Renal physiology Flashcards
1
Q
Osmolarity
A
Solute concentration: nº of osmoles (Osm) per liter = Osm/L
2
Q
What is the difference between osmolarity and tonicity?
A
Osmolarity refers to both penetrating and non-penetrating solutes, tonicity refers too only non-penetrating solutes
3
Q
Osmosis
A
Movement of water from areas of high osmotic pressure (hyperosmotic - more concentrated relative to some other solution) to areas of low osmotic pressure (Hypoosmotic)
4
Q
Isosmotic
A
No difference in osmotic pressure
5
Q
What is the difference between osmotic regulators and conformers?
A
Osmotic regulators maintain a constant blood osmolarity, conformers follow isosmotic line (blood osmotic pressure = ambient osmotic pressure)
6
Q
Ion regulator
A
Maintenance of a constant concentration of inorganic ions in blood plasma
7
Q
Ionic conformer
A
Allows Concentration of a particular ion specie in blood plasma to match the concentration in environment
8
Q
Challenges to freshwater regulators
A
- External environment is hypo-osmotic to internal environment
- Constantly taking in water through osmosis
9
Q
Challenges to marine regulators
A
- External environment is hyperosmotic to internal environment
constantly losing water (faces dessication/constant water loss) - Constantly drinking seawater to compensate for water loss (this also causes a load up on ions that needs to be removed)
10
Q
Solutions for challenges faced by freshwater regulators
A
- Copious amounts of dilute urine counters water uptake (leads to ionic loss)
- Active uptake of ions through gills (active transport) to counter ionic loss/dilution
11
Q
What are the cell types in freshwater gills?
A
1) Pavement cells: 90% of gill epithelium, principally responsible for oxygen uptake
2) Mitochondria rich cells (MRCs): uptake of chloride, sodium, and calcium; partially under hormonal control, density and type can be changed in varying conditions
12
Q
Density of MRC’s in very “soft” freshwater (low calcium)?
A
This increases osmotic pressure for water to enter fish and dilute ionic concentrations (ion loss), thus MRC density is upregulated to counter challenges of ionic loss
13
Q
V-type (vacuolar) ATPase
A
Located on apical membrane of MRCs in freshwater gills. Transports H+ out of the cell which leaves the cell with a net negative charge
14
Q
Sodium channels
A
Located on apical membrane of MRCs in freshwater gills.
-negative charge of MRCs due to V-type ATPase attracts cations into the cell passively
15
Q
Na+ - K+ pump/ATPase
A
Located on the basolateral membrane of MRCs in freshwater gills, pumps 3Na+ out of MRC and 2K+ into MRC
16
Q
Potassium leak channels
A
Located on the basolateral membrane of MRCs in freshwater gills, helps maintain negative charge of MRC and low intracellular K+ concentration
17
Q
Electroneutral anion exchanger
A
Found on pavement cells and apical membrane of MRC’s, exchanges a bicarbonate ion for a chloride ion (driven by buildup of bicarbonate which causes a driving force for bicarbonate efflux)
18
Q
Cystic fibrosis transmembrane regulator (CFTR)
A
Found on pavement cells and basolateral membrane of MRC’s, allows chloride ions to move from cells into the bloodstream
19
Q
Mutations in CFTR
A
- Results in cystic fibrosis
- Reduces chloride clearance from cells, this maintains a higher than normal electronegative potential in the cells
- This also reduces extracellular removal of cations (cations also build up inside the cells)
- This causes increased mucosal buildup (higher osmotic pressure causes water to enter cells) - leads to respiratory and digestive difficulties
20
Q
Calcium co-transporter and calcium-ATPase
A
Moves calcium out of the cell based on driving force for sodium (sodium influx-attraction to negative charge) or ATP breakdown
21
Q
What are the effects of drinking seawater?
A
- Water in the gut will be hyperosmotic to blood plasma (will cause water to be drawn out of the lood plasma by osmosis and sodium and chloride ion diffusion into blood plasma)
- Net result is veery concentrated blood plasma
22
Q
What are the adaptations of marine fish for the effects of drinking seawater?
