The Genitourinary System

Question 1

What is the kidney?

On an average day, how much water and salt do we consume?

Answer 1

The central regulator of homeostasis

Take in more water and salt than we need as we do not know how much we will need in a given day (e.g. hot or cold day, amount of exercise, etc.). This means the excess must be excreted (e.g. as urea)

Question 2

Out of Na⁺, K⁺, H₂O and urea - which of these do not have a pump?

Answer 2

H₂O and urea

Question 3

What are the 5 main functions of the kidney?

Answer 3

Excretion of metabolic products - urea, uric acid, creatinine etc.

Excretion of foreign substances - drugs, etc.

Regulation of body fluids, electrolytes and acid-base balance

Control BP

Secrete hormones - erthropoietin, renin, etc.

Question 4

What is the anatomical structure of the kidneys? Fill in the missing labels below:

Answer 4

Cortex = outer layer

Medullary region = inside the cortex

Renal artery = blood in

Renal vein = blood out

Ureter = urine out

Question 5

What are the different blood vessels in the renal blood supply, starting from the renal artery, through the glomerular capillaries to the renal vein? Fill in the flow diagram below:

Answer 5

Question 6

What is the function of the bladder and urethra? How do they work together and urine fills the bladder?

What are the functions of the: detrusor muscle, trigone, internal sphincter, external sphincter, bulbourethral gland and label them on the diagram below:

Answer 6

The kidney eventually produces urine, which travels down the ureter to the bladder

The bladder fills to a particular volume (around 200ml) - and then is detected by the detrusor muscle and trigonal region

Detrusor muscle = contracts to build pressure in the bladder to support urination

Trigone = stretching in this region signals to the brain for the need of urination

Internal sphincter = involuntary control to prevent urination

External sphincter = voluntary control to prevent urination

Bulbourethral gland = produces thick lubricant that is added to watery semen to promote sperm survival

Question 7

What is the structure of a functional unit (nephron) in the kidney? Fill in the missing labels on the diagram below:

Answer 7

Question 8

What is the function of the nephron at the start, during the PCT?

What are some key properties of the epithelial cells to support this function?

Answer 8

A level notes - look at picture

As solutes are pumped out of the filtrate / reabsorbed, the glomerular filtrate has a lower osmolarity, so water follows and moves out as well

Around the same amount of solute and water are reabsorbed

Many mitochondria - provides ATP for all the pumps (to pump out the solutes)

Question 9

What is the function of the nephron for the Loop of Henle?

How are the epithelial cells of the loop of henle adapted to its functions?

Answer 9

Counter current system - a level notes

Again, epithelial cells contain many mitochondria in the thick ascending limb of the loop of henle to provide ATP for the pumping activity

Epithelial cells of the thin decending loop of henle = passive movement of water = low mitochondria density as less ATP is needed

Question 10

What is the function of the nephron in the distal convoluted tubule (DCT) and collecting duct (CD)?

How can the amount of water reabsorption be varied in the DCT / CD?

Answer 10

Put in A level notes

Vary water reabsorption in the CD via vasopressin and aquaporins - it also changes the osmolarity surrounding the loop of Henle affecting the concentration gradient for reabsorption of water

Question 11

What are the properties of the cells found in the DCT and CD?

What are the 2 cell types in the CD and their properties?

Answer 11

Epithelial cells of the DCT = rich in mitochondria

Principal cells = low mitohondrial density; Intercalated cells = rich in mitochondria

Question 12

What are the 2 types of nephron?

What are the 3 depths of the nephron?

Which is more abundant that the other?

Answer 12

1. Superfiial nephron; 2. Juxtamedullary nephron
2. Cortex; 2. Outer medulla; 3. Inner medulla
   10: 1 ratio for superficial to juxtamedullary

Question 13

Why is the cortex of the kidney granular looking, whereas the medulla of the kidney is striated looking?

Where is the glomerulus found in comparison to the rest of the nephron?

Answer 13

All the glomerulae are found in the cortex (not organised into a pattern); all the Loops of Henle are found in the medulla

Almost always found near the early DCT

Question 14

What does GFR stand for? How is GFR regulated?

What is the juxtaglomerular apparatus? What is its function?

What are the 3 types of cells found in the juxtaglomerular apparatus?

