Lecture 32: Na+/water reabsorbtion, control of body water, urine Flashcards
What are the three locations where water is reabsorbed and what percentage in each location?
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
How is regulated (facultative) water reabsorption regulated?
▪ accounts for 2-8% of total water reabsorption
▪ regulated by anti-diuretic hormone (ADH)
▪ tight epithelia
▪ only transcellular reabsorption
Is bulk water reabsorption regulated?
▪ accounts for 92% of total water reabsorption
▪ not regulated – automatic!!
▪ via leaky epithelia
▪ trans-and paracellular water reabsorption
What percentage of water is usually excreted in the kidneys?
It varies based on the body’s requirements and hydration levels.
Excretion
▪ < 1 - 6% of filtered load is excreted
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?
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
How is Regulated sodium reabsorption regulated?
▪ accounts for 7-8% of total sodium reabsorption
▪ regulated by aldosterone (RAAS)
What is water reabsorption in the proximal tubule primarily driven by?
▪ Water reabsorption in the proximal tubule (67% of the filtered load) is driven by Na+ reabsorption (osmotic gradient)
Why is the sodium gradient very important for reabsorption?
Other molecules, eg. glucose tag along this gradient and is how they can be reabsorbed
How does chloride enter the proximal tubule epithelium?
via the paracellular pathway (electrical gradient following Na+)
What is the descending part of the nephron loop permeable to?
permeable to water only
What is the ascending part of the nephron loop permeable to?
permeable to sodium only
In juxtamedullary nephrons, what do the different permeability’s in the descending/ascending nephron loop generate?
A Hyper-Osmotic Medullary Gradient (HOMG)
(crucial for the kidneys’ ability to concentrate urine and maintain water and electrolyte balance in the body)
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.
C
Where is changes in the plasma (ECF) osmolarity detected?
detected by osmoreceptors in hypothalamus
What would occur if total body weight decreased? eg. dehydrated?
(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
Where is ADH produced and where is it released from?
Produced in the hypothalamus and is released from the posterior pituitary glands
What would happen if total body weight increased? eg. hyper hydrated?
(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
What would occur if there was a Decrease in blood volume/loss of isosmotic fluid (water + Na+)? eg. vomiting
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
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.
C
What possible condition would you have if there was blood in the urine?
haematuria, UTI damage to filtration barrier
What possible condition would you have if there was glucose in the urine?
glucosuria, diabetes mellitus
What possible condition would you have if there was protein in the urine?
proteinuria, glomerulonephritis damage to the filtration barrier
What can be found in normal urine (just know the basics)?
▪ 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
What could be found in pathological urine?
▪ Glucose (glucosuria, diabetes)
▪ Protein, especially albumin
(proteinuria)
▪ Blood: red blood cells/erythrocytes (haematuria)
▪ Haemoglobin (haemoglobinuria)
▪ White blood cells/leucocytes
▪ Bacteria (infection)
Who has lower pH urine, vegetarians or high meat diet?
Meat eaters have a lower pH urine
Vegetarians more basic urine
What would occur if there was an increase in blood volume/gain of isosmotic fluid (water + Na+)? eg. salty diet
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
