Download Lecture 13: Kidney

Lecture 13: Renal Physiology
• Does your GI tract absorb into blood all nutrients? ions? water?
1. Now imagine that you drink water: 1 glass, 2, 3, 4, 5 glasses (1L).
Does the additional 1 liter of water stay inside
the blood vessels (18% increase of volume from 5.5L)?
– No. Water will leak into interstitial fluid, so that the load will be
redistributed between the vascular system and the tissues. One
becomes 1kg heavier but blood volume is not affected significantly.
2. Now imagine that you drink isosmotic fluid: 300mOsm NaCl (one
soup spoon of table salt per liter of water).
Does the GI absorb all the ions? water?
Does the additional liter of water stay inside the blood vessels?
–
Yes. Salt will not leave the blood vessels since there is no concentration
gradient between the blood and extracellular fluid. Water will stay
with salt, i.e. in blood vessels. One becomes 1kg heavier and blood
volume is increased by 18% (assuming no kidney function).
3. Recall that after losing 1L of blood, arterial blood pressure (Pa)
is decreased so much that one can collapse (blood donation =
0.5L). Would you collapse if you drink 1L of Gatorade?
–
No. Because kidneys can normally transfer extra salt and water into
the bladder as fast as we drink and absorb it.
• Kidneys receive 1 L/min = 20% of
cardiac output at rest.
• 20% of this volume (i.e. 200
mL/min = a glass of water/min) is
filtered into the internal kidney
tubules and, if necessary, kidney
would dispose almost all of this
amount into the bladder.
Homeostasis of ions and water
feedback
control
Kidney processes:
1. Filtration
2. Reabsorption
3. Secretion
[H+], [Na+], [K+],
blood volume=const
controlled variables
Sensors in kidney and
elsewhere
•
•
•
Normal plasma Na+ levels: 135 to 145
•
mmol/L.
Very low Na+ (less than 125 mmol/L) 
•
nausea, vomiting, headache, short-term
memory loss, confusion, lethargy, fatigue,
loss of appetite, irritability, muscle
weakness, muscle cramps, seizures,
•
decreased consciousness or coma.
Very high Na+ (greater than 157 mmol/L) 
seizures and coma.
Normal plasma K+ levels: 3.5 to 5.0 mmol/L
(98% of K+ is inside cells).
Very low K+ (less than 3 mmol/L)  muscle
weakness, muscle pain, tremor, muscle
cramps, constipation; flaccid paralysis and
hyporeflexia.
Very high K+ is a medical emergency due to
the risk of potentially fatal abnormal heart
rhythms.
• Kidney is a waterproof bag
with one high pressure
tube in (==renal artery) and
two low pressure tubes out
(==renal vein and ureter)
ultrafiltrate
1
 Out
 In
urine
2
3
urine is
ultrafiltrate of
blood from
which
nutrients and
necessary ions
were removed
In kidney there are three processes:
1. Filtration: Blood ultrafiltrate is pushed
into renal tubes.
2. Reabsorption: All glucose, all a.a. and
necessary amount of ions are
reabsorbed back into blood.
Whatever left is excreted as urine.
3. Secretion: some substances are
actively secreted into urine.
Is everything that is filtered excreted?
Was everything that excreted filtered?
cortex
• The building blocks of kidney
are called nephrons from G.
nephros=kidney
• ~1,000,000 nephrons in each
kidney are organized radially
from medulla to cortex
Renal tubule
medulla
Starts with renal corpuscle
(initial filtering component) =
a glomerulus + a Bowman's
capsule
Ends with
collecting
duct
2
1
Renal corpuscle
Glomerular
capillaries
4
3
5
ultrafiltrate
Process 1: Filtration
• renal artery 3
subdivides to form
afferent arterioles 4
• Afferent arteriole
subdivides into a
smaller number of
very short
capillaries called
glomerular
capillaries 5
• The tuft formed of
capillary loops is
called glomerulus
Glomerular
capillaries
Capillary types
Glomerular
capillaries
Fenestrations
~ 60 nm wide
• Glomerular capillaries are
very leaky.
• All substances smaller
than 40nm can leave
glomerular capillaries
and go into Bowman’s
capsule (the initial part of
kidney tubules).
• What substances are
smaller than 40nm?
