Renal Circulation | Renal Blood Flow | Renal Autoregulation | Renal Physiology

Summary

This video from Bite Size Med explores the intricacies of renal circulation, highlighting how kidneys maintain high blood flow to form urine. It details the branching pattern from renal arteries to glomeruli and the unique counter-current mechanism involving vasa recta. The video explains how renal blood flow is regulated through afferent and efferent arterioles, emphasizing the role of filtration fraction and the autoregulation mechanisms, including the myogenic response and tubuloglomerular feedback. It concludes with methods for measuring renal plasma flow and blood flow, providing a comprehensive understanding of this vital physiological process.

Takeaways

• 🧬 The kidneys receive about 25% of the cardiac output, highlighting their high blood flow requirement.
• 🔄 The renal circulation follows the standard circulatory pathway with unique adaptations, including the renal artery branching into segmental, interlobar, arcuate, and interlobular arteries.
• 🌀 The cortex of the kidney is better perfused than the medulla, which is crucial for the filtration function performed by the glomeruli located in the cortex.
• 💧 The afferent arteriole enters the glomerulus, and the efferent arteriole exits it, with the glomerular capillaries being responsible for blood filtration.
• 🔁 The efferent arteriole forms a peritubular capillary network around the nephron, which then drains into the venous system, reversing the typical circulatory flow.
• 🔄 The renal circulation features a portal system where blood from the glomerular capillaries forms another capillary network around the tubules.
• 📉 The renal blood flow is regulated by the renal vascular resistance, primarily controlled by the afferent and efferent arterioles.
• 🌡️ The glomerular filtration rate (GFR) is influenced by the hydrostatic and oncotic pressures within the glomerulus, which can be adjusted by the dilation or constriction of the arterioles.
• 🔧 The kidneys autoregulate their blood flow through mechanisms like the myogenic response and tubuloglomerular feedback, maintaining a stable GFR despite changes in blood pressure.
• 📏 GFR can be maintained by adjusting the resistance in the afferent and efferent arterioles, which in turn affects the hydrostatic pressure in the glomerulus.

Q & A

What percentage of cardiac output do the kidneys receive?

The kidneys receive around 25% of the cardiac output.

What is the function of the glomerular capillaries in renal circulation?

The glomerular capillaries filter blood, separating plasma to begin the process of urine formation.

What happens to blood after it passes through the glomerulus?

After blood passes through the glomerulus, the efferent arteriole carries it to another capillary network called the peritubular capillaries, which surrounds the nephron.

What is a portal system, and how does it apply to the renal circulation?

A portal system involves blood passing through two consecutive capillary networks. In renal circulation, blood from the glomerular capillaries enters the peritubular capillaries instead of draining directly into veins, making it a portal system.

What is the primary regulator of renal blood flow?

Renal blood flow is regulated by the resistance in the afferent and efferent arterioles, which affects the pressure within the glomerular capillaries and ultimately the glomerular filtration rate (GFR).

How does afferent arteriole dilation affect GFR?

When the afferent arteriole dilates, more blood flows into the glomerulus, increasing the hydrostatic pressure in the capillaries and leading to a higher GFR.

What is the role of the juxtaglomerular apparatus?

The juxtaglomerular apparatus helps regulate blood pressure and GFR. It includes the macula densa, which senses sodium chloride levels, and juxtaglomerular cells, which produce renin when blood pressure or sodium chloride is low.

What is the function of renin in the renin-angiotensin-aldosterone system (RAAS)?

Renin converts angiotensinogen to angiotensin I, which is then converted to angiotensin II. Angiotensin II constricts the efferent arteriole, increasing glomerular capillary pressure and GFR.

How does tubuloglomerular feedback maintain GFR?

When the macula densa detects low sodium chloride levels, it signals the juxtaglomerular cells to release renin and also dilates the afferent arteriole. These actions increase blood flow and pressure in the glomerulus, maintaining GFR.

What is the myogenic mechanism in renal autoregulation?

