Acid Base Physiology | Part Three | Renal Regulation | Acidification of Urine | Renal Physiology

Summary

This Bite Size Med video delves into the kidney's role in acid-base balance regulation. It explains the Henderson-Hasselbalch equation, the importance of bicarbonate and carbon dioxide as the main buffer pair, and how kidneys respond to changes in pH by adjusting hydrogen ion secretion and bicarbonate reabsorption. The video also covers the nephron's function in bicarbonate reabsorption and hydrogen ion secretion, the role of urinary buffers like phosphate and ammonia, and how kidneys compensate for respiratory and metabolic acid-base disturbances.

Takeaways

• 🧠 The kidneys play a crucial role in regulating acid-base balance in the body alongside buffers and the lungs.
• 🔍 The Henderson-Hasselbalch equation (pH = pK + log [base]/[acid]) is fundamental in understanding pH regulation, where changes in hydrogen ion concentration affect pH levels.
• 🌡 A normal blood pH is around 7.4, indicating a neutral to slightly alkaline state.
• 🔄 The bicarbonate-carbon dioxide buffer pair is the most significant in maintaining pH balance, with the kidneys regulating bicarbonate levels.
• 💧 The kidneys respond slowly but strongly to changes in hydrogen ion concentrations, managing high or low bicarbonate levels by adjusting hydrogen ion secretion and bicarbonate production.
• 🔁 Bicarbonate reabsorption primarily occurs in the early part of the nephron, where hydrogen ions are secreted, and bicarbonate is reabsorbed through secondary active transport.
• ⏳ Hydrogen ion secretion mainly takes place in the late distal convoluted tubule and collecting duct, involving primary active transport mechanisms.
• 🧪 The kidneys use urinary buffers, such as phosphate and ammonia buffers, to manage the excretion of hydrogen ions and maintain acid-base balance.
• 🔄 In response to respiratory acidosis, the kidneys reabsorb more bicarbonate and excrete more hydrogen ions to compensate for the increased CO2 levels.
• 🔄 Conversely, in respiratory alkalosis, the kidneys reduce bicarbonate reabsorption and hydrogen ion excretion to counteract decreased CO2 levels.
• 🔄 The kidneys can compensate for all four primary acid-base disturbances, demonstrating their critical role in the body's acid-base homeostasis.

Q & A

What are the three regulatory systems of acid-base balance mentioned in the script?

The three regulatory systems of acid-base balance are buffers, the lungs, and the kidneys.

What does the Henderson-Hasselbach equation represent?

The Henderson-Hasselbach equation represents the pH of a solution, which is calculated as pH = pK + log([base]/[acid]).

What is the normal pH of blood, and what does a change in pH indicate?

The normal pH of blood is around 7.4. An increase in hydrogen ion concentration reduces the pH, indicating acidosis, while a decrease in hydrogen ion concentration raises the pH, indicating alkalosis.

What is the most important buffer pair in the body, and what is its pK value?

The most important buffer pair in the body is the bicarbonate (HCO3-) and carbon dioxide (CO2) pair, with a pK of 6.1.

How do kidneys regulate bicarbonate concentration?

Kidneys regulate bicarbonate concentration by excreting more hydrogen ions and producing new bicarbonate when there are high hydrogen ion concentrations and low bicarbonate, and by reducing hydrogen ion secretion and excreting more bicarbonate when there are low hydrogen ions and high bicarbonate.

In which parts of the nephron does bicarbonate reabsorption and hydrogen ion secretion primarily occur?

Bicarbonate reabsorption primarily occurs in the early part of the proximal convoluted tubule (PCT), while hydrogen ion secretion occurs more in the late distal convoluted tubule (DCT) and collecting duct.

How does the sodium-hydrogen exchanger contribute to bicarbonate reabsorption?

The sodium-hydrogen exchanger contributes to bicarbonate reabsorption by using the energy from the sodium-potassium ATPase to exchange sodium for hydrogen ions, which helps drive the reabsorption of bicarbonate.

