urinary systems and electrolyte balance

Question 1

bodily fluid compartments: intraellular fluid(ICF)

Answer 1

inside the cell
The ICF is separated from the interstitial fluid (IF) just by cell membranes. The IF is separated from blood plasma by endothelium of blood capillaries.
Since water is 60% of total body weight, it has to be divided amongst the body fluid compartments.
ICF is the larger compartment and contains 2/3rd of the total. The remaining 1/3rd is the ECF.
The ICF and ECF are in osmotic equilibrium. To maintain this equilibrium water shifts between these two compartments.

Question 2

bodily fluid compartments: extracellular fluid(ECF) and adipose tissue

Answer 2

ECF is outside the cell

The ECF is divided into two separate compartments, interstitial fluid (found between cells in ordinary tissues) and blood plasma (part of the blood apart from the red blood cells and white blood cells).

These three compartments in an animal interact and affect one another

The ECF is further divided into IF and plasma. IF accounts for 3/4th of the ECF volume, and the remaining is the plasma.

All solutes and water that enter, leave via the ECF.

• Even though water makes up 60% of the total body weight, it varies amongst individuals due to the amount of adipose tissue (lipid rich cells).
• Since adipose tissue is low in water content and increase in adipose tissues leads to a decrease in the total body weight attributed to water.

Question 3

capillaries

Answer 3

• IF and plasma are divided by capillary walls. Movement between plasma and IF is isosmotic, which means water moves freely.
• Capillaries have thin walls; you will achieve filtration or reabsorption depending on the pressure that is present, such as hydrostatic or osmotic pressure.
• Osmotic pressure in human plasma is 300 mOsm.
• IF and plasma have similar osmotic pressures, however plasma is 1.5 mOsm higher than IF, this minute difference will not disturb the isosmotic state.
• Remember between the ICF and ECF water cannot move freely as the osmolarity inside a cell is different to external environment and thus they require transporters.

Question 4

electrolyte composition

Answer 4

• IF and plasma are isosmotic, which means they have a similar concentration of anions and cations.
• On the other hand, ICF have very different concentrations.
• difference in concentration across seawater, freshwater and terrestrial animals, as these three environments pose different challenges to these organisms.
• If an animal lives in freshwater, the surroundings have a low concentration of solutes, suggesting that the animal will be hyperosmotic to the environment. For sea water animals it is the exact opposite.

Question 5

osmoregulation

Answer 5

• Major sites of ion and water exchanges are skin (sweat), respiratory system (dry and wet during breathing), digestive tract (water and fluid absorption) and excretory system (urine, faecal matter).
• Osmoregulation is the movement of water and solutes to maintain an isosmotic state.
• Sponges and cnidarians carry out this process with the lack of a circulatory system as they are in direct contact with water (bulk-flow), hence becomes easier for them to regulate and exchange.
• The wall of the sponge is full of pores that propel water into the spongocoel and out through the osculum.

Question 6

osmoregulation of freshwater fish-

Answer 6

• The outer covering of fish known as the integument is impermeable to water, therefore lack direct contact and exchange with the external environment.
• Fresh water fish are surrounded by an environment low in salt ions, however it has a higher concentration of salts in its body, and thus hyperosmotic to the environment.
• The salts from the freshwater fish will eventually be lost to the ambient environment via the gills, and at the same time there will be a large influx of water.
• FW fish take in a lot of water, which has to be lost.
• note: water move from a region of higher water potential to a region of low water potential.
• Movement across compartments is a necessary mechanism to resupply cells or tissues with needed raw materials, to void waster and to maintain a proper composition of body fluids.
• Goldfish in water take up to 30g/day, which is essentially 1/3rd of it body weight. A large influx like this can bloat the fish and dilute its blood.
• The fish must continuously spend energy and expel the water, but at the same time this process causes the goldfish to lose a lot of salts to the external environment.
• The fish must spend a lot of energy to take up those lost salts. This is done via active transport. The transporters that are in place take up Na+ and Cl- and loose bicarbonate and H+ (electroneutral), with the help of ATP.

