The Structural Basis of Kidney Function

Question 1

What is the function of urine

Answer 1

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

Question 2

Which hormones do the kidneys release

Answer 2

Renin
Erythropoietin
1,25-OH vitamin D

Question 3

What are the 4 main stages of urine production in the kidney

Answer 3

Reabsorption
Creation of hyper-osmotic extracellular fluid
Adjustment of ion content of urine
Concentration of urine

Question 4

Describen the reabsorption process in urine production

Answer 4

Material to be retained is reabsorbed in the proximal convoluted tubule
Includes ions, glucose, amino acids, small proteins, water etc.

Question 5

Describe the creation of hyper-osmotic extracellular fluid in urine production

Answer 5

Main function of the loop of Henle and vasa recta

Countercurrent mechanism

Question 6

Describe the adjustment of ion content of urine in urine production

Answer 6

Principally a function of stalk convoluted tubule

Controls amounts of sodium, potassium, hydrogen and ammonium

Question 7

Describe the concentration of urine in urine production

Answer 7

Occurs at collecting tubule
Movement of water down osmotic gradient into the extracellular fluid
Controlled by vasopressin

Question 8

What are the components of the renal corpuscle

Answer 8

Bowman’s capsule
Glomerulus consists of capillaries
Podocytes associated with glomerulus

Question 9

Describe the filtration barrier of the kidney

Answer 9

fenestrae (“windows”) in capillary endothelium
specialised basal lamina
filtration slits between foot processes of podocytes
allows passage of ions and molecules

Question 10

Describe the drainage of the kidney

Answer 10

Drains into minor calyx at papilla of medullary pyramid

Minor and major calyces and pelvis have urinary epithelium

Question 11

What are the functions of the proximal convoluted tubule

Answer 11

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 12

What are the structural features of the proximal convoluted tubule

Answer 12

Cuboidal epithelium
Sealed with (fairly water-permeable) tight junctions
Membrane area increased to maximise rate of resorption
(brush border at apical surface
interdigitations of lateral membrane)
Contains aquaporins
Prominent mitochondria (high energy

Question 13

What are aquaporins

Answer 13

membrane protein mediating transcellular water diffusion

Question 14

Describe what occurs in the descending thin tubule of the loop of Henle

Answer 14

(Simple squamous epithelium)
Sits in hyper-osmotic fluid
Passive osmotic equilibrium/movement (aquaporins present)

Question 15

Describe what occurs in the ascending thick limb of the loop of Henle

Answer 15

sodium and chloride are actively pumped out tubular fluid
Low permeability to water
Hypo-osmotic tubular fluid and hyper-osmotic extracellular fluid

Question 16

Give the structural features of the wall of the ascending thick limb

Answer 16

Cuboidal epithelium 
Few microvilli
Lots of ion pumps
Water-impermeable tight junctions
Membranes lack aquaporins
Prominent mitochondria

Question 17

Describe the vasa recta

Answer 17

Blood vessels arranged in a loop
Blood in rapid equilibrium with extracellular fluid
Loop structure stabilises hyper-osmotic sodium

Question 18

What are the structural features of the distal convoluted tubule

Answer 18

Site of osmotic re-equilibration
Adjustment of ions
Cuboidal epithelium, few microvilli (no brush border)
Invaginated lateral membrane with sodium pumps
Lots of large mitochondria
Specialisation at macula densa

Question 19

What is osmotic re-equilibration controlled by in the distal convoluted tubule

Answer 19

Vasopressin

Question 20

What is adjustment of ions in the distal convoluted tubule controlled by

Answer 20

Aldosterone

Question 21

What are the structural features of the medullary collecting duct

Answer 21

Simple cuboidal epithelium
Cell boundaries do not interdigitate 
Little mitochondria (little active pumping) 
Basolateral membrane has aquaporin-3 
Tight junctions

Question 22

Describe what occurs in the medullary collecting duct

Answer 22

1. Filtrate passes through the medulla (hyper osmotic extracellular fluid)
2. Water moves down osmotic gradient to concentrate urine

Question 23

Describe the juxtaglomerular apparatus

Answer 23

Endocrine specialisation
Secretes renin to control blood pressure via angiotensin
Senses stretch in the arteriole wall and chloride in the tubule
Next to the afferent arteriole

Question 24

What are the cellular components of the juxtaglomerular apparatus

Answer 24

macula densa of distal convoluted tubule
juxtaglomerular cells of afferent arteriole
Packed with pro-renin

Question 25

Describe the filtrate in the kidneys

Answer 25

Enters through the glomerulus Consists of all components with a molecular weight less than 50,000 Isotonic (-cellular components) and will enter the proximal tubule

Question 26

Why do loops of Henle have different lengths

Answer 26

Differing concentrations of ions in the medulla

Question 27

What is the concentration of urine determined by

Answer 27

Rate of water movement depends on AQP2 in the apical membrane (Vasopressin secretion)