BIOL 2420 Lecture Notes - Lecture 5: Proximal Tubule, Renal Glucose Reabsorption, Electrochemical Gradient
26 Apr 2018
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Unit 7 – Lecture 5
Reabsorption
- 180 L fluid/day passes from glomerular capillaries into the tubules
- only 1.5 liters are excreted in the urine
o more than 99% of the fluid entering the tubules must be reabsorbed into the blood as
filtrate moves through the nephrons
- most reabsorption takes place in the proximal tubules
o a smaller amount takes place in the distal segments of the nephrons
- regulated reabsorption in the distal nephron allows kidneys to return ions and water to the
plasma selectively (as needed to maintain homeostasis)
- why filter 180L of fluid per day when you only excrete 1%?
o Foreign substances are filtered into the tubule but not reabsorbed into the blood
▪ High filtration per day helps filter out these foreign substances
▪ Once filtered into the lume of oa’s apsule, it is o loger part of the
od’s iteral eiroet
• The lumen of the nephron is external environment, and anything in the
filtrate is destined to leave the body in the urine unless there is a tubule
mechanism for reclaiming it
o Small nutrients (glucose and citric acid cycle intermediates) are
filtered but the proximal tubule very efficiently reabsorbs them
o Filtering ions and water into the tubules simplifies their regulation
▪ A portion of filtrate that reaches the distal nephron is not needed to maintain
homeostasis is passed on into the urine
▪ High GFR = excretion can occur rapidly
▪ If ions and water are needed for homeostasis then they are reabsorbed
Reabsorption May Be Active or Passive
- Active transport is used for reabsorption from the tubule lumen to the ECF
o Filtrate floig out of Boa’s apsule ito the proial tuule has the sae solute
concentrations as ECF
o To move solute out of the lumen, the tubule cells must use active transport to create
concentration or electrochemical gradients
o Water osmotically follows the solutes as they are absorbed
- Figure 19.8 – Reabsorption overview
o Active transport of Na+ from the tubule lumen to the ECF creates a transepithelial
electrical gradient where the lumen is more negative than the ECF
o Anions follow the positively charged Na+ out of the lumen
o Removal of Na+ and anions from lumen to ECF dilutes the luminal fluid and increases
the concentration of the ECF – water leaves the tubule by osmosis
- Loss of volume from the lumen increases the concentration of solutes (K+, Ca2+, urea) left behind
in the filtrate
o The same amount of solute in a smaller volume equals higher solute concentration
▪ Once luminal solute concentrations are higher than solute concentrations in the
ECF, solutes diffuse out of the lumen if the epithelium of the tubule is
permeable to them
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