40nm
RBC (7.5µm)
human nail human hair
thickness thickness
(40-140µm)
= 1mm (millimeter)
ovum
(140µm)
= 1µm (micrometer)
WBC
(10-12µm)
Neurons
(10-20µm)
axon
diameter
(1µm)
= 1nm (nanometer)
synaptic vesicle
diameter
(0.05µm)
• where on this scale is human nail thickness? human hair thickness?
RBC? neuron size? axon diameter? synaptic vesicle?
thrombocytes
Scanning electron
micrograph of T lymphocyte
(right), a platelet a.k.a
thrombocyte (thromb- + cyte, "blood clot
cell")(center) and a red
blood cell
Capillary types
Fenestrations
~ 60 nm wide
Glomerular
capillaries
• RBC (7.5µm) are not filtered
• Glucose, a.a., ions, waste products
including urea are filtered into
Bowman’s capsule
• What about proteins that are
smaller than 40nm? (Albumin=lipid
carrier, part of lipoprotein;
Globulins=clotting factors, peptide
hormone carriers, antibodies;
Fibrinogen=blood clotting)
• You do not want to loose proteins
circulating in blood into kidney
tubules.
• How would YOU change
morphology to keep proteins inside
capillaries?
Specializations of glomerular capillaries aimed to
reduce protein filtration
Efferent arteriole
• Capillaries are covered by podocyte cells.
• Podocyte cells have a number of protruding foot processes.
• Space between foot processes makes filtration slits.
Podocyte cells with protruding foot processes. Filtration slits make a sieve.
Glomerular
capillaries
width of
filtration
slits
~ 40 nm
-
-
-
-
-
-
-
-
• Filtration slits make a sieve that
prevents molecules larger than
8nm to escape into kidney
tubules.
• In addition, podocytes are
negatively charged and proteins
are also negatively charged so
that podocytes repel proteins
that further reduces the ability
of proteins to escape into
kidney tubules.
• Conclusion: very small filtration
of proteins, even when proteins
are smaller than 8nm
2 Space inside
Bowman’s capsule
1
capillary
podocyte
1
capillary
1 capillary
red blood cell
•
•
•
•
F = endothelial lining is highly fenestrated (F).
P = podocytes extend primary processes (P1) that give rise to numerous foot processes (P2)
FS = filtration slits
The system is very efficient. E.g. serum albumin (steroid hormone carrier) has 3.55nm molecular radius. Less
than 1% is filtered into renal tubules
2
1
Renal corpuscle or
200
L/day
ultrafiltrate
Glomerular
capillaries
4
3
5
• Filtration summary:
• 200mL/min = 200
L/day is filtered into
the internal kidney
tubules
• Filtered: glucose,
a.a., ions, waste
products including
urea
• NOT filtered: RBC,
most molecules
bigger than 8nm,
most proteins
120 L/day in proximal tubule
5
3
200
L/day
Renal corpuscle or
ultrafiltrate
2
Glomerular
capillaries
1
6
4
Process 2. Reabsorption:
• the renal tubule is
continuous with
Bowman’s capsule.
• The epithelial cells of
the tubule wall differ
in composition and
function along the
tubule:
• Segments:
–3 Proximal convoluted
tubule
–4 Loop of Henle
–5 Distal convoluted
tubule
–6 Collecting duct
Reabsorption mechanism:
1. a.a.: co-transport with
Na+ on luminal side,
facilitated diffusion on
blood vessel side
2. glucose: same
ultrafiltrate
• a.a. reabsorption cannot be saturated.
• normally all glucose is reabsorbed. In
diabetes blood [glucose] is so high that
the absorption mechanism is saturated
 not all glucose is reabsorbed
 some glucose ends up in urine
• Urea is not reabsorbed at all
• Substances NOT regulated by kidney:
– a.a. and glucose (reabsorb all)
– urea (excrete all)
– (NB: absorption is NOT regulation!!!)
K+
2
• Substances regulated by kidney:
Amino
acids
1
Amino
acids
Amino acids
– Na+ (reabsorb only as much as
necessary)
– K+ (reabsorb only as much as necessary)
– H+ (reabsorb only as much as necessary)
– H2O (reabsorb only as much as
necessary)
• We will look at regulation of Na+ and
H2O
120 L/day in proximal tubule
1
Renal corpuscle or
70%
200
L/day
Na+
ultrafiltrate
Na+ 2
30%
Na+
3
4
• 2.1 Regulation of
Na+
• Bulk active
reabsorption in the
proximal 1
convoluted tubule
(70%): co-transport
with Glucose and
a.a.
