Urine formation

Summary

This script offers an in-depth look at urine formation, focusing on the nephron's role as the kidney's filtration unit. It explains the journey of blood through the nephron, detailing the filtration process in the glomerulus and the subsequent reabsorption along the tubules. The script also touches on the body's ability to regulate the reabsorption of substances like water and sodium through hormonal control, ultimately highlighting how only 1% of filtered substances become urine, with the rest being reabsorbed or secreted back into the bloodstream.

Takeaways

• 😀 The nephron is the functional unit of the kidney responsible for filtering blood and forming urine.
• 🌀 Blood enters the nephron through the afferent arteriole, which leads to the glomerulus where filtration occurs.
• 🔎 The glomerulus is a capillary bed that filters blood due to its high pressure, typically around 50 millimeters of mercury.
• 🚫 Large molecules like proteins and blood cells do not pass through the filtration membrane due to size and charge.
• 💧 Approximately 120 milliliters of filtrate are produced per minute in each kidney, totaling 240 milliliters for both kidneys.
• ⏳ The body reabsorbs 99% of the filtrate back into the bloodstream through various parts of the nephron.
• 🔄 The proximal convoluted tubule reabsorbs 65% of the filtrate, the loop of Henle 15%, the distal convoluted tubule 15%, and the collecting ducts 4-5%.
• 🌡 Hormones like antidiuretic hormone and aldosterone can regulate the amount of water and sodium reabsorbed, respectively.
• 📉 Urine formation is the result of glomerular filtration, tubular reabsorption, and tubular secretion, with only about 1% of the filtrate becoming urine.
• ➕ Tubular secretion is the process by which substances from the blood are secreted into the nephron for excretion in urine.

Q & A

What is the functional unit of the kidney?

The functional unit of the kidney is the nephron, which is responsible for the filtration process.

What is the role of the glomerulus in the nephron?

The glomerulus is a capillary bed within the nephron where the initial filtration of blood occurs, separating plasma from blood cells and larger proteins.

What is the purpose of the tubules in the nephron?

The tubules in the nephron are responsible for further processing of the filtrate, including reabsorption of necessary substances back into the blood and secretion of waste products.

Why is the blood pressure higher in the glomerulus compared to other capillary beds?

The blood pressure in the glomerulus is higher, at about 50 millimeters of mercury, to facilitate the filtration process and push substances from the blood plasma into the nephron.

How much of the blood plasma is filtered at the glomerulus per minute?

Approximately 20% of the blood plasma, which is about 120 milliliters per minute, is filtered at the glomerulus in both kidneys combined.

What is the total amount of filtrate produced by the kidneys in a day?

The kidneys produce approximately 172,800 milliliters or about 172.8 liters of filtrate per day.

How much of the filtrate actually becomes urine, and why?

Only about 1% of the filtrate becomes urine, as 99% of it is reabsorbed back into the blood through the tubules of the nephron.

What is the role of the afferent and efferent arterioles in the nephron?

The afferent arteriole brings blood into the glomerulus, and the efferent arteriole carries the blood away from the glomerulus, with the peritubular capillaries surrounding the tubules to facilitate reabsorption.

What is tubular reabsorption and where does it primarily occur?

Tubular reabsorption is the process by which filtered substances are returned to the bloodstream. It primarily occurs in the proximal convoluted tubule, where 65% of the filtrate is reabsorbed.

What is tubular secretion, and how does it differ from reabsorption?

Tubular secretion is the process where substances are actively transported from the blood into the tubules to be excreted in the urine. It differs from reabsorption as it adds substances to the filtrate rather than returning them to the blood.

How does the body regulate the balance between filtration, reabsorption, and secretion to control urine output?

The body regulates urine output by adjusting the rates of filtration, reabsorption, and secretion through hormonal controls such as antidiuretic hormone and aldosterone, which can increase water and sodium reabsorption respectively.

