Tubular Function Flashcards
How much filtrate can a healthy individual produce?
180L/day
What is the renal tubule composed of?
PCT, loop of Henle, DCT, collecting ducts
What is the primary site of reabsorption?
PCT - proximal convoluted tubule
What structure of the PCT allows reabsorption?
it is long, convoluted and has microvilli allowing for larger surface area and more absorption
What are all portions of the nephron closely associated with?
peritubular capillaries (vasa recta)
When substances are reabsorbed what is their pathway of travel?
tubule lumen > tubule wall > interstitial space > peritubular capillaries
When substances are secreted what is their pathway of travel?
peritubular capillaries > interstitial space > tubule wall > tubule lumen
What substances are reabsorbed?
Sodium Chloride Calcium Phosphate Potassium Bicarbonate, Water Glucose Amino acids, Urea (sometimes)
What substance is always secreted?
creatinine
How does water move across the membrane?
via protein pores in the phospholipid bilayer (aquaporins)
How do amino acids and glucose move down their concentration gradient?
via sodium co-transporters
How much filtrate is passively reabsorbed?
50% (90L)
Where is the sodium potassium pump located?
basolateral membrane of epithelial cells
Where do the sodium co-transporters get energy?
secondary active transport from energy generated by Na+/K+ pump
How does the sodium/potassium pump allow reabsorption of the remaining filtrate?
- Water follows Na+ wherever it goes - osmosis
- Glucose and amino acids co-transported with Na+
- Chloride and negative ions follows Na+
What happens to glucose reabsorption in diabetic patients?
high levels of glucose in lumen overwhelm the co-transporters therefore not all of the glucose would be reabsorbed and ultimately cause the glucose to be excreted from the body via the urine
What % of nutrients are reabsorbed at the PCT?
100%
What % of water is reabsorbed at the PCT?
65%
What % of negative ions are reabsorbed at the PCT?
65%
What is the remaining filtrate that travels to the loop of Henle?
60L
What are the 3 sections of the loop of Henle?
thick ascending limb
thin ascending limb
thin descending limb
Where are the aquaporin channels in the loop of Henle?
thin descending limb - epithelial cells
Where is water impermeable in the loop of Henle?
thick ascending limb
thin ascending limb
Where is there sodium/potassium pumps in the loop of Henle?
thick ascending limb
Why can water not follow sodium?
no aquaporin channels in thick ascending limb
What pump in the thick ascending limb transports Cl-, Na+, 2 K+?
NKCC2 Transporter
Where does the NKCC2 get energy?
secondary active transport via sodium/potassium pump
What is the interstitial space in the loop of Henle?
medulla
Where does anti-durietic drug furosemide work?
binds to NKCC2 pump and inhibits it
What conditions are created in the medulla as sodium is actively reabsorbed from the filtrate? (water prevented from following)
medulla becomes highly salty causing water in the descending limb to leak into medulla through aquaporin channels following rules of osmosis
What happens to the concentration of the filtrate as it descends the thin descending limb?
no active sodium pumps in descending limb therefore urine becomes more concentrated
What happens to the concentration of the filtrate as it ascends the thick ascending limb?
concentration of (sodium) filtrate decreases
What is the process in which urine is concentrated called?
counter current multiplication
Where does the reabsorbed water (and salt) go in the medulla?
return to bloodstream via the vasa recta
What is the role of the DCT?
Fine-tuning - site of fluid volume and electrolyte regulation
Where is the target site for several hormones that regulate water and sodium levels?
the DCT
What hormones work in the DCT and what is their role?
- Anti-diuretic hormone (ADH) – Increases water reabsorption
- Aldosterone – Increases Na+ reabsorption
- Atrial natriuretic hormone (ANH) – Promotes Na+ secretion
Why is filtrate hypotonic as it enters the DCT?
it has more water and solute than surrounding interstitium (100mosmoles vs 3000mosmoles in normal bodily fluids)
Is there aquaporin channels in DCT?
No aquaporins present (in the absence of hormonal regulation)
What happens to urine in the DCT in the absence of external hormonal regulation?
In the absence of external hormonal regulation a large volume of dilute (hypotonic) urine is produced
What does low fluid, low blood volume cause?
low blood pressure
What detects this change?
pituitary gland
Where does ADH (vasopressin) act?
epithelial cells lining DCT and collecting ducts
What does ADH cause?
Insertion of aquaporins allows water to be reabsorbed from the filtrate
small volume of concentrated urine produced
What is the vasopressin receptor?
AVPR2 (vasopressin receptor 2)
Once vasopressin binds, what occurs?
activates signal transduction within cell
signal enters nucleus and stimulates transcription factors
which translate proteins that create aquaporin channels that are inserted into membrane and allow water to leave filtrate, pass through tubule wall and into interstitial space to eventually return back to blood
What hormone can a fall in blood volume also stimulate?
aldosterone
What is the action of aldosterone?
Aldosterone acts on distal and collecting tubules
Upregulates activity and insertion of Na+K+ pumps and channels
small volume of concentrated urine produced
Why does water leave due to Na+ pump activity?
osmosis
Where does aldosterone bind?
diffuses through cell membrane and binds directly to transcription factors that up regulate activity and insertion of Na+/K+ pumps
When is ANP released?
increase in blood pressure
How does ANP work?
counteracts effects of ADH and aldosterone
by removing aquaporin channels
and decreasing activity and insertion of Na+/K+ pumps
What urine is produced in ANP?
large volume of dilute urine
What % of liquid is excreted as urine?
1-2%
What fraction is reabsorbed in the PCT?
2/3
The kidney uses sodium handling in various different ways to support reabsorption of a large volume of filtrate. In which section of the kidney is the majority of sodium reabsorbed?
Proximal convoluted tubule
In the Loop of Henle, the thin descending limb……
Is freely permeable to water
What is the primary purpose of counter current multiplication?
water reabsorption
What cellular response does ADH signalling have on epithelial cells lining the distal convoluted tubule?
Insertion of aquaporins
By what process is sodium reabsorbed across the basolateral surface of epithelial cells lining the proximal convoluted tubule?
active transport
