Lecture 32: Na+/water reabsorbtion, control of body water, urine

Question 1

What are the three locations where water is reabsorbed and what percentage in each location?

Answer 1

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

Descending limb of the nephron loop:
▪ 25% of filtered load reabsorbed

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

Question 2

How is regulated (facultative) water reabsorption regulated?

Answer 2

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

Question 2

Is bulk water reabsorption regulated?

Answer 2

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

Question 3

What percentage of water is usually excreted in the kidneys?

Answer 3

It varies based on the body’s requirements and hydration levels.

Excretion
▪ < 1 - 6% of filtered load is excreted

Question 4

What are the four locations where sodium is reabsorbed in the nephrons and what is the relative percentages for each section? How much is excreted?

Answer 4

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

Ascending limb of the nephron loop:
▪ 25% of filtered load reabsorbed

Distal convoluted tubule
▪ 5% of filtered load reabsorbed

Collecting duct (CD):
▪ 2-3% of filtered load reabsorbed

Excretion:
▪ < 1% of filtered load is excreted

Question 5

How is Regulated sodium reabsorption regulated?

Answer 5

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

Question 6

What is water reabsorption in the proximal tubule primarily driven by?

Answer 6

▪ Water reabsorption in the proximal tubule (67% of the filtered load) is driven by Na+ reabsorption (osmotic gradient)

Question 7

Why is the sodium gradient very important for reabsorption?

Answer 7

Other molecules, eg. glucose tag along this gradient and is how they can be reabsorbed

Question 8

How does chloride enter the proximal tubule epithelium?

Answer 8

via the paracellular pathway (electrical gradient following Na+)

Question 9

What is the descending part of the nephron loop permeable to?

Answer 9

permeable to water only

Question 10

What is the ascending part of the nephron loop permeable to?

Answer 10

permeable to sodium only

Question 11

In juxtamedullary nephrons, what do the different permeability’s in the descending/ascending nephron loop generate?

Answer 11

A Hyper-Osmotic Medullary Gradient (HOMG)

(crucial for the kidneys’ ability to concentrate urine and maintain water and electrolyte balance in the body)

Question 12

Water reabsorption in the kidney:
A. mainly occurs in the thick ascending limb of the
nephron loop (TAL).
B. is independent of sodium reabsorption.
C. is facilitated by glucose reabsorption.
D. is facilitated by sodium secretion.

Answer 12

C

Question 13

Where is changes in the plasma (ECF) osmolarity detected?

Answer 13

detected by osmoreceptors in hypothalamus

Question 14

What would occur if total body weight decreased? eg. dehydrated?

Answer 14

(Makes you pee less)

Decrease in TBW: (less ECF volume)

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 Collecting duct cells: increasing water permeability

This Increases water reabsorption (driving force = HOMG)

Decrease in urine volume

ECF osmolarity returns to normal

Question 15

Where is ADH produced and where is it released from?

Answer 15

Produced in the hypothalamus and is released from the posterior pituitary glands

Question 16

What would happen if total body weight increased? eg. hyper hydrated?

Answer 16

(makes you pee more)

Osmoreceptors in the hypothalamus are inhibited which will decrease the release of ADH

This will result in less water reabsorption in the collecting duct (less aquaporins) and therefore more water is excreted in the urine

This helps maintain plasma osmolarity stable and a cell volume that is stable

Question 17

What would occur if there was a Decrease in blood volume/loss of isosmotic fluid (water + Na+)? eg. vomiting

Answer 17

Decrease in blood volume/loss of isosmotic fluid (water + Na+)

Detected by pressure receptors in the kidney

Activation of RAAS
Increased release of aldosterone from the adrenal gland (produced in adrenal cortex)

Increased sodium channels in apical membrane of DCT or CD

Increased sodium and water reabsorption

Blood volume returns to normal

Question 18

Normal urine:
A. has a volume of 20 L per day.
B. does not contain hydrogen ions (H+).
C. contains sodium and potassium.
D. tastes sweet.

Answer 18

C

Question 19

What possible condition would you have if there was blood in the urine?

Answer 19

haematuria, UTI damage to filtration barrier

Question 20

What possible condition would you have if there was glucose in the urine?

Answer 20

glucosuria, diabetes mellitus

Question 21

What possible condition would you have if there was protein in the urine?

Answer 21

proteinuria, glomerulonephritis damage to the filtration barrier

Question 22

What can be found in normal urine (just know the basics)?

Answer 22

▪ Water: 95-98% of urine is water →1.5 L/day
▪ Creatinine (muscle metabolism)
▪ Urea (amino acid breakdown)
▪ Uric acid (purine breakdown)
▪ H+
(hydrogen ions)
▪ Na+ (sodium), K+ (potassium)
▪ Medications (anti-viral, diuretics)
▪ toxins

Question 23

What could be found in pathological urine?

Answer 23

▪ Glucose (glucosuria, diabetes)
▪ Protein, especially albumin
(proteinuria)
▪ Blood: red blood cells/erythrocytes (haematuria)
▪ Haemoglobin (haemoglobinuria)
▪ White blood cells/leucocytes
▪ Bacteria (infection)

Question 24

Who has lower pH urine, vegetarians or high meat diet?

Answer 24

Meat eaters have a lower pH urine

Vegetarians more basic urine

Question 25

What would occur if there was an increase in blood volume/gain of isosmotic fluid (water + Na+)? eg. salty diet

Answer 25

Detected by cardiac muscle cell receptors

Release of ANP which makes you pee more

This will be concentrated urine, losing more Na+ and
more water than normal

Restores homeostasis by decreasing ECF volume