What is the role of carbonic anhydrase in the nephron?

Carbonic anhydrase in the nephron plays a role in catalyzing the conversion of carbon dioxide and water into carbonic acid, which then dissociates into hydrogen ions and bicarbonate ions, facilitating the secretion of hydrogen ions and reabsorption of bicarbonate.

What are the two urinary buffers mentioned in the script, and how do they help in acid-base regulation?

The two urinary buffers mentioned are the phosphate buffers and the ammonia buffers. They help in acid-base regulation by binding with hydrogen ions to form titratable acids, which can be excreted, thus helping to maintain acid-base balance.

How does the kidney compensate for respiratory acidosis and alkalosis?

In respiratory acidosis, where the partial pressure of carbon dioxide (PCO2) is high, the kidneys compensate by reabsorbing more bicarbonate and excreting more hydrogen ions. In respiratory alkalosis, where PCO2 is low, the kidneys reduce bicarbonate reabsorption and hydrogen ion excretion.

What happens in the kidney during metabolic acidosis and alkalosis?

During metabolic acidosis, where blood hydrogen ion levels are high and bicarbonate is low, the kidney removes excess hydrogen ions and retains more bicarbonate. In metabolic alkalosis, the kidney does the opposite, reducing hydrogen ion secretion and bicarbonate reabsorption.

Outlines

00:00

🧠 Understanding Kidney Regulation of Acid-Base Balance

This segment delves into the renal mechanisms for maintaining acid-base balance. It begins by outlining the three main regulatory systems: buffers, lungs, and kidneys. The Henderson-Hasselbalch equation is introduced to explain the relationship between pH, pK, and the ratio of base to acid concentration. The kidneys' role is highlighted as the slowest but most potent system, managing high hydrogen ion concentrations and low bicarbonate levels by excreting more hydrogen ions and producing new bicarbonate. The process of bicarbonate reabsorption and hydrogen ion secretion in the nephron is detailed, with a focus on the proximal convoluted tubule, thick ascending limb, distal convoluted tubule, and collecting duct. The segment explains how bicarbonate is reabsorbed in exchange for chloride and how hydrogen ions are secreted, particularly in the late distal convoluted tubule and collecting duct. The concept of urinary buffers, including phosphate and ammonia buffers, is introduced to illustrate how the body handles fixed acids.

05:01

🌡️ Acid-Base Regulation and Urinary Buffers

The second paragraph expands on the body's production of volatile acids like carbonic acid and carbon dioxide, as well as fixed acids from protein and phospholipid catabolism. It discusses how these acids are buffered by urinary buffers, specifically the phosphate and ammonia buffers. The phosphate buffer system involves inorganic phosphates that bind with hydrogen ions to form titratable acids, which are then reabsorbed by the proximal convoluted tubule. The ammonia buffer system is described, detailing how amino acids are metabolized in the liver to form glutamine, which is then converted into ammonium ions and bicarbonate in the proximal convoluted tubule. The role of the collecting ducts in allowing ammonia to diffuse into the tubular lumen and form ammonium chloride, which is excreted, is explained. The paragraph concludes by discussing how the kidneys can compensate for different acid-base disturbances, such as respiratory acidosis or alkalosis, and metabolic acidosis or alkalosis, by adjusting bicarbonate reabsorption and hydrogen ion excretion.

Keywords

💡Acid-Base Balance

Acid-base balance refers to the maintenance of stable blood pH levels, typically around 7.4, through various physiological processes. This balance is crucial for the proper functioning of the body's cells and organs. In the video, acid-base balance is maintained by three regulatory systems: buffers, the lungs, and the kidneys. The kidneys play a key role in this balance by regulating the levels of hydrogen ions and bicarbonate.

