L12 Renal System Flashcards
What is the equation for the amount of urine excreted?
Amount filtered - amount reabsorbed + amount secreted
What is the role of reabsoprtion?
Movement of solutes/fluid out of the filtrate and into the capillaries via epithelial transport mechanisms
What is epithelial transcellular transport?
1) epithelial transcellular transport
- substances cross apical and base lateral membranes of the tubule epithelial cells
What is the paracellular transport pathway?
Substances pass through the cell-cell junction between two adjacent cells
How does the solute choose what epithelial transport route to take?
Depends on
- electrochemical gradient
- permeability of epithelial junctions
In the PCT, how are microvilli specialised for function?
Microvilli on the apical surface maximise surface area available for reabsorption
What is responsible for the synthesis of proteins in the PCT?
ER, Golgi apparatus, lysosomes and vacuoles are responsible for the synthesis of membrane proteins
What is the role of interdigitations of the basolateral membrane?
They shorten the distance to the mitochondria which is beneficial in active transport
Outline the sodium ion reabsorption at the PCT
Happens passively at the apical membrane - down an electrochemical gradient
Ion exchange with other positively charged ions
Co-transport with essential solutes e.g. glucose, amino acid actively at the basolateral membrane
How is H2O reabsorbed at the PCT?
Though a paracellular route via osmosis
How is glucose transported at the PCT?
Co-transport at the apical membrane
A carrier at the basolateral membrane
How is urate reabsorped at the PCT?
Organic anion transporters
Paracellular route
Passively transcellular route
Secretion via organic anion transporters
How are low molecular weight proteins/amino acids reabsorbed at the PCT?
Endocytosis at the apical membrane
Breakdown in lysosomes
Release of amino acids
Define diabetes mellitus
Excessive glucose concentration saturates the number of carriers and excess glucose appears in urine
What happens during secretion at the PCT?
Transfer of molecules from the peritubular capillaries into the tubule - active process
Transport of organic anions and cations
Why does osmolarity change throughout the nephron?
1) isometric fluid leaving the PCT becomes more concentrated in the descending limb
2) removal of solute in the thick ascending limb creates hypo osmotic fluid
3) permeability to water and solutes in the distal tubule and collecting duct is regulated by hormones
4) final urine osmolarity depends on reabsorption in the collecting duct
Outline the permeability of the ascending and descending limb
Descending
- permeable to water
- impermeable to solutes
Ascending
- impermeable to water
Outline the permeability of the collecting duct
Permeable to urea
Impermeable to H2O
How is concentrated urine formed?
H2O reabsorption needs to be increased
1) anti-diuretic hormone makes collecting duct permeable to H2O - H2O is passively reabsorbed driven by the osmotic gradient in the medullary interstitium
2) countercurrent systems maintain osmotic gradient in the medullary interstitium
What are the properties of the countercurrent exchange system?
- two flows moving in opposite directions
- vessels anatomically very close together
- passive transfer of molecules from one vessel to another to another
What is the countercurrent multiplier system?
The countercurrent exchange is enhanced by active transport of solutes
E.g. loop of henle and vasa recta
How and why is the osmotic gradient maintained?
Maintained for the reabsorption of H20
Done by preventing the reduction is osmolarity of medullary interstitium
What role do the vasa recta and the collecting duct play in countercurrent exchange?
The H2O is reabsorbed from the collecting duct
H2O is removed from the medulla by the ascending vasa recta
Results in concentrated urine (high osmolarity)
What happens when H2O is not reabsorbed from the collecting duct?
Due to the absence of vasopressin
Results in dilute urine - low osmolarity
How is renal adjustment carried out?
Directly by excreting or reabsorbing H+
Indirectly by excreting or reabsorbing HCO3-
What is acidosis?
Alpha intercalated cells in the collecting duct excrete H+
Reabsorb HCO3-
What is alkalosis?
Beta intercalated cells in the collecting duct excrete HCO3-
Reabsorb H+
What happens at different place during countercurrent multiplier exchange?
Descending limb of loop
- H2O re absorption
- increased filtrate osmolarity
Ascending Iimb of loop
- active solute re absorption
- decreased filtrate osmolarity
Descending limb of vasa recta
- H2O reabsorption
- solute uptake
- increased blood osmolarity
Ascending limb of vasa recta
- H2O reabsorption
- decreased blood osmolarity