Answer 14

Glomerular filtration rate - regulated by controlling how much liquid is coming into the glomerulus and renin production

It is located next to the glomerulus, between the afferent arteriole and the distal convoluted tubule of the same nephron. It is responsible for regulating GFR

1. Macula densa (found in the DCT) = regulates GFR through the tubulo-glomerular feedback mechanism; 2. extraglomerular mesangial cells; 3. juxtaglomerular cells = produce renin to regulate BP

Question 15

How does glomerular filtration work?

What are the different processes that can occur within the nephron?

Answer 15

Put in A level notes (basement membrane, podocytes etc.) - A passive process where fluid is driven through the glomerular capillaries into the Bowman’s capsule due to the hydrostatic pressure of the heart, which pushes small enough molecules through the semipermable membrane of the glomerular capillaries. It is impermeable to cells and proteins

Glomerular filtration, reabsorption, secretion, excretion - different substances undergo these processes in different amounts

Question 16

What is meant by the terms hydrostatic and oncotic pressures?

How do these 2 pressures interact in the glomerulus?

Answer 16

In the glomerulus:

Hydrostatic pressure = fluid exerts this pressure (pressure comes from BP / heart contracting) so solutes and fluid are pushed out into the glomerular filtrate

Oncotic pressure = cells and protein are left behind in the glomerulus, creating an osmotic gradient, drawing the fluid back across the semi-permeable membrane

Question 17

What is HP_gc?

What is HP_bw?

What is π_gc?

So what is the equation to calculate P_uf (filtration pressure)?

Answer 17

HP_gc = hydrostatic pressure in the glomerular capillaries - drives fluid into the glomerular filtrate

HP_bw = hydrostatic pressure in the bowman’s capsule - pulls fluid back into the glomerulus (HP_gc works against this)

π_gc = oncotic pressure of plasma proteins in the glomerular capillaries - pulls fluid back into the glomerulus

P_uf = HP_gc - HP_bw - π_gc

Question 18

What is GFR?

How is GFR calculated?

Answer 18

Amount of fluid filtered from all the glomeruli into the bowman’s capsule in the kidneys per unit time (mL/min) - sum of filtration rate of all functioning nephrons

GFR = P_uf x K_f

Where P_uf = filtration pressure, and K_f = ultrafiltration co-efficient taking into account membrane permeability and surface area available for filtration

Question 19

What are the 3 main factors that can be altered to affect / change GFR?

What are healthy GFRs in males and females respectively?

What occurs with GFR in renal disease?

Answer 19

Change in surface area, permeability or pressure

Males = 90-140mL/min; Females = 80-125mL/min

GFR falls - things that should be excreted via the kidneys remain in the plasma and build up over time

Question 20

What are the names of the 2 mechanisms to regulate GFR?

Answer 20

1. Myogenic mechanism
2. Tubulo-glomerular mechanism

Question 21

What is the myogenic mechanism to regulate GFR?

Answer 21

1. Arterial pressure increases - this may be due to exercise, an adrenaline rush, etc. This in turn will increase the hydrostatic pressure of glomerulus (HP_gc), increasing GFR
2. Afferent arteriole stretches
3. Afferent arteriole contracts - this is to resist the force of stretching due to the increased BP
4. Vessel resistance increases - as the diameter of the arteriole decreases
5. Blood flow reduces
6. GFR returns to normal

Question 22

What is the tubulo-glomerular feedback mechanism to regulate GFR?

Answer 22

1. Increase / decrease in GFR
2. Increase / decrease in salt absorption in the Loop of Henle - due to increase / decrease in GFR respectively
3. Change in salt absorption is detected by the macula densa cells - due to change in their intracellular osmolarity, which changes their size
4. increase / decrease in ATP and adenosine discharge
5. The discharge of those molecules causes the afferent arteriole to constrict / dilate respectively
6. GFR stabilises

After a certain point - results in renin prduction and secretion, leading to angiotension II production (for further arteriole constriction) - to try and retain water

Question 23

What is meant by the term ‘renal clearance’?

Why is this important?

What is the equation to determine clearance rate?

Answer 23

The volume of plasma that is completely cleared of of the substance per unit of time e.g. 0.05L of plasma is completely cleared of urea per minute

This concept can be used to: 1. calculate GFR, 2. calculate renal plasma flow (RPF), 3. understand the excretory route of a substance (filtration, secretion, reabsorption, etc.)

Question 24

Label on the diagram below all the different rates:

Rate in = ?

Answer 24

OE = out for excretion

OR = out for recirculation

Rate in = rate out

Rate in = OE + OR

Rate in = RPF (renal plasma flow rate) = OE + OR = rate out

GFR = OE (if the substance is freely filtered, and there is no secretion or reabsorption)

RPF = OE (if this substance is completely secreted, and not reabsorbed)

Question 25

What are theideal properties of a molecule to practically measure GFR?

