Renal Medicine Flashcards
How is risk of kidney failure reduced?
Early detection via screening for CKD risk factors
Why is mean life expectancy of patients on RRT measured after 90 days?
In the first 3 months people withdraw from the registry, some die or leave for other reasons.
Glomerular filtration rate
Volume of fluid filtered from the glomerular capillaries in a specified period of time
GFR for young male and female
Young male = 130 ml · min-1·1.73 m-2
Young female = 120 ml · min-1·1.73 m-2
When does GFR decline?
Stable until age 40 then declines at 1 ml · min-1·1.73 m-2
‘At age 80 mean GFR is approx half that of a young adult’
What is the gold standard to measure GFR?
Inulin
Isohexol
Gold standard is to use a compound that is not reabsorbed or secreted
What measures GFR in kidney donors for transplantation?
Isotropic GFRs
In clinical practice how is GFR measured and why?
Creatinine
- From muscle cells
- Closest to an ideal endogenous
- Steady production means steady state in plasma dependent upon excretion
- Small changes at good function = large changes in GFR
Why isn’t urea used to measure GFR?
Less reliable
Less vulnerable to change
Most basic renal investigation
URINE DIPSTICK
looks at- protein, blood, glucose, leucocytes, nitrites and pH
Non visible haematuria
From anywhere in renal tract
Not normal thing to have
UTI to bladder carcinoma to glomerulonephritis
Presence in our setting suggestive of an ‘active sediment’ i.e. glomerular or tubular origin
Microscopy
-Casts -> Red cell casts
Albuminuria
Signal of damage
Albumin is the most common protein present in the urine in health and in disease
Albumin is the most prominent plasma protein and we filter 1% in normal kidney health
Reabsorbed by proximal tubular epithelium
If excess filtration by glomerulus or decrease in reabsorption you can develop albuminuria
Not all proteinuria is albumin
Albuminuria and urine dipstick
Urine dip is semi-quantitative
- Specific for the identification of albuminuria
- Other proteins may not be detected by this method
Besides GFR what is cardiovascular risk associated with?
ACR level
Treatment targets for CKD?
Cause of CKD
Blood pressure targets
Proteinuria
What stimulates RAAS ?
Renin released from granular cells of the renal juxtaglomerular apparatus (JGA) in response to one of three factors:
- Reduced sodium delivery to the distal convoluted tubule detected by macula densa cells.
- Reduced perfusion pressure in the kidney detected by baroreceptors in the afferent arteriole.
- Sympathetic stimulation of the JGA via β1 adrenoreceptors.
RAAS summary
Renin -> angiotensinogen = angiotensin 1
angiotensin 1 + ACE enzyme -> angiotensin 2
angiotensin 2 causes:
- increase aldosterone secretion -> more Na absorption, K excretion and water retention
- vasoconstriction and increased BP
- vasopressin secreted from pituitary -> water absorption
- increased sympathetic activity
Kidney disease is one of the most common complications of…
Type 2 diabetes
Key risk marker of CKD
Albuminuria is the key risk marker
–> ACE-inhibition (or ARB) is essential
Patients with A1 proteinuria = ≤140/90
A2 or A3 = ≤130/80
Why would increased reabsorption of glucose via SGL2 lead to high intraglomerular pressure and hyperfiltration?
Long term consequences of hyperglycaemia on kidney
What is the key point to intervene
Incipient- can be reversed
At overt- more proteinuria, more difficult
What is recommended in the multifactorial intervention strategy for DKD (diabetic kidney disease)?
What is the future of treatment for DKD?
SGLT2 inhibition
CASE: CKD patient
62 year old Caucasian female
PMH
- Hypertension in both her pregnancies (24 and 26 year old children)
- -> No proteinuria during
- Hip replacement
- Ex smoker
- AKI post hysterectomy 4 years ago
Hypertension – 165/88
GFR 36 mL/min (GFR was 38 mL/min 12 months ago)
Urine ACR 60 mg/mmol, no haematuria
Drugs – amlodipine 5mg, allopurinol, citalopram
Treatment?
CKD G3bA3 presumed secondary to hypertension
High risk of renal progression- 11% 5 year of RRT
CVS risk is relatively high for the long term and current BP is putting her at higher risk for renal progression
Simply decision – BLOCK RAAS
Target BP <130/80 for kidneys and a systolic nearer 120 ideally for CVS risk
CASE: CKD patient
75 year old male born in India with
- Type 2 diabetes (retinopathy)
- AKI Nov 12
- CKD stage G3bA3 – GFR 36 mL/min
HbA1C 58 mmol/mol
Urine ACR 230 mg/mmol
BP 148/74
DH – Metformin 1g BD, Amlodipine 10mg, atenolol 100mg, Indapamide 2.5mg, ramipril 10mg, atorvastatin 10mg
Treatment?
5 year risk of RRT = 18.5%
I want his BP at least below 130/80
Put him on an SGLT2-I
- Currently canagliflozin has an extended license that covers him
- Dapagliflozin soon…
Diagnosing AKI
Delta is key (change in creatinine)
Raised serum creatinine +/- reduced urine output
Serum Creatinine produced at constant rate from muscles and filtered in glomeruli
But:
Dependent on muscle mass / age / race / gender
Increasing tubular secretion when renal function poor
Inaccurate reflection of GFR
Drugs e.g. trimethoprim interfere
KDIGO staging system for AKI
CASE:
74 year old woman presents with 10 days of urinary symptoms (dysuria, smelly urine)
PMH of hypertension on enalapril
Creat on admission 550, K 5.5
Exam
- Clinically dry
- BP 80/60, P110
- Temp 39.2
Does she have AKI and which stage?
So what is her mortality risk?
Yes as her creatinine was 105 3 days prior to admission
Stage 3 AKI
36% in hospital mortality
Phases of AKI
At what point should RRT be started?
RRT should be initiated once AKI is established and unavoidable
but before overt complications have developed
Ideal is NOT to delay initiation until the onset of life threatening complications
Subjective interpretation of symptoms
- Some patients accommodate to them
- Medications can mimic
Quantitative measurements
- Estimation of GFR
- -> MDRD and Cockcroft-Gault overestimate GFR in CKD 4 & 5
Current practice is to initiate on clinical factors rather than GFR alone
- European best practice guidelines suggest initiate before GFR is less than 6 and consider to do so at 8-10
Pros and cons of RRT
PROS
Avoids:
metabolic abnormalities
and problems of volume overload
CONS Exposes patient to potential of: Venous thrombosis Bacteraemia Haemorrhage from anticoagulants Plus some will recover without ever developing an absolute indication
Indications for acute dialysis
Hyperkalaemia refractory to medical therapy
- K+ > 6.5 with ECG changes
Severe Acidosis pH < 7.25, HCO3 <15
Fluid overload
- despite high-dose furosemide appropriate
Symptomatic uraemia: urea > 35
- Pericarditis, encephalopathy
Hyperkalaemia in ECG
Early changes of hyperkalemia include tall, peaked T waves with a narrow base, best seen in precordial leads ; shortened QT interval; and ST-segment depression.
New hyperkalaemia drugs
Iatrogenic hypoglycaemia:
Multifactorial with a low pre-treatment blood glucose being a consistent risk factor
Novel potassium binders:
Patiromer and sodium zirconium cyclosilicate
Approved by NICE for the treatment of life-threatening hyperkalaemia
Efficacy in the acute setting has not yet been reported.
