2 The Structural Basis of Kidney Function

Question 1

Q: Which process is the kidney key in? What are the functions of the kidney? (6) How is it sensitive to the body’s needs? (2)

Answer 1

A: homeostasis- constancy of the internal environment

(creating urine)

• filtration of blood plasma (close to aorta= system under high pressure)
• selective reabsorption of contents being retained (PCT)
• tubular secretion of some components
• concentration of urine as necessary (collecting tubule)
• endocrine function: signals for the rest of body eg renin, erythropoietin, activated vitamin D

hormones, nerves

Question 2

Q: Why may someone who drinks lots of water get urine infections?

Answer 2

A: constantly drinking loads and body is always removing and during night, still removing but no input -> concentrated urine = puts you at higher risks of getting an infection

Question 3

Q: Describe the gross structure of the kidneys. (3) Urine path? Blood supply variation? (2)

Answer 3

A: cortex on outside = disorganised structure

medullary pyramids are striated and regular

^ come down into papilla -> urine -> expelled into renal pelvis -> ureter

• high blood supply where there’s filtration AKA outer
• lower where tubes come down… ^^

Question 4

Q: What’s the result of a car accident/ lots of blood loss? (3)

Answer 4

A: bp drops

outer cortex is preserved

medulla is part of kidneys that die -> can lead to death

Question 5

Q: What’s the renal cortex structure? (2) Why? Medulla?

Answer 5

A: disorganised/ granular

lots of glomeruli hence not regular structure

striated look since- lots of tubules present

Question 6

Q: Describe filtration.

Answer 6

A: 1. blood goes through glomerulus

2. filtered under pressure = hyperfiltration
3. all components of filtrate =

Question 7

Q: What’s the structure of the glomerulus? (3) All?

Answer 7

A: 1. fenestrated endothelium = leaky = specific for filtration

2. basement membrane = specialised to act as filter too
3. outside= podocytes (cells of bowmans)= sit on arterioles

3 fenestrated layers

Question 8

Q: What are the components of the renal corpuscle? (3) Blood supply? (2) Filtration barrier made of? (3) Result? Where does the filtrate drain? (2)

Answer 8

A: Bowman’s capsule (collect filtrate)
glomerulus consists of capillaries
podocytes associated with glomerulus

from afferent arteriole, exit to efferent arteriole
glomerular capillaries at high pressure due to pressure difference (due to differences in diameter)

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

Question 9

Q: Does reabsorption need energy? Where does it occur? What’s retained? (5)

Answer 9

A: yes

Material to be retained is reabsorbed in proximal convoluted tubule

Includes ions, glucose, amino acids, small proteins, water, etc

Question 10

Q: What’s the main function of loop of Henle and vasa recta (blood vessels)? mechanism? Where in the kidney is the loop of Henle?

Answer 10

A: Creation of hyper-osmotic extracellular fluid
Countercurrent mechanism= concentrated twice during once cycle

medulla

Question 11

Q: What is the function of the distal convoluted tubule? Eg? (4) What does it form?

Answer 11

A: Adjustment of ion content of urine // fine tuning

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

passes own glomerulus to form juxtaglomerular apparatus

Question 12

Q: Which part of the nephron determines the concentration of urine? Describe. Controlled by? 2 example urine values?

Answer 12

A: collecting tubule

Movement of water down osmotic gradient into extracellular fluid
Controlled by vasopressin (=ADH, antidiuretic hormone)-> affects aquaporins

specific gravity of 1= urine with same concentration or SG above 1.0303

Question 13

Q: Describe the specialised structure of the proximal convoluted tubule. (7) What type of epithelium?

Answer 13

A: -lots of mitochondria- reflect high energy requirement

• lots of vesicles containing what has been reabsorbed -> needs to be taken up
• 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 - membrane protein mediating transcellular water diffusion

Cuboidal epithelium

Question 14

Q: What’s the ratio between PCT and DCT?

Answer 14

A: 1:1

but appear to have more P as they are longer- deal with more fluid
D are shorter

Question 15

Q: What’s the role of the proximal tubule? Describe (5).

Answer 15

A: Reabsorption of 70% of glomerular filtrate BY…

Na+ uptake by basolateral Na+ pump
Water and anions follow Na+
Glucose uptake by Na+/glucose co-transporter (active movement)
Amino acids by Na+/amino acid co-transporter (active movement)
Protein uptake by endocytosis (since they’re larger)

Question 16

Q: What’s the structure of the loop of henle? (2) What else is in the same area?

Answer 16

A: descending limb= thin
ascending= thicker

lots of capillaries= vasa recta= reabsorbing content of extracellular fluid

Question 17

Q: What’s the structure of the descending limb of the loop of henle? (2) What occurs here? Driving force?

Answer 17

A: (thin tubule)
Simple squamous epithelium

Passive osmotic equilibrium (aquaporins present)= passive water movement

hyperosmotic extracellular environment created by ascending limb

Question 18

Q: What’s the structure of the ascending limb of the loop of henle? (3) What occurs here? (3) Result? (2) Controls? (2)

Answer 18

A: Cuboidal epithelium, few microvilli
High energy requirement - prominent mitochondria

1. Na+ and Cl- actively pumped out of tubular fluid
2. Very water-impermeable tight junctions
3. Membranes lack aquaporins - low permeability to water (don’t want water to follow)

Results in hypo-osmotic tubular fluid (diluted), hyper-osmotic extracellular fluid

• activity of descending limb
• collecting duct

Question 19

Q: What’s the structure of the vasa recta? Role? (2) Reason?

Answer 19

A: Blood vessels also arranged in loop

Blood in rapid equilibrium with extracellular fluid
Loop structure stabilises hyper-osmotic [Na+]= too high is toxic

Question 20

Q: What’s the structure of the distal convoluted tubule? (4) Site of? Explain. Control?

Answer 20

A: -Cuboidal epithelium, few microvilli

• Complex lateral membrane interdigitations with Na+ pumps
• Numerous large mitochondria
• Site of osmotic re-equilibration (control by vasopressin) (determines how much fluid is absorbed)

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

Specialisation at macula densa, part of juxtaglomerular apparatus

Question 21

Q: What’s the structure of collecting ducts? (4) What do they drain into? Describe. (2)

Answer 21

A: -have aquaporins in apical and basal membrane (different ones)

• simple cuboidal epithelium
• good tight junctions
• 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 22

Q: What does the medullarly collecting duct pass through? What occurs here? (2) Control of both steps?

Answer 22

A: Passes through medulla with its hyper-osmotic extracellular fluid

1. 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
2. Basolateral membrane has aquaporin-3
   - not under control

Question 23

Q: How does the juxtaglomerular apparatus work? What else affects this? (2)

Answer 23

A: -JM cells secrete renin to control blood pressure via angiotensin (ANG II -> vasoconstriction -> increased pressure)

Senses stretch in arteriole wall and [Cl-] in tubule

Question 24

Q: What makes up the endocrine part of the nephron? (3) Describe.

Answer 24

A: juxtaglomerular apparatus

• macula densa= modified cells of distal convoluted tubule
• juxtaglomerular cells around afferent arteriole sense pressure and can release renin