Which is the ideal molecule that could be used to practically measure GFR?

What is the common molecule used practically to measure GFR?

Answer 25

Ideal properties = measure a freely filtered molecule in the urine that is neither secreted nor reabsorbed in the kidneys (as GFR = OE). So GFR can be measured by measuring renal clearance of this molecule

Ideal = inulin

Common = creatinine

Question 26

What is good about using creatinine clinically? What are some issues with creatinine?

Why is creatinine concentration unchanged between the afferent and efferent arterioles?

Answer 26

Creatinine does not need to be infused, muscles are continuously producing it - large volumes of urine can be collected over long periods of time. However, muscle wasteage raises creatinine levels and a small amount of creatinine is secreted in the nephron

Because it it freely filtered

Question 27

What is RPF?

What is PAH? How does PAH pass through the kidneys?

How and why can RPF be calculated using RAF?

Answer 27

Renal plasma flow (RPF) rate - volume of plasma delivered to the kidneys per unit time

Para aminohippurate (PAH) - all the PAH is removed from the plasma passing through the kidneys through filtration and secretion

Because all of the PAH that enters the kidney is excreted, so the renal clearance of PAH must equal RPF

Question 28

What is FF?

What is the equation to calculate FF?

What is considered a normal FF?

Answer 28

Filtration fraction (FF) - usually calculated as a percentage and represents the proportion of the fluid reaching the kidneys that passes into the renal tubules

FF = [GFR] / [RPF] x 100

Between 15 - 20%

Question 29

What are the 2 types of transport mechanisms?

What are the 3 different types of passive transport?

What are the 2 different types of active transport and their subtypes?

Why might some people argue diffusion down the electrochemical gradient is secondary active rather than passive?

Answer 29

1. Passive transport, 2. Active transport

Passive = diffusion, osmosis, electrochemical gradient

Active: primary = pumps, endocytosis; secondary = movement of one solute along its electrochemical gradient provides energy for the movement of another solute against the oncentration gradient - symport (bring something else with it), antiport (pump something out in the opposite direction)

Electrochemical gradient only exists due to pumps creating that different in electrical potential

Question 30

What are the passive transport mechanisms in the renal tubules?

What are the active transport mechanisms in the renal tubules?

Answer 30

All 3 passive mechanisms take place in the nephrons

Both, primary and secondary active mechanisms are in the nephrons. Na⁺ /K⁺ ATPase pump = lowers Na+ concentration in the cell, so Na+ moves down the concentration gradient, the movement can be used to bring something else in / pump something else out

Question 31

What is the paracellular pathway? Which molecule often uses this pathway?

What is transcellular reabsorption? How does water use this pathway? How does Na⁺ use this pathway?

Answer 31

Substances, other than travelling through cells, also go between cells, known as the paracellular pathway. Often water uses the path, and with it other solutes also use this pathway

Transcellular = through the cell. As the membrane is made of lipids, water requires a protein channel (e.g. aquaporin) to travel through cells. Most of the Na+ reabsorption requires Na⁺/K⁺ ATPase

Question 32

In which part of the nephron is most of the substances reabsorbed?

What substances are reabsorbed in the early PCT?

What substances are secreted in the early PCT?

Answer 32

PCT - around 70%, including water as it follows the substances via osmosis

Reabsorbed = 67% Na+, 67% Cl-, 90% HCO3-, 100% glucose, 67% water, 100% amino acids, 50% urea

Secreted = drugs, ammonia, bile salts, prostaglandins, vitamins (folate and ascorbate)

Question 33

How is the Na⁺ and HCO₃^- reabsorbed in the PCT?

How does angiotensin II regulate Na⁺ reabsorption?

Answer 33

Carbonic anhydrase is present in the tutbular fluid - it converts H⁺ and HCO₃^- into H₂O and CO₂ so CO₂ can diffuse into the cells

Inside the cells carbonic anhydrase then coverts back the H₂O and CO₂ into H⁺ and HCO₃^-. The H⁺ is then exhcanged for an Na⁺ molecule via an antiportar, so H⁺ exits the cell whilst Na⁺ enters it

To prevent HCO₃^- build up, it is goes through a symporter where both, Na⁺ and HCO₃^- leave the cell

Increasing the number of Na⁺/H⁺ antiportars

Question 34

How does angiotensin II work on the PCT when Na⁺ levels are low / blood volume is low?