New hyperkalaemia drugs
Iatrogenic hypoglycaemia:
Multifactorial with a low pre-treatment blood glucose being a consistent risk factor
Novel potassium binders:
Patiromer and sodium zirconium cyclosilicate
Approved by NICE for the treatment of life-threatening hyperkalaemia
Efficacy in the acute setting has not yet been reported.
Approach in mild, moderate and svere hyperkalaemia
Protect the heart
10 ml 10% calcium gluconate
Use large IV access and give over 5 min
Repeat ECG
Consider further dose after 5 min if ECG changes
If pre treatment blood glucose <7 mmol/L give 10% glucose at 50ml/hr for 5 hours to avoid HYPOGLYCAEMIA
Salbutamol use for hyperkalaemia
Recommend 10-20mg nebulised salbutamol if K >6.5 and consider if >6
Combination of salbutamol with insulin-glucose is more effective than either treatment alone
Caution in tachycardia or ischaemic heart disease
Not for use as a monotherapy
40% have a K decline of <0.5 mmol/L
Potassium binders
Recommend that Sodium Zirconium Cyclosilicate is used as an option in the emergency management of acute life-threatening hyperkalaemia (serum K+ ≥ 6.5 mmol/l).
Sodium Zirconium cyclosilicate 10g TDS for 72 hours
- non-absorbed potassium binder that exchanges H+ and Na+ for K+ and ammonium ions throughout the entire gastrointestinal tract
Sodium zirconium
SZC provides a potential option for treating severe acute hyperkalaemia
Rapid onset of action within 1 hour.
The median time to normalisation of serum K+ is 2.2 hours and SZC lowers serum K+ by 1.1 mmol/l within 48 hours
Undergoing evaluation as an adjunct to insulin-glucose in treatment of acute hyperkalaemia for normalisation of serum K+ in phase 2 trials
The 4 M’s
Monitor Patient
Observations and EWS, regular blood tests, fluid charts, urine volume, daily weights
Maintain Circulation
Hydration, resuscitation, oxygenation
Minimise kidney insults
Nephrotoxic medications, surgery or high risk interventions, iodinated contrast and prophylaxis, hospital acquired infection
Manage the acute illness
Sepsis , heart failure, liver failure
Obstructive uropathy
Flow of urine blocked:
Prostatic obstruction causes 25% of AKI
Single remaining kidneys at high risk
Can still produce significant amounts of urine
Delay in correction (catheter or nephrostomy) compromises renal function permanently
NICE = when nephrostomy indicated it should be done as soon as possible and within 12 hours of diagnosis
Obstructive uropathy
Flow of urine blocked:
Prostatic obstruction causes 25% of AKI
Single remaining kidneys at high risk
Can still produce significant amounts of urine
Delay in correction (catheter or nephrostomy) compromises renal function permanently
NICE = when nephrostomy indicated it should be done as soon as possible and within 12 hours of diagnosis
Complications of declining GFR and ESRF
Haematological Bone CVS Other - Immunological - Malnutrition
How does GFR dropping lead to bone mineral disease?
Reduced GFR -> retention of phosphate and reduction of vit D synthesis -> reduced calcium -> directly increases PTH (parathyroid hormone) -> tries to make more vit D but cannot so vicious cycle
-? long term hyperparathyroidism and bone disease
Phosphate toxicity
Mechanisms of renal anaemia
Anemia is a common complication of chronic kidney disease. Although mechanisms involved in the pathogenesis of renal anemia include chronic inflammation, iron deficiency, and shortened half-life of erythrocytes, the primary cause is deficiency of erythropoietin (EPO).
Renal anaemia and iron status
Iron metabolism involves storage and transfer to the bone marrow for erythropoiesis
Serum ferritin = acute phase reactant
- Low = absolute deficiency
TSAT (serum Fe/TIBC *100%)
- Some acute phase reactivity
- Diurnal variations
% hypochromic red cells
- Sensitive and early marker
Hepcidin
- Acute phase protein from the liver
- Blocks iron absorption from GI tract by controlling surface expression of FPN1
What is ERSF (end stage renai failure)?
Renal failure that requires renal replacement therapy
Kidney
- Excretes toxins
- Sodium and water balance
- Acid base balance
- Homeostasis
- Endocrine
- - 1α vitamin D hydroxylation, erythropoietin - Metabolic
Objective criteria to start RRT
Uncontrollable hyperkalaemia
Uncontrollable fluid overload
Uncontrollable acidosis
Uraemic pericarditis
Uraemic encephalopathy or neuropathy
Symptomatic uraemia
–> Nausea, vomiting, anorexia, pruritus, hiccoughs, akathisia, malaise, pleuritic chest pain
When is dialysis inappropriate?
Unacceptable impact on quality of life
Patient choice
Imminent death
Dialysis may or may not increase lifespan
Co-morbidities e.g. cardiovascular
Maximum conservative care
Still actively managed and need regular review
Avoid:
- Inappropriate drugs and doses
Optimise
- Haemoglobin
- Salt and water balance
- Acidosis
- Control symptoms of uraemia
Supportive care
- Care package
- Palliative care teams
3 aspects of RRT
Haemodialysis
Peritoneal dialysis
Transplant
Haemodialysis
Blood passes down one side of a highly permeable membrane
Water and solutes pass across the membrane
Solutes up to 20,000 daltons
Drugs & electrolytes
Infuse replacement solution with physiologic concentrations of electrolytes
Peritoneal dialysis
peritoneal dialysis involves pumping dialysis fluid into the space inside your abdomen (tummy) to draw out waste products from the blood passing through vessels lining the inside of the abdomen
Peritoneum used as the membrane
- Solute and water exchange between peritoneal capillary blood and dialysate fluid
- Membrane= vascular wall, interstitium, mesothelium and adjacent fluid films
Small molecules transfer by diffusion
Fluid movement determined by osmosis
- Dialysate dextrose concentration
- Solvent drag for middle sized molecules
Modes of peritoneal dialysis
- Intermittent PD (IPD)
Original form
For 24h twice a week using rapid 1-2hr exchanges - Continuous ambulatory PD (CAPD)
3-5 exchanges over 24hr
Introduced in 1980s when worked out that 4x2 litre exchanges over 24hr with a 2 litre UF would maintain the urea around 20 - Automated (APD)
Automatic cycling machine to perform rapid exchanges overnight
Peritoneal vs Haemodialysis
PD
Mechanical
- Catheter insertion
- Leaks
Infection
- Peritonitis
- Bacterial
- Fungal
- Mycobacterial
- Tunnel
- Exit site
Sclerosing peritonitis
Chemical peritonitis
Protein leak
HD Vascular access - Mechanical complications - At insertion/formation - Longterm - Infection
Extracorporeal circulation
- Hypotension
- Prothrombotic
- bioincompatibility
Deceased donors
Donation after brain death (DBD)
Donation after circulatory death (DCD)
What determines who gets the kidney?
