The role of the kidney in acid-base balance - renal physiology
Summary
TLDRThe kidneys maintain acid-base balance through two main mechanisms: reabsorbing bicarbonate and secreting hydrogen ions. Blood is filtered in nephrons, where plasma moves into renal tubules. These tubules reabsorb water and electrolytes while leaving waste and acids. Key processes include bicarbonate reabsorption and hydrogen ion secretion, utilizing enzymes like Carbonic anhydrase. Hydrogen ions are expelled through sodium-hydrogen countertransport and ATP-dependent pumps. Buffers like ammonia and phosphate systems prevent urine pH from dropping too low. This complex system ensures the body's pH balance is maintained efficiently.
Takeaways
- 🔄 The kidneys maintain acid-base balance by reabsorbing bicarbonate from the urine back to the blood and secreting hydrogen ions into the urine.
- 🔬 Kidneys filter blood through millions of nephrons, which start with a glomerulus for blood filtration.
- 💧 The renal tubule reabsorbs water and electrolytes, leaving waste products and acids behind.
- 🛠 The renal tubule consists of several segments: the proximal convoluted tubule, the Loop of Henle, the distal convoluted tubule, and the collecting duct.
- 🔬 Brush border cells in the tubules have microvilli to increase surface area for solute reabsorption.
- 🧪 Bicarbonate reabsorption involves binding with hydrogen to form carbonic acid, which is split into water and carbon dioxide by Carbonic anhydrase Type 4.
- 🧪 Carbonic anhydrase Type 2 reverses the reaction, forming bicarbonate and hydrogen, which are then transported back into the blood.
- 💡 Hydrogen ions are secreted into the urine using sodium-hydrogen counter-transport in the proximal tubule and ATP-powered pumps in the distal tubule and collecting ducts.
- 🛡 The ammonia buffer system uses ammonia from amino acid breakdown to combine with hydrogen, forming ammonium which helps prevent urine pH from dropping too low.
- 🔶 The phosphate buffer system uses monohydrogen phosphate to bind hydrogen ions, forming dihydrogen phosphate which is excreted in urine.
- 🩺 Kidneys play a crucial role in maintaining the blood's pH balance through bicarbonate reabsorption and hydrogen ion secretion using various buffer systems.
Q & A
How do kidneys maintain acid-base balance in the body?
-Kidneys maintain acid-base balance by reabsorbing bicarbonate (HCO3-) from the urine back into the blood and secreting hydrogen ions into the urine.
What are nephrons and what is their function in the kidneys?
-Nephrons are tiny functional units in the kidneys that filter blood. They consist of a glomerulus where blood filtration starts and renal tubules that reabsorb necessary substances and leave waste products behind.
Describe the process of blood filtration in the kidneys.
-Blood enters the kidneys and is distributed to nephrons. In the glomerulus, about one-fifth of the plasma is filtered into the renal tubule, which then reabsorbs water and electrolytes while leaving waste products and acids.
What role do brush border cells play in the renal tubules?
-Brush border cells line the renal tubules and have microvilli on their apical surface to increase surface area for solute reabsorption. They help reabsorb bicarbonate and other solutes back into the blood.
Explain the bicarbonate reabsorption process in the proximal convoluted tubule.
-In the proximal convoluted tubule, bicarbonate binds to hydrogen ions to form carbonic acid, which is then split into water and carbon dioxide by Carbonic anhydrase Type 4. These diffuse into cells and are converted back to bicarbonate and hydrogen ions by Carbonic anhydrase Type 2.
How are hydrogen ions secreted into the urine?
-Hydrogen ions are secreted into the urine through two mechanisms: sodium-hydrogen counter transport in the proximal tubule and a hydrogen ATPase pump in the distal tubule and collecting ducts.
Why can't the urine pH drop below 4.5?
-The urine pH can't drop below 4.5 because the tubules cannot maintain such a low pH. To prevent this, the urine uses chemical buffers like the ammonia and phosphate buffer systems to bind hydrogen ions.
What is the ammonia buffer system and how does it work?
-The ammonia buffer system involves the breakdown of amino acids like glutamine into ammonia (NH3) in the proximal convoluted tubule cells. Ammonia combines with hydrogen ions to form ammonium (NH4+), which is excreted in the urine, helping to remove hydrogen ions from the body.