Question 7

aquaporins - freshwater fish

Answer 7

• Porins are similar to ion channels but permit the passage of large molecules.
• Aquaporins are water channels in the plasma membrane, each aquaporin molecule transport 3 billion water molecules/second.
• If a plasma membrane lacks aquaporins, water crosses the membrane 5-50 times slower.
• They serve significant physiological roles such as urine formation, production of aqueous humor of the eye, secretion of tears and sweat.
• They can be transcellular (through the cell membrane) or paracellular (across different compartments).

Question 8

cell volume regulation - fresh water fish

Answer 8

• Cells control their volume by transporting solutes across the plasma membrane, causing changes in osmotic pressure that induce movement of water.
• Water flows into a region where there is a higher solute concentration.
• If there is an imbalance in water content and the cell swells, the transport mechanisms will come into place to rectify this.

Question 9

preformed water

Answer 9

• Whenever you ingest any food, you are also ingesting water.
• The epithelium of a hummingbird consists of a single layer of cells bearing microvilli on the apical membrane.
• Dissolved sugar molecules such as glucose and fructose must cross the epithelium from the intestinal lumen to the blood.
• This is ingested preformed water, different from metabolic water.

Question 10

metabolic water

Answer 10

• Metabolic water is formed when organic food molecules are aerobically catabolized as shown by the above reaction (glucose oxidation).
• The significance les in the amount of water lost during this reaction.
• However, there is definitely a net water gain.
• Water is not only gained during drinking, but your cells are producing water.

Question 11

Water Loss: Respiratory, Urinary and Faecal route

Answer 11

• When you breathe in you gain water and when you breathe out you lose water.
• Air entering the nose is warmed and humidified by heat. The nasal passages are cooled by evaporative water loss, leading to a flow of cool air.
• During expiration, the air is cooled and leads to a loss of water, wetting the nasal passage.
• Kidneys are regulatory rather than excretory organs. However, it is clear that the excretory function of the kidney is central to their role to regulate the composition and volume of body fluids.
• Water loss also takes place through the faecal route, food is ingested that contains preformed water, and is excreted through this route.

Question 12

example: desert kangaroo rats

Answer 12

• Desert kangaroo rats have been shown to conserve water better than lab rats.
• An experiment was conducted where these rats were given 0 preformed water and they were given barley grain.
• They made metabolic water to survive.
• Interestingly these rats had a net gain of metabolic water compared to the lab rats.
• More concentrated urea, less water loss and drier faeces.

Question 13

regulation: blood plasma- forms of regulation

Answer 13

3 forms of regulation:
1. Osmotic- is the regulation of osmotic pressure of an organism’s body fluids, detected by specialized receptors to maintain homeostasis of the organism’s water content.
2. Ionic- Maintenance of the concentration of various ions in the bod fluids relative to one another. The urinary system plays a key role in this process.
3. Volume- Cell volume regulation is an important homeostatic function, defining not only cell shape but balance between the ICF and ECF.

Question 14

environmental challenges

Answer 14

• Animals face sporadic challenges introduced by the environment to their regulatory system.
• Exchanges of ions and water between an animal and its environment can be obligatory and regulated.
• Obligatory exchanges cover responses of an animal to factors that are beyond their physiological control (physical factors)
• Regulated exchanges are physiologically controlled and required for maintaining the internal homeostasis.

Question 15

osmoregulation- types of osmoregulation

Answer 15

two types, an osmotic regulator and an osmotic conformer
* A perfect regulatory won’t follow the trend of the isosmotic line. However, the osmotic conformer will follow the trend of the isosmotic line.
* Conformers tend to have the same osmotic pressure as the externa environment whereas regulators keep osmolarity constant regardless of changes in the external environment.
* A disadvantage for the conformers is that the cells may not have the ideal solute concentration for metabolism.
* The disadvantage for regulators is that they utilize too much to keep the internal solute concentration constant.