National computer program based on: Blood group Tissue match Waiting time Age of the donor and potential recipient Location of patient relative to the kidney
Long term complications after kidney transplant
Immunosuppression side effects eg cancer + infection
CVD- MI, HBP, diabetes, high cholesterol
Chronic rejection and kidney loss
Recurrent disease
Key features of nephrotic syndrome
Albumin below 30g/L
Proteinuria >3g/24 hrs
+/- oedema (usually)
Podocytes and nephrotic syndrome
Nephrotic syndrome is a disorder of the glomerular filtration barrier, and central to the filtration mechanism of the glomerular filtration barrier is the podocyte
When podocytes get injured they don’t repair
Complications of nephrotic syndrome
Thromboembolism
- reduced level of anticoagulants antithrombin 3, plasminogen, protein C and S bc of urinary losses
- increased platelet activation
Infection - hypogammaglobulinaemia
Hyperlipidaemia
Nutrition
AKI
Minimal change disease
Pattern of injury rather than a specific disease
Mechanisms unknown
- Pertubation of T cell biology with the secretion of permeability factors into the circulation
- Modern focus on dysregulation of podocyte CD89 (B7.1)-lymphocyte CTLA-4 axis
Treating minimal change
Steroids are first line
Can be secondary eg to Hodgkins, non Hodgkins, etc
Focal Segmental Glomerulosclerosis
It’s a lesion not a specific disease
Lots of pathogenetic and aetiologic heterogeneity
Primary and viral or drugs – diffuse and generalised foot process effacement
Secondary = <50% of glomerular surface area
Familial or Virus asociated
Medication: Heroin-nephropathy Interferon-a Lithium Pamidronate/alendronate Anabolic steroids
Pathophysiology of Focal Segmental Glomerulosclerosis (FSGS)
Adaptive structural-functional responses likely mediated by glomerular hypertrophy or hyperfiltration
- > Reduced kidney mass
- > initially normal kidney mass
Nonspecific pattern of FSGS caused by kidney scarring in glomerular disease
- > Focal proliferative glomerulonephritis (IgAN, LN, pauci-immune focal necrotizing and crescentic GN)
- > Hereditary nephritis (Alport syndrome)
- > Membranous glomerulopathy
Primary vs secondary FSGS
Secondary
- Viral
- Cancer
- Lupus
- Drugs
Primary caused by autoimmune reaction of a circulating antibody directed to a specific antigen on the podocyte surface
Anti-GBM disease
Target antigen = alpha-3 chain of type IV collagen
Clinical syndrome encompasses a spectrum ranging from mild or no renal involvement to rapidly progressive glomerulonephritis
Combination of glomerulonephritis and pulmonary hemorrhage is commonly referred to as Goodpasture syndrome
Triggers for anti-GBM disease
Respiratory infections (eg, influenza)
Inhaled toxins (eg, hydrocarbons, gasoline vapors, hypercarbic oxygen, tobacco, hairspray) may trigger pulmonary involvement.
Individuals with Alport syndrome post transplantation
Strong positive association with the HLA-DR15 haplotype, particularly the DRB1*1501 allele
Main functions of kidney
Control volume & composition of body fluids
To get rid of waste material from body
Acid-Base balance
As an endocrine organ – EPO (erythropoietin), Renin & Vit D
Why is the nephron circulation unusual?
Within the BC is a tuft of blood vessels known as the glomerulus, which has an unusual arrangement in that it enters as an artery (afferent arteriole) and also leaves as an artery (efferent arteriole), before forming the peritubular capillaries around/adjacent to the tubule. Blood and the tubule again meet up along the peritubular capillairies - very important point.
Hence nephron is unusual in that it has 2 sets of capillaries i.e glomerular & peritubular.
Bowman’s capsule and glomerulus function
Filters large amounts of plasma
Structure of glomerulus
The glomerulus consists of a clump of capillaries & Bowman’s capsule
Glomerular filtrate
same composition as plasma except
No cells, v. little protein
Why is urine composition not the same as plasma?
Selective modification of filtrate as it passes through tubule
GF formed at 120ml/min
Urine flow ~1ml/min
Urine formation begins when large amounts of fluid that is virtually free of protein is filtered from the glomerular capillaries into the Bowman’s capsule. In essence the GF is an ultrafiltrate of plasma.
Modification of glomerular filtrate
The filtration process at the glomerulus is relatively non-selective. Modification occurs is along the tubule by the process of reabsorption and secretion of water and various solutes.
Blood enters afferent arteriole- filtered non selectively at 120ml/min → leaves glomerulus via efferent arteriole → goes to peritubular capillaries → reabsorbed at 119ml/min → also substances secreted in here
Reabsorption in kidney
From tubular lumen INTO peritubular plasma
For a substance to be reabsorbed it must first cross the luminal membrane – diffuse through the cytosol – across the basolateral membrane and into the blood (transcellular transport). Vice versa for secretion.
Secretion in kidney
From peritubular plasma INTO tubular lumen
What happens to wanted/unwanted substances?
Clearing unwanted substances by excretion into urine & Returning wanted substances by reabsorption into blood
Co transport vs Counter transport
Co-transport of Na and glucose i.e. because Na moves into cell down it’s concentration gradient i.e. high outside and low inside – creates lots of energy. This can pull other substances along with it - called cotransport. One form of secondary active transport. For Na to pull another substance with it needs a coupling mechanism – carrier protein. E.g. with glucose. (symport or antiport)
Counter-transport is when substance to be transported along with Na binds to carrier protein from inside of cell and comes out.
Segments of renal tubule
The tubule is divided into 7 segments which can be distinguished by differences in structure and function.
Throughout it’s length the nephron is comprised of a single layer of epithelial cells resting on a basement membrane.
2 types of nephron
Cortical nephron (85%) SHORT LoH
Juxta-medullary nephron (15%) LONG LoH
2 types of nephron
Cortical nephron (85%) SHORT LoH
Juxta-medullary nephron (15%) LONG LoH
Differences between cortical and juxtamedullary nephrons
Length of the loops of Henle.
→ Cortical nephrons have short-reach loops that just penetrate the boundary between the inner and outer zones of the medulla. These loops do not extend into the medulla.
→ Juxtamedullary nephrons have long-reach loops that penetrate deep into the medulla. (better at concentrating urine) In humans about 15 per cent of nephrons are juxtamedullary and 85 per cent are cortical.
Vasculature
→ Cortical nephrons – entire tubular system is surrounded by and extensive network of capillaries
→ Juxtamedullary nephrons – long efferent arterioles extend from glomeruli to outer medulla and divided into specialised capillaries (vasa recta) that extend downward into medulla and lie side by side with loops of Henle
Proximal convoluted tubule
High capacity for reabsorption – special cellular characteristics:
→ highly metabolic, numerous mitochondria for active transport
→ extensive brush border on luminal side large surface area for rapid exchange
Right after Bowman’s capsule
Function of proximal convoluted tubule
Major site of reabsorption
High [Na] in tubule but low inside cell hence → movement of Na down it’s conc gradient at luminal membrane also aided by greater intracellular negative potential (-70mv).
As Na diffuses down it’s electrochemical gradient, energy is released which drives another substance – the energy generated from Na moving into the cell is ultimately generated by the primary active transport of Na moving out of the cell at the basolateral membrane i.e. the Na-K-ATPase keeps the cytoplasmic [Na] lower than tubular [Na] and maintains the electrochemical gradient for passive Na transport across luminal membrane.
Located in the luminal and basolateral membranes are enzymatic and protein carriers, primary and secondary active transport systems, which together with its permeability characteristics, make the proximal tubule the major site of reabsorption of the glomerular filtrate.
About 65 % of the filtrate including all essential nutrients are reabsorbed by the proximal tubule.
Glomerular filtrate is protein free but some small proteins (<60kD) get through. These proteins are taken up by endocytosis → degraded by lysosomal enzymes amino acids and simple sugars reabsorbed into plasma
By the end of the early proximal tubule essentially all of the glucose and amino acids and much of the HCO3- have been reabsorbed. HCO3- is preferentially reabsorbed relative to Cl- and the concentration of Cl- rises in the tubular fluid. This establishes a Cl- concentration gradient from lumen to peritubular fluid and, as Cl- moves passively down its concentration gradient, the lumen acquires a positive electric charge relative to the peritubular fluid and Na+, as illustrated in the figure, moves passively along the gradient with Cl-. This passive component of Na+ reabsorption occurs primarily along the paracellular path.
