Reabsorption

Question 1

Overview of reabsorption in the nephron tubule (and collecting duct): PCT

Answer 1

• (R) of ~65% water, Na+, K+, Ca2+
• 90% HCO3
• 100% organic nutrients
• ~50% urea reabsorbed

Question 2

Overview of reabsorption in the nephron tubule (and collecting duct): LOH

Answer 2

• (R) of 15% water
• 20-30% Na+, Ca2+
• 10% HCO3

Question 3

Overview of reabsorption in the nephron tubule (and collecting duct): DCT

Answer 3

• variable water (~15%)
• variable (R) of Na+ + Ca2+
  (under hormonal control)

Question 4

Overview of reabsorption in the nephron tubule (and collecting duct): CD + papillary duct

Answer 4

CD
- variable (R) of water, K+, Na+, H+ and HCO3

Papillary duct
- ~50% urea (R)

Question 5

Proximal convoluted tubule - micro-villi

Answer 5

Proximal convoluted tubule: - epithelial cells are large, square shaped + big nuclei + also have micro-villi

• Micro-villi are increasing the inside surface area = allowing greater amount of surface area to come into contact with the filtrate = sign of getting a lot of reabsorption
• Most reabsorption takes place in the proximal convoluted tubule = great reabsorber

Question 6

What is trans-epithelial transport

Answer 6

= utilising pumps and channels

• solutes are (R) via this transport
• the (R) is highly specific

Question 7

Secondary active transport +

Osmosis

Answer 7

• utilises energy indirectly from “other” concen. gradients e.g. glucose uses the [Na+] gradient
• cotransport, counter-transport, antiport

Osmosis

• diffusion of water, follows concen gradients, especially (Na+) = uses aquaporins
• solutes are “water magnets”

Question 8

Tubular (R) includes trans-epithelial transport:

Answer 8

• neph tubule wall made up of a single layer of epithelial cells = aka tubular epithelium
• on one side of the epithelial cell is the neph tubule lumen, containing tubular fluid + other side is the interstitial fluid
• (R) = transport of material from the tubular fluid across the epithelium to the IF and then into the blood in the peritubular capillary
• the cell mem facing tubule lumen (luminal/apical mem) contains dif channels to the CM facing IF (basolateral mem)

Question 9

What does tubular (R) rely on?

Answer 9

• Tubular (R) relies on active transport of Na+
• The first step of tubular (R) is a problem. The TF + the IF are basically the same thing (both made from capillary filtration).
• = no concen gradients. What is the driving force?
• Tubular (R) relies on the large [Na+] gradient b/w the extracellular fluid + the intracellular fluid of the epithelial cells generated by the Na+-K+ATPase
• This [Na+] gradient is crucial in driving all (R) in the nephron tubule
  → Blockade of the Na+-K+ATPase w/ the drug ouabain stops all tubular (R)

Question 10

Na+ reabsorption via primary active transport

Answer 10

1. Na+ is pumped out the basolateral side of cell by the Na+K+ATPase into the IF, reducing intracellular [Na+]
   - the [Na+] in the IF then moves into the blood stream
2. Na+ enters cell through membrane proteins, moving down its electrochemical gradient
   - The [Na+]tf is higher than that in the [Na+]i –> pressure of [Na+] that drives diffusion of Na+ from the TF into the epithelial cells via Na+ channels

Question 11

This (R) of Na+ from the tubular fluid is utilised in 2 ways:

Answer 11

• i. it sets up electrochemical gradients for other solute to be reabsorbed.
• ii. the diffusion of Na+ can be used indirectly to drive the movt of other solutes (secondary active transport)

Question 12

(i) (R) of Na+ sets up electrochemical gradients that drive (R) of other solutes

Answer 12

1. Na+ is (R) by active transport
2. Electrochemical gradient drives anion (R)
3. Water moves by osmosis, following solute (R)
4. Concentrations of other solutes increase as fluid vol. in lumen decreases (K+, Ca2+, urea). Permeable solutes are (R) by diffusion.

Question 13

(ii) secondary active transport of glucose

Answer 13

• 100% of glucose is (R) in the PT, against its concen gradient via cotransport w/ Na+
• The energy from the diffusion of Na+ (going w/ its [Na+] gradient) is sufficient to “drag” the glucose w/ it, via the sodium & glucose cotransporter (SGLT)
• Na+-cotransport in the PT is used to reabsorb other organic solutes, such as amino acids
• A similar process is used to secrete H+ into the TF, via a Na+-H+ exchanger (an antiport).

Question 14

(R) of HCO3- in the PT

Answer 14

• (R) of bicarbonate from nephron is important for acid-base balance.
• However, bicarbonate cannot cross the apical membrane of the PT.

How to reabsorb??

• CO2 can freely diffuse across the apical membrane into the epithelial cell.
• So the bicarbonate combines with H+ using the carbonic anhydrase reaction in reverse

Question 15

(R) in LOH

Answer 15

• The two limbs of the LoH have very dif permeability characteristics

Descending limb

• is permeable to water but impermeable to ions
• selectively reabsorbs water

Ascending Limb

• is permeable to ions but impermeable to water
• selectively reabsorbs solute (Na+, K+, Cl-, Ca2+)
• especially via Na+-K+-2Cl- cotransporter (ATP pump)
• Because of the NaK2Cl pump more solute is reabsorbed than water
  → TF is more dilute (hypo-osmotic) when it reaches the DCT

Question 16

DCT + CD (R)

Answer 16

• provide “fine tuning” of (R) under hormone control

(R) of H2O

• regulated by vasopressin (ADH) in CD (& DT)
• regulation of no. of water pores (aquaporins) in epithelium

(R) of Na+

• Two paths for Na+ (R), one is constitutive (Na+-Cl- symport),
• the other regulated by aldosterone (Na+-K+ antiport).

(R) of Ca2+

• constitutive: 65% Ca2+ reabsorbed in PT, 20% in LoH
• In DT Ca2+ (R) is regulated by parathyroid hormone (PTH).
• A decrease in plasma [Ca2+] → secretion of PTH.
• PTH causes a Ca2+ channel to be inserted into apical membrane that ↑Ca2+ (R) (similar to the insertion of Na+ channel by aldosterone)