Question 16

realistic positions of animals in osmoregulation patterns

Answer 16

• Animals display gradations or mixtures of osmotic regulation and conformity. Among animals that are Osmoregulators the, the regulation is limited to ranges of external osmotic pressure.
• From the chart above regardless of the ambient osmotic pressure the shrimp is an almost perfect Osmoregulator.
• The mussel is an osmotic conformer.
• Crab is a perfect example of how regulators can conform. This is usually when FW animals face brackish waters, they regulate in FW but conform in SW.
• Remember the way an animal regulates depends in the environment in lives. Furthermore, if they do ever change their regulation, it is because they are migrating into a different environment. All these are forms of adaptation.

Question 17

ionic regulation

Answer 17

• As stated previously solutes contribute largely to the osmolarity and determine the osmotic gradient across membranes, and hence the direction of water movement. .
• The image above highlights the solute composition of selected animals to illustrate the importance of various solutes.
• Extracellular space of most animals is dominated by Na+ and Cl-. SW is mainly Na+ and Cl-, followed by reduced levels of K+, Mg2+ and Ca2+.
• Ionconformers have high levels of Na+ and Cl- close to that of SW. whereas Ion regulators they have low levels of Na+ and Cl-.
• Osmoconformers have the same osmolarity as SW but maintain a different solute profile, much like that of an Osmoregulator.

Question 18

volume regulation

Answer 18

• Hemolymph is the circulating fluid of an open circulatory system. In an open circulatory system, hemolymph flows through blood vessels.
• Hemolymph of FW crabs is hyperosmotic to the surrounding water. Osmosis allows water to move in which is eventually lost as urine.
• Key point- Even though Volume, Osmotic and ionic regulation are distinct processes, they are all integrated in one organism.

Question 19

SW V FW

Answer 19

• Some aquatic animals live in environment that are uniform and stable in their water-salt composition, and these are animals that live in the open ocean.
• The salinity of water is calculated as the number of grams of dissolved inorganic matter in a Kg of water.
• SW is 34g/Kg, and its osmotic pressure is nearly 1000 mOsm.
• FW is defined as water having salinity less than 0.5g/Kg, and has an osmotic pressure of 0.5-15 mOsm.
• This is all a contribution of the concentration of solute in the water.
• Slide 10 highlights the osmotic pressures of FW animals in comparison to the ambient osmotic pressure (river water). This indicates that all these animals are hyperosmotic to river water, as their internal solute concentration is higher than that of river water.

Question 20

FW challenges

Answer 20

• Volume regulation- A constant influx of water into the organism due to an osmotic gradient.
• Osmotic regulation- The water that enters, dilutes the blood and reduced osmotic pressure within the ECF.
• Ion Regulation- Due to excretion of excess water ions are constantly lost to the external environment.

Question 21

FW animals - ion regulation

Answer 21

• For a FW fish, the uptake of ions from a highly concentrated solution requires the use of energy, and therefore FW fish reabsorb ions from their kidneys.
• Freshwater animals are hyperosmotic to the ambient environment, which means gain of water and loss of ions.
• The more rapidly water is taken up, the faster it is lost by diffusion, and the more energy it spends to carry counteract these processes.

Question 22

FW animals - rate of exchange determinants

Answer 22

• Three factors determine the rate of exchange: permeability, surface area to volume ratio and magnitude of gradient.
• FW animals have a great influx of water and they void this by excreting copious amounts of urine.
• Because urine production balances osmotic water gain, rate of urinary water excretion resembles the rate of water influx

Question 23

FW animals - U:P ratio

Answer 23

• The urine of FW animal is hypoosmotic (low concentrations of Na+ and Cl-) to their blood. This is defined as the U:P ratio (Urine to plasma).
• If U:P is less than 1, this signifies that the blood osmotic pressure is high due to lots of urine production.
• When U:P is less than 1 for an ion, it signifies that the large amounts of urine production tend to raise plasma concentrations for that particular ion.
• Therefore, the kidneys not only solve the problem of volume regulation by excreting urine but aid in ionic and osmotic regulation by maintaining high osmotic pressures and an increased ion concentration in the blood.

