Tubular Reabsorption and Secretion Flashcards
1
Q
What ions can use paracellular pathway to transport between cells?
A
- Ca2+
- Mg2+
- K+
2
Q
Describe sodium reabsorption
A
- Na+ enters cells from filtrate at luminal membrane and is actively transported out by Na/K-ATPase pump in basolateral membrane
- diffuses into peritubular capillaties
- generates strong electrochemical and osmotic gradient which favours passive diffusion at luminal face by cotransport/facilitated diffusion through channels
- pump maintains intracellular concentrations at low levels and K+ that is pumped in is almost immediately diffused out into interstitial fluid by leaky channels
3
Q
Describe the reabsorption of water, ions and nutrients
A
- due to high osmotic gradient from Na+ transport, water moves by osmosis into peritubular capillaries aided by aquaporins
- as water leaves, the concentration of solutes in filtrate increases causing them to follow the concentration gradients into peritubular capillaries
4
Q
What ions move by secondary transport?
A
- glucose
- amino acids
- lactate
- vitamins
- cations
5
Q
Why are some substances not reabsorbed?
A
- lack carriers
- non-lipid soluble
- too large to pass through membrane pores
6
Q
What substances are not reabsorbed?
A
- urea (can be reclaimed)
- creatinine
- uric acid
7
Q
Describe reabsorption in the PCT
A
- absorbs all glucose, lactate and amino acids through cotransport with na+
- 65% Na+ reabsorbed through Na/K-ATPase
- water (by osmosis)
- filtered bicarbonate
- chloride (by passive transport and paracellular diffusion by electrochemical gradient)
- potassium (mainly paracellularly)
- Mg2+ and Ca2+ (passive down electrochemical gradient)
- Ca2+ and P4O3- by hormone control
- HCO3- through cotransport with Na+
- urea and lipid solutes through passive diffusion from electrochemical gradient and osmosis
- small proteins (endocytosed by tubule cells and digested to amino acids)
8
Q
Describe reabsorption in the ascending limb of the loop of Henle
A
- Na+, Cl- and K+: active transport by Na-K-Cl cotransporter in thick portion, paracellular transport and Na/H exchanger
- Ca2+ and Mg2+: passive transport by electrochemical gradient and paracellularly
9
Q
Describe reabsorption in DCT
A
- most is based on body’s needs and is regulated by hormones
- Na+: primary active transport requiring aldosterone
- Ca2+: PTH-mediated active transport by ATP-dependent Ca2+ transporter, basolateral Na/Ca exchanger
- Cl-: diffusion driven by electrochemical gradient, cotransport with Na+
- water: osmosis dependent on ADH
10
Q
Describe the permeability differences in the nephron and its importance
A
- beyond PCT permeability of tubule epithelium changes
- water reabsorption not coupled to solute reabsorption
- water can leave descending loop of Henle by not ascending limb where aquaporins are scarce
- permeability differences play role in kidneys ability to form concentrate or dilute urine
11
Q
What is the importance of tubular secretion?
A
- disposes of substances not already in filtrate like drugs
- eliminates undesirbale substances or end products that have been reabsorbed by passive processes (urea/uric acid)
- gets rid of excess K+
- controls blood pH
12
Q
Describe secretion in the proximal tubule
A
- ammonium ion and other nitrogenous waste
- secretes H+ and reabsorbs HCO3- (to maintain pH)
13
Q
Describe secretion at the ascending limb of the loop of Henle
A
- Na+ and Cl- pumped out of thick portion
- contribute to high osmolality of interstitial fluid in inner medulla
- unlike descending limb, epithelium is not permeable to water resulting in dilute filtrate
14
Q
Describe secretion at the DCT
A
- H+ might be secreted and in presence of aldosterone more Na+ is reabsorbed
- very low water permeability
15
Q
Describe secretion at the collecting duct
A
- K+, H+ and/or HCO3- reabsorbed or secreted as urine travels back into medulla
- dependent on what is required to maintain blood pH
- wall of medullary region permeable to urea and some diffuses out to contribute to high osmolality of inner medulla
