The role of the kidney in acid-base balance - renal physiology

Summary

The 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

00:00

🩺 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.

05:02

💧 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.

Keywords

💡Acid-Base Balance

The regulation of the body's pH level by maintaining a balance between acids and bases. The kidneys play a crucial role in this process by reabsorbing bicarbonate and secreting hydrogen ions. This balance is essential for normal bodily functions and is highlighted in the video through the kidney's reabsorption and secretion mechanisms.

💡Nephron

The functional unit of the kidney that filters blood and forms urine. Each nephron consists of a glomerulus and a renal tubule, where various substances are reabsorbed or secreted. The video details how nephrons are involved in maintaining pH balance and filtering waste from the blood.

💡Glomerulus

A cluster of capillaries within the nephron where blood filtration begins. The glomerulus allows plasma to pass into the renal tubule, initiating the process of urine formation. The video describes how about one-fifth of the plasma leaves the glomerulus and enters the renal tubule for further processing.

💡Renal Tubule

A series of tubes in the nephron where reabsorption and secretion occur. The renal tubule includes the proximal convoluted tubule, Loop of Henle, and distal convoluted tubule. The video explains how these segments work together to reabsorb important substances and secrete waste products, playing a key role in pH balance.

💡Bicarbonate Reabsorption

The process by which the kidneys reabsorb bicarbonate (HCO3-) from the urine back into the blood. This is crucial for maintaining the blood's pH level. The video explains how bicarbonate reabsorption occurs in the proximal convoluted tubule and involves various cellular mechanisms.

💡Hydrogen Ion Secretion

The process by which the kidneys secrete hydrogen ions (H+) into the urine. This helps to remove excess acid from the blood. The video describes how this occurs in both the proximal and distal convoluted tubules and involves specific pumps and transporters.

💡Carbonic Anhydrase

An enzyme that catalyzes the conversion between carbonic acid and bicarbonate and hydrogen ions. This enzyme is vital in the process of bicarbonate reabsorption and hydrogen ion secretion. The video mentions types 2 and 4 of this enzyme and their roles in the renal tubule.

💡Ammonia Buffer System

A system in the kidneys that helps buffer excess hydrogen ions in the urine. Ammonia (NH3) combines with hydrogen ions to form ammonium (NH4+), which is excreted in the urine. The video highlights how this system allows the urine to carry more hydrogen ions without becoming too acidic.

💡Phosphate Buffer System

A buffering mechanism that uses phosphate to neutralize hydrogen ions in the urine. Monohydrogen phosphate (HPO4 2-) combines with hydrogen ions to form dihydrogen phosphate (H2PO4-), which is excreted. The video explains how this system supports the kidneys in maintaining pH balance.

💡Proximal Convoluted Tubule

The first segment of the renal tubule where most reabsorption of water, electrolytes, and bicarbonate occurs. The video details how the proximal convoluted tubule reabsorbs substances from the filtrate, playing a critical role in acid-base balance.

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