Fanconi’s syndrome
All PCT re-absorptive mechanisms defective
Glucose, amino acids, Na+, K+ etc all found in urine
Many causes – inherited, acquired eg malignancy, exogenous factors eg medications like valproate
Structure of Loop of Henle
LoH consists of 3 functionally distinct segments :
Thin Descending
→thin epithelial cells, no brush border, few mitochondria & low metabolic activity
Thin Ascending
Thick Ascending
→ thick epithelial cells, extensive lateral intercellular folding, few microvilli, many mitochondria high metabolic activity
Function of loop of Henle
LoH critical role in concentrating/diluting urine
»adjusting rate of water secretion/absorption
Thin descending: Very permeable to water
20% of filtered water reabsorbed in descending LoH
Thin/Thick ascending: Virtually impermeable to water
LOH creates a zone within the medulla where the tissue fluid osmolality is high. Loop diuretics act here causing 20% of filtered Na to be excreted, by blocking Na-transport out of LoH
Medullary osmotic gradient
- LoH creates an osmolality gradient in medullary intersitium
- Collecting Duct traverses medulla: urine concentrated as water moves out by osmosis
A high solute concentration (high osmotic pressure) is generated and maintained in the medullary interstitium and the tubule fluid becomes hypotonic.
How is counter current multiplication done by the loop of Henle
LoH has 2 parallel limbs arranged so that tubular fluid flows into descending limb into medulla and out of medulla through the ascending limb ie. Flow of fluid is in opposite directions = countercurrent
The fluid that enters descending limb from proximal tubule has osmotic concentration approx. equal to that of plasma = 300mosm/kg.
The ascending limb is impermeable to water (as already said) but reabsorbs solutes partic NaCl. Hence as tubular fluid travels up ascending limb it becomes more dilute – whilst the solute is accumulating in the intersitial fluid around the loop raising it’s osmolality.
On the other hand the descending limb is freely permeable to water, thus the hyperosmotic ISF causes water to leave the descending limb. This leads to osmotic gradient between ascending and descending limb of 200mOsm/kg. This effect is multiplied by the entry of new fluid into the descending limb which pushes fluid from around the loop to the ascending limb.
Thus a continuous osmolality gradient is created the top of the loop (cortex) of about 300mOsm/kg to a peak of 1200mOsm/kg at the bottom of the loop (medulla). Though not all of this is due to salt accumulation as we’ll see later. Fluid that leaves the LoH is hypo-osmotic with ref to plasma (~100mOsm/kg)
Thin ascending limb permeable to Na & Cl, but Thick ascending limb actively pumps Na & Cl out of tubular fluid
Reabsorption in thick ascending LoH
The thick ascending LoH reabsorbs approx 25% of filtered Na and also can compensate partially for any failure by PCT to reabsorb Na.
Na enters cell via Na:K:2Cl cotransporter (or symporter), the driving force of which is large electrochemical difference favoring entry of Na into the cell. To exit, Na is transported actively via Na-K-ATPase, while K and Cl cross into the peritubular fluid passively.
The inhibition of the Na:K:2Cl transporter by loop diuretics, results in inhibition of net NaCl reabsorption and increased excretion of these ions along with water.
By disrupting the reabsorption of these ions, loop diuretics prevent the generation of the medullary osmotic gradient. Without such a concentrated medulla, water has less of an osmotic driving force to leave the collecting duct system, ultimately resulting in increased urine production.
Why is gradient not disrupted by movement of water?
Vasa recta also acts as a counter current exchange system
Vasa recta acting as a counter current exchange system
VR also acts as counter-current exchange system
VR freely permeable to solutes & H20
As it descends into medulla H20 diffuses out and salts diffuse in
Reverse occurs as it ascends
Blood flow in VR is low ~5% of renal blood flow » minimizes solute loss from interstitium & maintains medullary interstitial gradient
Alteration of blood flow in VR can change gradient
The vasa recta delivers O2 and nutrients to cells of the loop of Henle. The vasa recta, like other capillaries, is permeable to both H2O and salts and could disrupt the salt gradient established by the loop of Henle. To avoid this, the vasa recta acts as a counter-current multiplier system as well. As the vasa recta descends into the renal medulla, water diffuses out into the surrounding fluids, and salts diffuse in. When the vasa recta ascends, the reverse occurs.
As a result, the concentration of salts in the vasa recta is always about the same, and the salt gradient established by the loop of Henle remains in place. Water is removed by VR, so doesn’t dilute longitudinal osmotic gradient. The medullary blood flow in the VR is slow, which is sufficient to supply the metabolic needs of the tissue, but minimize solute loss from the medullary interstitium.
Reabsorbed Na+ in desc.VR carried to inner medulla equilibrating with ISF → ↑ regional osmolarity
Na+ in asc.VR returns to systemic circulation. Amount of solute in asc.VR is product of flow rate & concentration
e.g. If blood flow in VR increases then solutes are washed out of the medulla and its interstitial osmolality is decreased. If blood flow is decreased then the opposite happens.
Distal Convoluted Tubule& Connecting/Collecting Tubule
DCT:
1st part (macula densa) linked to juxtaglomerular complex
Provides feedback control of GFR & tubular fluid flow in the same nephron
2nd part very convoluted
Connecting Tubule:
Connects end of DCT to collecting duct – mainly in outer cortex
Overlap in functional characteristics with 2nd part of DCT
Function of DCT
Solute reabsorption continues, w/out H2O reabsorption
High Na+,K+-ATPase activity in basolateral membrane
Very low H2O permeability
Further dilution of tubular fluid
Anti-diuretic hormone (ADH) can exert actions
Role to play in acid-base balance via secretion of NH3
Collecting duct structure
Collecting ducts formed by joining of collecting tubules
cuboidal epithelia, very few mitochondria
2 types of cells:
→ Intercalated cells
- Involved in acidification of urine and acid-base balance
→ Principal cells
- Role to play in Na balance & ECF volume regulation
Final site for processing urine
Made very permeable to H2O by ADH*
Also permeable to urea*
*contribute to counter-current mechanism
The journey of ADH/vasopressin and the collecting duct
Your body makes ADH in the hypothalamus and stores the hormone in your pituitary gland. ADH then concentrates the urine by triggering the kidney tubules to reabsorb water back into your bloodstream rather than excreting water into your urine. The single most important effect of antidiuretic hormone is to conserve body water by reducing the loss of water in urine. The most important variable regulating antidiuretic hormone secretion is plasma osmolarity, or the concentration of solutes in blood.
Osmolarity is sensed in the hypothalamus by neurons known as an osmoreceptors, and those neurons, in turn, simulate secretion from the neurons that produce antidiuretic hormone. When plasma osmolarity increases above the threshold, the ever-alert osmoreceptors recognize this a the cue to stimulate the neurons that secrete antidiuretic hormone. Secretion of ADH is mediated primarily by hypothalamic osmoreceptors, which monitor changes in plasma osmolality. ADH secretion can also be regulated by volume receptors and arterial baroreceptors. (Covered by Prof G. Sagnella in regulation of ECF volume etc).
