Solute and Water Handling Along the Nephron Flashcards
Which is the longest segment of the nephron?
PCT
What are the functions of the PCT?
Reabsorption of most of water
Reabsorption of most salts
Reabsorption of all organic nutrients
Secretion of H+ and organic ions
Which ions are reabsorbs at the PCT?
Na+, Cl-, K+, HCO3-, Ca2+, PO42-, SO42-
How is water reabsorbed at the PCT?
by Iso-osmotic/isotonic reabsorption
Which Organic molecules are reabsorbed at the PCT?
sugars (glucose), amino acids, vitamins usually completely reabsorbed
Lipophilic solutes (e.g.) urea, passively and partially reabsorbed
What is the TF/P?
tubular fluid] / [plasma] (or the TF/P ratio) compares the concentration of the fluid in the lumen of the nephron at a given point along the nephron relative to the plasma concentration of that solute.
What does it mean for TF/P = 1?
If a solute is freely filtered, then its concentration in the filtrate in Bowman’s capsule, and thus at the beginning of the PCT, is the same as that in the plasma and the TF/P ratio = 1.0.
A solute is being reabsorbed at the same rate as water
What does it mean for the TF/P<1
means the concentration of the solute in the filtrate is less than that in plasma.
This occurs when a solute is reabsorbed faster than water (quantity decreases more than volume).
The TF/P ratio decreases for all reabsorbed organic solutes like sugars and amino acids and for bicarbonate.
TF/P ratio = 0 when the solute is completely reabsorbed.
PCT as water volume decreases in the PCT lume
What does it mean for the TF/P>1
means the concentration of the solute is greater in the tubular fluid at that point, relative to the plasma (but doesn’t necessarily mean the solute was secreted into the nephron…)
Remember, ~65% of the water volume was reabsorbed in the PCT, therefore, if TF/P still = 1, then solutes must have also been reabsorbed at the same rate as water (i.e., ~65% reabsorbed).
How are solutes reabsorbed and water absorbed in the PCT?
Na+ is passively reabsorbed down its electrochemical gradient maintained by Na/K ATPase (primary active transport)
Water and other solutes follow salt (coupled secondarily active transport)
What maintains the sodium gradient in the PCT?
Na+ gradient maintained by the activity of
the basolateral Na/K ATPase
Which solutes are directly coupled to Na transport?
Glucose
Some amino acids
Phosphate, lactate, other metabolic intermediates
Hydrogen ions (H+) [note that this is a Na/H exchange]
all with their on transporters
How is water reabsorbed in the PCT and loop of henle?
the reabsorption of solutes of leads to decrease in osmolarity promotes water reabsorption through AQP1
How does anions CL- and K+ or reabsorbed from the PCT? How do lipophilic solutes are reabsorbed from the lumen?
moves paracellularly pulled along with water
Lipophilic solutes (like urea) diffuse across the epithelium transcellularly
What is the process by which glucose is reabsorbed?
SGLT2- pulls glucose into cytoplasm (apical surface)
Glucose builds up in the cytoplasm. It passively passes through the basolateral membrane via the GLUT2 transporter
Na gradient is maintained by the Na/K+ transporter across the basolateral membrane