• Fine Na+ regulation
in distal convoluted
tubule 2 and
collecting duct 3
(30%)
120 L/day in proximal tubule
200
L/day
Na+ 2
channels
• 2.1 Fine tuning of Na+
reabsorption in distal
convoluted tubule and
collecting duct include
regulation of:
Na+
K+
ultrafiltrate
1
1. number of Na+-K+
pumps 1
2. number of Na+ channels
on luminal membrane 2
Amino
acids
Amino
acids
•
Both are regulated by
Aldosterone
2.1.1 Na+ reabsorption: Aldosterone
• Aldosterone is a steroid hormone produced
by adrenal gland (steroid hormones act on
nucleus to promote or inhibit protein
production).
• Aldosterone increases synthesis of Na+-K+
pumps and Na+ channels in the cells of distal
convoluted tubules and collecting duct.
• Aldosterone absent  2% of filtered
Na+ is not reabsorbed but excreted.
Na+ 2
channels
• This is equivalent to 35g of NaCl
Na+
per day
• High Aldosterone 
almost all Na+ is reabsorbed
• Eat high Na+ diet  low Aldosterone
• Eat low Na+ diet  high AldosteroneAmino
acids
• What regulates Aldosterone?
K+
1
2.1.1 What regulates Aldosterone?
• Aldosterone secretion by adrenal gland is directly
stimulated by a hormone Angiotensin II.
1
Liver secretes
Angiotensinogen into
blood stream, so that
Angiotensinogen is in high
and stable concentration
3
Small polypeptide
Angiotensin I (“A I”) is
cleaved by Renin
AI
Angiotensinogen
Angiotensin Converting
Enzyme
A II
Renin is an enzyme
2
Na+
Depending on
needs,
Kidney (juxtaglomerular
cells) release Renin into
blood stream.
4
A I is converted into A II by
angiotensin converting
enzyme (ACE) on the luminal
surface of capillaries
particularly in the lungs
• ↑Renin  ↑A II  ↑ Aldosterone  ↑ #Na+
channels and ↑ #Na-K pump  ↑ Na+ reabsorption
• Thus: ↑Renin  ↑ Na+ reabsorption  ↑Pa
• Renin is released by juxtaglomerular cells:
Renal
corpuscle
2
• How is Renin release regulated?
1
• Three inputs to juxtaglomerular
cells:
1. SNS (external regulations):
fight-or-flight  ↑SNS 
↑Renin  ↑ Na+ reabsorption
 ↑ Pa
2. Juxtaglomerular cells function
as baroreceptors: if ↓Pa  ↓P
3 part of distal
convoluted tubule
in afferent arteriole  ↑Renin
 ↑Na+ reabsorption  ↑Pa
Kidney
3. Osmoreceptors in macula densa
located near the start of distal
internal
convoluted tubule. Macula
regulation
densa senses [Na+] in the
tubular fluid flowing past it:
↓[Na+]  ↑Renin  ↑Na+
reabsorption
2.1.1 Renin / A II
/ Aldosterone
• In addition, Angiotensin II is a potent
constrictor of arterioles all over the body:
• ↓Pa  ↑Renin 
 ↑A II  constriction of arterioles ↑Pa
↑Aldosterone ↑Na+ ↑Pa
• If Angiotensin II levels are continuously elevated, it
could be one cause of hypertension.
• Why continuous hypertension is bad?
• What is the first line of defense against high blood
pressure?
• ACE inhibitors (angiotensin converting enzyme
inhibitors): lisinopril, benazepril, captopril…
2.1.2 Atrial Natriuretic Factor (ANF)
• ANF (small 28 a.a. peptide) is secreted by
the cells of cardiac atria in response to distention
which would result from increase of blood volume.
