Genitourinary System

Question 1

What are the main parts of the kidneys?

Answer 1

Cortex, Major Calyx, Minor Calyx, Medulla, Ureter, Renal Vein, Renal Artery

Question 2

What are the functions of the kidneys?

Answer 2

1. Excretion of metabolic products, e.g., urea, uric acid, creatinine.
2. Excretion of foreign substances, e.g., drugs.
3. Homeostasis of body fluids, electrolytes & acid-base balance.
4. Regulates blood pressure
5. Secretes hormones, e.g., erythropoietin, renin.

Question 3

Describe pathway of blood flow in the kidneys

Answer 3

Renal artery -> Segmental artery -> Interlobar Artery -> Afferent arteriole -> Glomerular capillaries -> Efferent arteriole -> Peritubular capillaries -> Interlobular vein -> Arcuate vein -> Interlobar vein -> Renal vein

Question 4

What are the functions of the peritubular capillaries?

Answer 4

1. Provide oxygen and nutrients to nephron
2. Help in reabsorption of diff substances across nephrons and take them away into circulation
3. Help in the secretion of different substances into the tubular fluid.

Question 5

Describe anatomy of bladder and urethra

Answer 5

Detrusor muscle: Contracts to build pressure in the urinary bladder to support urination.
Trigone: Stretching of this triangular region to its limit signals the brain about the need for urination.
Internal sphincter: Involuntary control to prevent urination.
External sphincter: Voluntary control to prevent urination.
Bulbourethral gland: Produces thick lubricant which is added to watery semen to promote sperm survival.

Question 6

Describe the structure and features of the nephron

Answer 6

Glomerulus contained within Bowman’s capsule which leads onto proximal convoluted tubule. The epithelial cells in this area are rich in in mitochondria as lots of reabsorptive action. The thin descending loop of Henle and thin ascending section are both low in conc of mitochondria. Thick ascending is rich in mitochondria and so is the distal convoluted tubule. Principal cells in collecting duct are have a low conc of mitochondria but intercalated cells are rich in mitochondria.

Question 7

Describe the types of nephrons

Answer 7

Two types: superficial nephron and juxtamedullary nephrons. Superficial nephrons are ten times more abundant. These have their Bowman’s capsule in the outer cortex and their loop of Henle extends only up to the outer medulla, with only the thin section of the loop descending into the medulla. Juxtamedullary nephrons have their Bowman’s capsule close to the border of the outer medulla and their loop of Henle is much longer, extending deep into the inner medulla. The location of the Bowman’s capsule and convoluted tubules in the cortex gives it a granular appearance while the collecting ducts and loop of Henle in the medulla gives it a striated appearance.

Question 8

What are the constituents and functions of the juxtaglomerular apparatus?

Answer 8

Constituents: Macula densa found in the distal convoluted tubules, Extraglomerular mesangial cells and Juxtaglomerular cells found on the afferent arteriole. A function of the Macula Densa is GFR regulation through tubulo-glomerular feedback mechanism. Juxtglomerular cells are responsible for renin secretion for regulating blood pressure.

Question 9

What are the 4 renal processes and where do they occur?

Answer 9

Glomerular filtration occurs solely in the glomerulus. Reabsorption and secretion takes place over the entire length of the nephron. Excretion occurs from the collecting duct where filtrate forms urine. Different substances undergo a different combination of these processes in different amounts.

Question 10

What kind of process is glomerular filtration and whar is the filtration barrier?

Answer 10

It is a passive process as fluid is ‘driven’ through the semipermeable glomerular capillaries into the Bowman’s capsule space by the hydrostatic pressure of the heart. The filtration barrier is size and charge dependent as it is highly permeable to fluids and small solutes but impermeable to cells and proteins.

Question 11

Describe the filtration barrier in the glomerulus

Answer 11

The capillary endothelium is fenestrated and each fenestrae is 70nm in diametes, allowing water, ions and small proteins to pass. It is lined with the glomerular basement membrane which is lined by negatively charged proteins and most plasma proteins are negatively charged so prevents movement from the bloodstream. The Bowman’s space is lined by podocytes which have interlocking projections forming a slit diaphragm. It is thin & porous so only water & small solutes can pass.

Question 12

What pressures drive glomerular filtration?

Answer 12

As the blood flows in the capillaries, there is hydrostatic pressure exerted by the fluid in the blood which pushes fluid and solutes out while the solutes in the bloodstream and plasma proteins exert an oncotic pressure pulling fluid into the bloodstream from the surrounding tissue. The same is true for the fluid and solutes of the interstitial fluid. Oncotic pressure is what leads to osmosis – bigger proteins will create greater oncotic pressure.

Question 13

How is the net ultrafiltration pressure calculated and what variables are taken into account?

Answer 13

HPgc = Hydrostatic pressure in glomerular capillaries 
HPbw = Hydrostatic pressure in bowman’s capsule 
πgc = Oncotic pressure of plasma proteins in glomerular capillaries

Puf = HPgc – HPbw – πgc

Question 14

How does ultrafiltration occur in terms of participating pressures?