Outlines

00:00

🔬 Understanding Nephrons and Urine Formation

This paragraph introduces the process of urine formation in the kidneys, focusing on the nephron as the functional unit responsible for filtration. It explains the blood supply to the nephron, particularly the role of the glomerulus as the site of filtration. The nephron's tubules are highlighted as the pathway for filtrate, which, if not reabsorbed, becomes urine. The paragraph also reviews the structure of the nephron, including the glomerular capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. The importance of the peritubular capillaries, which facilitate the reabsorption of filtrate back into the blood, is emphasized.

05:01

💧 Glomerular Filtration and Cardiac Output

The second paragraph delves into the specifics of glomerular filtration, detailing how blood plasma is forced through the filtration membrane due to high pressure within the glomerulus. It clarifies that large molecules like proteins and blood cells are too big or repelled by the filtration membrane's charge to pass through. The paragraph then connects this process to cardiac output, explaining that the heart pumps approximately 70 ml of blood per beat, resulting in about 5 liters per minute. Of this, 20% reaches the kidneys, and 60% of that volume is plasma, which is subject to filtration. The calculation shows that 120 ml of filtrate is produced per minute by both kidneys, leading to a significant daily volume that is mostly reabsorbed, leaving only about 1.7 liters as urine.

10:02

🔁 Tubular Reabsorption and Its Regulation

This paragraph discusses the reabsorption process, where the majority of the filtrate is returned to the bloodstream through the nephron's tubules. It outlines the percentages of reabsorption at different segments of the nephron: 65% in the proximal convoluted tubule, 15% in the loop of Henle, another 15% in the distal convoluted tubule, and 4-5% in the collecting ducts, totaling nearly 99% reabsorption. The paragraph also touches on how the body can adjust these percentages through hormonal regulation, such as the release of antidiuretic hormone to increase water reabsorption or aldosterone to enhance sodium reabsorption.

15:06

➡️ Tubular Secretion and Urine Composition

The final paragraph introduces tubular secretion, a process where substances from the blood are actively secreted into the nephron's tubules, contributing to urine composition. It contrasts this with reabsorption and emphasizes that secretion involves moving substances from the blood into the tubules, which are on the path to excretion. The paragraph concludes by summarizing that urine formation is a complex process involving filtration, reabsorption, and secretion, with the latter two processes being adjustable to maintain the body's homeostasis.

Keywords

💡Nephron

A nephron is the functional unit of the kidney, responsible for filtering the blood and producing urine. In the video, the nephron is described as the 'filtration unit' where blood is filtered, and it is composed of a glomerulus, tubules, and other structures. The script emphasizes the nephron's role in creating filtrate, which is the initial fluid that may become urine after further processing.

💡Glomerulus

The glomerulus is a network of capillaries located at the beginning of the nephron where blood filtration begins. The script describes it as a 'ball of yarn' structure where blood vessels branch into a capillary bed. It is the site of high pressure filtration, which is crucial for separating waste and excess substances from the blood.

💡Filtrate

Filtrate refers to the fluid that has been filtered from the blood and contains waste products and excess substances. In the video, filtrate is mentioned as the substance that is contained within the tubules of the nephron after passing through the glomerulus. It is the precursor to urine, which, if not reabsorbed, will continue through the nephron and be excreted as urine.

💡Afferent Arteriole

The afferent arteriole is the blood vessel that carries blood towards the glomerulus. The script labels it as the first part of the blood supply to the nephron, emphasizing its role in delivering blood to the capillary bed where filtration occurs. It is essential for maintaining the high pressure needed for effective filtration.

💡Efferent Arteriole

The efferent arteriole is the blood vessel that carries blood away from the glomerulus. According to the script, it continues and hugs all the tubules of the nephron, forming the peritubular capillaries. This structure plays a role in the reabsorption of filtered substances back into the blood.

💡Proximal Convoluted Tubule

The proximal convoluted tubule is the first section of the nephron's tubular system, located after the glomerular capsule. The script explains that it is where a significant portion (65%) of the filtrate is reabsorbed back into the bloodstream. This process is critical for the body's ability to conserve water and essential nutrients.