A
- later parts of the intestine actively transport sodium and chloride ions out of the gut into the blood
- increases water reabsorption
- excess ions are removed in the gills (NKCC cotransporter causes chloride buildup in MRC to produce a driving force for chloride to leave the cell, negative charge attracts sodium ions to the apical membrane which enters environment via a paracellular pathway)
23
Q
Challenges faced by marine birds/reptiles
A
- body is hyposmotic to seawater
- results in water loss and salt loading due to ingestion of hyperosmotic water (must remove excess solutes)
24
Q
how do marine birds/reptiles remove excess solutes?
A
salt glands (secretory cell uses NKCC and paracellular pathway to converge salt ions into lumen of secretory tubule for excretion)
25
marine osmoconformers
Marine osmoconformers are organisms that maintain an internal salt concentration equal to their surrounding environment. They do not actively regulate their osmotic balance but instead allow their body fluids to fluctuate with changes in external salinity. Examples include many invertebrates like jellyfish and sea stars.
26
humidic animals
Humidic animals are species that require high humidity to maintain proper physiological functions, such as water balance and respiration. They are often found in moist environments and may struggle to survive in dry conditions. Examples include many amphibians like frogs and salamanders.
27
xeric animals
Xeric animals are species adapted to dry, arid environments. They have specialized mechanisms to conserve water, such as reduced water loss, efficient excretory systems, and behavioral adaptations to avoid dehydration. Examples include desert reptiles like lizards and camels.
28
evaporative water loss (EWL)
Evaporative water loss (EWL) is the loss of water from an organism's body due to evaporation from surfaces like the skin, respiratory tract, or mucous membranes. It is a critical process for temperature regulation but can lead to dehydration if not properly managed, especially in hot or dry environments.
28
glomerulus
The glomerulus is a network of capillaries in the kidney where filtration of blood occurs. It is part of the nephron and plays a key role in filtering out waste, excess substances, and water from the blood to form urine.
28
U/P > 1
A U/P ratio > 1 indicates that the urine is more concentrated than the plasma. This suggests that the kidneys are actively reabsorbing water and solutes, often in response to dehydration or the need to conserve water. It is seen in animals with highly concentrated urine.
29
what is the difference between humidic and xeric animals?
Humidic animals need moist environments to prevent water loss (e.g., amphibians).
Xeric animals are adapted to dry environments, conserving water efficiently (e.g., desert reptiles, camels).
29
U/P < 1
A U/P ratio < 1 means urine is less concentrated than plasma, indicating the kidneys are excreting more water relative to solutes. This occurs in conditions of excess hydration.
30
excretory water loss
Excretory water loss is the water lost through urine and feces as the body removes waste products and excess substances. It varies based on hydration, diet, and kidney function.
31
what contributes to the large hydrostatic pressure outside the tubule?
After blood goes through the glomerulus it exists through the efferent arteriole, which has a smaller diameter. This resistance increases pressure in the glomerular capillaries promoting filtration
31
U/P = 1
A U/P ratio = 1 means urine is isosmotic to plasma, indicating no net concentration or dilution of solutes. This occurs when the kidneys excrete water and solutes in the same proportion as the blood plasma.
31
what forms the ultra-filter in Bowman’s capsule?
1. Fenestrated capillary endothelium (glomerulus)
2. Basement membrane (prevents large molecules from passing)
3. Podocyte filtration slits (selectively allow small molecules through)
32
What is definitive urine?
Definitive urine is the final urine excreted after filtration, reabsorption, and secretion in the nephron. It reflects the body's regulation of water, ions, and waste.
32
What is primary urine?
Primary urine is the initial filtrate formed in Bowman’s capsule after blood plasma passes through the glomerular filter. It contains water, ions, glucose, and waste, but no large proteins or cells. It's isosmotic to blood pressure
33
What is filtration pressure?
Filtration pressure is the net force driving fluid from the glomerulus into Bowman’s capsule. It results from the balance of hydrostatic pressure, osmotic pressure, and capsular pressure.
34
What is glomerular filtration rate (GFR)?
GFR is the rate at which fluid is filtered from the glomerulus into Bowman’s capsule. It depends on filtration pressure, permeability, and surface area of the glomerular membrane.
35
How do chronic hypertension and diabetes mellitus affect the kidneys?
Both chronic hypertension and diabetes mellitus can damage the glomeruli and blood vessels in the kidneys, leading to glomerulosclerosis, reduced glomerular filtration rate (GFR), and kidney dysfunction over time. These conditions increase the risk of chronic kidney disease.
36
What are the loops of Henle?