💡Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a fundamental concept in understanding acid-base chemistry. It is represented as pH = pK + log([base]/[acid]). The equation is used to calculate the pH of a solution based on the concentrations of its acidic and basic components. In the context of the video, it is used to explain how changes in hydrogen ion concentration affect blood pH, leading to acidosis or alkalosis.

💡Bicarbonate (HCO3-)

Bicarbonate is a crucial buffer in the blood and plays a significant role in maintaining acid-base balance. It is part of the bicarbonate-carbon dioxide buffer system, which is the most important buffer pair in the body. The video explains that bicarbonate is regulated by the kidneys, which reabsorb it or allow its excretion to manage acid-base balance.

💡Nephron

The nephron is the functional unit of the kidney, responsible for filtering blood and producing urine. It consists of several parts, including the glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. The video discusses how different parts of the nephron are involved in the reabsorption of bicarbonate and secretion of hydrogen ions.

💡Carbonic Anhydrase

Carbonic anhydrase is an enzyme that catalyzes the conversion of carbon dioxide and water into carbonic acid, which then dissociates into hydrogen ions and bicarbonate ions. This enzyme plays a central role in the regulation of acid-base balance, as explained in the video, by facilitating the formation and dissociation of carbonic acid in the nephron.

💡Hydrogen Ion Secretion

Hydrogen ion secretion is a process by which the kidneys eliminate excess hydrogen ions from the body to prevent acidosis. The video describes how hydrogen ions are secreted into the tubular lumen of the nephron, particularly in the late distal convoluted tubule and collecting duct, through primary active transport mechanisms.

💡Bicarbonate Reabsorption

Bicarbonate reabsorption is the process by which the kidneys retain bicarbonate in the blood to prevent alkalosis. The video explains that most of the bicarbonate reabsorption occurs in the proximal convoluted tubule, where it is co-transported with sodium, and later in the nephron, it is exchanged for chloride.

💡Intercalated Cells

Intercalated cells are specialized cells found in the collecting duct of the nephron. They are involved in the regulation of acid-base balance by secreting hydrogen ions and reabsorbing bicarbonate. The video mentions type A alpha intercalated cells, which use hydrogen ATPase and hydrogen potassium ATPase to actively transport hydrogen ions into the tubular lumen.

💡Urinary Buffers

Urinary buffers are substances in the urine that can bind to hydrogen ions, thus helping to regulate the urine's pH. The video discusses two main urinary buffers: phosphate buffers and ammonia buffers. These buffers are crucial for the excretion of fixed acids and maintaining overall acid-base balance.

💡Ammonia Buffer System

The ammonia buffer system is a key mechanism for regulating acid-base balance, particularly in chronic acidosis. The video explains how ammonia is produced in the proximal convoluted tubule from glutamine and then excreted in the urine as ammonium, which binds with hydrogen ions to form ammonium chloride, thereby helping to excrete excess hydrogen ions and regulate pH.

💡Respiratory Acidosis/Alkalosis

Respiratory acidosis and alkalosis are conditions resulting from imbalances in the respiratory system's ability to regulate carbon dioxide levels. The video describes how changes in the partial pressure of carbon dioxide (PCO2) affect blood pH and how the kidneys compensate for these changes by adjusting bicarbonate reabsorption and hydrogen ion secretion.

Highlights

The kidneys regulate acid-base balance through three regulatory systems: buffers, lungs, and kidneys.

The Henderson-Hasselbalch equation is introduced to explain the pH of the solution.

Acidosis occurs with an increase in hydrogen ion concentration, and alkalosis with a decrease.

The normal blood pH is around 7.4, with bicarbonate and carbon dioxide being the most important buffer pair.

The kidneys manage high hydrogen ion concentrations and low bicarbonate by excreting hydrogen ions and producing new bicarbonate.

The nephron's role in bicarbonate reabsorption and hydrogen ion secretion is explained.

Eighty percent of bicarbonate is reabsorbed in the early part of the proximal convoluted tubule (PCT).