Question 24

FW animals - role of gills

Answer 24

• Gills are a suitable organ for osmoregulation. Gills of various species are active not only in gas exchange but also in ion transport, excretion of nitrogenous wastes, and maintenance of acid-base balance.
• Gills play a central role in regulating osmotic stress. From the image above, the epithelium separates the blood from external water, which consists of several cell types: mucous cells, pavement cells and chloride cells.
• The epithelium of the lamella consists of pavement cells and mitochondria, best suited for respiratory exchange.
• The epithelium covering the gills have chloride cells, mitochondria and enzymes that assist with salt transport.

Question 25

FW fish - active ion transport of Na+ and Cl-

Answer 25

• A significant way by which FW animals replenish lost Na+ and Cl- is active transport.
• Active ion transport takes place in the gills. This requires ATP, which means this places demands on an animal’s energy resources.
• The mechanisms that pump Na+ and Cl- from the external environment into the blood are typically different from each other. The Cl- pump typically exchanges HCO3 for Cl- (electroneutral).
• The Na+ pump exchanges H+ (protons) for Na+ or Nh4 (ammonium ions).
• The HCO3 and H+ are produced by anaerobic catabolism, being formed by metabolically formed CO2 and H2O.
• In mammals, internal Na+ homeostasis is maintained through Na+ reabsorption via a variety of Na+ transport proteins with mutually compensating functions, which are expressed in the nephrons.
• Na+ homeostasis is achieved through the skin gill ionocytes, namely Na+/H+ exchangers. Expressing H+ ATPase rich cells and Na+ and Cl- Co-transporters.

Question 26

mairne animals-

Answer 26

• Let’s assume the ICF has an osmotic pressure of 300 mOsm. Kindly remember that we previously stated that SW has an osmotic pressure of 1000 mOsm.
• Osmotic pressure is governed by the solute concentration.
• The osmotic gradient between the marine animal and the SW is 700 mOsm.
• Let’s assume, FW animals has an ICF osmotic pressure of 300 mOsm. The osmotic pressure of the ambient environment in FW is very low. Hence a smaller osmotic gradient in comparison to marine animals.
• Therefore, since the osmotic gradient is higher in marine animals, the challenges are equally that high.
• In this case water moves in the opposite direction.
• SW has a very high salt concentration. Let’s assume a marine animal is drinking water, with this there is going to be a great influx of salt. The only choice the animal has is to expel all that excess salt and, in the bargain, it loses water as well.
• Marine animals try to reabsorb as much water as possible and lose as much of salt as possible to maintain their ICF osmotic pressure.
• Not all animals have gills. With respect to gills, substances can move in and out as they are permeable to water.
• Organisms that live on land have adapted and are able to secrete excess salt through glands. Some marine animals have glands on their body, seabirds have them in nasal passages.
• Here is a great example of the dogfish, which are hyperosmotic in SW (one of a kind!). Ideally marine fish have a lower salt concentration in their blood in comparison to SW, dogfish are the exact opposite.
• Even though the dogfish has a lower concertation of inorganic ions like Na+ and Cl- the Urea and TMAO are contributing to its increased osmotic pressure and hence the influx of water from the external environment.
• Kindly observe the rectal gland secretion. Since the hyperosmotic dogfish is gaining an influx of water Na+ and Cl- from the SW will follow. The dogfish loses this excess salt through its salt glands.
• Integuments are poorly penetrable to water. Marine fish drink water with a price of gaining excess salt. Because they are conserving water they will from small amounts of isosmotic urine. Principal role of the kidneys in marine fish is to get rid of excess salt.
• Animals are classified on their ability to tolerate changes in external osmolarity.
• Stenohaline animals can tolerate a narrow range of salt concentration.
• Euryhaline animals can tolerate wide variation in osmolarities.

