Renal biology

Question 1

What are the main functions of the kidneys ?

Answer 1

• ultrafiltration
• formation of concentrated urine
• acid base balance

Question 2

Where are the kidneys located ?

Answer 2

• they are at the abdominal level
• the right kidney is lower because the liver pushes against it

Question 3

What supplies and removes blood from the kidneys ?

Answer 3

• the renal artery supplies the kidneys with blood
• the renal vein removes blood from the kidneys

Question 4

Where does filtration take place ?

Answer 4

• filtration takes place in the renal pyramids

Question 5

Describe the path of the ultrafiltrate that forms in the renal pyramid

Answer 5

the ultrafiltrate that forms moves into the minor calyx and then into the major calyx and then into the renal pelvis and out through the ureter

Question 6

Why does the kidney need a high blood supply ?

Answer 6

the kidneys need constant vasoactivity (high turnover of blood) in order to carry out functions

Question 7

How much blood does the kidney receive ?

Answer 7

the kidney receives 25% of the cardiac output of the body

Question 8

What is found within renal pyramids

Answer 8

• renal pyramids contain specialised cells known as nephrons
• there are 1 to 1.5 million nephrons per kidney
• there are 8 to 10 renal pyramids per kidney

Question 9

Describe the structure of nephrons

Answer 9

• they have straight and convoluted structures
• they are tightly packed because of the convoluted segments
• each nephron is about 5 to 6 cm

Question 10

Describe the structure of the Bowman’s capsule

Answer 10

• the kidney contains a closed end known as a Bowman’s capsule
• the Bowman’s capsule contains invaginations of capillaries known as the glomerulus
• the space surrounding the glomerulus is known as the Bowman’s lumen
• the Bowman’s capsule and glomerulus together is known as the renal corpuscle

Question 11

Within the renal pyramid, where does the filtrate move from ?

Answer 11

the filtrate moves from the glomerulus to the Bowman’s lumen and to the proximal convoluted tubule

Question 12

What provides the nephron with its function ?

Answer 12

• each segment of the nephron is lined with simple epithelium
• this is what provides the nephron with its function

Question 13

What are the 2 functions of the proximal convoluted tubule ?

Answer 13

• reabsorption of 75% of the filtrate back into the interstitial fluid
• secretion : the reverse mechanism where the contents of the interstitial fluid moves back into the tubular fluid

Question 14

Describe the structure of the loop of Henle

Answer 14

• has an epithelial cell lining
• the descending limb of the loop of Henle = leaky epithelial cell lining because important for water reabsorption
• the ascending limb of the loop of Henle = thick epithelial lining because Na+ and Cl- reabsorption and it’s impermeable to water

Question 15

How much of the filtrate moves into the distal convoluted tubule and collecting duct ?

Answer 15

8 - 12%

Question 16

Which mechanism takes place in the DCT and collecting duct ?

Answer 16

fine tuning

Question 17

How does blood enter and leave the renal corpuscle ?

Answer 17

• one of the capillaries of the renal corpuscle is the afferent arteriole and one is the efferent arteriole
• blood enters through the afferent arteriole and moves into the glomerular tufts and moves out of efferent arteriole

Question 18

What is the juxtaglomerular filtrate ?

Answer 18

the intersection between the afferent arteriole and distal convoluted tubule is the JGA

Question 19

What are podocytes ?

Answer 19

• podocytes have specialised foot like processes
• they surround the capillaries and are known as the visceral layer of the Bowmans capsule

Question 20

What makes up the filtration barrier ?

Answer 20

• capillary endothelium
• glomerular basement membrane - podocyte foot like processes (face the lumen of the Bowman’s capsule)

Question 21

What controls what can pass through the filtration barrier ?

Answer 21

1) size selectivity
- proteins which are less than 60KDA can pass through but more than 60 cannot pass through (less than 1.6 nm can pass but more than 3.6 nm cannot pass )
e.g. haemoglobin can not pass through barrier as it is too big

2) charge selectivity
- the basement membrane contains glycoproteins which are negatively charged and so negatively charged substances cannot pass through the filtration barrier

Question 22

How does the filtration barrier only allow certain substances through ?

Answer 22

• there are pores between the endothelial cells of the capillaries ~ 100 nm in size
• there are pores between the podocytes which are 6 nm in size
• this forms a mesh that only allows certain substances through

Question 23

What determines the direction of filtration and rate of filtration ?

Answer 23

• Starling forces affect rate of filtration and they take place in several different directions
• force 1 is hydrostatic pressure and force 2 is oncotic pressure
• hydrostatic pressure moves in both directions from the capillary lumen to the lumen of the bowman’s capsule and from the lumen of the bowman’s capsule to the lumen of the capillary
• proteins which are left behind in the glomerulus will create the oncotic pressure which is in the direction of the lumen of bowman’s capsule to lumen of capillaries
• the hydrostatic pressure and oncotic pulling pressure will govern the direction of flow
• filtration takes place from the capillaries into the lumen of bowman’s capsule

Question 24

What is plasma clearance ?

Answer 24

• plasma clearance is the amount of solute cleared per unit of time
• plasma clearance is a good indicator of kidney function

UV/P ratio :
- concentration of solute left in urine (U)
- concentration of solute in the plasma (P)
- the volume being filtered across the filtration barrier into urine (V)

Question 25

What is the glomerular filtration rate ?

Answer 25

- inulin is a plant polysaccharide which is not absorbed or secreted by the body - inulin is filtered freely across the filtration barrier and into the PCT - it is a good indicator of glomerular filtration rate because amount of inulin being filtered is equal to the amount being cleared - the reference GFR = 125 ml per minute

Question 26

What is renal plasma flow ?

Answer 26

- PAH has 2 mechanisms : filtration and secretion - it is not freely filtered by the filtration barrier so some will be secreted and this gives a clearance of 600 ml per minute - this is a good indication for renal plasma flow

Question 27

Why is the clearance of glucose 0 ml per minute ?

Answer 27

- glucose is involved in filtration and reabsorption - the clearance of glucose is 0 ml per minute because it is all reabsorbed back into the blood - if glucose is found in the urine it suggests diabetes

Question 28

Why do bicarbonate ions need to be reabsorbed into the PCT ?

Answer 28

they are important in acid base balance

Question 29

How are bicarbonate ions reabsorbed ?

Answer 29

1) Na+/K+ ATP pump on the basolateral surface of the epithelial cell will push Na+ into the interstitial fluid and bring K+ into the cell creating a low Na+ concentration within the cell 2) A transporter on the apical surface of the epithelial cell will bring Na+ into the cell and remove H+ into the tubular fluid 3) The H+ ions bind to the HCO3 - ions in the tubular fluid and this takes place because of carbonic anhydrase which is a brush border enzyme 4) This enzyme is important to convert H+ and HCO3 - into water and CO2 5) These can freely diffuse into the cell 6) Carbonic anhydrase II present in the cell carries out the reverse reaction 7) HCO3 - ions in the cell will now move into the interstitial fluid by a transporter which allows Cl- to move into the cell 8) HCO3- will move into the blood to maintain the pH of the blood