Urine Formation 1
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89 Terms
What is glomerular filtration?
The process in which water and small solutes in blood plasma pass from the glomerular capillaries into the nephron’s capsular space
Where does glomerular filtration take place?
The renal corpsucle
Where does filtrate collect after passing through the filtration membrane?
The capsular space
How many layers make up the filtration membrane?
3
What is the first layer of the filtration membrane?
The fenestrated epithelium of the glomerular capillaries
What does ‘fenestrated’ mean?
It contains many tiny pores for filtration
Why is the fenestrated capillary endothelium important?
It allows water and small solutes to pass while preventing blood cells from entering the filtrate
What cannot pass through the fenestrated capillary endothelium?
Blood cells
What is the second layer of the filtration membrane?
Basement membrane
What is the basement membrane made of?
Proteoglycan gel
What size molecules are excluded from the basement membrane?
8 nm and up
Why does the basement membrane repel some molecules?
The negative charge of the proteoglycans
Besides size, what else can determine if a molecule can pass through or not?
Electrical charge
What is the third layer of the filtration membrane?
Filtration slits
What cells form the filtration slits?
Podocytes
What shape are podocytes typically compared to?
An octopus
What re the extensions of podocytes called?
Foot processes or pedicels
What do the podocyte foot processes wrap around?
Glomerular capillaries
What forms between neighboring foot processes?
Filtration slits
What is unique about the filtration slits?
They have a negative charge, helping repel certain molecules
Why are glucose and amino acids found in filtrate?
They are small enough to pass through the filtration membraneHo
How does the concentration small solutes in the filtrate compare to blood plasma?
It is about the same
What substances can generally not pass through the filtration membrane because they are protein-bound?
Iron, calcium, and thyroid hormones
Why can’t protein-bound substances pass through the filtrate?
The proteins they’re attached to are too big to pass through
Why are blood cells usually absent from urine?
Because they are too big to pass through the fenestrated capillary membrane
Why are plasma proteins absent from filtrate?
They are too big to pass and through and are repelled by the negative charge of the membrane
If glucose is filtered into the nephron, does that mean it will be excreted into urine?
No; normally all filtered glucose is reabsorbed by the PCT
What layer of the filtration membrane contains podocytes?
Filtration slits (visceral layer of Bowman’s capsule)
Blood first passes through the ________ epithelium of the glomerular capillary
fenestrated
List the three layers of the glomerular filtration membrane in order
Fenestrated epithelium → Basement membrane → Filtration slits
What substances are normally not found in the filtrate?
Blood cells, plasma proteins, and protein-bound molecules
What is the glomerular filtration rate?
The amount of filtrate formed per minute by both kidneys combined
How much filtrate is produced per every 1 mmHg of net filtration pressure?
About 12.5 mL/min
What is the filtration coefficient?
A measure of how easily a fluid crosses the filtration barrier
What two factors determine the filtration coefficient?
Permeability of the filtration barrier and surface area available for filtration
How does the filtration coefficient generally compare between men and women?
It is 10% lower for women
What is tubuloglomerular feedback?
Feedback mechanism in which the glomerulus receives info about the composition of fluid in the nephron tubule and adjusts filtration accordingly.
What are the three main functions of tubuloglomerular feedback?
Adjusts filtration to regulate tubular fluid composition
Stabilizes nephron performance
Compensates for blood pressure regulation
What is the juxtaglomerular apparatus?
A specialized structure where the nephron loop contacts the afferent and efferent arterioles at the vascular pole of the renal corpuscle
What structure begins at the juxtaglomerular apparatus?
Macula densa
What is the macula densa?
A patch of densely packed sensory cells in the distal nephron that monitors the NaCl concentration of tubular fluid
What happens to the NaCl concentration in the nephron when the GFR is high?
It increases because filtrate moves through the nephron quickly
What do Macula Densa cells absorb when the GFR is high?
Na, K, and Cl
What happens to the macula densa cells after they absorb more NaCl
Water follows osmotically and the cells swell
What do swollen macula densa cells release?
ATP
What does ATP become?
Adenosine
What is adenosine’s role in tubuloglomerular feedback?
Acts as a paracrine messenger that signals nearby granular (juxtaglomerular) cells
What are granular (juxtaglomerular) cells?
Modified smooth muscle cells located around afferent arterioles
What affect does adenosine have on the afferent arteriole?
It causes the afferent arteriole to constrict
Why does constricting the afferent arteriole lower GFR?
It decreases bloodflow in the glomerulus, reducing filtration
What type of feedback is tubuloglomerular feedback?
Negative feedback
Why is tubuloglomerular feedback important?
It prevents excessive filtration and helps maintain a stable GFR despite changes in blood pressure
If the macula densa detects a high NaCl concentration, what does this indicate?
The GFR is too high
The ________ detects NaCl concentration in the tubular fluid
Macula Densa
ATP released by the macula densa is converted to ______.
Adenosine
Adenosine causes the ________ to constrict.
Afferent Arteriole
What does RAAS stand for?
Renin-Angiotensin-Aldosterone System
What is the purpose of RAAS?
Increase blood pressure and restore blood volume
What triggers RAAS?
A significant drop in blood pressure
What receptors detect a drop in blood pressure?
Baroreceptors in the aorta and carotid arteries
What happens when baroreceptors detect a drop in blood pressure?
They send signals to the brain stem, activating the sympathetic nervous system
What do sympathetic nerve fibers stimulate?
Granular cells in the kidney
What enzyme to the granular cells release?
Renin
What is renin?
AN enzyme released by the kidneys in response to low blood pressure
What substance does renin act on?
Angiotensinogen
What is angiotensinogen
A protein circulating in the blood plasma
What does renin do to angiotensinogen?
It converts it into angiotensin I
What does ACE stand for?
Angiotensin-Converting Enzyme
Where is ACE primarily found?
In the lungs and kidneys
What does ACE do?
Convert Angiotensin I into Angiotensin II
Why is Angiotensin II important?
It restores blood pressure and blood volume through multiple mechanisms
What effect does Angiotensin II have on blood vessels?
It is a potent vasoconstrictor
What happens to blood pressure under vasoconstriction?
It increases
What does angiotensin II constrict in the kidney?
Efferent arterioles
What does constricting the efferent arteriole help?
It raises glomerular pressure and helps maintain GFR even with low blood pressure
How does efferent arteriole constriction increase water reabsorption?
It lowers blood pressure in the peritubular capillaries, allowing more sodium and water to move from the nephron back into the blood stream.
What gland release aldosterone?
The adrenal gland
What stimulates the adrenal gland to release aldosterone?
Angiotensin II
What is the purpose of aldosterone?
It increases sodium and water reabsorption
Where does aldosterone primarily act?
The DCT and collecting ducts
What effect does angiotensin II have on the PCT
Directly increases water and sodium reabsorption
What hormone is released from the posterior pituitary gland during RAAS activation?
Antidiuretic Hormone (ADH)
What stimulates ADH release?
Angiotensin II
What is the function of ADH?
It increases water reabsorption in the collecting ducts
Besides hormones, what behavior does angiotensin II stimulate?
Thirst
Why is thirst important in RAAS?
Drinking more water helps restore blood volume and pressure
Why is RAAS activated?
To help restore blood volume and blood pressure when there is a drop
Why does angiotensin II constrict the efferent arterioles instead of the afferent?
Constricting the efferent arteriole helps maintain pressure inside the glomerulus, allowing GFR to continue despite low blood pressure
How does aldosterone help raise blood pressure?
It causes kidneys to reabsorb more sodium, the water follows which increases blood volume and pressure