L24: Renal System II Flashcards
urinary excretion of a substance depends on
its filtration, reabsorption, secretion
glomerular filtration
from glomerulus to Bowman’s capsule
reabsorption
from tubules to peritubular capillaries
secretion
from peritubular capillaries to tubules
excretion
from tubules out of the body
filtered quantities of a substance are generally
larger than the amounts of the substances in the body
reabsorption of waste products
is relatively incomplete, large fractions are excreted
reabsorption of most useful plasma components
is complete
reabsorbed substance must move
across two barriers: the tubule epithelium and the capillary endothelium
primary barrier to reabsorption
epithelial cells lining the renal tubules
___ limit the passage of substances between cells
tight junctions between the renal tubule epithelial cells
transport can be
active or passive
barrier for secretion are
the same as for reabsorption
substances such as ___ move from the peritubular capillaries into the tubular lumen
hydrogen ion, potassium, organic anions
tubular secretion is an important mechanism for
disposing of drugs and drug metabolites, eliminating undesired substances that have reabsorbed by passive processes, removing excess K+, controlling blood pH
glomerular filtration is driven by
hydrostatic pressure in the glomerular capillaries
glomerular filtration is opposed by
hydrostatic pressure in Bowman’s space and the osmotic force due to the proteins in the glomerular capillary plasma
under normal conditions,
losses of salt and water equal gains
urinary loss regulation
matching of losses and gains of salt and water
Na+ reabsorption
an active process occurring in all tubular segments except the descending limb of the loop of Henle
water reabsorption
is by osmosis and is depended upon Na+ reabsorption
Na+ is actively transported
across the basolateral membrane of renal tubule cells establishing an osmotic gradient for water reabsorption
active transport of Na+ out of renal tubule cell keeps
intracellular Na+ lower than the renal tubular fluid Na+
active transport of Na+ out of tubule cell forms a concentration gradient and favors
the movement of Na+ from the lumen of the renal tubule into the renal tubule cell
Na+ moves from the tubular fluid into the tubular epithelial cell
through Na+ channels or via cotransport with other molecules such as glucose
active reabsorption of sodium and other solutes in the proximal tubule creates
an osmotic gradient
because of the osmotic gradient
water follows the solutes
reabsorption of water creates a concentration gradient for permeating solutes
such as urea to move from tubular fluid to the peritubular capillaries
water permeability
varies from segment to segment and depends largely on the presence of water channels, called aquaporins
water channels
aquaporins (AQP)
aquaporins are highly expressed
in the proximal tubule resulting in a high water permeability
cortical and medullary collecting ducts
are the only tubular segments in which water permeability is under physiological control
___ stimulates the insertion of aquaporin (AQP2) water channels
vasopressin (ADH)
AQP2 channel pathways is present
in a PKA-dependent manner
AQP2 are absent in the collecting duct apical membranes unless
ADH is active
___ is not secreted
Na+
__ is freely filterable at the glomerulus
Na+
__ entry into the cell from tubular lumen is
always passive either through channels or contransport
__ increases water permeability
ADH
___ is a major determinant of the ability to survive with limited water intake
the ability of kidneys to produce hyper osmotic urine
a maximal osmolarity that kidneys can produce
1400 mOsmol
normal osmolarity of blood
300 mOsmol
urinary concentration takes place
as tubular fluid flows through the medullary collecting ducts
the interstitial fluid surrounding these ducts
very hyperosmotic
in the presence of ADH
water diffused out of the ducts into the interstitial fluid to return to the capillaries
the descending limb of the loop of Henle
is permeable to water, impermeable to solutes
the ascending limb of the loop of Henle
is impermeable to water, permeable to solutes
differences in the transport of water and solutes establish
osmotic gradient in the medullary interstitial fluid
osmolarity differences
exist at each horizontal level and are multiplied as the fluid goes deeper into the medulla
the loop of Henle is
a countercurrent multiplier system
osmoreceptors in the hypothalamus
monitor the osmolarity of extracellular fluid
osmolarity increases,
ADH secretion is stimulated and increases water reabsorption
osmolarity decreases
ADH secretion is inhibited, which decreases water reabsorption and increases water excretion
__ also influence ADH secretion
baroreceptors that detect blood volume and blood pressure
baroreceptors in atria
detect and respond to changes in blood volume
baroreceptors in carotid sinus
respond to changes in blood pressure
blood volume and pressure decreases,
the frequency of action potentials in baroreceptors decreases, ADH secretion increases, water reabsorption increases, blood volume and pressure increases
blood volume and/or pressure increases
ADH secretion decreases, water reabsorption decreases, blood volume and pressure decreases
