2. Structural basis of kidney function

Question 1

How is fluid lost from the body?

Answer 1

Exhalation
Urine
Sweat

Question 2

Functions of the kidneys

Answer 2

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

Sensitive to body needs via hormones and nerves

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

Question 3

Appearance of cortex and medulla

Answer 3

Cortex: Granular
Medulla: Striated

Question 4

What happens to the kidneys if someone goes into shock?

Answer 4

Body will try to reduce blood supply to bowels and kidneys

If someone goes into renal failure because of blood loss: pyramids die 1st as have worst blood supply

Question 5

Describe filtration in urine production

Answer 5

Blood passing through glomerulus is filtered

Filtrate consists of all components <50 000 molecular weight

Question 6

How is high pressure maintained in the capillaries of the glomerulus?

Answer 6

Renal artery comes off aorta

Short distance= high pressure

Question 7

Describe the size of afferent and efferent arterioles. What is the consequence of this?

Answer 7

Afferent arteriole: Big
Efferent arteriole: Small
Creates a pressure gradient, this squeezes the fluid out through the filtration system

Question 8

3 components that allow filtration in the glomerulus

Answer 8

High SA: many capillaries with fenestrated endothelium
Specialised basal lamina
Filtration slits between foot processes of podocytes: allows passage of ions and molecules <50 000

Question 9

Composition of fluid entering proximal tubule

Answer 9

Isotonic: exactly the same concentration as the blood (minus the cellular components)

Question 10

What are the components of the Renal corpuscle?

Answer 10

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

Question 11

Where does drainage of the renal corpuscle occur?

Answer 11

At urinary pole of corpuscle

Drains to proximal convoluted tubule

Question 12

Describe reabsorption in urine production

Answer 12

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

Question 13

Describe the structural features that of the proximal convoluted tubule

Answer 13

Cuboidal epithelium
Sealed with (fairly water-permeable) tight junctions
Many vesicles as reabsorbing and sending things across
High SA to maximise rate of resorption:
Brush border at apical surface
Interdigitations of lateral membrane
Contains aquaporins: membrane protein mediating transcellular water diffusion
High blood supply
Many mitochondria: as requires lots of energy

Question 14

Function of proximal convoluted tubule

Answer 14

Reabsorption of 70% of glomerular filtrate
Na+ uptake (basolateral Na+/K+ ATPase creates gradient)
Water and anions follow Na+
Glucose uptake by Na+/glucose cotransporter
Amino acids by Na+/amino acid cotransporter
Protein uptake by endocytosis

Question 15

Describe creation of hyper-osmotic extracellular fluid in urine production

Answer 15

Main function of loop of Henle and vasa recta (blood vessels)
Countercurrent mechanism

Question 16

Describe the descending thin tubule of the loop of Henle

Answer 16

Sits in hypo-osmotic fluid, so fluid is pulled in
Passive osmotic equilibrium (aquaporins present)
Allows fluid and ions to cross membrane
Simple squamous epithelium

Question 17

Describe the ascending thick limb of the loop of Henle

Answer 17

Na+ actively pumped out of tubular fluid
Very water-impermeable tight junctions (fluid becoming more concentrated in tubule, don’t want water going back in)
Membranes lack aquaporins: low permeability to water
Results in hypo-osmotic tubular fluid, hyper-osmotic extracellular fluid
Cuboidal epithelium, few microvilli
High energy requirement so many mitochondria

Question 18

Describe the vaso recta

Answer 18

Blood vessels also arranged in loop
Blood in rapid equilibrium with extracellular fluid
Loop structure stabilises hyper-osmotic [Na+]

Question 19

Describe Adjustment of ion content of urine in urine production

Answer 19

Principal function of distal convoluted tubule

Controls levels of Na+, K+, H+, NH4+

Question 20

Describe the distal convoluted tubule

Answer 20

Site of osmotic re-equilibration (controlled by vasopressin)
Adjustment of Na+/K+/H+/NH4+ (controlled by aldosterone)
Cuboidal epithelium, few microvilli
Complex lateral membrane invaginations with Na+/K+ pumps
Numerous large mitochondria
Specialisation at macula densa, part of juxtaglomerular apparatus

Question 21

Why are more proximal convoluted tubules seen in a slice than distal convoluted tubules?

Answer 21

Distal are shorter in length as it doesn’t have as much work to do

Question 22

Describe concentration of urine

in production of urine

Answer 22

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

Question 23

Describe the medullary collecting duct

Answer 23

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 pituitary hormone vasopressin)
Basolateral membrane has aquaporin-3, not under control
Simple cuboidal epithelium
Cell boundaries don’t interdigitate
Little active pumping so fewer mitochondria

Question 24

Where does the medullary collecting duct drain?

Answer 24

Drains into minor calyx at papilla of medullary pyramid

Question 25

Epithelium of minor and major calyces and pelvis

Answer 25

Minor and major calyces and pelvis have urinary epithelium

Question 26

Where is the juxtaglomerular apparatus located?

Answer 26

Next to afferent arteriole where high pressure blood entering glomerulus

Question 27

Function of juxtaglomerular apparatus

Answer 27

Endocrine function
Senses stretch in arteriole wall and [Cl-] in tubule
Secretes renin to control BP via angiotensin

Question 28

Cellular components of juxtaglomerular apparatus

Answer 28

Macula densa of distal convoluted tubule

Juxtaglomerular cells of afferent arteriole

Question 29

What does low Na+ in the distal convoluted tubule result in?

Answer 29

Increase in renin
Causes angiotensinogen to become angiotensin I and then II Causes to produce more aldosterone (retain fluid) and vasoconstriction to increase BP

Question 30

What is the MOA of ACE inhibitors as anti-hypertensives?

Answer 30

Inhibit ACE
Stop angiotensin II being produced
People get rid of fluid
Blood vessels open up and BP decreases