Question 27

function of the urinary system

Answer 27

Removal of waste products
Produced by cellular metabolism

Regulation of volume and solute concentration of blood plasma
Blood volume;pH
Elimination of waste products into the environment

Question 28

components of urinary system

Answer 28

Kidneys: Urine production
- Excretion and regulation

Urethra: urine transfer to the bladder

Bladder: urine storage
-elimination
Urethra: urine release
-elimination

Question 29

kidneys-location, adrenal(suprarenal) glands

Answer 29

location:
Either side of the midline
On the posterior abdominal wall
T12-L3(left slightly higher than right)
in terms of peritoneum: Lie behind the parietal peritoneum = retroperitoneal organs
Surrounded by fat (adipose tissue)

adrenal glands:
Not apart of the urinary system
Location: on superior pole of kidneys
Cortex
- corticosteroids; aldosterone
- sodium and water retention, BP inc. and volume
Medulla
- adrenaline and noradrenaline
‘fight or flight’ response

Question 30

protection of kidney

Answer 30

Fat, fascia, rib 11 and 12
Renal capsule(collagen rich)
Perinephric fat(adipose tissue)
Renal fascia(collagenous membrane)
(^provide: stability, support and cushioning)
Suspensory fibres(stability)

Question 31

structure of kidney

Answer 31

150g - 10cm long, 5cm wide, 2.5cm thick
Indented ovoid - bean shape
Hilum: structures enter and leave the kidney
Large blood supply-20-25% of cardiac output

Congenital anomalies of kidneys:
Horseshoe kidney(not always bad)

Renal lobe contains the functional unit(nephron)

Question 32

renal lobe

Answer 32

Renal cortex
- Proximal and distal parts of the nephron and collecting ducts (medullary rays)

Renal medulla (pyramids)
- Loop of Henle and collecting ducts (collection and delivery of urine)

Question 33

ureter

Answer 33

Hollow muscular tubes,propel urine from the kidneys to the bladder
25-30cm in length
Located in both the abdomen and pelvis
Retroperitoneal
Congenital anomalies:
duplex ureter (bifid ureter) - 1 in 125 people

Enter the bladder obliquely
When the bladder is full
- prevent backflow of urine into the ureters
- compression closes off the ureters acting as valves

Question 34

urinary bladder

Answer 34

• Apex - points toward the pubic symphysis
• Fundus - is opposite the apex and formed by the posterior wall

Usually a capacity of about 0.75 litres
In pathology, may be very different:
Obstruction to outflow (e.g. prostate) leads to enlargement – volumes to 6-7 litres
World record (neurogenic bladder) – 21 litres - palpable at costal margins!

Question 35

female + male urethra

Answer 35

female:
4 - 5cms long – urinary tract infection much more common
Passes through the pelvic floor and opens anterior to the vagina

male:
20cm in length
S-shaped
4 parts:
Pre-prostatic
Prostatic
Membranous
Penile

Question 36

female and male urethra sphincters

Answer 36

female:
Internal urethral sphincter: Junction between the bladder and urethra
External urethral sphincter: Immediately inferior to the internal urethral sphincter

male:
Internal urethral sphincter: Junction between the bladder and urethra
- Prevents reflux of semen into the bladder
External urethral sphincter: Inferior to the prostate

Question 37

mammalian kidney

Answer 37

• Principal function of the kidney is to regulate the composition of body fluids (osmotic balance), excretion of waste in the form of urine, pH balance, hormone production (kidneys produce a hormone that stimulate RBC production) and regulate blood pressure.
• The main processes that contribute to the formation of urine are filtration, reabsorption, secretion and excretion.

Question 38

structure and function of kidneys

Answer 38

• Water and solutes leave the arterioles and enter the lumen of the Bowmans capsule forming an ultra-filtrate.
• The renal tubules are line with cells so that ion and water can move freely, passing through this cell layer (Reabsorbed).
• Certain type of molecules are secreted from the IF into in the proximal tubule (PT) to be excreted as waste.
• The glomerular filtrate contains all the constituents of the blood except blood cells and proteins.
• Filtration in the glomerulus is soo extensive that 15-25% of water and solutes are removed from the plasma.
• The filtrate that is produced, is at a rate of 180L/day. However, we don’t drink that much water to compensate for the loss, this means that almost all the ions and water is reabsorbed into the blood stream.
• Shown from the image above, there are several biological structures that are involved in filtration.
• There are two arterioles efferent and afferent. Afferent transports blood towards the glomerulus and efferent transports the blood away from the glomerulus.
• Inside the glomerulus you have a network of capillaries with thin walls allowing easy movement of substances. These loops are also referred to as glomerular tufts. The key here is the constant blood supply.

