urine formation Flashcards
name the 3 Basic Processes of Urine Formation
Glomerular FILTRATION
Tubular REABSORPTION
Tubular SECRETION
explain glomerular filtration process of urine formation
Blood pressure forces water and most solutes in blood plasma across the walls of the glomerular capillaries
This produces the glomerular filtrate
Glomerular Filtration depends on what 3 main pressures
Net filtration pressure (NFP; total pressure promoting filtration) =
- Glomerular blood hydrostatic pressure (HP) (hydrostatic pressure = pressure exerted by a fluid)[promotes filtration], minus
- Capsular HP [opposes filtration], minus
- Blood colloidal osmotic pressure (BCOP; due to proteins in plasma) [opposes]
the first main pressure promotes filtration out whereas the other 2 oppose filtration
what is glomerular filtration rate
The pressure exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity = hydrostatic pressure
explain how efferent arteriole diameter is greater than the efferent arteriole diameter
Efferent has higher resistance so increases BP upstream in glomerulus (55mmHg cf. 33 in capillaries elsewhere)
explain mesangial cells function and location
Mesangial cells (intraglomerular) contract to decrease surface area of capillaries, so glomerular filtration decreases
Positioned between capillaries
what is Glomerular Filtration Rate (GFR)
Amount of filtrate formed in all renal corpuscles of both kidneys each minute
adult 105-125ml/min (female-male)
what is the daily glomerular filtration rate in L/Day
150-180 L/day
but since 99% returned to blood, only 1-2 L excreted/day
what keeps the glomerular filtration rate constant and how
Homeostasis keeps GFR relatively constant by renal autoneural & hormonal regulation
- angiotensin II increases BP
- whereas ANP decreases BP
(BP = Blood Pressure. ANP = Atrial Natriuretic Peptide)
what can cause a decrease in glomerular filtration rate
Severe blood loss decreases GFR
But, high BP does not increase it much
what does Atrial Natriuretic Peptide (ANP) do?
and what happens when theres an increase/decrease in BP/volume
Opposes the Renin-Angiotensin system
An increase in BP/volume
ANP release
- Decrease renin, ADH & aldosterone
= Decrease in BP/volume
what happens If glomerular capillaries become damaged/permeable
Plasma proteins enter glomerular filtrate, leads to
Decreased BCOPressure, leads to
H2O drawn out of blood, leads to
Increased Net FP, leads to
Decreased blood volume & increased interstitial fluid volume, leads to
Oedema
what is oedema
build-up of fluid in body tissues, especially in feet and ankles
explain the tubular reabsorption basic process of urine formation
Filtered fluid flows along the renal tubule and through the collecting duct
Tubule and duct cells return ~99% of filtered water and useful solutes to the blood flowing through the peritubular capillaries.
tubular reabsorption is what kind of process
Selective process (to reclaim back into blood)
in tubular reabsorption transport across membranes may be ……
Active primary:
- Uses ATP
- e.g., Na+, bicarbonate ions (stabilises blood pH)
Active secondary:
- Uses ion’s electrochemical gradient
- e.g., glucose & amino acids
Passive
- e.g., urea, lipid soluble nonpolar components, H2O
Transport maximum (Tm) reflects what
number of carriers available to transport substance (except for Na+)
explain the tubular secretion basic process of urine formation
As fluid flows along the tubule and through the collecting duct, the tubule and duct cells remove substances from the blood in the peritubular capillaries and transports them into the fluid in the renal tubules e.g.,
- Wastes
- Drugs (e.g., penicillin, morphine)
- Excess ions
how does tubular secretion act as a back up
to remove material that was not filtered out in the glomerulus
what 2 tubular processes act as hormonal regulation
Hormonal regulation (angiotensin II & aldosterone) of both tubular reabsorption & secretion.
Rate of urinary excretion of any solute =
rate of glomerular filtration
+ rate of secretion
minus rate of reabsorption
(tubular process 1+3-2= rate of urinary excretion)
what is the filtered reabsorbed and excreted values of urea daily
54g filtered
24g reabsorbed
30g excreted
what is the filtered reabsorbed and excreted values of glucose daily
162g filtered
162g reabsorbed
0g excreted
what will happen if the tubule filtration reabsorption and secretion is disrupted
die in a few days as has immediate effects on composition of circulating blood
show the Production of Dilute & Concentrated Urine
Dilute urine = 65mOsm/kg
Concentrated urine = 1200mOsm/kg
extracellular fluid = ~300mOsm/kg
the production of dilute and concentrated urine is controlled by what
antidiuretic hormone
- ADH, also called vasopressin
what happens if antidiuretic hormone (ADH) is absent
urine contains high ratio of water to solutes (= dilute urine), as principal cells of collecting duct IMPERMEABLE to water
what happens if antidiuretic hormone (ADH) is present
much water reabsorbed back into blood (= conc. urine) due to osmotic gradient maintained by:
- Differences in solute & water permeability & reabsorption in different sections of long loops of Henle & collecting duct
- Countercurrent flow in neighbouring desc & ascend limbs of loop
what happens if there is too little water in the blood
its returned to normal by:
1. hypothalamus detects too little water in blood
2. pituitary gland releases ADH
3. kidneys maintain blood water level
4. so less water is lost in urine (urine more concentrated)
5. blood water levels return to normal
what causes regulation of water reabsorption by ADH
negative feedback
DIURESIS =
PRODUCTION of urine
ANTIdiuresis =
reduced production of urine, hence ADH results in concentrated urine (less urine volume as water reabsorbed).
what process happens is water conc is too high
some stimulus disrupts homeostasis by increasing blood water concentrations
this triggers osmoreceptors in hypothalamus this input sends nerve impulses to the control centre where the hypothalamus and posterior pituitary release ADH.
this output goes to effectors where cells in DCT and collecting duct become more permeable to water, which increases water reabsorption
this then causes an increase in blood water conc
this then returns to homeostasis when response brings plasma osmolarity ack to normal
the release of ADH is stimulated or inhibited by what
Na in extracellular fluid
explain the process of how Na in extracellular fluid stimulated or inhibits ADH release
high Na conc detected in bloodstream
then, pituitary gland(posterior lobe) stimulated to release ADH
then, ADH carried to kidney by the blood
then, increased reabsorption of water from kidney back into the blood stream
H2O then dilutes Na in the blood
this then lowers Na conc
what happens in the collecting duct is ADH is present
If ADH present, water out of collecting duct (by diffusion), so concentrated urine produced
explain the countercurrent multiplier system
The more salt actively pumped out of asc limb, the more conc will become the fluid passing through the desc limb into the asc limb
i.e., by pumping out solutes, the asc limb indirectly conc fluid being delivered to asc limb by the desc limb
As desc permeable to water but asc not & water move out by osmosis