At a cellular level, Binding of AVP to V2-receptors stimulates the synthesis of aquaporin-2 water channel proteins and promotes cAMP-dependent trafficking of aquaporin 2 water channels to the luminal membrane of principal cells allowing back diffusion of water down its concentration gradient.
Vasopressin via V2 receptors also activates urea transporters in the distal nephron to facilitate urea reabsorption and urea recycling, which allows maximization of sodium reabsorption in the thick ascending limb, supporting the axial hyperosmotic gradient drawing water from the distal nephron (see next slide).
Collecting duct and urea
Urea filters freely through glomerulus and passes down the tubule. Unlike cortical collecting tubule, the medullary collecting duct is permeable to urea. As water is reabsorbed from the CD (say in the presence of ADH) the urea is concentrated so that it moves out of the CD and is absorbed into the surrounding capillaries and also into the intersitium of the medulla where it contributes to the osmotic gradient around the LoH.
In fact increasing levels of urea in kidney is a sign of pre-renal failure because reabsorption is enhanced. Monitored using blood urea nitrogen test (BUN). Can see that urea reabsorption would increase during dehydration.
ADH and diuresis/anti-diuresis
tubular fluid which enters the CD system is always hypo-osmotic and its concentration or further dilution as it traverses the CD depends on the water permeability of the duct, which is determined by the action of ADH. In the presence of ADH water permeability is increased. ADH acts by inducing synthesis and insertion of water channels (aquaporins) into the luminal membrane.
Water is reabsorbed along the osmotic gradient and the urine osmolarity approaches that of the medullary ISF at the tips of the long loops of Henle. Water reabsorption increases the CD urea concentration and ADH increases the duct permeability to urea and therefore its reabsorption is increased.
Other solutes, particularly Na+ and Cl-, continue to be reabsorbed in the CD and this further solute reabsorption serves to maintain the medullary hyper-osmolarity and thus facilitates the reabsorption of water in the presence of ADH. The final urine volume may be as low as 0.5 to 1.0 % of the filtered load and its concentration as high as 1400 mOsm/L.
What is GFR?
GFR (glomerular filtration rate) is how much filtrate is removed from blood each min and not how much blood passes through glomerulus per minute
How is glomerular filtrate formed?
By passive ultrafiltration of plasma across the glomerular membrane, as described by Starling’s principle of capillary fluid filtration.
A net pressure drop across the glomerular membrane drives the ultrafiltration process.
At what rate is glomerular fluid formed?
The glomerular filtration rate (GFR) is set by-
(i) Intrinsic control:
Autoregulation:
1. Bayliss Myogenic Response
- (vasoconstriction of afferent arterioles)
2. Tubulo-glomerular feedback (TGF) - (change in flow of plasma osmolality conveyed to distal proximal capillaries)
(ii) Extrinsic control:
renal sympathetic vasoconstrictor nerve activity
What drives GFR?
Net pressure drop
Favouring filtration:
- Glomerular capillary pressure (PGC) = 60mmHg
Opposing filtration:
- Hydrostatic pressure in Bowman’s space (PBS) = 15mmHg
- Osmotic force of plasma proteins (ΠGC) = 29mmHg
net effect → PGC – PBS – ΠGC = 16mmHg (net filtration pressure)
Filtration across the capillaries is determined by opposing forces.
How do blood pressure and oncotic pressure change going around tubule?
From afferent arterioles → tubules etc = blood pressure drops
vs oncotic pressure that increases as you go around tubule
So in glomerulus BP higher than oncotic pressure = favouring filtration
How does glomerular membrane sort solutes?
Glomerular membrane sieves out solutes from plasma by molecular size
eg urea + glucose are small molecules so are present in both GF and plasma - freely filtered
But big proteins like albumin not present in GF- too big
Epithelium of Bowman’s capsule is made up of…
Podocytes
Podocytes have pedicels or “little feet” that interlock like a zipper to form a filtration membrane with slits (filtration slits). Filtration slits are covered with a thin fibrous membrane (slit diaphragm). Acts as second layer of filter
Glomerulus structure
Glomerulus: Ball-like network of capillaries (glomerular capillaries). Supplied by an afferent arteriole. Drained by a smaller efferent arteriole (which leads to a second network of capillaries after the glomerulus).
Endothelial cells of the glomerular capillary wall have fenestrations or pores. Fenestrated wall acts as a filter, keeping blood cells and proteins in blood and allowing water and small solutes to filter out of blood (see it better in next slide)
Mesangial cells — support cells between the capillaries
Bowman capsule (glomerular capsule) . Surrounds glomerulus like a hollow cup. Inner (visceral) wall of capsule adheres to outer walls of glomerular capillaries. Cells are spider-like cells called podocytes (lit. “foot cells”)
Podocytes have pedicels or “little feet” that interlock like a zipper to form a filtration membrane with slits (filtration slits). Filtration slits are covered with a thin fibrous membrane (slit diaphragm). Acts as second layer of filter
Do them fenestrae have a membrane?
No. Because they’re WEAK
When you look at the EM of the glomerular barrier, you’ll see that there is no membrane in the fenestrae
ultra-high magnification shows that the filtration slits are subdivided in very narrow pores created by a cytoskeletal arrangement of proteins nephrin and podocin.
How does the glomerular membrane do its sieving action?
Three layers of increasing fineness. If the filtration slits breakdown then protein and RBCs get through
If proteins are leaking shit is going down. ie the filtration slits or BM is damaged
What determines GFR
GFR determined by combination of factors:
→ hydrostatic and oncotic pressures across capillary membranes
→ permeability of capillary filtration barrier & surface area available
Change in any of these factors will change GFR
Hence GFR important clinical indicator of functioning of nephrons (renal function)
Inulin method
Gold standard for measuring GFR but NOT used clinically
Inulin (not insulin)
An inert polysaccharide, MW ~5,000
Filters freely through the glomerular membrane
Not absorbed, secreted or metabolised
How is inulin used to measure GFR?
Set up IV infusion to infuse inulin into patient
→ wait till inulin reaches steady state
→ filtration of inulin through BC calculated by:
Rate of filtration = plasma conc of inulin [Pin] x GFR
→ inulin travels through then excreted into urine
→ urine collected and volume measured and concentration of inulin in urine:
Rate of entry into bladder = conc of inulin in urine [Uin] x urine flow rate (V)
V = flow rate/collection time
Bc inulin not absorbed or secreted can clearly see rate of filtration through glomerular membrane per min = rate of entry into bladder per min
Pin x GFR = Uin x V
use this to work out GFR MUST KNOW
Calculation for GFR
Pin x GFR = Uin x V*
Where:
GFR = glomerular filtration rate; ml.min-1
Pin = plasma inulin concentration; mg.ml-1
Uin = urine inulin concentration; mg.ml-1
V* = urine flow rate; ml.min-1
Renal clearance definition
Renal clearance of a substance is the volume of plasma that is completely cleared of the substance by the kidney per unit of time (expressed in ml/min)
Cs = Us x V / Ps
Comparison of clearance
Clearance of Inulin (GFR) ~ 125ml/min in adult male and 10% less in females
Substance with clearance = inulin (= GFR)
e.g. antibiotics (streptomycin/gentomycin))
Substance with clearance < inulin (< GFR)
Not filtered freely e.g. albumin
Reabsorbed from tubule e.g. glucose
Substance has clearance > inulin (>GFR)
Secreted into tubule e.g. PAH (para-aminohippuric acid)
Hence, comparison of clearance values to inulin (in effect GFR) gives you information about the renal handling of a substance and whether the kidneys are filtering properly
Cons of inulin method
Drawbacks inulin method:
- prolonged infusion
- repeated plasma samples
- difficult routine clinical use
Why do we use creatinine for GFR measurement in clinical practice?