Question 39

the glomerulus - filtration

Answer 39

• From the image above the glomerulus is acting like a filtration barrier, which consists of three significant layers. Endothelial cells basement membrane and the podocytes. This is the filtration barrier.
• The function is to keep blood and protein into the body and allow the passage of small molecules into the urine. The podocytes are modified epithelial cells that provide structural support.
• The glomerulus has a size selectivity, which means that molecules less than 1.8 nm can be easily filtered (water, sodium, insulin and glucose). Molecules more that 3.6 nm are not filtered (haemoglobin).
• Furthemore the glomerulus has a charge selectivity, where negatively charged molecules cannot pass that easily, as all the three layers shown above contain negatively charged glycoproteins.

Question 40

hydrostatic and oncotic pressure

Answer 40

• Filtration of blood to form the filtrate in the lumen depend on certain forces. Hydrostatic pressure (Hp) and oncotic pressure (Op).
• Hydrostatic pressure- is the pressure that the fluid exerts on the walls of the compartment, either the walls of the capillary or the Bowmans capsule (pushing force).
• Oncotic pressure- is the pressure exerted by the plasma protein on the walls of the compartment in which they are contained (pulling force).
• Glomerular filtration rate (GFR)- the rate at which the filtrate is generated. 120 ml/min and a 180 l/day.
• Clearance- The amount of fluid cleared completely of a certain substance.

Question 41

fluid movement and starling forces

Answer 41

• To understand fluid movement and direction of flow you have to understand starling forces.
• The hydrostatic pressure in a capillary is the major driving force pushing fluids from the blood into the lumen of the Bowmans capsule.
• If the Hp is greater in the capillary than the Bowmans capsule, fluid will be pushed out.
• I did mention that during filtration plasma proteins were left behind due to a size selectivity across the glomerulus. Due to the presence of proteins, the blood has a greater osmotic pressure than the lumen of the Bowmans capsule. This pressure is termed as the oncotic pressure.
• The oncotic pressure present in the capillaries tend to pull fluids back in. The balance between the forces influences rate and direction of fluid movement.

Question 42

net filtration pressure

Answer 42

• Pressure favouring filtration minus the pressures opposing filtration, is your net filtration pressure.
• To make this simpler it is the glomerular capillary pressure (hydrostatic pressure within the capillaries) minus the Intracapsular pressure (hydrostatic pressure within the lumen of the Bowmans capsule). This will give you the net hydrostatic pressure. The net hydrostatic pressure minus the colloid osmotic pressure (oncotic pressure). This will give you the net filtration pressure.
• The reason we calculate the GFR is because, It can be used to identify if an individual is suffering from kidney problems.

Question 43

juxtaglomerular apparatus

Answer 43

• The JGA is a specialized region which is significant for sensing the blood pressure/flow into the kidney and producing hormones such as renin.
• Renin is a hormone, significant in blood pressure regulation and fluid balance.
• The JGA is a region where the afferent arterioles come into contact with the distal tubule.
• On the outside of the afferent arterioles you have presence of JG cells that can sense blood pressure.
• At the point of contact with the distal tubule, modified cells in the distal tubule, Macula densa cells sense changes in flow and an Na+ concentration of the intertubular fluid.
• function of these cells(when systemic blood pressure decreases) there is a decreased stretch of the JG cells which release renin, renin will increase blood pressure back to normal.
• function of these cells (when the filtrate has a decreased flow rate) the macula densa cells sense this, leading to vasodilation of the afferent arteriole and renin secretion by the JG cells. Renin will return flow rate back to normal
• There is connection between, blood pressure, osmolarity, blood flow and Na+ concentration. This is the purpose of the JGA.

