Lecture 32 - Sodium and water reabsorption, control of body water and urine composition

Question 1

There are three important places where water is reabsorbed in the nephron:

Answer 1

Proximal convoluted tubule (PCT):
67% of filtered load reabsorbed

Descending limb of the nephron loop:
25% of filtered load reabsorbed
Absorbs water,
Ascending absorbs sodium.

Collecting duct (CD):
2 - 8% of filtered load reabsorbed

Excretion
< 1 - 6% of filtered load is excreted

Question 2

Types of water reabsorption:

Answer 2

Bulk (obligatory) water reabsorption

Regulated (facultative) water reabsorption

Question 3

Bulk (obligatory) water reabsorption

Answer 3

accounts for 92% of total water reabsorption
not regulated – automatic!!
leaky epithelia
trans-and paracellular water reabsorption

Question 4

Regulated (facultative) water reabsorption

Answer 4

accounts for 2-8% of total water reabsorption
regulated by anti-diuretic hormone (ADH)
tight epithelia
only transcellular reabsorption

Question 5

Types of sodium absorption:

Answer 5

Bulk sodium (Na+ ) reabsorption
accounts for 92% of total sodium reabsorption

Regulated sodium (Na+ ) reabsorption
accounts for 7-8% of total sodium reabsorption
regulated by aldosterone (RAAS)

Question 6

what drives and regulates body water homeostasis:

Answer 6

Distribution of body water

Osmolarity of solutions

Changes in blood osmolarity

Reabsorption of water and sodium in the nephron

Effects of osmotic changes on the kidney

Effects of volume changes on the kidney

Question 7

what is water reabsorption in the proximal tubule (67% of the filtered load) is driven by?

Answer 7

Na+ reabsorption (isosmotic!!)

Transporters such as the sodium-glucose cotransporter use the sodium gradient to reabsorb solutes (like glucose)

Question 8

What type of epithelia is present in the PCT?

Answer 8

Leaky

Question 9

How does chlorine diffuse across?

Answer 9

via the paracellular pathway

Question 10

How is water diffused across?

Answer 10

Water follows chlorine by the paracellular (between cells) and the transcellular (across cells) pathways

Question 11

What are the steps of the Transcellular pathway in the PCT?

Answer 11

1. Na+ and Glucose in (apical)
2. Glucose transporter out (basolateral)
3. Na/K ATPase out/in (basolateral)
4. K+ channel out (basolateral)

Question 12

What are the steps of the paracellular pathway in the PCT?

Answer 12

Chlorine:
1. Electrical driving force made in peritubular fluid by K+ and Na+
2. Cl- flows through leaky tight junctions by electric gradient

Water:
1. Osmotic gradient made
2. Water flows through paracellular and aquaporins (transcellular)

Question 13

Descending in the nephron loop is permeable to what?

Answer 13

Water

Question 14

Ascending in the nephron loop is permeable to what?

Answer 14

Sodium

Question 15

The descending loop is what type of epithelia?

Answer 15

leaky epithelium, so water (H2O) is reabsorbed from the nephron into the peritubular fluid

Question 16

What does the Hyper-Osmotic Medullary Gradient (HOMG) show?

Answer 16

The deeper the nephron loop in the medulla = the higher the osmolarity gradient

Question 17

How is water reabsorbed in the collecting duct:

Answer 17

Regulated (facultative) water reabsorption

tight epithelia

only transcellular reabsorption

regulated by anti-diuretic hormone (ADH)

Question 18

How is sodium reabsorbed in the DCT and collecting duct:

Answer 18

Regulated sodium reabsorption

regulated by aldosterone (RAAS)

Question 19

How is ADH secreted?

Answer 19

TBW changes alter plasma (ECF) osmolarity

detected by osmoreceptors in hypothalamus

stimulates pituitary gland to secrete more/less ADH

Question 20

What does ADH do?

Answer 20

alters permeability of collecting duct (CD) cells, so water is retained/excreted to balance the initial change in TBW

Question 21

ADH causes ____ water reabsorption

Answer 21

Increased, more water out of nephron into the blood

Question 22

What are the chain of effects caused by ADH?

Answer 22

Decrease in TBW

Increase in ECF osmolarity

Detected by osmoreceptors in the hypothalamus

Increase in release of ADH from the posterior pituitary

Insertion of aquaporins in apical membrane of CD cells: Increased water permeability

Increase in water reabsorption Decrease in urine volume

ECF osmolarity returns to normal