Clinically use creatinine for GFR measurement
Advantages:
An intrinsic inert substance
Released at ~steady level in plasma from skeletal muscle
No infusion needed
Freely filtered
Not reabsorbed in the tubule
BUT
Disadvantages:
Some secreted into the tubule Ccr> GFR
~150 ml/min (rather than 125ml/min)
*Trimethoprim (antibiotic) – competitive inhibitor of creatinine secretion
How is creatinine measured in clinical practice?
Creatinine is released into plasma at fairly constant rate by skeletal muscle as long as muscle mass remains constant – no infusion needed
Blood samples are collected for measurement of plasma creatinine concentration and the patient is provided with an appropriate container and preservative and instructed to collect all urine excreted over the next 24 hours. The urine volume and creatinine concentration is measured and the clearance is calculated.
It is NOT reabsorbed but a small amount is secreted so really the amount excreted slightly exceeds the amount filtered, but because there’s usually slight error in estimation of plasma [creatinine] these 2 errors cancel each other out and creatinine clearance provides a reasonable estimate of GFR
(because most people use serum[cr] rather than plasma
Trimethoprim competitively inhibits renal tubular creatinine secretion and may cause an artificial increase in serum creatinine, particularly in patients with a pre-existing renal insufficiency - however, GFR is unchanged.
Trimethoprim and creatinine
Trimethoprim competes with Crn for same transporters that secrete Crn from tubular blood into urine» increase in serum levels of Crn
Why does creatinine need to be taken up from blood against a large concentration gradient?
The most part (up to 94%) of Cr is found in muscular tissues. Because muscle has virtually no Cr-synthesizing capacity, Cr has to be taken up from the blood against a large concentration gradient by a saturable, Na+- and Cl−-dependent Cr transporter that spans the plasma membrane.
The daily demand for Cr is met either by intestinal absorption of dietary Cr or by de novo Cr biosynthesis. The muscular Cr and PCr are nonenzymatically converted at an almost steady rate (∼2% of total Cr per day) to creatinine (Crn), which diffuses out of the cells and is excreted by the kidneys into the urine.
Who should avoid creatinine supplementation?
Subjects with impaired renal function and those at risk should avoid Cr supplementation. While in normal subjects Cr supplementation only slightly increases serum [Crn], a considerably more pronounced increase may be seen in patients with renal dysfunction that is potentially associated with an increase in Crn-derived uremic toxins
eGFR
eGFR is estimated Glomerular Filtration Rate:
Using blood tests, age, sex, and sometimes other information to estimate the GFR from the MDRD equation*
186 x (Creat/88.4)-1.154 x (Age)-0.203 x (0.742 if female) x (1.210 if black)
*MDRD equation improvement on original Cockcroft-Gault equation
Superseded by CKD-EPI Adults (NICE recommendations)
Levey AS, Stevens LA, et al. A New Equation to Estimate Glomerular Filtration Rate. Ann Intern Med. 2009; 150:604-612.
This isn’t as good as measuring it (i.e. 24h urine collection), but is much simpler as it requires just one blood test.
It is being used increasingly to spot kidney disease earlier than would be possible using just creatinine measurements
CKD-EPI Adults
GFR = 141 × min (Scr/κ, 1)α × max(Scr/κ, 1)-1.209 × 0.993Age × 1.018 [if female] × 1.159 [if black]
Where:
Scr = serum creatinine in mg/dL or µmol/L
κ = 0.7 for females and 0.9 for males
α = -0.329 for females and -0.411 for males
Min indicates the minimum of Scr/κ or 1
Max indicates the maximum of Scr/κ or 1
The equation does not require weight, as the results are normalised to 1.73m2 body surface area (i.e. accepted average surface area)
NKDEP recommends reporting GFR values ≥60ml/min/1.73m2 as such and not an exact number (with reference to CKD stages)
CKD-EPI is more accurate than MDRD, less biased at GFR ≥60ml/min/1.73m2 & performs better in people >75years
International consensus developing to move away from inclusion of race in the calculation. Statement paper due out this year
CKD stages
The stages of CKD (Chronic Kidney Disease) are mainly based on measured or estimated GFR.
There are five stages but kidney function is normal in Stage 1, and minimally reduced in Stage 2.
Why is significant error possible with eGFR?
It is only an estimate.
A significant error is possible.
eGFR is most likely to be inaccurate in people at extremes of body type, for example malnourished, amputees, etc.
It is not valid in pregnant women or in children
Race: Some racial groups may not fit the MDRD equation well. It was originally validated for US white and black patients. For Afro Caribbean black patients, eGFR was 21% higher for any given creatinine in the MDRD study.
Glucose handling by kidney- what happens when this is measured experimentally?
Glucose and inulin infused together and plasma [gluc] & [inulin] held at steady level, then urine collected.
From our mathematical formulas able to calculate GFR (using inulin) and then glucose filtration rate (GFRxplasma[gluc])
Rate of glucose filtration (red line) increases hand in hand with plasma [glucose] and until a plasma [gluc] of ~10-15mM is reached (renal threshold-orange line) there is NO glucose in the urine, so the PCT has reabsorbed the entire load (carrier mechanism with Na).
Up until this point the re-absorption line has shadowed the filtration line (shown in green). However above this threshold glucose starts to appear in the urine (blue line) and then the reabsorption line falls short of the filtration line. Above a plasma [gluc] of 22mM the reabsorption line plateaus. This is known as the Transport Maximum (Tm) for glucose i.e. the rate at which the carrier mechanism is fully saturated.
This explains why normal urine has no glucose and also why in many diabetic patients whose plasma [gluc] lie above the renal threshold have glycosuria (i.e. pass glucose in their urine).
NB: Appearance of glucose in urine (at threshold) occurs before the transport maximum is reached, because different nephrons have different Tm so some will start excreting glucose in urine before others.
Glucose transport in tubule
Glucose carried uphill against concentration gradient via specific Na-glucose co-transporter (SGLT2) at luminal end
Glucose exists at basolateral end by facilitated diffusion driven by high [glucose] in cell
long explanation:
Combination of active & passive transport at different sides i.e. once side have active transport and on the other passive transport either by simple diffusion or facilitated diffusion.
High [Na] in tubule to low inside cell hence have movement of Na down it’s conc gradient at luminal membrane also aided by greater intracellular negative potential (-70mv).
As Na diffuses down it’s electrochemical gradient, energy is released which drives another substance – in this instance glucose uphill against it’s concentration gradient across the luminal membrane into the cells (Na-glucose symport via a specific carrier protein) – the energy generated from Na moving into the cell is ultimately generated by the primary active transport of Na moving out of the cell at the basolateral membrane i.e. the Na-K-ATPase keeps the cytoplasmic [Na] lower than tubular [Na] and maintains the electrochemical gradient for passive Na transport across luminal membrane.
Glucose just exits out at basolateral membrane by faciliated diffusion driven by the high [glucose] in the cell. Also known as SGLT2 (sodium-glucose cotransporter). Genetic defect in this protein – familial renal glycosuria just like similar defect in intestinal protein SGLT1 – glucose-galactose malabsorption. SGLT2 inhibitors now used to treat diabetics.
What is responsible for the majority of glucose reabsorption?