The process of hydrogen ion secretion and bicarbonate reabsorption in the PCT is detailed.

The late nephron, including the distal convoluted tubule (DCT) and collecting duct, is involved in hydrogen ion secretion.

Type A alpha intercalated cells in the DCT and collecting duct are responsible for hydrogen ion secretion.

The urinary system buffers hydrogen ions using phosphate and ammonia buffers.

The role of phosphate buffers in forming titratable acid and their reabsorption in the PCT is discussed.

Ammonia buffers are upregulated in the liver and play a crucial role in long-term chronic acidosis.

The kidneys compensate for respiratory acidosis by reabsorbing more bicarbonate and excreting more hydrogen ions.

In respiratory alkalosis, the kidneys reduce bicarbonate reabsorption and hydrogen ion excretion.

The kidneys also compensate for metabolic acidosis and alkalosis by adjusting hydrogen ion and bicarbonate levels.

The kidneys are responsible for acidification of urine and overall regulation of acid-base balance.

Transcripts

hi everyone welcome to bite size med

where we talk about quick bite size

concepts in basic medical sciences for

study and rapid review

this video is on how the kidneys

regulate acid-base balance

just a quick recap there are three

regulatory systems of acid-base balance

buffers the lungs and the kidneys

the henderson-hasselbach equation gives

a ph of the solution

that's ph equals pk plus the log of the

concentration

of base over acid

if there's an increase in the hydrogen

ion concentration

the ph reduces and there's acidosis

if there's a lower hydrogen ion

concentration the ph

rises and that's alkalosis the normal ph

of blood is around 7.4

the most important buffer pair is the

bicarb carbon dioxide pair

they have a pk of 6.1 so the ph

is equal to 6.1 plus the log of the

bicarb concentration

over .03 into the partial pressure of

arterial

carbon dioxide this bicarbonate is

regulated by the kidneys

the kidneys are the slowest to respond

but they're the strongest

they manage high hydrogen ion

concentrations and low bicarbonate

by excreting more hydrogen ions and

producing new bicarbonate

similarly low hydrogen ions and high

bicarbonate is managed by reducing

hydrogen ion secretion

and excreting more bicarbonate so first

let's see how these two get exchanged in

the nephron

normally hydrogen ions are secreted and

bicarbonate is reabsorbed

so this is the nephron with the

glomerulus the proximal convoluted

tubule

the loop of henle with its thin

descending limb thin

ascending limb thick ascending limb the

distal convoluted tubule

and the collecting duct except for the

thin descending limb and the thin

ascending limb

all other parts of the tubule are

involved so the proximal convoluted

tubule

the thick ascending limb the dct and the

collecting duct

so we're going to look at it in two

parts bicarbonate reabsorption first

and then hydrogen ion secretion eighty

percent of the bicarb gets reabsorbed in

the early pct

carbon dioxide enters the cell binds to

water and by carbonic anhydrase

becomes carbonic acid that dissociates

into the hydrogen ion and the bicarb ion

the hydrogen ion gets secreted into the

tubular lumen

by a sodium hydrogen exchanger

this is a secondary active transport the

energy for this

comes from the sodium potassium atpase

on the basal lateral membrane

which pushes three sodium out and two

potassium in

so now the sodium concentration in the

cell is low which pulls sodium along its

gradient

in exchange for the hydrogen ions

bicarbonate

gets reabsorbed with sodium as a

co-transport

while in the later nephron it gets

exchanged for chloride

so the hydrogen ion that's secreted it

binds to bicarbonate which has been

filtered from the glomerulus and so is

in the tubular lumen

and by carbonic anhydrase they form

carbonic acid

that then dissociates into carbon

dioxide and water

and the carbon dioxide diffuses back

into the cell

so what's happening here for every

bicarbonate that's getting reabsorbed

one hydrogen ion is getting secreted the

bicarbonate gets into the bloodstream

but the hydrogen ion

binds to the filtered bicarbonate in the

lumen