The loops of Henle are U-shaped sections of the nephron that create a concentration gradient in the kidney, essential for water and salt reabsorption. They consist of a descending limb (permeable to water) and an ascending limb (permeable to ions but not water). This process helps concentrate urine and conserve water.
37
What are descending thin segments in the nephron?
The descending thin segments of the loop of Henle are highly permeable to water but not to solutes. As filtrate moves through this segment, water is reabsorbed, concentrating the filtrate.
37
What is the ascending thin segment in the nephron?
The ascending thin segment of the loop of Henle is impermeable to water but permeable to ions like sodium and chloride. It allows ions to passively diffuse out, contributing to the creation of a concentration gradient in the kidney.
38
What is the ascending thick segment in the nephron?
The ascending thick segment of the loop of Henle actively transports ions (such as sodium, potassium, and chloride) out of the filtrate using ATP-dependent processes. This segment is impermeable to water and plays a key role in creating the osmotic gradient that allows for water reabsorption in the kidney. Active transport of NaCl
39
What is the single effect in the context of the nephron?
The single effect refers to the active transport of ions (such as sodium and chloride) out of the ascending limb of the loop of Henle, creating an osmotic gradient. This process helps establish the concentration gradient that allows for water reabsorption in the descending limb and collecting ducts.
40
What is countercurrent multiplication?
Countercurrent multiplication is the process in the loop of Henle where the flow of filtrate in the descending and ascending limbs runs in opposite directions. This mechanism, along with active ion transport in the ascending limb, creates an osmotic gradient in the kidney medulla, which allows for efficient water reabsorption and the concentration of urine.
40
What is diuresis?
Diuresis is the increased production of urine, often caused by factors such as fluid intake, diuretic drugs, or conditions like diabetes. It helps eliminate excess water or solutes from the body.
41
What is antidiuretic hormone (ADH)?
Antidiuretic hormone (ADH), also known as vasopressin, is a hormone produced by the pituitary gland that promotes water reabsorption in the kidneys by increasing the permeability of the collecting ducts to water. It helps regulate fluid balance and blood pressure by reducing urine output.
42
What is urea?
Urea is a waste product of protein metabolism, formed in the liver through the urea cycle. It is excreted in urine by the kidneys and serves as a major nitrogenous waste in mammals. Urea helps with the kidney’s ability to concentrate urine and conserve water.
42
What are areas with high permeability to urea in the nephron?
Areas with high permeability to urea include the descending limb of the loop of Henle, the medullary collecting duct, and parts of the proximal convoluted tubule. These areas allow urea to be reabsorbed or secreted, contributing to the kidney's ability to concentrate urine and maintain water balance.
42
What are baroreceptors?
Baroreceptors are sensory receptors that detect changes in blood pressure. They are located in the carotid sinuses and aortic arch. Baroreceptors help regulate blood pressure by sending signals to the brain to adjust heart rate and blood vessel constriction.
42
What are osmoreceptors?
Osmoreceptors are sensory receptors that detect changes in blood osmolarity (solute concentration). They are located primarily in the hypothalamus and help regulate water balance by triggering the release of antidiuretic hormone (ADH) to conserve water when osmolarity increases.
43
What is the vasa recta?
The vasa recta surrounds the loop of Henle, maintaining the renal medulla’s osmotic gradient via countercurrent exchange, allowing water and solute reabsorption while preserving urine concentration and water balance.
43
What is the Aquaporin water channel containing vasopressin (AQWCV)?
Aquaporin water channels (AQWCV) are membrane proteins that allow water to pass through cells. When vasopressin (ADH) is present, it triggers the insertion of aquaporins into the collecting duct of the nephron, increasing water reabsorption. This helps conserve water and concentrates urine.
43
What are areas with low permeability to urea in the nephron?
Areas with low permeability to urea include the thick ascending limb of the loop of Henle, the distal convoluted tubule and cortical and outer renal medulla. These segments are impermeable to urea, preventing its reabsorption and helping maintain the osmotic gradient in the kidney.
44
What is the urea transporter protein UT-A1?
UT-A1 is a urea transporter in the kidney’s collecting ducts, aiding urea reabsorption into surrounding tissue. This process helps concentrate urine and maintain water balance, particularly under the influence of ADH (vasopressin).
44
So in fresh water there’s going to be —– of water. Osmosis is favoring water moving —— because the osmotic pressure inside the organism is —— than it is in the surrounding environment. So free water is going to move ——.