💡Loop of Henle

The loop of Henle, also known as the nephron loop, is a U-shaped structure in the nephron that plays a key role in concentrating the urine. The script describes it as being composed of a thin descending limb and a thick ascending limb. It is involved in the countercurrent multiplier system, which helps in the reabsorption of water and the concentration of urine.

💡Distal Convoluted Tubule

The distal convoluted tubule is a part of the nephron that follows the loop of Henle. As mentioned in the script, it is involved in the reabsorption of additional filtrate, specifically 15% of the filtered load. This segment also plays a role in the regulation of ion concentrations in the body.

💡Collecting Duct

The collecting duct is the final part of the nephron where the remaining filtrate passes through before it becomes urine. The script indicates that a small percentage (4-5%) of the filtrate is reabsorbed here, and it is also the site where substances can be secreted from the blood into the urine, contributing to the final composition of the urine.

💡Tubular Reabsorption

Tubular reabsorption is the process by which filtered substances are returned to the bloodstream from the nephron's tubules. The script explains that approximately 99% of the filtrate is reabsorbed, with different percentages occurring at various segments of the nephron. This process is essential for the body to maintain proper electrolyte balance and hydration.

💡Tubular Secretion

Tubular secretion is the process by which substances are secreted from the blood into the nephron's tubules, contributing to the formation of urine. The script mentions that this process can occur at different parts of the nephron and involves the transfer of substances such as urea into the tubules to be excreted.

Highlights

The nephron is the functional unit of the kidney responsible for filtration.

Blood supply to the nephron is crucial for oxygen delivery and filtration.

The glomerulus is a capillary bed where filtration begins.

Filtrate from the blood contains what has been filtered out and is destined to become urine if not reabsorbed.

The afferent arteriole brings blood into the glomerulus.

The efferent arteriole carries blood away from the glomerulus to the peritubular capillaries.

The glomerular capsule is the initial part of the nephron where filtration occurs.

The proximal convoluted tubule is the first section of the tubule where reabsorption begins.

The loop of Henle is responsible for concentration and dilution of the filtrate.

The distal convoluted tubule plays a role in fine-tuning the composition of the filtrate.

The collecting duct gathers filtrate from multiple nephrons and contributes to the final concentration of urine.

Glomerular filtration rate is approximately 120 ml per minute in both kidneys combined.

Cardiac output contributes to the volume of blood filtered by the kidneys.

Only 20% of cardiac output reaches the kidneys, equating to 1 liter of blood per minute.

Of the blood plasma filtered, 20% becomes filtrate, amounting to 120 ml per minute.

The kidneys filter approximately 175 liters of filtrate per day, yet produce only about 1.7 liters of urine.

Tubular reabsorption reclaims 99% of the filtrate back into the bloodstream.

Tubular secretion allows the nephron to secrete substances from the blood into the filtrate.

Hormones such as antidiuretic hormone and aldosterone regulate the reabsorption process.