1. ANF is a powerful vasodilator
2. ANF acts on the distal convoluted
tubule and collecting duct to inhibit
Na+ reabsorption  ↓blood volume
3. ANF acts on renal blood vessels to increase filtration
(dilates afferent arterioles and constricts efferent
arterioles)  more blood plasma is filtered  more
Na+ can be excreted  ↓blood volume
ultrafiltrate
120 L/day in proximal tubule
1
Renal corpuscle or
70%
200
L/day
Na+
ultrafiltrate
Na+ 2
30%
Na+
3
4
• 2.1 Regulation of
Na+ summary
• Bulk active
reabsorption in the
proximal convoluted
tubule (70%): cotransport with
Glucose and a.a.
• Fine Na+ regulation
in distal convoluted
tubule and
collecting duct
(30%): Renin  A II
 Aldosterone, ANF
120 L/day in proximal tubule
bulk1
Na
reabsorption
Renal corpuscle or
200
L/day
+
ultrafiltrate
Na+ 2
fine
Na
tuning
+
3
4
• 2.2 Regulation
of water
reabsorption
• Bulk water
reabsorption in
the proximal 1
convoluted
tubule.
• Fine water
regulation in the
distal convoluted
tubule2 and
collecting duct 3
ultrafiltrate
High osmolarity
H2O
K+
H2O
Amino
acids
Amino
acids
Amino acids
• 2.2.1 H2O bulk reabsorption
• On the peritubular capillary
side:
– Na/K pump  ↑Na+
– ↑[K+] due to K+ leaking back
via K+ channels
– ↑[glucose]
– ↑[a.a.]
• Increased osmolarity on the
peritubular capillary side
• Tight junction between cells
of proximal tubule are leaky
to H2O  H2O leaks inside the
kidney following osmolarity
gradient and then into
peritubular capillaries
Case 1
Fluid inside collecting duct:
100 mOsm (a lot of water, very little salt)
100 mOsm
ultrafiltrate
H2O
100 mOsm
H2O
100 mOsm
a lot of water
excreted into
bladder
medulla
cortex
Case 2
100 mOsm
300 mOsm
300 mOsm
H2O
600 mOsm
600 mOsm
H2O
1,200 mOsm 1,200 mOsm
H2O
medulla
very little water
excreted into bladder
2.2.2 H2O fine regulation in distal
conv. tubule & collecting duct
• Case 1. drink a lot of water:
• distal convoluted tubule and
collecting duct are generally
impermeable to water  water
in not absorbed, but excreted.
• Case 2. water needs to be
conserved:
• collecting duct becomes very
leaky to water  water leaks
into kidney following osmolarity
gradient and then into
peritubular capillaries.
• The permeability of collecting
duct is regulated by insertion of
water channels
Lumen of the
collecting duct
Peritubular capillaries
tight junctions
High osmolarity
Low
osmolarity
• The permeability of collecting duct is regulated by insertion
of water channels
• How water channels are regulated?
• By antidiuretic hormone (ADH, also known as vasopressin)
ADH
• How is ADH regulated?
• ADH is produced by a group of hypothalamic neurons and released from
posterior pituitary
ADH
• ADH release is stimulated by:
1. ↓of firing of baroreceptors in atria and pulmonary vein
2. ↑ of blood osmolarity  ↑of firing of hypothalamic
osmoreceptors (they also generate the sense of thirst)
• Why not position baroreceptors in aorta?
• The receptors in aorta would measure Pa which does not
change much; on the contrary blood pressure in atria and
pulmonary veins is directly proportional to blood volume.
• Case 1: Eat salty food  ↑osmolarity  ↑ADH  ↑water
reabsorption  ↑blood volume  ↓ blood osmolarity
• Case 2: Hemorrhage  ↓ blood volume  ↓ firing of
baroreceptors  ↑ADH  ↑water reabsorption  ↑
blood volume
120 L/day in proximal tubule
bulk1
Na
reabsorption
Renal corpuscle or
200
L/day
+
ultrafiltrate
Na+ 2
fine
Na
tuning
+
3
4
• 2.2 Regulation of
water reabsorption
summary
• Bulk water
reabsorption in the
proximal
convoluted tubule.