There’s going to be —– of ions. Again, there’s a —— concentration of ions inside the organism than there is outside the organism. So those ions are going to want to move ——–
In freshwater, organisms tend to gain water by osmosis because the osmotic pressure inside the organism is higher than the surrounding environment. Water moves into the organism. Additionally, ions will tend to move out since there is a higher ion concentration inside the organism than outside.
45
In salt water. It’s the exact opposite. Animals are going to be constantly —- water, because again, more free water in the ——. So it’s going to move towards the —–. And you’re going to get an —— of ions because the ions ———-
In saltwater, animals tend to lose water due to osmosis, as the environment has more free water. Water moves towards the higher concentration of solutes inside the organism. At the same time, there is an accumulation of ions because the ion concentration is higher inside the organism compared to the surrounding environment.
46
U/P ratio in humans
From 0.1-4
47
Is the loop of Henle part of the nephron?
Yes, the loop of Henle is part of the nephron. It connects the proximal convoluted tubule to the distal convoluted tubule and plays a crucial role in concentrating urine and maintaining water and salt balance.
48
How is the Active NaCl Transport in loop of Henle?
In the ascending thick segment of the loop of Henle, NaCl (sodium chloride) is actively transported out of the filtrate using the Na+/K+/2Cl− symporter. This process requires ATP and helps create a high osmotic gradient in the kidney's medulla, enabling water reabsorption in the descending limb and collecting ducts.
49
How would you explain urea permeability in the kidneys?
It is high in areas like the proximal convoluted tubule and medullary collecting duct (via UT-A1), but low in the ascending limb and distal tubule, helping the kidney concentrate urine and conserve water.
50
What is the blood supply to the renal medulla?
The renal medulla receives blood supply through the vasa recta.The vasa recta is crucial for maintaining the osmotic gradient in the medulla, which is essential for concentrating urine and regulating water balance in the kidneys.
51
Regarding osmoregulation, describe the “osmoconformer” strategy
An osmoconformer is an animal that matches the environment while maintaining cellular function.
52
Regarding osmoregulation describe the “osmoregulator” strategy
An osmoregulator is an animal that maintains tight control of its internal environment regardless of outside conditions.
53
Are osmoregulation and osmoconformation mutually exclusive?
No. An organism can have some mechanisms under regulatory control and some not.
54
What are some mechanisms under regulatory control?
Osmotic regulation (maintaining total dissolved solutes for osmo pressure), ionic regulation, volume regulation
55
What is one way to identify an osmoconformer versus an osmoregulator on a graph?
Plot blood osmotic pressure and ambient osmotic pressure. An osmoconformer will have a visible increasing slope, an osmoregulator will remain relatively constant. This also shows those with mixed systems.
56
What are the challenges of freshwater regulators?
Freshwater regulators live in a hypo osmotic environment and thus they struggle to maintain needed ions and rid excess water.
57
What are the challenges of marine regulators?
Marine regulators live in a hyperosmotic environment and thus they struggle to rid excess ions and keep water.
58
What is the main surface of aquatic ion exchange?
The gills. They have a high surface area to meet oxygen demands and thus are also key to the intake/loss of water and ions.
59
What are the important cells of the gill?
Pavement cells which make up most of the gill’s surface, Mitochondria rich cells (mRc) which maintain internal [solute], and chloride cells/ionophores. Accessory cells are also important but exclusively found in marine fish.
60
What is the role of a pavement cell in a gill?
It is the main site of oxygen exchange and makes up 90% of the gill.
61
What is the role of an MRC in a gill?
MRCs deal with ion loading challenges and will uptake Chloride, Sodium, and Calcium ions in freshwater and remove Chloride and Sodium ions in saltwater. It is under hormonal control partially and the density and type of MRCs can change depending on the external environment.
62
What is one way MRCs change in response to different conditions?
In soft water there is low calcium, thus calcium from within the organism will want to diffuse out because of concentration gradients. Thus, the organism will have more and larger MRCs to take up more calcium because of the calcium that is lost.
63
What role does active transport play in MRCs?
In freshwater MRCs actively transport ions into the organism, and in saltwater MRCs actively transport ions out of the organism. This is active transport because it is against a concentration gradient.
64
Describe the role of V-type ATPase in the gills.
In freshwater gills, it constantly pumps out protons using ATP on the apical membrane of MRCs. This attracts sodium ions due to a net negative charge.