Transcripts

so let's have a quick look at how the

blood in our body moving through into

the kidneys gets filtered and ultimately

forms urine so we're going to have a

look at urine formation so a couple of

things you need to go through the

previous videos and understand what a

nephron is and understand how the blood

supply gets to the nephron and how that

blood supply starts to become filtered

okay so just as a very quick recap we're

going to go through some functional

anatomy and we're going to name

different aspects of this nephron so

remember this is the functional unit of

the kidney this is the filtration unit

is the nephron and what you can see is a

blood supply coming through now this

blood supply is it here to give this

nephron oxygen this blood supply is

coming in because here is a capillary

bed now remember this capillary bed is

called the glomerulus remember

glomerulus means ball of yarn so you've

got a blood vessel coming in that

branches into this gun Miralles

capillary bed and this is where

filtration happens and remember the

tubules here of the nephron contain

what's just been filtered from this

blood and so this is called filtrate and

if this filtrate remains in the nephron

all the way through it comes out as

urine okay so urine formation has to do

with this filtration process here okay

and what's happening to this filtrate

all the way through the nephron that's

what we're going to discuss today but

before we begin let's label some things

so that you know exactly where we are so

first thing is that we have blood coming

into or towards this nephron remember

this is called the afferent arteriole

and afferent arteriole comes down into a

capillary bed which we call the

glomerulus

the glove mirrorless that continues down

into not a vein remember but an artery

another arterial which are called the

efferent arterial and you can see that

the efferent arteriole continues and it

basically hugs all the tubules of the

nephron and we call this peri tubular

capillaries okay why does this capillary

bed here hug the tubules of the nephron

because remember what gets filtered into

the nephron doesn't necessarily stay in

the nephron 99% of this filtrate that

gets into the nephron actually gets

thrown back into the blood and it gets

thrown back into this peritubular

Network okay so that's labeling the

blood supply let's label the nephron now

I'm not going to write the labels down

because I'll have to rub them out but

what you can see is the first part here

we call the glomerular capsule okay so

that's the glomerular capsule looks like

a little pac-man the first portion of

the tube you'll closest to the capsule

is called the proximal convoluted tubule

proximal because it's closest to

convoluted because usually you'll see it

as these squiggly wavy lines but I've

just drawn here straight for simplicity

sake

so proximal convoluted sugar you've got

the loop of Henle also known as the

nephron loop which is made up of the

thin descending loop of Henle and the

thick a sending loop of Henle then you

have the distal convoluted tubule and

then you have the collecting duct okay

now when it comes to your information

what you're going to find is couple

things that urine formation is actually

equal to a couple of things your

information is equal to glove Marilyn

filtration what that means is remember

our ultimate goal is your information

here so we're going to talk about how we

can get from here to here firstly we

need to start off with glomerular

filtration let's write that as number

one so you know from previous videos

that as the blood comes through that

there is a relatively high pressure here

the glomerulus that now remember that at

most capillary beds that pressures

around about thirty twenty to thirty

millimeters of mercury so what's that

mean there's a hydrostatic pressure are

pushed behind the blood pushing stuff

out of the capillaries at the tissues of

your body but once you get here to the

kidneys this capillary bed is different

and the push is greater it's fifty

millimeters of mercury okay so you have

this very strong push pushing substances

within the blood plasma throat okay now

remember proteins do not go through and

cells do not go through why while cells

are too big so white blood cells red

blood cells and platelets they're all

too big to move through this filtration

membrane or I should say filtration

membranes proteins

well the proteins are actually small

enough to get through but there is a

negative charge at this filtration

membrane and remember proteins are

negatively charged and like charges

repel each other so in a healthy

filtration membrane proteins are

repelled and stay in the blood okay

first thing so filtration now off the

blood that comes down through into here

what you'll find is so think about

cardiac output right so your heart will

contract and relax contraction will exit

as it does this it pushes out blood

right so every time it contracts it

pulls it pushes out around about 70 ml

of blood from one ventricle so left

ventricle squirt 70 mils of blood and it

does that about 72 times a minute

so if it squirts out 70 mils in one go

times 72 in a minute that gives you

around about five liters a minute so

your heart pushes out from one ventricle

five liters a minute that's the cardiac

output and that five liters a minute

goes to the whole body right so only a

fraction of that's going to get to the

kidneys 20% of that will get to a

kidneys what's 20% of 5 blenders 1 litre

so 1 litre of blood goes to the kidneys

every minute

now that 1 litre of blood is made up of

all these different components is made

up of cells

it's about proteins it's made up of

blood plasma and so forth what gets