so there's no net secretion of hydrogen

ions but there is net reabsorption of

bicarbonate

so that's bicarbonate reabsorption now

hydrogen ion secretion

is more in the late dct and collecting

duct

in the type a alpha intercalated cells

carbon dioxide enters the cell combines

with water to form carbonic acid

which then dissociates into a hydrogen

ion in the bicarb ion

the hydrogen ion here gets actively

pushed into the tubular lumen

by a hydrogen atpase and a hydrogen

potassium atpase

this is primary active transport because

it uses atp

versus the pct where hydrogen was

secreted as a part of secondary active

transport

and here the bicarbonate gets reabsorbed

in exchange for chloride

when the secreted hydrogen ions is more

than the bicarbonate in the filtrate

excreting it as an ion itself will lower

the urinary ph

that number can go down to as low as 4.4

this is the limiting ph it cannot go

lower than that

the body produces volatile acids like

this carbonic acid and carbon dioxide

but also catabolism of proteins and

phospholipids

produces fixed acids which cannot

dissociate

but these have to get excreted as well

but the hydrogen also needs to get out

so how it gets buffered by two urinary

buffers

the phosphate buffers and the ammonia

buffers

the secretion of a hydrogen ion with the

urinary buffer

is called a titratable acid

the phosphate buffer involves inorganic

phosphates

they are filtered by the glomerulus so

they're in the tubular lumen

they bind to the hydrogen ions to form

this titratable acid

phosphates do get reabsorbed by the

proximal convoluted tubule

but because more water is reabsorbed

there the phosphate concentration in the

lumen increases

now this bicarbonate that is generated

is new

it's not just replacing what was

filtered since the hydrogen ion joined a

buffer that is not bicarbonate

so we've effectively created a new

bicarbonate molecule

but since phosphate does get reabsorbed

the amount here is small

and that's why there's an ammonia buffer

amino acids get metabolized in the liver

and form glutamine

in the pct glutamine gets taken up by

diffusion down its concentration

gradient

by glutaminase it becomes glutamate and

by glutamic dehydrogenase

it forms alpha-ketoglutarate each of

these steps

releases an ammonium ion and further

metabolism of this alpha-ketoglutarate

produces two bicarbonate ions so

in the end what we need to know is

there's two ammonium ions

and there's two bicarbonate ions the

bicarb gets reabsorbed and this is new

bicarb

ammonium shifts out of the cell by

counter transport with sodium

it binds with chloride and gets excreted

as

ammonium chloride

the collecting ducts are permeable to

ammonia

so ammonia can diffuse into the tubular

lumen

bind with the hydrogen ion that's been

secreted to form ammonium

but ammonium cannot go through the

aluminal membrane

so it gets conveniently trapped in the

lumen this is called diffusion trapping

so it binds to chloride forms ammonium

chloride and gets eliminated

it's this ammonia buffer system that can

be up regulated

when needed like in long-term chronic

acidosis

so secretion of hydrogen ions and

reabsorption of bicarbonate go together

so what can affect this first the

partial pressure of carbon dioxide

if it's high like in respiratory

acidosis

by the henderson hasselbach equation the

ph will be low

so the kidney compensates by reabsorbing

more bicarbonate

and excreting more hydrogen ions so it

compensates for respiratory acidosis

if the pco2 is low like in respiratory

alkalosis

the ph is high and the kidneys

compensate

by reducing bicarb reabsorption and

reducing hydrogen ion excretion

and that's how it compensates for

respiratory alkalosis

if the blood hydrogen ion levels are

high and the bicarb is low

like in metabolic acidosis the kidney

will compensate by removing the excess

hydrogen ions

and keeping more bicarbonate and it does

the opposite

in metabolic alkalosis so the kidneys

can compensate for all four primary

acid-base disturbances

and that is how the kidneys are

responsible for acidification of urine

and also regulate acid-base balance

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