The myogenic mechanism involves smooth muscle cells in the vessel walls. When the vessel is stretched due to increased pressure, calcium influx causes the muscle to contract, which constricts the vessel and keeps blood flow constant.

Outlines

00:00

🧘‍♂️ Renal Circulation Basics

This paragraph introduces the fundamental aspects of renal circulation. The kidneys receive a significant portion of the cardiac output, approximately 25%, highlighting their high blood flow requirement. The standard circulatory pathway is outlined, with arteries branching into arterioles, capillaries, and then veins. The renal artery and vein are detailed, with the renal arteries branching from the aorta and the renal veins draining into the inferior vena cava. The renal circulation is described as similar but with unique features, such as the renal artery entering the kidney at the hilum and branching into segmental, interlobar, arcuate, and interlobular arteries. The kidneys' structure, with an outer cortex and inner medulla, is mentioned, with the cortex being better perfused than the medulla. The nephrons, located in the cortex, are responsible for filtering blood through the glomerulus, afferent, and efferent arterioles. The glomerular capillaries filter blood, while the efferent arterioles form peritubular capillaries around the nephron. These capillaries eventually drain into the venous system, completing the circulatory pathway in reverse order through interlobular veins, arcuate veins, interlobar veins, and segmental veins, which finally drain into the renal vein.

05:01

🌡️ Regulation of Renal Blood Flow

This section delves into the regulation of renal blood flow and the factors affecting glomerular filtration rate (GFR). It explains how the renal blood flow is influenced by the pressure difference over resistance, with the renal arterial and venous pressure difference driving the flow and renal vascular resistance being the limiting factor. The afferent and efferent arterioles are identified as the primary regulators of flow due to their high resistance. The effects of prostaglandins and angiotensin II on the GFR are discussed, illustrating how dilation or constriction of these arterioles can increase or decrease filtration, respectively. The importance of the glomerular filtration barrier and the forces that control filtration, including hydrostatic and oncotic pressures, are highlighted. The paragraph also covers the autoregulation of renal circulation, which maintains a stable GFR despite changes in renal pressure. Two mechanisms of autoregulation are described: the myogenic mechanism, where vessel stretch triggers smooth muscle contraction to maintain flow, and tubuloglomerular feedback, where the juxtaglomerular apparatus senses changes in sodium chloride levels and adjusts the GFR accordingly.

10:02

🌡️ Measuring Renal Blood Flow

The final paragraph focuses on the measurement of renal blood flow. It explains that renal plasma flow can be measured using para-amino hippuric acid (PAH) clearance, which provides an estimate of the effective renal plasma flow. The renal blood flow is calculated as the renal plasma flow divided by one minus the hematocrit, providing a complete picture of the renal circulation. The paragraph concludes with a call to action for viewers to engage with the content by giving a thumbs up and subscribing to the channel.

Keywords

💡Renal Circulation

Renal circulation refers to the blood flow to and from the kidneys, which is crucial for their function in filtering blood and forming urine. The video explains how blood enters the kidneys through the renal artery and exits through the renal vein, with key roles for various arteries and capillary networks within the kidneys. The structure of renal circulation is essential for maintaining proper filtration and reabsorption processes.

💡Glomerulus

The glomerulus is a network of fenestrated capillaries located within the kidney's nephron. It is responsible for filtering blood as it enters the nephron, allowing the formation of filtrate while keeping larger molecules, like proteins, within the blood. The video describes the afferent arteriole bringing blood into the glomerulus, and the efferent arteriole taking blood away, marking the start of urine formation.

💡Afferent and Efferent Arterioles

The afferent arteriole brings blood into the glomerulus, and the efferent arteriole carries blood away. These two arterioles play a crucial role in regulating blood flow and filtration pressure within the kidney. The video discusses how changes in their diameter influence glomerular filtration rate (GFR), with afferent constriction reducing GFR and efferent constriction increasing GFR under certain conditions.

