Lecture 29 - The nephron

Question 1

The nephron

Answer 1

Microscopic functional unit of the kidney
Approximately 1 million per kidney (2 million because there are 2 kidneys)
Responsible for urine formation

Question 2

Types of nephron

Answer 2

Cortical nephrons
85% of nephrons are this type (main types)
Lie mainly in cortex - some parts go into the medulla but majority of it sits out in the cortex

Juxtamedullary nephrons
Extend deep into medulla
Important for the formation of concentrated urine

Question 3

The nephron and associated structures

Answer 3

Function:
Selectively filter blood
Return to blood anything to be kept
Carry waste away for storage and expulsion

Each nephron is composed of (regardless of whether it is cortical or juxtamedullary):
A glomerular capsule
Renal tubules
A collecting duct

Each nephron is associated with:
A glomerulus
Peritubular capillaries

Question 4

Glomerular capillaries

Answer 4

Specialised for filtration

Thin walled single layer of fenestrated endothelial cells (to allow for filtration, so that things can leak out of it whilst still being selective) )

Fed and drained by arterioles - fed by afferent arterial and blood drains away through an efferent arteriole and the whole reason for this is that we can control via dilating or constricting Bia these vessels to control pressure of blood and flow of blood which means that we have some control over filtration

Blood pressure here is tightly regulated

Question 5

Peritubular capillaries

Answer 5

Specialised for absorption

Wrapped around renal tubules

Carries filtered blood from the efferent arteriole

Receives reabsorbed filtrate from the nephron

Can secrete into nephron

Vasa recta
Extensions that from the nephron loops deep into the medulla
Only found with juxtamedullary nephrons
The hairpin turns slow the rate of blood flow, which helps maintain the osmotic gradient required for water reabsorption.

Question 6

The renal corpuscle

Answer 6

Glomerulus enclosed by the glomerular capsule
Where capillary and nephron meet
Site of the filtration barrier

Question 7

Glomerular capsule

Answer 7

First part of the nephron (aka Bowman’s capsule)

Two layers:
Outer parietal layer of simple squamous cells - thin layer of simple squamous cells that stops the filtrate from leaking out into the surrounding tissue
Inncer visceral layer of podocytes

Between the two layers is the capsular space (aka Bowman’s space/urinary space) which receives filtrate

Filtrate = what we take out of the blood that might become urine (some of it definitely becomes urine) is going to go first before it travels down the rest of the nephron tube

Question 8

Podocytes

Answer 8

Surround the glomerular capillaries

Very branched, very specialised epithelium

Branches from intertwining food processes called pedicels
By interdigitating, they can wrap around glomerular capillaries and this becomes incredible important because the capillaries are fenestrated and anything that is small enough to makes its way through the fenestrations is able to be filtered but actually we want to limit this size in order to become more selective so pedicels reduce the space that things can pass through so it is more selective

Filtration slits form between pedicels

Filtered blood (filtrate) goes through these slits and passes into capsular space

Question 9

Filtration barrier

Answer 9

Aka blood-urine barrier/glomerular capsular membrane

Lies between the blood and capsular space

Allows free passage of water and small molecules (small molecules like glucose and salts can pass through)

Restricts passage of most proteins
Job is to restrict what goes through based on size

RBCs are not filtered into nephron

Question 10

Three layers of the filtration barrier

Answer 10

Fenestrated endothelium of glomerular capillary

Fused basement membrane - anchors them together and you do not want these structures to come apart because you won’t get filtering going on and stuff that shouldn’t be getting across is getting across

Filtration slits between the pedicels of the podocytes

Question 11

Which two structures form the renal corpuscle?

Answer 11

Glomerulus surrounded by the glomerular capsule

Question 12

What happens after filtration?

Answer 12

Not everything that is filtered is excreted
Some filtrate is reabsorbed
And some of what wasn’t filtered is secreted into the nephron

So urine = filtered-reabsorbed + secreted

Question 13

Proximal convoluted tubule (PCT)

Answer 13

(2nd part)

Bulk reabsorption (first chance to take back anything that we have filtered)

Located in cortex
Surrounded by peritubular capillaries

Structure:
Cuboidal epithelial cells
Dense microvilli (brush border) on luminal membrane
Highly folded basolateral membrane
Many mitochondria for active transport
Leaky epithelium (because we want paracellular pathway reabsorption as well, things passes between the cells as well as across the cells)

Close to the renal capsule and it is wiggly in appearance

Question 14

Nephron loop

Answer 14

Aka the loop of Henle
(3rd part)

Loops down into the medulla
Length is important in production of dilute/concentrated urine

Surrounded by vasa recta (juxtamedullary nephrons only)

Structure:
Thick descending limb - like PCT
Thin descending limb - simple squamous epithelium
Thin ascending limb - simple squamous epithelium
Thick ascending limb - Like DCT

Different areas have different permeabilities to water and sodium

Question 15

Distal convi=oluted tubule (DCT)

Answer 15

(4th part)
Fine tuning

Cuboidal epithelium, but thinner than PCT

Structure
Few microvilli = no brush border
Fewer mitochondria
Reabsorption regulated mainly by aldosterone

Question 16

Collecting duct

Answer 16

5th part
Fine tuning

Filtrate from several DCTs drains into one collecting duct, which empty at papilla

Structure
Wall of simple cuboidal epithelium
Principal cells - reabsorption
Intercalated cells - acid/base balance

Reabsorption regulated by ADH and aldosterone

Question 17

Histology of the nephron

Answer 17

Renal corpuscle
Squamous epithelium of parietal layer of Bowman’s corpuscle
Bowman’s/capsular space
Glomerulus

PCT
Fuzzy lumen due to long microvilli

DCT
Clear lumen

Question 18

Juxtaglomerular apparatus (JGA)

Answer 18

Specialised zone in every nephron/ next to nephron

Located where DCT lies against afferent arteriole

Both vessels have specialised cells:
DCT - Macula densa cells. Chemoreceptors. Detect sodium levels (how much sodium in the filtrate and this will tell us how much sodium we will loose in the urine and it also gives us an idea of water loss
Afferent arteriole - juxtaglomerular cells. Mechanoreceptors. Detect blood pressure (detect stretch within the blood vessel)

Controls glomerular filtration rate, ensuring system working at full capacity

Stabilise blood pressure

Question 19

Flow of filtrate/urine through the nephron and kidney

Answer 19

Side note - filtrate when passing through the nephron and use the word urine when it has left the nephron and this is because filtrate can still be changed a little

Flow of filtrate
Blood comes into the glomerus and filtration through the 3 parts of the filtration barrier - across the fenestrated endothelium with the capillary, across the fused basement membrane, and across the filtration slits between the pedicels of the podocytes and into the capsular space. Then into the proximal convoluted tubule where we have bulk of the reabsorption going on and then into the nephron loop and then to the distal convoluted tubule and collecting duct where we have fine tuning of the filtrate then down to the papilla of the medullary pyramids and at this point the fluid coming out of the papilla is urine, and goes into the minor calyx which joins with another minor calyx to form a major calyx and then the major calyces merge together to form the renal pelvis which then narrows at the hilum to form the ureter