms of the action of ANP downstream of cGMP:
-causes a receptor mediated increase in cGMP which dilates the afferent glomerular arteriole increasing hydrostatic pressure in glomerulus, increasing GFR (which increase Na+ delivery to the kidney)
-decrease Na+/Cl- cotransporter activity in distal tubule
-decreases ENaC and Na+/K+ ATPase activity in the cortical collecting duct
The net effect is an increase in renal Na+ excretion in the urine hence its a natriuretic

Question 39

Prostaglandins

Answer 39

Both PGE2 and PGI2 (prostacyclin) are produced tonically
They both increase Na+ excretion (i.e are natriuretic)
If this tonic system is inhibited this will lead to a fall in prostaglandins and hence Na+ retention
In people with normal renal function this effect of NSAIDs may be insignificant but it becomes more important in renal failure

Question 40

Dopamine

Answer 40

Synthesised in the kidneys mainly by epithelial cells in the proximal tubule in part from sympathetic nerve terminals
Dopamine tonically acts via both D1 receptors to increase cAMP and decrease the activity of the Na+/H+ exchanger in the proximal tubule (this exchanger is very important for Na retention in this segment)
Leads to increased Na+ excretion (again natriuretic)