How is glucose reabsorbed in the early PCT?

Answer 34

When there is low Na⁺ levels in the blood, or low blood volume, renin is secreted. This causes angiotensin II secretion, which increases the number of antiportars, so more Na⁺ is pumped out into the tubular fluid and water follows. This causes greater Na⁺ and water reuptake

Glucose reabsorption dependent on the Na⁺/glucose symporter. Na⁺ is pumped out of the other side of the cell into the blood, causing Na⁺ to move down the electrochemical gradient from the tutbular fluid into the cell, bringing with it a glucose molecule. As the glucose concentration in the cell is higher than in the blood, it diffuses through a glucose transporter (GLUT 2)

Question 35

What are the substances reabsorbed in the Loop of Henle?

How does salt and water reabsorption take place in the Loop of Henle?

How does the fluid change from the descending to the ascending limb?

Answer 35

Reabosorbed = 25% Na⁺, 25% Cl^-. 15% H₂O

Descending limb = impermeable to NaCl, permeable to water

Ascending limb = impermeable to water, permeable to NaCl

Put in A level notes on the counter current system

More salts are reabsorbed than water, so the fluid coming in down the descending limb is hyper-osmolar, and the fluid going out of the ascending limb is hypo-osmolar

Question 36

What are the different pumps involved in the Loop of Henle?

Answer 36

In the thick ascending limb there is Na+ and Cl- reabsorption

Na⁺/K⁺ ATPase pump exchanges one Na+ out of the cell for one K⁺ entering the cell

There is also an Na⁺/K⁺/Cl^- symporter, than brings in those salts into the cell

The K⁺ and Cl^- go travel through a symporter into the blood, down the electrochemical gradient

There is also a paracellular pathway for the cations

Question 37

What substances are reabsorbed in the early DCT?

What are the different transporters / pumps involved in the early DCT?

Answer 37

Reabsorbed = Na⁺, Cl^-, Ca²⁺

For Na⁺ and Cl^- reabsorption, the Na⁺/K⁺ ATPase pump pushes out one Na⁺ into the blood and brings in one K⁺ into the cell. Na⁺ and Cl^- enter the cell from the tubular fluid via the Na⁺/Cl^- symporter. The K⁺/Cl^- symporter then pushes out a K⁺ and Cl^- into the blood

Active calcium reabsorption occurs using the Na⁺/K⁺ ATPase pump that pushes out an Na⁺ into the blood and brings in one K⁺ into the cell. Na⁺ and Ca²⁺ enter the cell from the tubular fluid via the Na⁺/Ca²⁺symporter. Then via an Na⁺/Ca²⁺antiporter, the Ca²⁺ exits the cell into the blood, and an Na⁺ enters

Question 38

The distal DCT and CD contain which 2 types of cells? What are their functions?

Answer 38

Principal cell = Na⁺ reabsorption, H₂O reabsorption, K⁺ secretion

Intercalacted cell = maintains acid base balance

Question 39

What substances are reabsorbed in the distal DCT and CD? What substances are secreted in the DCT and CD?

What are the different transporters / pumps involved?

Answer 39

Reabsorbed = Na⁺, water, HCO₃^-, and H⁺; Secreted = HCO₃^-, and H⁺

Na⁺ and water are reabsorbed as the Na⁺/K⁺ ATPase pump pushes out one Na⁺ into the blood and brings in one K⁺ into the cell. This lowers the concentration of Na⁺ in cell, so Na⁺ moves into the cell from the tutbular fluid, and K⁺ moves into the tubular fluid as there is a higher concetraion of K⁺ inside the cell than in the tutbular fluid. Water follows behind the Na⁺, travelling from the tubular fluids through aquaporins into the cell, and through more aquaporins into the blood

Question 40

What is the primary hormone that regulates Na⁺ reabsorption and what is the primary hormone that regulates H₂O reabsorption? And how?

Answer 40

Aldosterone - regulates Na⁺ reabsorption by changing the number of Na⁺ channels and Na⁺/K⁺ ATPase pumps

ADH - regulates water by altering the number of aquaporins

Question 41

How is the acid base balance maintained in the DCT?

What is the difference between an alpha-intercalated cell and a beta-intercalated cell?

Answer 41

Using a proton (H⁺) pump and a Cl^- antiporter

Alpha = HCO₃^- reabsorption, H⁺ secretion

Beta = HCO₃^- secretion, H⁺ reabsorption