Structural basis of kidney function

Question 1

what is the standard amount of fluid we should have

Answer 1

2 Litres for a 70 kg Man

Question 2

How can ADH kill you?

Answer 2

• ADH makes you thirsty

- once you have too much ADH, thirst stops so they become overly dehydrated which kills the person

Question 3

What are the functions of the kidneys?

Answer 3

Production of urine:

- Filtration of blood plasma
- Selective re-absorption of contents to be retained
- Tubular secretion of some components
                   - Concentration of urine as necessary

-Sensitive to body needs via hormones, nerves

Endocrine function - signals to rest of body (hormones include renin, erythropoietin, 1,25-OH vitamin D):

Question 4

State some endocrine functions of the kidneys.

Answer 4

Production of erythropoietin
Hydroxylation of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol by 1 alpha hydroxylase (in other words: production of calcitriol)
Activation of the renin-angiotension system by secreting renin

Question 5

Describe the filtration of the blood plasma

Answer 5

• Lots of tubules
• outer cortex is very granulated
• medulla is striated structure
• filteration takes place and end up in the major calyces
• comes out of the ureter.

Question 6

What part of the kidney dies first if someone is going through kidney failure?

Answer 6

• The kidneys are clinically important because it receives a lot of blood supply.
• When someone is going through kidney failure, the amount of blood is reduced to the kidneys and the bowel.
• The part to die first of the kidney is the pyramid as it receives the most minimum amount of blood and the cortex receives the most.
• This can be seen in people’s urine

Question 7

What causes the granular structure and the striated structure?

Answer 7

• Granular because it is not a regular structure

- in the medulla: have very straight tubules so the medulla looks very different.

Question 8

What is the renal corpuscle?

Answer 8

Bowman’s capsule
glomerulus consists of capillaries
podocytes associated with glomerulus

Question 9

Describe filtration and the components filtered?

Answer 9

-Blood passing through glomerulus is filtered,
-Filtrate consists of all components
-at urinary pole of corpuscle
drains to proximal convoluted tubule

Question 10

Describe what happens to people with uncontrolled hypertension?

Answer 10

• Renal artery comes off the aorta
• very short distance
• high pressure structure
• these arterioles have a lot of pressure
• hypertension: capillaries get damaged
• proteins in the urine
• because filter is damaged

Question 11

Describe some features of the glomerulus.

Answer 11

The capillaries are fenestrated. There is a specialised basal lamina that filters the blood.

Question 12

What are the 3 components of the filter?

Answer 12

1. High SA, lots of capillaries
2. Inner surface of capillaries is fenestrated endothelium, so acts like a sieve.
3. Modified basement membrane allows blood to filter.

Question 13

How many layers of filtaration on the glomerulus. Describe another feature which is used for filtration?

Answer 13

There are 3 layers of filtration on the glomerulus.

Outside of the capillaries, there are podocytes which provide filtration.

Question 14

Describe some features of the glomerulus.

Answer 14

The capillaries are fenestrated. There is a specialised basal lamina that filters the blood.

Question 15

what type of fluid leaves after filtaration

Answer 15

ISOTONIC.

Question 16

State the five stages of urine production.

Answer 16

Ultrafiltration 
Selective Reabsorption 
Creation of hyper-osmotic ECF 
Adjustment of ion concentration in urine 
Final adjustments of urine concentration

Question 17

Where does most reabsorption take place?

Answer 17

Proximal convoluted tubule (70% reabsorbed)

Question 18

State some features of the proximal convoluted tubule and what happens at this structure

Answer 18

Larger diameter than the distal convoluted tubule
Brush border
Abundant mitochondria
CUBOIDAL epithelium sealed with tight junctions
Aquaporins
Sealed with (fairly water-permeable) tight junctions
Membrane area increased to maximise rate of resorption
brush border at apical surface
interdigitations of lateral membrane

Functions
Reabsorption of 70% of glomerular filtrate
Na+ uptake by basolateral Na+ pump
Water and anions follow Na+
Glucose uptake by Na+/glucose co-transporter
Amino acids by Na+/amino acid co-transporter
Protein uptake by endocytosis

Question 19

What mechanism creates the hyperosmotic extracellular fluid?

Answer 19

Countercurrent mechanism

Question 20

Describe features of the descending and ascending limbs of the loop of Henle.

