Urinary System Flashcards
Processes that regulate urine formation
- glomerular filtration (passive)
- tubular absorption (active or passive)
- tubular secretion (active or passive)
Glomeruli
- capillary endothelium
- basement membrane
- glomerular epithelial cells (podocytes) with foot processes
Evaluation of glomerular filtration
- excretion of substances that pass freely thru the glomeruli –> size <2.4 - >3.4, positive charged passes more freely
- albumin is 3.5 nm and negatively charged –> not present in urine of cats, cattle, horses, in low conc in dogs
Glomerular filtration rate
Fluid that goes from plasma to glomerular filtrate
- assessed by rate some substances are cleared from the plasma
- GFR depends on renal plasma flow
- RPF depends on blood volume, cardiac output, # of functional glomeruli, constriction/dilation of afferent/efferent arterioles
- other: intracapsular hydrostatic pressure, oncotic pressure
The ideal biomarker for GFR
- not bound to protein
- pass freely through filtration barrier
- neither excreted nor absorbed by renal tubules
- insulin, iohexol, manitol nearly meet the criteria
- creatinine is secreted very little and meets criteria
- imagining can be used
Function of renal tubules is assessed comparing ___________
The urinary excretion of a plasma substance to insulin
- if greater than insulin –> there is tubular secretion, and if lower there is resorption or does not pass freely though filtration barrier
Na
75% resorbed in proximal tubules
- passively resorbed in loop of Henle
- ADH stimulates absorption in the loop of Henle
- aldosterone stimulates absorption in the collecting ducts
Cl
75% resorbed in the proximal tubules
- passivley resorbed in the loop of Henle
- resorbed passively in distal nephron due to a gradient formed by Na
HCO3
90% conserved in the proximal tubules via H secretion
- collecting ducts type A intercalated cells increase resorption
- collecting ducts Type B intercalated cells decrease resorption when there is excess
K
Most is resorbed prior to distal tubules
- secreted by principal cells in collected ducts (promoted by aldosterone)
- movement into tubules enhanced by high flow and inhibited by low flow
- ADH promotes secretion in cortical collecting ducts
H
Secreted by type A intercalated cells in distal nephron
- aldosterone and acidemia promote secretion
- limited amount secreted in proximal tubule
- most renal secretion of H is within NH4 and either HPO4 or H2PO4
Ca
80-85% resorbed in proximal tubules and loop of Henle
- PTH promotes resorption
- vit D promotes resorption
PO4
85-90% resorbed in proximal tubule
- enhanced by hypophosphatemia and insulin
- inhibited by hyperphosphatemia and PTH
Mg2
Near all resorbed in the loop of Henle
- ADH, PTH, glucagon, calcitonin and B-agonists stimulate resorption
Glucose
All resorbed in proximal tubules
- mechanism involve transporter proteins that can be overwhelmed by high saturation of plasma glucose (glucosuria)
Proteins and amino acids
Nearly all resorbed in proximal tubules
- AA use 7 different transporters for 7 AA groups
- larger proteins (inclu albumin) enter the tubular cells by endocytosis and are degraded to amino acids
What are 3 important receptors mediating endocytosis?
Megalin, aminionless, and cubalin
Urea
60-65% resorbed in proximal tubules
- ADH enhance resorption in distal nephron
- responsible for 50% of hypertonicity of the medulla
Creatinine
Small amounts are secreted in proximal tubules (dogs)
- does not happen with horses and cats
Water
30% of renal perfusion flow becomes ultrafiltrate, 75% is passively resorbed in proximal tubules
- passively resorbed in the loop of Henle as it enters hypertonic medulla
- collecting ducts are permeable in presence of ADH thru aquaporin
_____ is imperbeable to water
Ascending loop of Henle
Concentrating ability
Ability to resorb water in excess to resorption of solutes
- will increase osmolarity
Diluting ability
Ability to resorb solutes in excess to resorption of water
- will decrease osmolarity
Isosthenuria
Urine osmolality is the same of plasma osmolality
- USG 1.007-1.013