💡Glomerular Filtration Rate (GFR)

GFR is the rate at which the kidneys filter blood to form urine. It is determined by the balance between hydrostatic and oncotic pressures in the glomerulus. The video details how factors like arteriole constriction, pressure changes, and plasma flow affect GFR, which is crucial for understanding kidney function. Maintaining a stable GFR is important for normal excretion and homeostasis.

💡Hydrostatic Pressure

Hydrostatic pressure refers to the pressure exerted by a fluid, such as blood, in a confined space, like the glomerulus. In the video, this pressure is essential in driving the filtration of blood through the glomerular capillaries. An increase in hydrostatic pressure leads to more filtrate being produced, which raises the GFR. Conversely, a decrease in pressure reduces filtration.

💡Peritubular Capillaries

These are the capillaries that surround the nephron tubules after blood leaves the glomerulus through the efferent arteriole. Their primary role is to facilitate reabsorption of water and solutes from the filtrate back into the blood. The video explains how the transition from filtration in the glomerulus to reabsorption in the peritubular capillaries helps regulate fluid balance.

💡Vasa Recta

The vasa recta are specialized capillaries that surround the loop of Henle in juxtamedullary nephrons. They have a crucial role in maintaining the counter-current exchange mechanism, which allows the kidney to concentrate urine. The video highlights their importance in deep medullary perfusion and in maintaining the osmotic gradient necessary for water reabsorption.

💡Tubuloglomerular Feedback

Tubuloglomerular feedback is a mechanism by which the kidney regulates GFR in response to changes in sodium chloride concentration in the distal convoluted tubule. The video explains how the macula densa senses low sodium chloride and signals the juxtaglomerular cells to release renin, thereby adjusting the constriction of arterioles to maintain stable filtration rates.

💡Renin-Angiotensin System

The renin-angiotensin system is a hormonal system that regulates blood pressure and fluid balance. Renin, secreted by the juxtaglomerular cells in response to low blood flow, converts angiotensinogen to angiotensin I, which is then converted to angiotensin II. The video describes how angiotensin II acts to constrict the efferent arteriole, increasing glomerular pressure and GFR.

💡Myogenic Mechanism

The myogenic mechanism is an autoregulatory process in the kidneys where increased blood pressure causes the smooth muscle in the afferent arteriole to constrict, thereby reducing blood flow and stabilizing GFR. The video explains this as one of the ways kidneys maintain stable filtration despite fluctuations in systemic blood pressure, ensuring proper renal function.

Highlights

The kidneys receive about 25% of the cardiac output, highlighting their high blood flow requirement.

Renal arteries branch into segmental, interlobar, arcuate, and interlobular arteries, demonstrating the kidney's unique circulatory pathway.

The cortex is better perfused than the medulla, correlating with the location of glomeruli in the cortex.

Afferent arterioles lead into the glomerulus, while efferent arterioles exit, forming a crucial part of the renal filtration process.

Glomerular capillaries are fenestrated, allowing for efficient blood filtration.

Efferent arterioles form peritubular capillaries that surround the nephron, aiding in reabsorption.

The renal circulation includes a portal system, a special feature where efferent arterioles form another capillary network.

Renal blood flow is determined by the pressure difference over resistance, a fundamental principle in renal circulation.

The glomerular filtration rate (GFR) is influenced by the hydrostatic and oncotic pressures within the glomerulus.

Afferent and efferent arterioles play a critical role in regulating GFR by adjusting their resistance.

Autoregulation of renal blood flow maintains a stable GFR despite changes in renal arterial pressure.

The myogenic mechanism and tubuloglomerular feedback are key autoregulatory mechanisms in the kidney.

Juxtaglomerular cells produce renin in response to low sodium chloride levels, affecting GFR.

Angiotensin II, formed from renin, constricts efferent arterioles to increase capillary pressure and GFR.

The macula densa senses changes in sodium chloride levels and influences afferent arteriole constriction or dilation.

Renal plasma flow can be measured using para-amino hippuric acid, providing insights into renal function.