SGLT2 responsible for 90% of glucose reabsorption in 1st part of PCT and SGLT1 for remaining in later part.
What protein should be inhibited in drugs aiming to reduce glucose levels in blood eg to treat diabetes?
SGLT2
-log10[H+] is the calculation for
pH
Volatile acid
oxidative metabolism produces CO2
CO2 + H2O ↔ H2CO3 (volatile acid) ↔ H+ + HCO3-
CO2 produced as a result of carb metabolism does not usually result in increase in H+ in plasma because it is excreted from body via lungs – hence the H2CO3 produced is known as a volatile acid. This is the major source of acid in the body.
Metabolism of dietary proteins generates what?
Non-volatile acids:
S-containing amino acids (cysteine, methionine) H2SO4
Lysine, arginine and histidine HCl
Sulphur containing aa produce sulphuric acid, which is non-volatile – these non-volatile acids need to be removed otherwise will get a net gain of H+
What 3 mechanisms control pH
Disturbance in [H+]/pH compensated for:
- ICF & ECF buffering systems
- Respiratory system
- Kidney
ICF and ECF buffering systems in pH control
The first line of defense consists of the intracellular and extracellular buffer systems. All buffer systems participate in accord with their pK and their quantity. Of particular importance is the CO2-HCO3- buffer system which is the major extracellular buffer system. The importance of this system is physiological in that the two components of the system, CO2 and HCO3- can be regulated independently.
Respiratory system in pH contorl
The second mechanism is the respiratory system which regulates the plasma Pco2 by controlling the excretion or retention of metabolically produced CO2 (which is the acid component of the CO2 - HCO3- buffer system) in response to changes in pH.
Kidney in pH control
The third mechanism is the kidney which plays a dual role; it regulates excretion or retention of HCO3- (the basic component of the CO2-HCO3- buffer system) and also regulates the regeneration of HCO3-
What is a buffer?
A solution that minimises changes in [H+] i.e. pH
What is pK?
The pK is the equilibrium constant of a reaction and a buffer solution most strongly resist changes in pH near the point where conc of base and acid are equal and therefore pH=pK so equilibrium has “room to move” on either side.
Range over which buffer is effective is about 1pH on either side of the pK.
Advantages of the buffer system?
At 6.1 the pKA of the CO2-HCO3- buffer is not close to the desired plasma pH of 7.4..
The unique physiological advantage of this buffer system is that the acid form (CO2) and salt form (HCO3-) can be regulated independently. Excretion or retention of CO2 is controlled by the lung and reabsorption and regeneration of HCO3- is controlled by the kidney.
A second advantage is that there is a readily available supply of CO2 from cellular metabolism. It is important to note that, while buffering is the first, and immediate, defense against changes in H+ concentration, the buffers are present in limited quantities. As buffer capacity is used, less is available to control pH. It is, therefore, necessary to have mechanisms to eliminate the excess H+ or base which caused the change in pH and to restore the buffer capacity to normal and this is the role of the renal and respiratory systems.
When does acid = base?
pK is the equilibrium constant of the reaction i.e. Numerically pH = pK then the concentration of acid=base of that buffer.
The linear portion of titration curves is most effective chemical buffering, extending 1pH unit either side of pK – so for bicarbonate system it is 5.1-7.1 and for phosphate it is 5.8-7.8
Bicarbonate buffer system
Most important is the [HCO3–]:[CO2] ratio
Plasma [CO2] proportional to partial pressure of CO2 (pCO2) in plasma
Constant to convert pCO2 (mmHg) to [CO2] mmol/L is 0.03, hence
Key points of buffers
pK=equilibrium constant of reaction
buffer solutions resist change in pH when [base]=[acid]
Buffer is most effective 1pH on either side of pK
At 6.1 the pK of CO2-HCO3 buffer not close to desired plasma pH of 7.4
Alveolar ventilation controls PCO2*
Kidneys control [HCO3–]ECF*
Buffers are in limited supply excess acid/base must be eliminated role of renal & respiratory systems
*independent regulation
How do kidneys control acid levels in blood
Kidneys control acid-base levels in blood by excreting H+ into tubular fluid, which is buffered to ultimately secrete an acidic/basic urine
Primary renal mechanisms involved in this are:
- Secretion of [H+] into tubular fluid
- “Re-absorption” and secretion of HCO3-
- Buffer systems within tubule that react with secreted [H+]
NH3: NH4+, HPO42-:H2PO4-, HCO3-:H2CO3
Renal control of [H+] and [HCO3-]
Kidney tubule cells:
CO2 & water → carbonic acid (bc of enzyme carbonic anhydrase)
The carbonic acid then dissociates into H+ and HCO3-, and Na+ moving down it’s conc gradient from tubular fluid into cell provides energy for secondary active secretion of H+ into tubule lumen. ATP provided energy for primary active secretion of H+ from cell into lumen.
With each H+ that is secreted, one HCO3- enters blood accompanied by Na+ which has been swapped for H+ - buffering in ECF
New HCO3- is generated only when H+ derived from intracellular H2CO3 is secreted into the tubule and buffered in the tubular fluid by a non-bicarbonate buffer.
Inhibitors of this enzyme, such as acetazolamide, will inhibit the formation of H+ for the acidification of the tubular fluid. When this occurs, reabsorption of HCO3- is inhibited leading to acidosis, loss of Na+ which is obligated to the unreabsorbed HCO3- and diuresis.
Where does reabsorption of bicarbonate occur?
Proximal convoluted tubule:
- 85-90% of filtered HCO3- “reabsorbed”
- Regeneration of bicarbonate
- Great capacity to secrete [H+]
How does reabsorption of bicarbonate occur?
occurs mostly in the proximal convoluted tubule
Filtered bicarbonate combines with secreted hydrogen ions forming carbonic acid. Carbonic acid then dissociates to form CO2 and water. This reaction is catalysed by carbonic anhydrase, which is present in the luminal brush border of the proximal tubular cells only. This CO2 readily crosses into the tubular cell down a concentration gradient.
Inside the cell the CO2 recombines with water, again under the influence of carbonic anhydrase, to form carbonic acid. The carbonic acid further dissociates to bicarbonate and hydrogen ions. The bicarbonate passes back into the blood stream whilst the H+ passes back into the tubular fluid in exchange for sodium. In this way, virtually all the filtered bicarbonate is reabsorbed in the healthy individual.
The H+ appears in the urine as water and the urine pH is not changed dramatically. The net result is the reabsorption of HCO3, a slight fall in tubular fluid pH (from 7.4 to 7.0), and no change in the PCO2 of tubular fluid.
In the proximal tubule H+ secretion occurs mainly via the Na/H counter-transporter
The buffer pair HCO3:H2CO3 is the predominant urinary buffer despite its low pK (6.1) because its concentration in the filtrate is much higher than the other buffer pairs and because the titration of HCO3 does not result in the accumulation of acid in the tubular fluid (The acid formed, H2CO3, leaves the tubule in the form of CO2).
This allows proton secretion to continue at a high rate, but it can reduce the tubular fluid pH only to a slight extent, and, therefore, it titrates relatively small amounts of the other buffers.
In what part of nephron is H+-ATPase pump more important?
Intercalated cells of late Distal tubule and Collecting duct
In distal part of nephron [HCO3-] is low and H+ react with other buffers
The H+-ATPase pump becomes more important in the later part of the nephron in allowing H+ to be secreted against a substantial [H+] gradient.