Question 44

tubule epithelial cells

Answer 44

• After the filtrate passes the glomerulus and enters the lumen of the Bowmans capsule, it passes the PT. This section of the nephron is specialized for transport and is the area where most reabsorption occurs.
• One of the important things to note is that the epithelial cells of the tubule are unique, they are not very permeable to lots of substances -> they have lots of transport mechanisms on either side of the cell to help regulate the movement of ions.
• The tubule epithelial cells have tight junctions which prevent paracellular transport, and contain a certain polarity allowing certain substances to move across, such as proteins and pharmaceutical agents.
• Kindly have a look at the image on slide 13, highlights the differences in the structure of the epithelial cells, with respect to its function.
• PT reclaims 80% of the filtered fluid and thus the epithelia have microvilli, lots of mitochondria and a large surface area.
• Loop of Henle reclaims 5-10% of the filtrate and plays a significant role in water conservation. One can observe the difference in epithelial cell structure between the descending and ascending limb, as they each play different roles in water conservation.

Question 45

ion reabsorption

Answer 45

• Reabsorption is critical in maintaining fluid and electrolyte balance in the system.
• Now we know that PT is the area where maximum reabsorption takes place.
• The other way primary urine is modified is through secretion.
• Secretion uses transporters found in the epithelial cells that line the lumen of the PT. H+, K+, toxins and pharmaceutical drugs move from the blood into the lumen of the PT. This process requires energy.
• Important thing to note is by the time the fluid reaches the end of the PT the osmolarity is 300 mOsm, isosmotic to the IF and plasma (no movement).

Question 46

cellular mechanisms in PT(proximal tubule)

Answer 46

• The wall of the tubule is once cell layer thick; this epithelium separates the lumen from the IF. These epithelial cells are specialised for transport, bearing a dense pile of microvilli on their luminal (apical) surfaces.
• They are tied together by leaky tight junctions.
• Figure 2 highlights the transport mechanisms and co-transporters involved in the epithelial membrane.
• In all sections of the kidney tubule, Na+ diffuses into the epithelial cells from the tubular fluid because there is an electrochemical gradient favouring this movement.
• In the early PT, the fluid is rich in glucose and amino acids, and much of Na+ entry into the cell occurs by means of co-transporters that bring about secondary active transport of glucose and amino acids.

Question 47

loop of henle

Answer 47

the ascending and descending limb, make up the Loop of Henle.
* The Loop of Henle is significant for water reabsorption. The descending limb (DL) is very permeable to water because it doesn’t have tight junctions.
* Epithelium cells of the DL have no active transport of solutes, highly permeable to water and impermeable to ion and urea.
* The ascending limb (AL) is the exact opposite, impermeable to water, permeable to ions and impermeable to urea. The thick segment of the AL has active transport of ions.

Question 48

control of acid:base balance

Answer 48

• The collecting duct is the final phase and you’ve got soo many ions moving in different directions. One of these ions are H+.
• The movement of H+ in either direction has a direct effect on pH, which either signifies an acidic or basic environment.
• In mammalian blood plasma and in the primary urine concentration of bicarbonate is high but concentration of protons is low
• Reabsorption of around 80% of bicarbonate takes place in the proximal convoluting tubule and continues in downstream sections of the nephron
• Protons are moved in the opposite direction, causing acidification of the intertubular fluid
• The final fine tuning of urine acidification and bicarbonate reabsorption takes place in the distal tubule and the collecting duct
• Alpha intercalated cells (acid-secreting) and beta intercalated cells (base-secreting) possess various sensors for bicarbonate, CO2 or proton concentration
• Signals from sensors modulate expression, abundance in the plasma membrane or activity of transporters, pumps and channels in these cells
• These processes are also under hormonal control (aldosterone, angiotensin II, etc.)

Question 49

countercurrent multiplication basic mechanism

Answer 49

• The main purpose of counter current multiplication is to create concentrated urine with the loop of Henle (LOH).
• the role of the DL is water reabsorption (water moves from the DL tubule out into the IF), and the role of the AL is to pump ions out into the IF via active transport.
• at the end of the PT the osmolarity is isosmotic (300 mOsm, as the PT is responsible of maximum reabsorption).
• the isosmotic fluid from the end of the PT enters the DL. The main function of the DL is water reabsorption. Therefore, because the osmolarity (concentration of Na+) in the IF is high, water will move out from the DL into the IF, equilibrating the osmotic pressure.
• Because water moves out of the DL, the Na+ concentration within the DL will increase.
• High concentration Na+ from the DL will move into the AL where the concertation of Na+ was initially low, due Na+ being transported into the IF via active transport.
• High concentration of Na+ in the AL will pump out Na+ into the IF.
• This is a constant cycle known as counter current multiplication, which creates a concentration gradient across the LOH. There is always a difference of 200 mOsm between the IF and tubular fluid.

