Nephron在线视频

Welcome to Clinical Histology.

I am Dr. Xie from SUMC.

In this session,

we will focus on the nephron,

the filtering unit of the kidney.

Firstly, we will take a closer look

at the kidneys and identify their nephrons.

Then, we will explore the components

and basic functions of the nephron.

We will also examine

the layers of the filtration membrane,

a vital structure for filtering blood

to produce urine.

The kidneys are two bean-shaped

fist-sized organs,

located in the retroperitoneal space,

one on each side of the spine.

They, together with the ureter,

bladder and urethra

constitute the urinary system.

Healthy kidneys filter about 90ml,

that is a half cup of blood,

every minute, removing wastes

and extra water to make urine.

As shown on the right,

the hemisected view of the kidney

reveals three distinct regions:

the outermost renal cortex,

the middle renal medulla,

and the inner renal pelvis.

The renal cortex and the renal medulla

make up the urine-forming portion

of the kidney.

The cone-shaped structures in

renal medulla are called renal pyramids,

which are made of parallel

bundles of small tubes,

we will see what these tubes

are in a moment.

The renal pelvis drains urine

made by the cortex and medulla

into the ureter.

Please note that renal cortex

houses 90-95% of kidney's blood vessels,

while fewer blood vessels

are found within the renal medulla.

How is the urine made?

As the urine-forming portion of the kidney,

the renal cortex and medulla

contain over one million microscopic

filtering units called nephrons.

Each nephron is capable of

filtering blood and producing urine.

This diagram shows a single nephron,

which is composed of two major components:

the globe-shaped renal corpuscle,

and a long, meandering tube of epithelium

called the renal tubule.

The renal tubule extends

from the corpuscle

and drains into the collecting duct.

The corpuscle filters

blood to form primary urine.

The primary urine then

flows through the renal tubule.

In the renal tubule, almost all of water,

along with electrolytes and nutrients

in the primary urine are reabsorbed

and returned to the blood circulation.

The remaining fluid

and wastes become secondary urine,

which flows out of the nephron tubule

into the collecting duct.

Please note the arrangement

of these components in the kidney:

the renal corpuscle

and the majority of the renal tubule

reside in the cortex,

whereas parts of the renal tubule

dip into the medulla

and form parallel tube bundles

with the collecting ducts.

Next,

we will zoom in to see more structural

details of the renal corpuscle and tubule,

which will help us understand

how exactly urine is formed.

Each renal corpuscle consists of two parts:

the glomerulus and the Bowman's capsule.

The glomerulus is actually

a tuft of looping fenestrated capillaries

that arise from the

afferent glomerular arteriole,

whereas the Bowman's capsule

is in fact the dilated, pouch-like,

proximal end of the renal tubule.

The glomerulus invaginates into the

Bowman's capsule to form the renal corpuscle.

Therefore,

the Bowman's capsule has two layers.

The parietal layer is composed

of squamous epithelial cells,

while the visceral layer consists

of modified epithelial cells

called podocytes,

which are in intimate contact

with the glomerulus.

This scanning electron micrograph

shows a glomerulus

surrounding by the podocytes.

We can see the podocytes

have long foot processes,

which is called the pedicels (or pedicles).

The pedicels wrap around glomerular capillaries

and interdigitate with each other,

leaving slits between them.

The slits are covered by slit diaphragms

which restrict

the passage of

large macromolecules

and ensure that they remain

in the bloodstream.

These weird podocytes are indispensable

components of the filtration membrane

for blood filtering.

The filtration membrane

is made up of three layers:

the fenestrated glomerular

capillary endothelium,

the podocyte as we discussed just now,

and the basement membrane between them.

The filtration membrane is actually

a barrier between the lumen

of glomerulus capillary

and the lumen of Bowman's capsule,

so-called Bowman's space.

Please pause the video and try to identify

where the filtration membrane is in this diagram.

Parts of the filtration membrane

are indicated

by maroon rectangles in this diagram.

I hope all of you can find them.

As the blood flows through

the glomerulus capillaries,

it is filtered by the filtration membrane.

Consequently,

the filtrate enters Bowman's space,

which is continuous

with the beginning of the renal tubule.

As mentioned above,

the renal tubule is a winding tube

responsible for further

processing of the primary urine.

It has three regions:

the proximal tubule constituted

by convoluted and straight sections,

the nephron loop,

and the distal tubule constituted

by convoluted

and straight sections as well.

They differ in their structures

and functions.

As shown in this figure,

the proximal tubule consists

of simple cuboidal epithelial cells

with central nuclei and prominent microvilli

that project into the lumen

to form a brush border.

This border greatly increases

the surface area for efficient reabsorption.

A great deal of secretion takes place

in the proximal tubule as well.

The proximal tubule continues

as the thin limb of the nephron loop,

also known as the loop of Henle,

which is hair-pin like structure

that extends deep into the medulla.

The nephron loop has two limbs:

the descending limb

travels down to the apex of renal medulla,

while the ascending limb climbs up

toward the renal cortex.

The descending limb is composed

of simple squamous epithelium,

the majority of the ascending limb

is made up of thicker

simple cuboidal epithelium.

The final segment is the distal tubule.

Like its proximal counterpart,

the distal tubule consists

of simple cuboidal epithelium.

However, the distal tubule

lacks a brush border.

For more detailed functions

of the renal tubules,

please check your textbooks or our

online clinical physiology sessions.

Glomerulonephritis refers to

inflammation of the glomeruli in the kidney.

It is characterized

by the leakiness of the filtration membrane,

which will lead to

proteinuria and/or hematuria

and finally complete renal failure.

Biopsy of the kidney

is an important way to diagnose

the different types of glomerulonephritis.

The left shows a normal glomerulus

surrounded by Bowman's capsule.

The Bowman's space

can be easily identified.

However, in the section on the right

from a glomerulonephritis patient,

the proliferation of fibrous tissue

has occupied the entire Bowman's space

and it is difficult

to clearly identify the glomerulus.

It seems that the glomerulus

has been compressed and replaced

by the fibrous tissue.

These abnormalities will destroy the

normal functions of the nephron

and cause renal failure.

Almost all the major functions

of the kidney are performed

by the nephrons.

In this session,

we have examined the structures

and basic functions of the nephron.

The nephron is composed

of a renal corpuscle and a tubule.

In the corpuscle, the pressure

in the glomerular capillaries,

forces water and small

solutes of blood plasma,

to go through the filtration membrane

into the Bowman's capsule.

From the Bowman's capsule,

primary urine enters the long renal tubule

for further processing.

The processed filtrate will finally

flow out of the nephron

and drain into the collecting duct.

Can you describe the components

of the nephron and the layers

of the filtration membrane now?

That's all for today.

Thank you for joining us!

Hope to see you soon!