How does the phosphate buffer system help in acid-base balance?
-The phosphate buffer system uses monohydrogen phosphate (HPO4 2-) to bind with hydrogen ions, forming dihydrogen phosphate (H2PO4 -). This process helps to neutralize hydrogen ions and maintain urine pH.
Summarize how the kidneys contribute to pH balance in the blood.
-The kidneys maintain pH balance by reabsorbing bicarbonate ions in the proximal convoluted tubule and secreting hydrogen ions in the proximal and distal convoluted tubules and collecting ducts. The ammonia and phosphate buffer systems help to manage excess hydrogen ions in the urine.
Outlines
🩺 Kidney Function in Acid-Base Balance
Kidneys maintain acid-base balance through reabsorption of bicarbonate (HCO3-) and secretion of hydrogen ions into the urine. They filter blood continuously, distributing it to millions of nephrons. Blood filtration starts in the glomerulus, where plasma enters the renal tubule. The renal tubule reabsorbs beneficial substances and leaves waste. It consists of several segments, each lined with brush border cells that aid in solute reabsorption through microvilli. The filtrate first passes through the proximal convoluted tubule, where bicarbonate is reabsorbed via a complex exchange process involving carbonic acid and various enzymes. The bicarbonate then enters the bloodstream, while hydrogen ions are secreted into the urine through different mechanisms.
💧 Hydrogen Ion Secretion and Buffer Systems
Hydrogen ions are expelled into the urine through two main mechanisms: sodium-hydrogen counter-transport in the proximal tubule and ATP-dependent pumps in the distal tubule and collecting ducts. The urine's pH cannot drop below 4.5, so chemical buffers are necessary. The ammonia buffer system involves the breakdown of amino acids into ammonia, which binds to hydrogen ions to form ammonium. This ammonium combines with chloride, reducing urine pH impact. The phosphate buffer system uses monohydrogen phosphate from plasma, which converts to dihydrogen phosphate when binding with hydrogen ions, subsequently excreted in urine.
Mindmap
Keywords
💡Acid-Base Balance
💡Nephron
💡Glomerulus
💡Renal Tubule
💡Bicarbonate Reabsorption
💡Hydrogen Ion Secretion
💡Carbonic Anhydrase
💡Ammonia Buffer System
💡Phosphate Buffer System
💡Proximal Convoluted Tubule
Highlights
Kidneys maintain acid-base balance by reabsorbing bicarbonate and secreting hydrogen ions.
Nephrons are the tiny functional units in kidneys that filter blood.
Glomerulus is the initial site for blood filtration in the nephron.
Renal tubules reabsorb essential substances like water and electrolytes, and excrete waste and acids.
Renal tubules have several segments: proximal convoluted tubule, Loop of Henle, distal convoluted tubule, and collecting duct.
Brush border cells in renal tubules aid in solute reabsorption through microvilli.
Bicarbonate reabsorption occurs primarily in the proximal convoluted tubule.
Carbonic anhydrase type 4 and type 2 enzymes facilitate bicarbonate reabsorption by converting carbonic acid.
Sodium bicarbonate co-transporters and bicarbonate chloride exchangers help move bicarbonate back into the blood.
Hydrogen ions are secreted into the urine via sodium hydrogen counter transport and ATP-powered pumps.
Urine pH is maintained above 4.5 by chemical buffers like the ammonia buffer system.
Ammoniogenesis breaks down amino acids into ammonia, which binds with hydrogen ions to form ammonium in urine.
Ammonium chloride in urine helps remove hydrogen ions without significantly lowering pH.
Phosphate buffer system also helps maintain urine pH by forming dihydrogen phosphate.
Overall, the kidneys play a crucial role in maintaining blood pH balance through various cellular and chemical processes.