• Fine water
regulation in the
distal convoluted
tubule and
collecting duct by
ADH / water
channels insertion
• Let us come back to the example we have started with:
• You drink a lot of isosmotic fluid (300mOsl = Gatorade)
↑ atreal
baroreceptors
firing
GI absorbs
all water
and all salts
↓ ADH
↑ ANF
↓ H2O
reabsorption
↑ blood
volume
↓ SNS activity
↓ Juxtaglomerular
cells firing
↑ filtration
↓ CO
↓ venomotor tone
↓ precapillary
sphincter tone
↓ renin  ↓A II
↓ Na+
reabsorption
↑ Na+ and H2O
excretion:
increased
urine volume
ultrafiltrate
• You eat a lot of salt (e.g. salt-dried fish):
movement of
water from ICF
 ECF  blood
GI absorbs
all salts
↑ blood
osmolarity
↑ hypothalamic
osmoreceptors
firing
↑ osmoreceptors
in macular densa
↑ ADH
↑ blood
volume
↑ thirst
↓ renin  ↓ Na+ reabsorption
see
previous
slide
Homeostasis of ions and water
feedback
control:
regulatory
mechanisms
for Na+: renin,
A I, A II,
Aldosterone;
ANF, ADH
Kidney
1. Filtration
2. Reabsorption
3. Secretion
[H+], [Na+], [K+],
blood volume
=const
controlled variables
I. Sensors:
1. SNS activity is function of Pa
2. baroreceptors function of Juxtaglomerular
cells
3. osmoreceptors in macular densa
4. osmoreceptors in hypothalamus
I. Sensors of blood volume:
1. in atria  ANF
2. in atria and pulmonary vein  ADH
Thirst quenched right away
• Your blood osmolarity is high and you feel thirsty. You
• take a sip of water…. and you are not thirsty any more.
• How does it work that you feel thirst quenched right away?
It takes 20 minutes for water to get absorbed into blood.
• “Thirst neurons anticipate the homeostatic consequences of eating and
drinking, 2016”: the brain thirst neurons in subfornical organ are firing like
crazy indicating that a mouse is really thirsty  mouse takes a sip of water
 thirst neurons decrease firing. Another sip  further reduction in firing.
• Basically these neurons are measuring the difference between the amount
of water needed and water that was consumed and update their firing rate
in real time.
• Zimmerman et al.:“thirst-promoting SFO [subfornical organ] neurons
respond to inputs from the oral cavity during eating and drinking and then
integrate these inputs with information about the composition of the blood.
This integration allows SFO [subfornical organ] neurons to predict how
ongoing food and water consumption will alter fluid balance in the future
and then to adjust behaviour preemptively.”
ultrafiltrate
1
2
3
Process 3. Active secretion into renal tubule (examples):
–
–
–
–
H+
K+
Organic anions: choline, creatinine
Foreign chemicals: penicillin
• Properties:
– Usually coupled to the reabsorption of Na+
– Primarily secretion occurs in the proximal convoluted tubule
(except K+)
– Secretion improves the efficiency of kidney to dispose of
substances at a higher rate than the filtered load.
Summary of kidney function
• Major functions:
• regulate water content
•
•
Regulate ionic composition of K+, Na+, H+
NB: [H+] (in mM)=24PCO2/[HCO3-] (in nM);
[HCO3-] is regulated by kidney
• Excretion functions:
•
•
•
•
•
urea from proteins (Note that kidney does not regulate urea,
kidneys just excrete all urea)
uric acid from nucleic acids
creatinine from muscle creatine
creatinine
bilirubin from Hb breakdown  gives color to urine
drugs, food additives, etc.
• Endocrine functions:
•
•
•
erythropoietin (regulates RBC production)
renin
active form of vitamin D
• During prolonged fasting:
•
kidney synthesize glucose from a.a. and release it to blood
(gluconeogenesis). Kidney can supply as much glucose as liver
at such times (done by cells of renal tubule).
urine
Nitrogen
excretion
• all animals have to
excrete nitrogen
waste from proteins
degradation
• Mammals excrete
nitrogen as urea
• Birds excrete nitrogen
as solid uric acid
• Excessive uric acid in
blood of humans has
been linked to gout,
an inflammatory type
of arthritis; and acute
mania. Reducing uric
acid improves
symptoms.
Guano
• In the USA about 9% of the population has had a kidney stone.
• The most common type of kidney stones contains calcium oxalate:
• At 3 millimeters, stones can cause blockage of the ureter. This leads to pain,
nausea, vomiting, fever, blood in the urine, pus in the urine, and painful urination.
• Larger stones can block ureter completely and cause dilation of the kidney.
• Formation: when urine contains more solutes than it can hold in solution, a seed
crystal may form and grow.