65
Describe the role of Na/K ATPase in the gills.
In freshwater gills, it exchanges 3 sodium to the blood for 2 potassium into the cell which leaks back out into the blood by leak channels, this further adds to the negativity. It is in the basolateral membrane of MRCs. It is the same in marine gills.
66
Describe the role of the electroneutral anion exchanger in the gills.
In freshwater gills, it exchanges bicarbonate waste for chloride. It’s in the apical membrane of pavement cells and MRCs. The binding of bicarbonate causes a conformation change that has a strong chloride binding site. It is either not present in marine gills or not worth mentioning.
67
Describe the role of the calcium co-transporter and calcium ATPase in the gills.
In freshwater gills, the calcium co-transporter relies on the gradient established by the NA/K ATPase to exchange a sodium into the cell for a calcium into the blood. It is on the basolateral membrane of MRCs. Calcium ATPase is also on the basolateral membrane and actively transports calcium into the blood. It has the same role in marine gills.
68
Why does drinking seawater dehydrate you?
Because the salt water is hyperosmotic to your blood plasma, and thus the water in your blood plasma is drawn into the salt water in your gut and sodium and chlorine will diffuse into the blood plasma.
69
How do saltwater fish differ from mammals in their ability to drink sea water?
They have active transport mechanisms for sodium and chloride into the blood plasma later in their intestine, this allows water retention but keeps a lot of excess ions which have to get removed via ion exchange mechanisms in the gills.
70
What is the role of the accessory cell in the gills?
Accessory cells in gills support tissue structure, regulate ion transport, assist immune defense, and maintain epithelial lining for gas exchange and osmoregulation.
71
What is the role of the NKCC cotransporter in the gills?
The NKCC cotransporter in gills facilitates the uptake of sodium, potassium, and chloride ions, crucial for osmoregulation and maintaining electrolyte balance in aquatic organisms.
72
Identify these gill ion transporters by whether they are present in marine, freshwater, or both types of fish. Also describe their function and location in a couple of words.
1. V-type ATPase
2. Na/K ATPase
3. Electroneutral anion exchangers
4. Cystic Fibrosis Transmembrane Regulators
5. Calcium co-transporters
6. Calcium ATPase
7. Paracellular sodium pathway
8. NKCC cotransporter
V-type ATPase
1. Location: Membranes of chloride cells.
Function: Acidifies compartments, helps in ion transport.
Fish Type: Both (helps in osmoregulation in both environments).
Na/K ATPase
2. Location: Basolateral membrane of gill cells.
Function: Pumps sodium out and potassium in, crucial for ion balance.
Fish Type: Both (important for osmoregulation in both environments).
Electroneutral anion exchangers
3. Location: Chloride cells.
Function: Exchange chloride for bicarbonate, aiding in pH balance.
Fish Type: Both (assist with ion regulation and pH homeostasis).
Cystic Fibrosis Transmembrane Regulators (CFTR)
4. Location: Chloride cells.
Function: Transport chloride ions across membranes.
Fish Type: Marine (important for salt excretion).
Calcium co-transporters
5. Location: Apical membranes of gill cells.
Function: Transport calcium and other ions to regulate calcium levels.
Fish Type: Freshwater (helps in calcium uptake due to low calcium levels in freshwater).
Calcium ATPase
6. Location: Membrane of chloride cells.
Function: Pumps calcium out of cells, maintaining calcium balance.
Fish Type: Both (important in both environments for calcium regulation).
Paracellular sodium pathway
7. Location: Between epithelial cells of gills.
Function: Passive sodium movement, helps in osmotic balance.
Fish Type: Marine (important for sodium excretion in marine fish).
NKCC cotransporter
8. Location: Chloride cells.
Function: Transports sodium, potassium, and chloride, aiding in ion balance.
Fish Type: Marine (primarily involved in salt excretion in marine fish)
73
How do salmon osmoregulate?
- Freshwater: Produce dilute urine, actively absorb ions in gills.
- Seawater: Drink seawater, excrete excess salt via gills, produce concentrated urine.
74
How does respiratory water loss work? What are some ways terrestrial organisms combat it?
Respiratory water loss: Loss of moisture during gas exchange.
Combat strategies: Reduced breathing, waterproofing, nocturnal activity, efficient kidneys.
75
What factor causes evaporative water loss to vary?