Answer 20

Descending
Thin, squamous epithelium 
Permeable to water 
Ascending 
Thick, cuboidal epithelium 
Few microvilli 
Prominent mitochondria (for active transport of ions) 
Na+ and Cl- are pumped out into the ECF 
Water impermeable

Question 21

State some features of the distal convoluted tubule. Which features are different from the proximal convoluted tubule?

Answer 21

Smaller diameter than the proximal convoluted tubule
Few microvilli
Numerous mitochondria
Complex lateral membrane interdigitations
Adjusts Na+, K+, H+ and NH4+ under the influence of aldosterone
Macula densa cells

Question 22

What is the role of the collecting duct?

Answer 22

Final adjustment of urine concentration

Question 23

Which transporters are involved in the movement of water into the cell via the apical membrane and out of the cell via the basolateral membrane?

Answer 23

Aquaporin 2 - apical membrane

Aquaporin 3 - basolateral membrane (not affected by vasopressin)

Question 24

What type of epithelium do the calyces and the renal pelvis have and what are its properties?

Answer 24

Urothelium - resistant to urine, ability to stretch, low permeability

Question 25

What does the juxtaglomerular apparatus respond to? What does it do?

Answer 25

Macula Densa - detects NaCl concentration
Juxtaglomerular Cells - detect stretch in afferent arterioles (renal perfusion pressure)
Leads to secretion of renin resulting in angiotensin II production

Question 26

describe the mechanism of reaborption?

Answer 26

Material to be retained is reabsorbed in proximal convoluted tubule
Includes ions, glucose, amino acids, small proteins, water, etc
Creation of hyper-osmotic extracellular fluid
Main function of loop of Henle and vasa recta (blood vessels)
Countercurrent mechanism
Adjustment of ion content of urine
Principally a function of distal convoluted tubule
Controls amounts of Na+, K+, H+, NH4+
Concentration of urine
Occurs at collecting tubule
Movement of water down osmotic gradient into extracellular fluid
Controlled by vasopressin (=ADH, antidiuretic hormone)

Question 27

Describe the countercurrent mechanism for the loop of henle?

Answer 27

Loop of henle becomes thin and there are different lengths of this. In the medulla there are different concentrations, there is a difference in its concentrating ability.
The vasa recta are the capillaries

Descending thin tubule
-not many mitochondria
Passive osmotic equilibrium (aquaporins present)
Simple squamous epithelium

Ascending thick limb
Na+ and Cl- actively pumped out of tubular fluid
Very water-impermeable tight junctions
Membranes lack aquaporins - low permeability to water
Results in hypo-osmotic tubular fluid, hyper-osmotic extracellular fluid
Cuboidal epithelium, few microvilli
High energy requirement - prominent mitochondria
Vasa recta
Blood vessels also arranged in loop
Blood in rapid equilibrium with extracellular fluid
Loop structure stabilises hyper-osmotic [Na+]

Question 28

What happens in the distal convoluted tubule/corticol collecting duct?

Answer 28

Site of osmotic re-equilibration (control by vasopressin)

Adjustment of Na+/K+/H+/NH4+ (control by aldosterone)

Cuboidal epithelium, few microvilli

Complex lateral membrane interdigitations with Na+ pumps

Numerous large mitochondria

Specialisation at macula densa, part of juxtaglomerular apparatus

Question 29

`what happens in the medullary collecting duct?

Answer 29

Passes through medulla with its hyper-osmotic extracellular fluid
Water moves down osmotic gradient to concentrate urine
Rate of water movement depends on aquaporin-2 in apical membrane
content varied by exo-/endocytosis mechanism
under control from the pituitary hormone vasopressin
Basolateral membrane has aquaporin-3, not under control
Duct has simple cuboidal epithelium
Cell boundaries don’t interdigitate
Little active pumping so fewer mitochondria

Drains into minor calyx at papilla of medullary pyramid
Minor and major calyces and pelvis have urinary epithelium

Question 30

Describe the juxtaglomerular apparatus

Answer 30

Endocrine specialisation
Secretes renin to control blood pressure via angiotensin
Senses stretch in arteriole wall and [Cl-] in tubule
Cellular components are
macula densa of distal convoluted tubule
juxtaglomerular cells of afferent arteriole

Question 31

How much distal is there

Answer 31

There are less distal convuluted tubule as it is shorter so you see more proximal than distal