PSIO202 Exam 4 - Urinary: Filtration, Absorption, Secretion Flashcards
What is the equation for rate of excretion?
rate of glomerular filtration + rate of secretion - rate of reabsorption
What is excretion?
process of eliminating waste products from the body, or products excreted from the body
What composes the filtration barrier?
basement membrane (basal lamina), fenestrated endothelium (pores), and podocytes (slits between the pedicels)
What do the pores of the filtration barrier filter/allow to pass?
filter blood cells
allows passage of any component of blood plasma
What does the basement membrane of the filtration barrier filter?
large proteins and negatively charged particles
What do the podocytes of the filtration barrier filter?
medium-sized proteins
What drives glomerular filtration pressure?
ratio of afferent efferent vessel diameter
For no filtration to occur, what must happen to the pressures?
CHP + BCOP must exceeed the GBHP
What is the equation for net filtration pressure, and what does it represent? What is normal NFP?
NFP = GBHP - (CHP + BCOP)
total pressure that promotes filtration
10 mm Hg
What is GBHP?
glomerular blood hydrostatic pressure, about 55 mm Hg
pressure inside the vessels pushing outward
What is CHP?
capsular hydrostatic pressure, about 15 mmHg
the pressure of the capsule on the blood vessels
What is BCOP?
blood colloid osmotic pressure, about 30 mm Hg
the pressure of what has already been filtered back on the blood vessels
What is NFP most reliant on? At what amount of this pressure will filtration stop?
GBHP, at 45 mm Hg GF will stop
What is the definition of glomerular filtration rate (GFR)?
amount of filtrate formed in all renal corpuscles of both kidneys per minute (average is 125 mL/min)
What happens if GFR is too high or too low?
too high: substances are lost (not reabsorbed) because fluid is passing through the nephron too fast
too low: waste products may not be removed from the body
To maintain a constant GFR, a consistent — must be maintained.
NFP, GBHP
What 3 ways is GFR regulated so that normal blood pressure does not stop filtration?
auto, neural, and hormonal
How does the renal corpuscle interact with the ascending loop of henle?
It faces the loop, and the macula dense cells of the loop interact with the juxtaglomerular cells of the afferent arteriole
What is the macula densa?
thickened part of ascending limb of loop of Henle
What are the juxtaglomerular cells?
modified muscle cells that line the afferent arteriole
What is the juxtaglomerular apparatus composed of?
macula densa + juxtaglomerular cells
What are mesangial cells?
contractile cells associated with the capillaries that affect surface area (increase surface area = increase filtration)
Describe the autoregulation of GFR.
myogenic mechanism - fast
increase pressure stretches afferent arteriole, smooth muscle contracts to reduce diameter of the afferent arteriole and return GFR to normal
tubuloglomerular feedback - slow
elevated BP causes fast fluid flow, Na+ and Cl- are not reabsorbed
macular densa detects high Na+ and Cl-, inhibits release of NO (vasodilator) from juxtaglomerular apparatus causing constriction and reduced GFR
Describe the neural regulation of GFR?
blood vessels in the kidney have sympathetic fibers, and when norepi is released, these neurons cause vasoconstriction in the afferent arterioles
SNS can override autoregulation
How does the SNS regulate GFR?
sympathetic nerves in the kidney vasoconstrict the afferent arterioles
stimulate renin release from juxtaglomerular cells
At rest, renal blood vessels are —— dilated.
maximally
With moderate sympathetic stimulation, afferent and efferent arterioles…
constrict equally, so GFR decreases slightly
With extreme sympathetic stimulation, what happens to the arterioles?
vasoconstriction of afferent arterioles predominates, greater blood flow to other tissues, GFR decreases substantially, and urine output decreases
Describe the hormonal regulation of GFR.
Atrial natriuretic peptide (ANP) -
high blood volume stretches atria, ANP released
relaxes glomerular mesangial cells which increase the capillary surface area and increase the GFR
Angiotensin I -
renin released from juxtaglomerular cells, causes ACE in the lungs, releases Angiotensin II
vasoconstricts afferent and efferent arterioles reducing the GFR
What is tubular reabsorption?
transfer of materials from the tubular fluid back into the blood
How much filtrate is reabsorbed, and where does it occur?
99% reabsorbed
mostly in PCT but fine-tuning in the rest of the nephron
What is tubular secretion?
transfer of materials from blood into the tubular fluid
What are the functions of tubular secretion?
control blood pH via secretion of H+, eliminate certain substances (NH4+, creatinine, K+, organic drugs)
What are the three main ways that things are reabsorbed?
active/passive processes, water follows by osmosis, and small proteins can move across by pinocytosis
Describe paracellular reabsorption.
between cells, 50% of reabsorbed material moves by diffusion in some parts
Describe transcellular reabsorption.
through a cell, material moves through both the apical and basal membranes by active transport
Na+/K+ ATPase pumps sodium from the tubule cell through the…
basolateral membrane only
How does water reabsorb?
osmosis only - 2 types
obligatory: water follows solutes being reabsorbed
facultative: under control of ADH
What reabsorption happens in the PCT?
all glucose, amino acids, lactic acid via Na+ symporters which pump Na+ out of the cell
most HCO3-, water, Na+
At the end of the PCT, the filtrate is still —— to blood, with an osmolarity of —–
isotonic, 300
In the PCT, reabsorption of nutrients is ——–, meaning that water….
isosmotic, water follows solutes
What secretion happens in the PCT?
Na+ antiporters secrete acid (H+) which is also part of pumping Na+ out of the cell
What is a renal threshold?
rate of transport is limited for symporters, so when blood concentration of a substance is exceeded, transport is limited. It stays in the filtrate instead of being reabsorbed and ends up in the urine.
What is the renal threshold for glucose, and what is it called if glucose ends up in the urine?
200 mg/dL
glycosuria
What is reabsorbed in the loop of henle?
descending: water
- as the loop gets lower, blood osmolarity increases so warer moves out
thin ascending: Na+ and Cl-
thick ascending: Na+, Cl-, and K+ via active transport
- K+ leaks back into the filtrate
- Na+ is pumped out on the basolateral side
- Cl- diffuses out of the cell
- cations move to the vasa recta (drawn to the negative charge in the capillary)
What is reabsorbed in the early DCT?
Na+ and Cl- via symporters
water via obligatory osmosis
When fluid enters the DCT, it is —— with an osmolarity of about…
hypotonic, 150 mOsm/L
Throughout the DCT, fluid becomes more— depending on —–.
hypotonic, hormones
What is reabsorbed/secreted in the late DCT and collecting duct?
HCO3-, K+, H+, water, and Na+
absorbed or secreted depending on pH of blood (but bicarb is typically reabsorbed while protons are secreted)
What types of cells are located in the late DCT and collecting duct, and what do they each do?
principal cells: reabsorb Na+ (and water if ADH present) and secrete K+
Intercalated cells: reabsorb K+ and HCO3- and secrete H+
How/why is dilute urine formed?
remove excess fluid from the blood by producing more dilute urine
decreased blood osmolarity inhibits ADH release, which increases the H2O lost in urine
What is the general change in filtrate osmolarity throughout the nephron?
increases moving down the loop of henle (as water moves out)
decreases moving up the loop of henle (as ions move out)
decreases in the collecting duct as water moves in
How/why is concentrated urine formed?
prevent water loss by excreting concentrated urine
increase blood osmolarity causes ADH release which increases aquaporin channels into the apical membranes of principa cells (more water reabsorption)