Transcripts

hello and welcome to bite size med this

video is on renal circulation

the kidneys excretory function results

in the formation of urine

and they do that from blood the kidneys

have a high blood flow

they get around 25 of the cardiac output

now the standard layout of a circulatory

pathway

involves arteries branching into

arterioles and then capillaries

followed by venules and veins the kidney

is the same

but different so there's a renal artery

and a renal vein yes

the renal arteries both right and left

they come off the aorta

and the renal veins drain into the

inferior vena cava

now first we're going to look at what

happens in between

the renal artery enters at the hilum of

the kidney

the first set of branches these are the

segmental arteries

they then pass between the pyramids and

these branches are the interlobar

arteries they then sort of arch over the

tops and form the arcuate arteries

before the arcuate arteries are the

interlobar arteries

and after that are the smaller

interlobular arteries

the kidneys have an outer cortex and an

inner medulla

the cortex is better perfused than the

medulla

the glomeruli of the nephrons there in

the cortex

so here the interlobular arteries they

form the afferent arterioles

the afferent arteriole enters the

glomerulus and what leaves the

glomerulus is the efferent arteriole

so in between we have the glomerular

capillaries

these are a bunch of fenestrated

capillaries that do the function of

filtering blood that's coming in

the efferent arterials they then form

peritubular capillaries around the rest

of the nephron

the peritubular capillaries then drain

into the venous system

so now the pathway is going to go in

reverse we start with the interlobular

veins

followed by the arcuate veins then the

interlobar veins

and the segmental veins now the

segmental veins are going to drain into

the renal vein

which exits at the hilum of the kidney

and drains into the inferior vena cava

peritubular capillaries they are in the

cortical nephrons

another type of nephron is the juxta

medullary nephron

these nephrons have long loops that

extend down into the deeper medulla

they have specialized capillaries that

go along with them in the same u

shape these are the vasa recta and they

are

important for the counter current

exchange in the counter current

mechanism

so you can see that the efferent from

the glomerular capillary network instead

of entering veins what did it do it

formed

another capillary network around the

tubules

so this is a portal system and that's

one of the special features of renal

circulation

the flow through circulation is the

pressure difference

over the resistance so here the flow

would be the renal blood flow

the pressure difference would be the

difference between renal arterial

and renal venous pressure and the

resistance would be renal vascular

resistance

of all the vessels the small vessels

like the interlobular arteries

the afferent and the efferent arterioles

they offer the highest resistance

so that's how you can regulate flow by

increasing the resistance in these

vessels

the renal blood flow would reduce

the renal plasma is filtered by the

glomerulus but only 20

normally gets filtered that's called the

filtration fraction

the fraction of the renal plasma that

got filtered by the glomerulus

the glomerulus is a set of capillaries

and like other capillaries what controls

filtration

that will be startling forces there are

two hydrostatic and oncotic pressures on

either side

but the most important one here is the

hydrostatic pressure in the capillaries

here that would be the glomerulus so

it's the hydrostatic pressure in the

glomerulus

this is the pressure from fluid or blood

itself

so it can be changed depending upon the

flow through the arterioles

the afferent and the efferent arterioles

by changing the pressure

and resistance of either of these the

amount of plasma that gets filtered that

changes as well

like if the afferent arterial is dilated

like say under the effect of some

prostaglandins

that means there's more renal plasma

flow that increases the capillary

hydrostatic pressure

so there's a higher filtration and

that's our gfr

so there's a higher gfr the opposite

would happen

if it gets constricted there will be a

lower renal plasma flow

lower hydrostatic pressure in the

glomerulus and so lower filtration

now if the efferent arterial gets

constricted

like under the influence of something

like angiotensin ii

that would increase the back pressure in

the capillaries

so there's a high capillary hydrostatic

pressure

so the gfr actually increases

this would be if there is a moderate

constriction of the afferent arterial