Evaporative water loss varies with temperature, humidity, and air movement.
76
How do we combat excretory water loss?
To combat excretory water loss, organisms produce concentrated urine and have efficient kidneys to minimize water loss while excreting waste.
77
What is the role of the mammalian kidney in osmoregulation?
The mammalian kidney regulates osmoregulation by filtering blood, reabsorbing water and ions, and producing concentrated or dilute urine to maintain water and salt balance.
78
What is the functional unit of urine concentration?
The functional unit of urine concentration is the nephron, specifically the loop of Henle, which creates a concentration gradient in the kidney to enable water reabsorption.
79
What drives the flow of fluid into Bowman’s capsule?
The flow of fluid into Bowman's capsule is driven by glomerular filtration pressure, which results from the blood pressure in the glomerulus forcing fluid and small solutes into the capsule.
80
Describe the ultrafilter of the renal corpuscle.
The ultrafiltrate in the renal corpuscle is formed when blood is filtered through the glomerulus's fenestrated endothelium, basement membrane, and podocytes, allowing small solutes to enter Bowman's capsule while blocking larger molecules.
81
Define the two types of nephrons and highlight their differences
- Cortical nephrons: Located primarily in the renal cortex, with short loops of Henle. They are involved in basic filtration and waste removal.
- Juxtamedullary nephrons: Located near the medulla, with long loops of Henle that extend deep into the medulla. They are crucial for concentrating urine and maintaining water balance.
82
How does the thickness of the renal medulla connect with an animal’s ability to concentrate urine?
A thicker renal medulla allows for a longer loop of Henle, which creates a stronger concentration gradient in the kidney. This enables better water reabsorption and the ability to produce more concentrated urine, helping animals conserve water, especially in arid environments.
83
How does permeability vary across the loop of Henle?
- Descending limb: Highly permeable to water, allowing water to be reabsorbed into the surrounding medullary interstitium.
- Ascending limb: Impermeable to water but actively transports sodium, potassium, and chloride ions, creating a concentration gradient in the medulla.
84
How does active transport in the thick ascending loop of Henle work?
In the thick ascending loop of Henle, the Na+/K+/2Cl− symporter actively transports sodium, potassium, and chloride ions into cells, creating a concentration gradient for water reabsorption.
85
How do the elasmobranchii osmoregulate?
Elasmobranchii (sharks, rays, skates) osmoregulate by maintaining high levels of urea and TMAO in their blood, matching the osmolarity of seawater to prevent dehydration. They also excrete excess salts via specialized rectal glands, helping balance internal and external salt concentrations.
86
Describe how countercurrent multiplication works.
Countercurrent multiplication creates a concentration gradient in the medulla by reabsorbing water in the descending loop and actively transporting ions in the ascending loop, enhancing urine concentration.
87
Describe how (and why) the permeability of the collecting duct changes.
The permeability of the collecting duct changes in response to antidiuretic hormone (ADH). In the presence of ADH, the duct becomes more permeable to water, allowing water reabsorption and concentrated urine formation. This helps conserve water, especially during dehydration. Without ADH, the duct remains impermeable to water, resulting in diluted urine.
88
What stimulates the release of ADH?
ADH release is stimulated by increased blood osmolarity (detected by osmoreceptors in the hypothalamus) or low blood volume/pressure (detected by baroreceptors). These signals prompt the release of ADH from the pituitary gland to conserve water.
89
How does Aquaporin-2 insertion and removal in the collecting duct work?
Aquaporin-2 (AQP-2) channels are inserted into the collecting duct membrane in response to ADH. ADH binds to receptors, triggering a signaling cascade that leads to the insertion of AQP-2 channels, increasing water permeability. When ADH levels drop, AQP-2 channels are removed from the membrane via endocytosis, reducing water reabsorption and leading to dilute urine.
90
How does urea permeability change across the nephron?
- Proximal tubule: Freely permeable to urea, allowing it to be reabsorbed.
- Loop of Henle: Low permeability to urea.
- Distal tubule and collecting duct: Permeability increases in the presence of ADH, allowing urea reabsorption to contribute to the osmotic gradient in the medulla for water reabsorption.
91
How does the blood supply to the renal medulla avoid losing water, gaining ions, and ruining the single effect?
The vasa recta uses countercurrent exchange to prevent water loss and ion gain, preserving the medullary osmotic gradient and the single effect.