Question 50

counter current multiplication with respect to osmolarity

Answer 50

maximum reabsorption takes place in the PT. This is why at the end of the PT, the osmolarity is isosmotic with the IF (300 mOsm, no movement).
* Now let’s say the filtrate is entering the DL and AL for the very first time.

• Naturally, if it is entering for the first time from the PT, it will have the same osmolarity as the IF (isosmotic). However, this is not an ideal scenario for creating concentrated urine!
• The aim is to create that 200 mOsm difference, and the way to accomplish that is by transporting Na+ into the IF, from the AL.
• Therefore Na+ is transported out from the AL into the IF creating a 200 mOsm difference. Due to the transport of Na+, the IF has an osmolarity of 400 mOsm and the filtrate in the AL has an osmolarity of 200 mOsm (200 mOsm difference).
  CONTINUESSSS …… LOOK AT LECTURE TO UNDERSTAND

Question 51

Vasa recta - NaCl

Answer 51

whatever moves from the LOH into the IF, moves into the blood.
* Vasa recta are specialized capillaries found around the JMN.
* These capillaries specialize in creating this hyperosmotic environment to draw water out.
* Descending vasa recta- Blood coming down from the cortex into the medulla is in contact with increased osmolarity IF. The Na+ will diffuse from the surrounding IF into the vasa recta.
* Ascending vasa recta- concentrated blood (increased osmolarity) will move towards the AL and will lose that concentrated Na+ to the diluted IF (Refer to counter current multiplication to understand this mechanism).

Question 52

Vasa Recta -H2O

Answer 52

• Descending vasa recta- Dilute blood flowing from the cortex into the medulla will lose water to the concentrated IF (increased osmolarity).
• Ascending vasa recta- Concentrated blood flowing towards the AL will gain water from the IF due to its increased osmolarity in the capillary.

Question 53

inner medulla and urea transport proteins

Answer 53

• Due to that increased concentration gradient across the nephron, there is constant water reabsorption, which creates concentrated urine.
• The steep concentration gradient also benefits the CD, which promotes water reabsorption from the CD.
• Furthermore, Urea also has the ability to move freely across the membranes. Urea can move across the AL, CD and the IF. Due to this movement and the constant influx of newly filtered urea, creates a concentration gradient, promoting water reabsorption from the DL.
• CD has special transport proteins. UT-A1 and UT-A3, expressed by epithelial cells of the CD, aid in transport of urea from the CD to the IF.
• Recent research has shown that vasopressin (antidiuretic hormone) upregulates expression of UT-A1 and UT-A3, increasing the rate of urea transport.

Question 54

hormone control and aquaporins

Answer 54

• A hormone known as vasopressin (antidiuretic hormone) is synthesized in the hypothalamus. This takes place when ADH is released by a neuronal cell body which lies in the posterior pituitary gland.
• In your hypothalamus you have the presence of osmoreceptors that are sensitive to osmolarity. An increased osmolality causes release of vasopressin.
• Angiotensin II also acts on receptors in the hypothalamus, causing vasopressin release. This is part of the renin-angiotensin system.
• Another significant stimulus for vasopressin release is from the heart. They have receptors known as baroreceptors, that sense change in blood pressure.
• When there is a decrease in water content in the blood, there is a reduced venus return. This is sensed by the receptors, leading to vasopressin release.
• Mechanism of vasopressin- When osmolarity in the IF increases (decrease in water content), Vasopressin is released.
• Vasopressin upregulates translocation of aquaporin (AQP-2) receptors on the apical side of the plasma membrane in the distal convoluted tubule and CD.
• On the basolateral side of the membrane, you always have presence of aquaporin-3 and 4.
• When osmolarity in the IF increases (decrease in water content), Vasopressin is released and upregulates translocation of AQP-2, leading to increased water reabsorption in the IF.