Functional histology of the kidney

Question 1

The Urinary System - Functions

what is the function of kidney?

what 3 things is it homeostasis of?

Answer 1

The function of the kidney is homeostasis of the blood.
In particular, homeostasis of:
• Plasma composition
o Regulated by excretion of water, ions and organic waste products into urine

• Blood pressure
o Through the enzyme renin

• RBC content
o Through the secretion of erythropoietin

The kidneys receive 25% of cardiac output.

Question 2

The Nephron

what is it? what happens to blood and components?

Answer 2

The nephron is the functional unit of the kidney, where blood is filtered at the molecular level to produce urine yet retain cells and large proteins. (also reabsorbing valuable small molecules)

• There are about 1 million nephrons in each human kidney

Question 3

Nephron parts and locations

all parts of nephon

what has the renal corpuscle? nephon? renal tubule?

Answer 3

1. Bowman’s capsule
2. Glomerulus
3. PCT
4. Loop of Henle thin arm
5. Loop of Henle thick arm
6. DCT
   a. Collecting tubule
7. Collecting duct
8. Papilla

Renal corpuscle = 1 + 2
Nephron = 1 to 6a (the functional unit)
Renal tubule = 3 to 6a (the tubule)

The renal corpuscle includes Bowman’s capsule and glomerulus. The renal tubule consists of PCT to Collecting duct.

Question 4

The Glomerulus - site of?
3 layers?
what is the filter size cut off?

Answer 4

The glomerulus is a knot of capillaries and the site of ultrafiltration

There are three layers of the glomerular filter

1. On top the endothelial layer with small pores
2. Then underneath there is a thick fused basal lamina
3. Filtration slits sit underneath formed by podocyte foot processes

The filter cut off is about 70kDa.
- This size means larger molecules are retained in the blood. Many proteins, nutrients and ions come through.

Question 5

The renal tubule - function

Answer 5

adjusting composition of ultrafiltrate, to recover nutrients, water etc, and to regulate plasma composition

Question 6

Proximal Convolute Tubule
structure + function

what is reabsorbed and how?

what happens to macromolecules and how are they reabsorbed?

what moves by passive flux?

3 main structure of the cell

Answer 6

Function is reabsorption from ultrafiltrate by:
• Active transport across membrane into cell, small molecules like Na+, glucose, Amino Acids

• By pinocytosis, macromolecules especially proteins, these are broken down in lysosomes and returned to blood
• By passive flux, water, Cl-

Structure of the cells:

• Long microvilli for high surface area for reabsorption, also contain lytic enzymes on surface to breakdown macromolecules
• Many lysosomes to break down and recycle macromolecules
• Many mitochondria to fuel active transport (seen especially near the basolateral sodium pumps)

Question 7

Loop of Henle (thin limb)
structure + function

what does it reabsorb and how?
desscribe structure

Answer 7

Function is to reabsorb water and salts from filtrate by passive flux across the epithelium by osmosis and concentration gradients

Structure is thin squamous epithelium to allow passive fluxes, minimum organelles.

Question 8

Distal Convoluted Tubule
structure + function

what does it exchange/reabsorb?
structure of cells
microvilli?
what particular organelle is abundant?

Answer 8

Function is homeostasis by regulated active transport and exchange of ions (Na+/K+/H+/HCO3-)

Structure of cell is cuboidal epithelium thicker than squamous to reduce passive fluxes and accommodate organelles.

There are few short microvilli.

Many mitochondria to fuel active transport. Mainly basal and can show as a pale or striped basal area in H&E stain.

Question 9

Collecting Duct and Collecting Tubule
structure + function

function? (2)
structure of cells? why?

Answer 9

Function is transport of urine to ureter and water homeostasis; passive reabsorption of water regulated by epithelial permeability.

Structure of cell is cuboidal to columnar epithelium to prevent passive flux of water (and urea). Also, specialised dense membranes where cell contacts other cell, function unclear.

Question 10

Juxtaglomerular apparatus

what does it release? effect of this?
where does it get the signal for this? what does that area detect?

Answer 10

Macula densa – sensing [Na+] in the DCT fluid, appears to signal to the..

Juxtaglomerular cells release renin, indirectly increases vascular tone and sodium reabsorption

Lacis cell – function unknown, maybe signalling between other two

Question 11

Ureter and bladder

what is it impermeable to?
how can it change? (describe structure)

Answer 11

Specialized to be impermeable to urine (noxious)

Changes appearance on stretching

Somewhat like stratified squamous epithelium when distended

but apical cells are biggest, and dome-shaped when not stretched: still much cytoplasm when stretched.

(In stratified squamous epithelium, basal cells are biggest, apical cells very flat.)

Question 12

Bladder distended vs contracted

describe structure when bladder is full - what do the patches protect? from what? what are they made up from?

contracted bladder
what happened to plaques?

Answer 12

Distended (bladder full):
These impermeable, rigid membrane patches (plaques) protect apical cells from toxic urine. They are made of transmembrane proteins called uroplakins.

Contracted (bladder empty):
The rigid plaques are invaginated forming irregular pits in the cytoplasm, allowing cell surface area to decrease.

Question 13

And the down-side of transitional epithelium?

Why are urinary infections quite common?
Why commoner in females?
Advice on prevention?

Answer 13

Cystitis

Why are urinary infections quite common?
Transitional epithelium highly impermeable - leukocytes of immune system cannot readily penetrate.

Why commoner in females?
Female urethra shorter. More risk of contamination, e.g. from anal region.

Advice on prevention?
Plenty of fluids.