• Prevention: drink lots of fluids so that more than two liters of urine is produced
per day, avoid soft drinks containing phosphoric acid (typically colas)
Renal failure

hemodialysis
The principle: diffusion of
solutes across a
semipermeable membrane
• What substances will you remove from blood?
• What substances you don’t want to remove from blood?
• stop here
Table of permselectivity for different
substances
Substance
Effective molecular
radius (nm)
Molecular mass
conc. in ultrafiltrate /
conc. in blood plasma
sodium
23
0.1
1.0
potassium
39
0.14
1.0
chloride
35,5
0.18
1.0
water
18
0.15
1.0
urea
60
0.16
1.0
glucose
180
0.33
1.0
sucrose
342
0.44
1.0
polyethylene glycol
1.000
0.70
1.0
insulin
5.200
1.48
0.98
lysozyme
14.600
1.90
0.8
myoglobin
16.900
1.88
0.75
lactoglobulin
36.000
2.16
0.4
egg albumin
43.500
2.80
0.22
Bence Jones protein
44.000
2.77
1.0
hemoglobin
68.000
3.25
0.03
serum albumin (steroid
hormone carrier)
69.000
3.55
<0.01
All proteins are
reabsorbed via
receptormediated
endocytosis.
Inside epithelial
cells they are
broken down into
single a.a. which
diffuse into blood
Nucleotides filtration
•
•
•
•
•
•
•
•
•
•
A nucleotide is a sugar, base, and phosphate. Nucleotides are molecules that, when joined, make
up the individual structural units of the nucleic acids RNA and DNA. In addition, nucleotides
participate in cellular signaling (cGMP and cAMP)
Nucleosides are glycosylamines consisting of a nucleobase (often referred to as simply base)
bound to a ribose or deoxyribose sugar via a beta-glycosidic linkage. Examples of nucleosides
include cytidine, uridine, adenosine,guanosine, thymidine and inosine.
Generally nucleosides are nucleotides (sugar covalently bonded to a nitrogenous base, such as
adenine, guanine, cytosine...) that lack phosphates. However, for the sake of technical
terminology, nucleotides are given classifications as nucleosides with a suffix describing the
number of phosphates present in a specific unit. For example, if a nucleotide has one phosphate,
it is a nucleoside monophosphate (NMP). If the nucleotide has two phosphates, then it is called a
nucleoside diphosphate (NDP), and for three, it is a nucleoside triphosphate (NTP). The
nucleotides that contain a ribose sugar are the monomers of RNA and those that contain a
deoxyribose sugar compose DNA.
GI: The GI tract digests nucleic acids into nucleotides, which are then digested by brush border
nucleotidases into sugar, base and phosphate, which are then absorbed. Normal human should
not have much in the way of free nucleotides in the blood.
Kidney: 1. Yes, adenosine and the like are filtered and reabsorbed. Interestingly, in the "old
days", we used to use the measurement of nephrogenous cAMP as an index of PTH activity
(before we had good serum PTH assays). PTH stimulates proximal tubule phosphate excretion
and distal tubular Ca reabsorption (as well as activating 1-OHase to convert 25D to 1,25D).
2. So here is the definite answer by a colleague who is both a nephrologist and a renal
physiologist:
Nucleotides are filtered and may also be secreted by the tubules. Much of them is known to be
degraded within the tubular lumen. They may also act on tubular cells by binding to receptors
on the apical membrane.
Small proteins and peptides are reabsorbed mostly in the proximal tubule through endocytosis.
3. This is from Robert Unwin - an expert in this area to whom Eric Cohen (a nephrologist at
MCW) referred the question.
I agree with you Eric. Some debate about circulating levels of nucleotides, given content in rbcs,
but our own data suggest very little ATP is filtered, so circulating levels of ATP at least must be
very low. Less know about its breakdown products, including adenosine, but you might think it
is higher. ATP can be measured in urine, but its source may be the bladder, rather than the
kidney.
Nucleotide cAMP
Nucleoside Adenosine
High blood pressure effect on kidney
• High blood pressure can damage
blood vessels in the kidneys,
reducing their ability to work
properly.
• When the force of blood flow is
high, blood vessels stretch so
blood flows more easily.
• Eventually, this stretching scars
and weakens blood vessels
throughout the body, including
those in the kidneys.