This secretion of H+ is rate limiting and the pH can fall to as low as as 4.5 in the collecting duct (though urine never becomes more acid than this).when maximal rates of H+ secretion are achieved. At this pH the rate of H+ back diffusion equals the rate of H+ secretion.
The urine is acidified in this process and the amount of H+ secreted in the regeneration of HCO3- can be estimated by measuring the amount of NaOH required to titrate the urine back to pH 7.4, hence the term titratable acid
The collecting tubule plays a substantial role in acidification of the urine. Usually the bicarbonate concentration of the tubular fluid reaching the collecting tubule is low and proton secretion can reduce the tubular fluid pH substantially. In doing so, phosphate and ammonia are titrated and acid is formed for excretion.
Phosphate buffer
Further H+ secreted into lumen buffered by HPO42-
Very effective buffer because pK=6.8 (close to pH of filtrate)
poor buffer in ECF because in low conc, but it is filtered at glomerulus and filtered load of phosphate exceeds its reabsorptive Tm and the excess phosphate becomes concentrated in its progress along the tubule. So very good buffer in tubular fluid, but also get a large amount reabsorbed in proximal tubules so not present in very high quantities.
Why is bicarbonate transport via secondary active Cl-HCO3 exchanger in phosphate buffer?
Large amounts of CA within intercalated cells of DT & CollDuct but NONE in luminal brush border. Bicarbonate transport is via secondary active Cl-HCO3 exchanger.
Ammonia buffer
Tubular epithelium produces NH3 from glutamine with the enzyme glutaminase
The ammonia buffer system plays an important role in the regeneration of HCO3-. Ammonium ions are produced in several tubular segments from glutamine which enters the tubular epithelial cells by an active mechanism. 2NH4 and 2 HCO3 produced from each glutamine molecule. Glutamine is metabolized to NH3 and alpha-ketoglutarate ion which is further metabolized to CO2 & H2O.
This is then hydrated to form H+ & HCO3 by CA. The H+ combines with NH3 forming NH4 which is secreted into the lumen by a sodium-driven secondary active antiporter.
What are the stages of urine buffering?
- Reabsorption of bicarbonate
- Acidification of phosphate
- Ammonia secretion
Change in pH corrected by what 3 mechanisms?
- Intra- and extra-cellular buffering
- Respiratory adjustment of ECF PCO2
- Renal adjustment of ECF [HCO3-]
How does the kidney sense extracellular acid-base status?
How are changes in pH, HCO3- and CO2 status detected?
Possible that there is no master pH sensor
Kidney has battery of molecules to scan epithelial cell environment to maintain acid-base homeostasis.
Some candidate molecular pH sensors have been identified i.e. acid/alkali-sensing receptors (GPR4), kinases (ErbB1/2), pH-sensitive ion channels and bicarbonate-stimulated adenylyl cyclase (sAC).
Role of respiratory system in pH
Chemosenstive area in medulla regulates respiration
Monitors [H+] of plasma via CSF indirectly
Charged ions can not cross BBB but CO2 does
→ so can be hydrated = H2CO3 which dissociates to produce H+ and HCO3-. Thus, elevated plasma PCO2 leads to a decreased CSF pH which stimulates pulmonary ventilation increasing respiratory excretion of CO2, decreasing PCO2 and returning ECF pH toward the normal range of 7.35 to 7.45.
A decrease in plasma PCO2 has the opposite effect on central regulation of ventilation and respiratory excretion of CO2
Metabolic acidosis
Characterised by low pH as a result of
ECF [H+] or ECF [HCO3-]
Caused by:
severe sepsis lactic acid
diabetes overproduction of 3-OH-butyric acid & other keto acids
diarrhoea loss of HCO3- from GI tract
Integrated renal and pulmonary compensation in METABOLIC ACIDOSIS
In ECF/ICF buffering system, the [HCO3] falls as it is used to mop up the H+. Assuming there is no respiratory disorder, the rise in [H+]/decreased pH stimulates respiration , by acting on the peripheral chemoreceptors, to cause hyperventilation and expel more CO2.
This respiratory compensation, decreases pCO2 and returns the pH to normal though HCO3 falls further. This respiratory compensation allows the pH to return towards normal because the ratio of HCO3 to CO2 rises
H+ + HCO3- H2CO3 CO2 + H2O
However because the buffering and hyperventilation are not fully effective in preventing a rise in [H+], the [H+] remains raised throughout the body. Renal compensation for met acidosis involves maximal conservation of filtered HCO3 and increased generation of new bicarbonate. To do this the kidney stimulates H+ secretion to increase HCO3- reabsorption.
Over days the kidney (except in renal failure) may be able to correct the disturbance by excreting the excess H+ and adding to the plasma the HCO3- that was lost as a result of the primary disturbance and as secondary consequence of the respiratory compensation. Once this has happened plasma [H+] returns to normal and ventilation is also normalized. Ammonium secretion also plays a major role in renal generation of new HCO3.
Metabolic alkalosis
Characterised by high pH caused by
ECF [HCO3-] or ECF [H+]
Caused by: Excessive diuretic (thiazide) use chronic loss of Cl-,Na+ & K+ → increased secretion of H+ Vomiting loss of H+ from GI tract Ingestion of alkaline antacids Hypokalemia
Integrated renal and pulmonary compensation in METABOLIC ACIDOSIS
What happens here is essentially opposite of what happens in metabolic acidosis. H+ in blood used up in trying to reduce increase in bicarb, and fall in H+ reduces the stimulation of peripheral chemoreceptors, ventilation is reduced and therefore less CO2 is expelled, so [CO2] rises.
This respiratory compensation therefore drives reaction further to the right so that more H+ is generated and [HCO3-] rises further. Thus pH returns to normal because ratio of HCO3:CO2 falls towards normal.
However kidney corrects this disturbance over several days. Rise in pH in tubule cells, reduces H+ secretion and HCO3- reabsorption, so allowing plasma [H+] to rise and correct the plasma HCO3 and finally removing the inhibitory effect on ventilation.
Respiratory alkalosis
Less CO2 enters cells and less HCO3 diffuses out into plasma so [HCO3] is reduced. Within days kidney compensates by reducing H+ secretion & decreasing HCO3- reabsorption
Talking here about pure acidosis & alkalosis – very often not like this. Can be mixed
Respiratory acidosis
caused by hypoventilation due to actions of drugs (anaesthetics/barbiturates), chronic emphysema, bronchitis, COPD (chronic obstructive pulmonary disease). These conditions impair the removal of CO2 from the lungs, hence it builds up in plasma.
Because CO2 enters into cells very rapidly and they contain CA get rapid rise in H+. Rapid ↑[H+] quickly buffered by proteins in plasma (within hours) ↑[HCO3-]. Limited by buffering capacity of blood. Within days kidney compensates by stimulating H+ secretion & increasing HCO3- reabsorption.
Simple acid-base disorder
Result of single primary disturbance with normal physiological compensatory response
Mixed acid-base disorder
Occurs in seriously ill patients, where two or more primary disorders may occur simultaneously
Net effect may be additive, with extreme alteration in pH (eg metabolic acidosis + respiratory acidosis)
Or may be opposite and nullify each other’s effect on pH (eg metabolic acidosis + respiratory alkalosis)
Siggaard-Anderson In-Vivo Nomogram: in mixed acid-base disorders
In mixed acid-base disorders there is a change in anion gap (AG)
The anion gap is the difference between primary measured cations (sodium Na+ and potassium K+) and the primary measured anions (chloride Cl- and bicarbonate HCO3-) in serum, plasma or urine.