but what if it's severe the renal blood

flow reduces by a lot

and plasma proteins they get stuck

accumulating in the glomerulus

the thing about the glomerular

filtration barrier is plasma proteins

can't get

through and plasma proteins are

responsible for oncotic pressure now

this becomes more

than the effect of the hydrostatic

pressure what does oncotic pressure do

it pulls fluid in the opposite direction

towards the capillary

so the gfr reduces when the constriction

is

severe versus when the constriction was

moderate

the gfr increases

by the time the plasma reaches the

peritubular capillaries

now the hydrostatic pressure in the

capillaries is lower than the

interstitium

what would that mean the direction of

flow is opposite

from the interstitium towards the

capillaries

that's reabsorption so this arrangement

helps filtration happen at the

glomerulus where the pressure is higher

and reabsorption happen at the tubules

where there's lower pressure

now this circulation is autoregulated

which means it's regulated by itself

over a wide range of pressure changes

otherwise if the renal pressure changes

just by a little then the gfr would also

change renal excretion would change

every time that happened

so gfr and renal plasma flow they go

together

by regulating the renal plasma flow we

can regulate the gfr

so how would we regulate the flow by

changing the resistance in the afferent

and the efferent arterioles

there are two mechanisms by which the

kidneys auto regulate their flow

one of them is the myogenic mechanism

myo means muscle so this is in reference

to the smooth muscle that's in the wall

of the vessels

when the pressure in the vessels

increases and the vessel stretches

calcium ions enter into the smooth

muscle cells

what does calcium and flux do it causes

smooth muscle to contract

and if the smooth muscle contracts the

vessel is going to constrict

so the vessel is resisting being

stretched and that's how it keeps the

flow constant

the second mechanism is feedback from

the renal tubule

to the glomerulus so it's called tibulo

glomerular feedback

this is by a group of structures right

here which is called

juxta glomerular apparatus now that has

three parts

the macula densa which is modified cells

of the distal convoluted tubule

there are extra glomerular mesangial

cells which are

outside the glomerulus and the modified

cells of the afferent arteriole

these are called the juxtaglomerular

cells or the jg cells

so together this entire thing it forms

the juxtaglomerular

the jg cells are the ones that produce

renin

the macula densa is a sensor it detects

flow rate through the tubular lumen and

the sodium chloride levels

when there's a low arterial pressure or

if the renal blood flow is low

that'll reduce the capillary hydrostatic

pressure in the glomerulus

if that's low that means there's going

to be less filtration

so there's less sodium chloride reaching

the distal convoluted tubule

and the macula densa it senses this and

then it tells the juxtaglomerular cells

that there's low sodium chloride so the

jg

cells they then produce renin what does

renin do

it converts angiotensinogen to

angiotensin 1

and then by the angiotensin converting

enzyme angiotensin 2 gets formed

this then acts on the efferent arterial

and constricts it

angiotensin 2 preferentially acts on the

efferent arteriole and remember what we

went over earlier what would happen if

the efferent arterial constricts

the capillary pressure in the glomerulus

that hydrostatic pressure

it increases so that means there's more

filtration now

so the gfr increases and the sodium

chloride levels get fixed

the macula densa also acts on the

afferent arterial

and dilates it so again that would

contribute to increasing the hydrostatic

pressure in the glomerulus

and increasing the gfr so despite a

change in the renal arterial pressure or

in the renal blood flow

the gfr got maintained

if the sodium chloride levels are high

then again the macula denso would detect

this

it releases adenosine which acts on the

afferent arteriole

and constricts it now what would happen

there's reduced renal plasma flow

there's low capillary pressure in the

glomerulus

so the filtration that's the gfr is

reduced

lastly how is renal blood flow measured

a renal plasma flow

is measured using para amino hyperic

acid by measuring its

clearance we can get the renal plasma

flow but pah is only like 90

cleared so it's just the effective renal

plasma flow that you can get with this

the renal blood flow is the renal plasma

flow over one minus the hematocrit

and that is renal circulation if this

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