urinary excretion - kidney to bladder
3 processes
- glomerular filtration
- tubular secretion
- tubular reabsorption
glomerular filtration
- glomerulus to capsular space
- bowmans space : inside glamarular capsule
tubular reabsorption
-tubular lumen to peritubular capillary
-peri= around peritubular= around capillary
tubular secretion
peritubular capillaries to tubular lumen
what processes make excretion
excretion = filtered + secreted - reabsorbed
-can alter the balance of 3 process to alter excretion
3 examples how to alter excretion
x : harmful substance or waste
y: useful substance; being regulated. most common
z : important substance for health, no excretion. nothing in urine
glucose in urine not normal, associated with diabetes
glomerular filtration
-kidneys try to maintain a relatively constant GFR
-1st chance to eliminate substance
-strong homeostatic mechanisms
glamarular filtration rate (GFR) (mL/min)
too high - substances pass too quickly and are not adequately reabsorbed
(better kidney function)
too low - nearly all substances reabsorbed and some waste products not adequately eliminated
(kidneys not working very well)
afferent arteriole
blood coming into glamurulus
efferent arteriole
blood coming out of glamarular
tubular reabsorption
- tubular lumen to peritubular capillaries
- mechanism for retaining substances
- proximal tubule largest contribution to reabsorption; fine-tuning further along tubules
tubular secretion
- peritubular capillaries to tubular lumen
- 2nd chance to eliminate substance after filtration (GFR)
- mechanism for eliminating undesired substances or end-products/removing excesses/ controlling blood acid-base balance
apical membrane
borders tubular lumen
basolateral membranes
borders tubular epithelial cell
glomerulus more or less efficient than other capillary beds at filtration?
more efficient
-180L/day in glomerulus vs. 4 L/day in systemic capillary beds
glomerular filtration membrane
- large surface area
- very permeable (with pores; 45 x leakier than typical capillary bed)
- glomerulus blood pressure is higher than blood pressure in systemic capillaries (60 mmHg vs. 15-35 mmHg)
glomerular filtrate
volume of fluid becoming filtrate (litres ~ 150-180 L in a day)
filtration fraction
fraction of fluid becoming filtrate (typical is 20%)
glomerular filtration rate (GFR)
amount of filtrate formed by both kidneys each minute (L/min; typical is 125 mL/min)
principle of filtration
force fluids and substances through a membrane by pressure
starling forces of net filtration pressure in glomerulus
Net glomerular filtration pressure = Pgc - Pbs - pi gc
Pgc = pressure of glomerular capillary blood (60)
Pbs= pressure of fluid in bowman’s space (15)
pi gc= osmotic force due to protein in plasma (29)
- 2 hydrostatic (away from itself) and 2 osmotic (toward itself)
- usually 1 osmotic left out because it is = 0 (osmotic force of the fluid)
- forces favour filtration over entire length
- healthy kidney will produce positive number
- 20% leaves bloodstream in glomerulus as glomerular filtrate
renal autoregulation
intrinsic mechanism within kidneys (doesnt require outside forces)
- protects the kidneys from elevated arterial pressure that would effect kidney
- blunts direct effects that changes in arterial pressure would otherwise have on GFR
- within a wide autoregulatory range can compensate for arterial pressure changes
autoregulation : keeping us in this homeostatic state
(renal autoregulation)
myogenic mechanism
-seconds
- smooth muscles in arterioles respond to changing systemic blood pressure
- alter vessel diameter / alter Pgc / alters NFP / alters GFR
2nd mechanism - tubuloglomerular feedback
- a little slower to respond than myogenic
- detect changes in sodium, chloride, water
- alter release of vasodilator nitric oxide (NO)
less nitric oxide = vasoconstrictor of afferent arteriole = decrease GFR
