The structural basis of kidney function

Question 1

Describe homeostasis

Answer 1

The constancy of the internal environment is the condition of free life
The ability to create our own internal environment- separate from the external environment

Question 2

What happens when too much ADH is secreted

Answer 2

Thirst centres no longer activated- thirst stops- dehydrated- die

Question 3

What happens to the food and drink once it enters the body

Answer 3

Some may be lost in faces as undigested residue before it enters the body

Body
Regulation of osmolarity, [Na+], [K+], pH, nitrogen etc
Control of body fluid volume

From here:
It can be exhaled (H20, CO2)
Lost in Urine
H2O, Na+,
K+, H+, urea

Lost in sweat- water and Na+

Question 4

Describe the functions of the kidneys

Answer 4

Function
Production of urine:
Filtration of blood plasma (glomerulus)
Selective reabsorption of contents to be retained (PCT)
Tubular secretion of some components (collecting duct)
Concentration of urine as necessary (DCT)

Question 5

Describe the endocrine function of the kidney

Answer 5

Sensitive to body needs via hormones, nerves

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

Question 6

Describe the consequences of shock on the kidney

Answer 6

Blood supply reduced to bowels and kidneys
Cortex- supply is good
Pyramids- supply is limited

part of kidney which dies first is the pyramid- may see a necrosis in the urine

Question 7

Why does the cortex appear granulated

Answer 7

due to the presence of the glomerulus in the cortex

Question 8

Describe the role of filtration in urine production

Answer 8

Blood passing through glomerulus is filtered

Filtrate consists of all components

Question 9

Describe the role of reabsorption in urine production

Answer 9

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

Question 10

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

Answer 10

Main function of loop of Henle and vasa recta (blood vessels)
Countercurrent mechanism creates the hyper-osmolar ECF

Question 11

Describe the role of adjusting the ion content in the urine in urine production

Answer 11

Principally a function of distal convoluted tubule- but also takes place in collecting duct
Controls amounts of Na+, K+, H+, NH4+

Question 12

Describe the role of concentrating the urine in urine production

Answer 12

Occurs at collecting tubule
Movement of water down osmotic gradient into extracellular fluid
Controlled by vasopressin (=ADH, antidiuretic hormone)

Question 13

Why does the medullar appear striated

Answer 13

Due to the loop of Henlé there

Question 14

How may proteins get in the urine

Answer 14

Hyperfiltration
High blood pressure damages the capillaries- destroying the filter- allowing larger particles such as proteins to enter the urine

Question 15

How is a pressure gradient created in the glomerulus

Answer 15

Afferent arteriole- big
efferent arteriole- small
creates a pressure gradient for fluid to be squeezed out

Question 16

Describe filtration in the glomerulus

Answer 16

§ Fenestrae exist in the capillary endothelium.
§ A specialised basal lamina exists- allowing substances to pass through
§ There are filtration slits between the foot processes of the podocytes which allows passage of ions and molecules ONLY less than 50,000 Da.

Question 17

What are the components of the renal corpuscle

Answer 17

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

Question 18

Describe the blood supply to the renal corpuscle

Answer 18

§ Blood supplied at the vascular pole of the corpuscle by the AFFERENT which leaves via the EFFERENT.
§ The glomerular capillaries are at a HIGH pressure.
§ The afferent arteriole is LARGER than the efferent arteriole

Question 19

What does the filtration barrier consist of in the renal corpuscle

Answer 19

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

Question 20

Describe the drainage of the filtrate fluid

Answer 20

at urinary pole of corpuscle

drains to proximal convoluted tubule

Question 21

Describe the characteristics of this filtrate

Answer 21

Fluid is isotonic with the blood- except for the absence of cellular components

Question 22

Describe the functions and actions of the PCT

Answer 22

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 23

Describe the structural features of the PCT

Answer 23

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 - membrane protein mediating transcellular
water diffusion (apical membrane)
Prominent mitochondria reflect high energy requirement

Question 24

Describe the role of the descending thin tubule

Answer 24

Water reabsorbed into ECF
Passive osmotic equilibrium (aquaporins present)
Simple squamous epithelium

Question 25

Describe the action of the ascending thick limb

Answer 25

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

Question 26

Describe the vasa recta

Answer 26

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

Question 27

Where is the DCT found

Answer 27

Back at the glomerulus

Question 28

Describe the structural features of the DCT

Answer 28

Cuboidal epithelium, few microvilli

Complex lateral membrane interdigitations with Na+ pumps

Numerous large mitochondria

Specialisation at macula densa, part of juxtaglomerular apparatus

No brush border

Question 29

Describe the functions of the DCT/ cortical collecting duct

Answer 29

Site of osmotic re-equilibration (control by vasopressin)

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

Question 30

Which is longer the DCT or PCT

Answer 30

DCT- doesn’t have as much to do

Question 31

Describe the structural features of the medullary collecting duct

Answer 31

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 32

Describe the functions of the medullary collecting duct

Answer 32

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

Question 33

Describe the formation of the juxta-glomerular apparatus

Answer 33

Loops back round to the glomerulus to form the juxta-glomerular apparatus (has macula densa cells).
Next to afferent arteriole- to sense blood pressure close to the site of its generation

Question 34

What happens if there is lots of Na+

Answer 34

No renin released- no aldosterone

less reabsorption of salt and water

Question 35

Describe the juxta-glomerular apparatus

Answer 35

§ Endocrine organ comprised of the macula densa in the distal convoluted tubule, glomerulus and afferent arteriole.
§ This organ can secrete renin to alter blood pressure by angiotensin and ADH.

Question 36

What does the juxta-glomerular apparatus respond to

Answer 36

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 37

What is the capacity of the bladder

Answer 37

§ The urine drains through the ureters into the bladder (capacity ~500ml).