filtered only plasma right because the

cells and proteins don't go through so

of that one liter what you'll find is 60

percent of it is plasma

so what's 60 percent of one liter six

hundred mils so you have six hundred

mils of blood plasma that's coming

through that can be filled in it now of

this how much of that six hundred mils

actually goes through twenty percent

okay so 20% of this six hundred mils

gets filtered through how what's twenty

percent of six hundred it's 120 mils so

every minute every minute in here you

have a hundred and twenty mils being

filtered through so that means you

create 120 mils of filtrate every minute

in both both your kidneys together so

not one nephron this is representing on

both your kidneys okay so 120 mils a

minute is what your kidney creates of

filtrate now think about that 120 miles

a minute now let's do some very quick

math how many minutes are there in an

hour 60 minutes so now okay how many

hours are there in a day 24 so 60 times

24 is

1440 minutes

okay 1440 minutes in a day but and you

have 120 mils per minute being mowed so

120 times 1440 and this is something

that I wrote down before

here's 172 800 mils per day your kidneys

create one hundred seventy two thousand

eight hundred mils of filtrate per day

that is nearly that's approximately 175

litres a day but you know that all this

100 that this is 175 litres you know PA

you know P at 175 litres a day you

actually pay out round about 1.7 liters

per day it's something 1% so of

everything that you filter through you

only pay 1% so let's just say here 175

litres per day or by the time it gets to

here it's only 1 point 7 litres a day so

what does that mean that means of

everything you've filtered 99% of it 99

percent of it goes back into your blood

back into your body okay so that means

that in order to create urine we need to

add something else to this equation we

said urine formation is equal to ik

lamella filtration which is 120 mils per

minute

-

tubular reabsorption right so this is

that's the tubules and it's going back

into the body so it's reabsorption not

absorption because you've already

absorbed this stuff before in the gr to

your reabsorbing it back into the body

okay so this is step two tubular

reabsorption so let's just leave this a

bit better here

step one filtration and this is step two

tubular reabsorption so tubular

reabsorption you've got all these to do

the proximal convoluted tubule you have

the loop of Henle and the distal

convoluted tubules are collecting that

in actual fact that happens all

throughout okay so if you'll have a look

here you'd find that and the proximal

convoluted tubule 65% of what's just

being filtered goes back into the body

and the proximal convoluted tubule 65%

okay at the loop of Henle that you'll

find is that 15% goes back into the body

okay so what's that so far certainly

only 80% okay here at the distal

convoluted tubule you'll find that

another 50% gets reabsorbed back into

the body that's 95% and here at the

collecting ducts four to five percent

that equals 99 percent to 100 percent

okay because remember 99 percent of what

just got filtered goes back into the

body 65 percent of it happens here at

the proximal convoluted tubule 15

percent happens at loop of Henle 15

percent happens at the distal convoluted

tubules and 45 percent happen at the

collecting ducts now the great thing is

your body can't alter this so it's not

always going to be 65 percent 1515 and 4

or 5 percent if you want to reabsorb

more this percentage can go up if you

want to reabsorb less it can go down and

vice versa with all these ok so for

example if you want to reabsorb all why

don't you can release 88 antidiuretic

hormone which makes you reabsorb more

water here at the collecting ducts if

you want to reabsorb more sodium into

your body you can release out dosterone

and this can tell the process of the

distal convoluted tubules to reabsorb

more sodium okay so your information is

equal to glomerular filtration

- tubular reabsorption so let me just do

some more room so does that make sense

why it's - because we remember 120 mils

per minute here but we need to - what's

going back 99 percent so now we have 1

point 2 mils coming through run 120 mils

take 1% of that is 1.2 mil because that

99% scrub back so now we only have by

the time we reach here where they have

one point two mils per minute which is

one point seven liters a day ok coming

through a one point two litres a day now

that's not the end of it because your

nephron can actually take some stuff

from the bodies not just give substances

to the blood it can receive substances

the blood and this is called this is

number three and this is called tubular

secretion now why is it called secretion

is because it's secreting substances

from the blood into the tube you'll

people get confused about reabsorption

or secretion and so forth whatever is in

the tube you need to remember is

essentially being outside the body this

is what's going to be pin out if it

stays in the tube so if it goes fund the

blood into into this tube you'll it's

secretion and this is number three

tubular secretion and again this can

happen at different stages or different

parts of the nephron okay

but usually it's not a huge amount okay

not a huge amount so what can we see we

can see when it comes to urine formation

it's made up of three different

components glomerular filtration which

is about 120 mils a minute chabela

reabsorption throwing back 99% of all

that stuff back into the body okay and

tubular secretion throwing out some

stuff this stuff is going to be for

example urea for example this is urea

can be thrown back into the tubules to

be excreted or to be paid out so this is

your information