Transcripts
foreign
s have two main ways to maintain
acid-base balance their cells reabsorbed
bicarbonate or hco3 minus from the urine
back to the blood and they secrete
hydrogen ions into the urine
by adjusting the amounts reabsorbed and
secreted they can balance the
bloodstream's pH
n now our kidneys filter blood
continuously by Distributing the blood
that comes into the kidney to millions
of tiny functional units called nephrons
each nephron is made up of a glomerulus
or a tiny clump of capillaries where
blood filtration starts
when blood passes through a glomerulus
about one-fifth of the plasma leaves the
glomerular capillaries and goes into the
renal tubule
the renal tubules function to reabsorb
the good stuff like water and
electrolytes and leave behind the bad
stuff like waste products in acid
the renal tubule is a structure with
several segments the proximal convoluted
tubule the u-shaped Loop of henle with a
thin descending limb and a thick
ascending limb and the distal convoluted
tubule which goes back up again before
emptying into the collecting duct which
collects the final urine
each of these tubules is lined by brush
border cells which have two surfaces one
is the apical surface that faces the
tubular Lumen and is lined with
microvilli
which are tiny little projections that
increase the cell's surface area to help
with solute reabsorption
the other is the basolateral surface
which faces the peritubular capillaries
which run alongside the nephron
so with bicarbonate reabsorption as the
filtrate leaves the glomerulus it first
goes through the proximal convoluted
tubule
at first this filtrate has the same
concentration of electrolytes as the
plasma came from
but when a molecular bicarbonate
approaches the apical surface of the
brush border cell it binds to hydrogen
that was secreted by the brush border
cell in exchange for a sodium ion
and this forms carbonic acid
at this point in an enzyme called
Carbonic anhydrase Type 4 that lurks in
the tubule in the microvilli kind of
like a shark swims along and splits the
carbonic acid into water and carbon
dioxide
unlike charged bicarbonate anions which
are stuck in the tubule the water and
carbon dioxide happily diffuse across
the membrane into cells
or Carbonic anhydrase type 2 facilitates
the reverse reaction combining them to
form carbonic acid which dissolves into
bicarbonate and hydrogen
a sodium bicarbonate co-transporter on
the basol lateral surface snatches up
the bicarbonate in a nearby sodium and
then shuttles both into the blood
alternatively a bicarbonate chloride
exchanger exchanges bicarbonate with
chloride leaving the bloodstream to
enter the cells
all this chemical trickery effectively
moves 99.9 percent of the filtered
bicarbonate that's in the tubule back
into the bloodstream
now hydrogen ions with their positive
charge don't want to naturally pass
through the cell membranes and into the
urine they need to be pushed out and
there are two mechanisms that do this
one mechanism is a sodium hydrogen
counter transport with this a carrier
protein in the apical wall binds a
hydrogen ion from the cell and a sodium
ion in the tubular fluid
as that sodium ions pushed in the
hydrogen ion gets pushed out kind of
like a revolving door
remember this is in the proximal tubule
but in the distal tubule and collecting
ducts there's another mechanism that
involves alpha intercalated cells
these cells have a different pump that
uses the energy of ATP to push hydrogen
ions into the tubule
the urine though Can Only Hold so many
free hydrogen ions because the pH starts
to drop really low and the tubules can't
maintain a urine pH below about 4.5
so to get around this limit and hold
more hydrogen ions the urine has
chemical buffers
which bind to the hydrogen ions and
prevent the pH from dropping too low
the most important is the ammonia buffer
system which the kidneys use a process
called ammonio Genesis
monogenesis starts when the proximal
convoluted tubule cells break down amino
acids like glutamine into ammonia or NH3
the ammonia is lipid soluble so it
diffuses freely into the tubule where it
combines with a hydrogen ion to form an
ammonium ion or nh4 Plus
ammonium combines with chloride in the
urine
and because ammonium chloride is only
weakly acidic the urine pH doesn't drop
much even though it has a lot of
hydrogen ions
most of this ammonium is lost in the
urine which helps the kidneys get rid of
a large amount of hydrogen
a second buffer system uses phosphate
monohydrogen phosphate or hpo42 minus
enters the tubule from the plasma
monohydrogen phosphates poorly
reabsorbed from the tubules so it ends
up concentrating there
it acts as a buffer by combining with
secreted hydrogen ions to form
dihydrogen phosphate or h2po4 minus
which is then peed out in the urine
alright as a quick recap
the kidneys help maintain pH balance of
the blood
in the nephron the proximal convoluted
tubule cells are able to reabsorb the
bicarbonate ions and cells in the
proximal as well as the distal
convoluted tubule and collecting ducts
secrete hydrogen ions that are carried
out into the urine using the ammonia and
phosphate buffer systems
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