Tubular Function Flashcards
Define reabsorption
Taken back up into the blood from renal tubule
What is reabsorbed 100%?
Glucose
What is reabsorbed 99%?
Sodium
What is reabsorbed 50%?
Urea
What is reabsorbed 0%?
Creatinine
What are the mechanisms of tubular reabsorption?
Passive diffusion, facilitated diffusion, active transport and osmosis
What are the two cellular pathways?
Transcellular and paracellular
Define transport maximum
Maximum rate a substance can be transported across a membrane = STATURATION KINETICS OF CARRIER-MEDIATED TRANSPORT
Difference between secretion and filtration?
Filtration refers to the passive process by which fluid and solutes are forced out of the glomerular capillaries and into Bowman’s capsule to form the initial filtrate.
Secretion refers to the active process by which substances are transported from the bloodstream into the renal tubules, primarily in the proximal tubules, distal tubules, and collecting ducts
Define renal threshold
Plasma concentration of substance = where it starts to be excreted in urine
What is the ideal vs actual renal threshold of glucose?
300 mg/dL = ideal but actually is 180 mg/dL
What happens because the renal threshold of glucose is lower than the Tm?
Glucose appears in the urine before the Tm is reached, which is not expected
Do all nephron have the same Tm for glucose?
No, some nephrons will begin to excrete glucose before other have reached their Tm
What is reabsorbed in the PCT?
More than 65% of filtered water and electrolytes 100% of amino acids and glucose are reabsorbed into blood
What is secreted in the PCT?
H+
How is the fluid described in the PCT?
Isotonic
What happens in the thin-descending loop of Henle?
20% of filtered water is reabsorbed = permeable to water and solutes
Describe thin-descending segment CELLS
Cells are thin = not very metbolically active
What are impermeable to water?
Thin and thick ascending segments of LOOP OF HENLE
What happens in the thick-ascending loop of Henle?
High levels of NaCl reabsorption occurs = hypo-osmotic
Describe the cells in the thick-ascening loop of Henle
Thick epithelial cells = high metabolic activity
What transporter is found in the thick-ascending limb of the loop of Henle?
NKCC co-transporter into cell ATPase = maintains low intracellular Na+ conc
What happens in the early distal tubule?
Reabsorption of most ions = Na+ , Cl-, Ca2+ and Mg2+ IMPERMEABLE to water
What is the early distal tubule referred to?
Diluting segment = because impermeable to water
What transporters are found in the early distal tubule?
NCC = pumps then into the cell and ATPase removes Na+
What controls water reabsorption in collecting duct?
Antidiuretic hormone
What are the two cell types found in collecting duct?
Principal cells
Type A intercalated cells
What transporters are found in principal cells?
BK (big K channel)
ROMK (renal outer medullary K channel)
EnaC
Na/K ATPase
What transporters are found in Type A intercalated cells?
H+ ATPase
H+/K+ ATPase
HCO3-/Cl-
What is the role of the principal cell?
K+ secretion into tubular lumen to be excreted
Aldosterone controls Na+ reabsorption
This can regulate blood pressure
What is the role Type A intercalated cells?
Maintain acid-base balance
Lower blood pH by secreting H+ into tubular lumen to be excreted
K+ reabsorption
What happens in the Medullary collecting duct?
Reabsorbs less than 5% of filtered water and Na+
What is the role of the medullary collecting duct?
Critical role in determining the urine output and concentration
Permeability to water is controlled by ADH
What are the factors regulating tubular reabsorption?
Hormonal factors
Aldosterone, Angiotensin II, ADH, ANP
Define the glomerulotubular balance
Ability of the renal tubules to adjust their reabsorption of sodium ions in response to changes in the rate of glomerular filtration
Particularly in proximal tubules
What do glomerulotubular balance and tubuloglomerular feedback do together?
Buffer the effects of spontaneous changes in GFR on urine output
What is the reabsorption of Na+ coupled with?
H+, glucose, amino acid and phosphate = either by co-transport or exchange
What transporters reabsorb glucose?
Glucose and Na+ bind SGLT-2 in apical membrane
Glucose enters cell as Na+ moves along electrochemical gradient
Na+ is pumped out of the cell
Glucose exits by facilitated diffusion via GLUT2
How are amino acids reabsorbed and where?
Proximal convoluted tubule
Transported by Na+/amino acid co-transporters
Amino acid leaves basolateral membrane via diffusion
How is potassium regulated?
Principal cells secrete K+ into tubular lumen to be excreted
Type A intercalated cells = reabsorb K+ via
K+/H+ ATPase
What can hyperkalemia and hypokalemia cause?
Hyper = fatal cardiac arrhythmias
Hypo = cardiac arrhythmias and muscle weakness which can be life threatening
How is low potassium regulated?
Increase activity of Type A intercalated cell
Increase H+ secretion (alkalosis) and K+ reabsorption
How is high potassium regulated?
Increase aldosterone = increased EnaC (apical) and Na+/K+ ATPase (basolateral)
Aldosterone = increases incorporation of K+ secreting channels into luminal membrane
What regulates calcium?
Parathyroid hormone
Where is calcium reabsorbed?
65% of filtered calcium reabsorbed in PCT
Reabsorbed at thick ascending limb and DCT under influence of PTH
What forms the countercurrent system?
Vasa recta and loop of Henle
What are the roles of vasa recta and loop of Henle in countercurrent mechanism?
Countercurrent multiplier = loop of Henle
Countercurrent exchanger = vasa recta
What is the role of the countercurrent mechanism?
Maintaining the osmotic gradient
What happens if the medulla of the kidney is not hypertonic?
Cannot reabsorb water, even with aquaporins in place
What two things flow in opposite directions in the countercurrent system, and why is this important?
Blood and solutes move in opposite direction = causing water to move in the correct direction