Answer 14

Fluid within the capillaries exerts a hydrostatic pressure by pushing fluid and solutes out of the bloodstream into the Bowman’s space, driven by pressure from the heart. However, the fluid in the Bowman’s space also exerts a hydrostatic pressure pushing fluid out of the space back into the bloodstream and this is supported by the oncotic pressure of the solutes within the bloodstream which also pull fluids back in. The oncotic pressure of solutes in the Bowman’s space is highly negligible as the filtered solutes are very small and hence oncotic pressure is also minimal.

Question 15

What is the glomerular filtration rate?

Answer 15

It is the amount of fluid filtered from the glomeruli into the Bowman’s capsule per unit time (mL/min) and sum of filtration rate of all functioning nephrons.

Question 16

How is glomerular filtration rate calculated?

Answer 16

GFR = Puf x Kf

Where Kf is an ultrafiltration coefficient (membrane permeability and surface area available for filtration).

Question 17

What is a healthy GFR and what affects this?

Answer 17

For healthy male & female adults, the GFR can be between 90-140 mL/min & 80-125 mL/min respectively.

Any changes in filtration forces or Kf will result in GFR imbalances. A fall in GFR is the cardinal feature of renal disease, with a build up of excretory products in the plasma.

Question 18

How is GFR regulated?

Answer 18

There are two mechanisms: The Myogenic mechanism and Tubulo-glomerular feedback mechanism.

Question 19

Describe the myogenic mechanism of GFR regulation

Answer 19

As arterial pressure increases, the afferent arteriole stretches, activating stretch receptors in the arteriole endothelium. This leads to a resultant smooth muscle contraction, decreasing the size of the vessel lumen and therefore limiting bloodflow into the glomerular capillaries as resistance is increased. Hence, GFR remains constant.

Question 20

What is the tubulo-glomerular feedback mechanism?

Answer 20

An increase in the GFR leads to increased NaCl in the loop of Henle and this change is detected by the Macula Densa cells of the distal convoluted tubule. It therefore increases the amount of ATP and adenosine discharged by to the afferent arteriole and this causes arteriole constriction. The increase in resistance to flow then controls arterial pressure in glomerular capillaries and stabilises GFR.

Question 21

What is renal clearance?

Answer 21

Renal clearance is the number of litres of plasma that are completely cleared of the substance per unit time. Renal clearance is only concerned with the excretory role played by the kidneys, i.e. rate of removal of a substance X from the blood and excretion through urine.

Question 22

What is the renal clearance equation?

Answer 22

𝐶=(𝑈 𝑥 𝑉)/𝑃

U = Concentration of substance in urine; V = Rate of urine production; P = Concentration of substance in plasma. Measured in mL/min.

Question 23

How can GFR be practically determined?

Answer 23

If a molecule is freely filtered and neither reabsorbed nor secreted in the nephron then the amount filtered equals amount excreted. Thus GFR can be measured by measuring renal clearance of this molecule. There are two such molecules: Inulin and Creatinine. However, inulin does not naturally occur in the body.

Question 24

What is the ideal molecule to measure GFR?

Answer 24

Ideal molecule is inulin which is a plant polysaccharide. It is freely filtered and neither reabsorbed nor secreted. Isn’t toxic and is measurable in urine and plasma. However not found in mammals so needs to be transfused.

Question 25

What is commonly used to measure GFR?

Answer 25

Creatinine is commonly used which is a waste product from creatine in muscle metabolism. Amount of creatinine released is fairly constant. If renal function is stable, creatinine amount in urine is stable. Low creatinine clearance or high plasma creatine may indicate renal failure.

Question 26

Why is creatinine not the ideal molecule?

Answer 26

It is freely filtered and not reabsorbed but a small amount is secreted into the nephron therefore its not a perfect molecule. However, the process for estimating creatinine in blood and urine can account for that to allow for GFR calculations.

Question 27

How is renal plasma flow determined?

Answer 27

If the total amount of a molecule entering the kidney equals amount excreted, then the renal clearance of this molecule equals the renal plasma flow (RPF).

Question 28

What molecule is used to determine renal plasma flow?

Answer 28

PAH (Para aminohippurate) is used as all the PAH is removed from the plasma passing through the kidney through filtration and secretion.

Question 29

What is filtration fraction, how is it calculated and what is a healthy range?

Answer 29

The ratio of the amount of plasma which is filtered, and which arrives via the afferent arteriole is defined by the Filtration fraction (FF). Its value ranges from 0.15-0.20 normally. Calculated as GFR/RPF.

Question 30

What are passive transport methods found in the renal tubules?

Answer 30

Diffusion, Osmosis and Electrical gradient difference. This is where charged ions can go to the side of the membrane which is oppositely charged.

Question 31

What are 2 methods of primary active transport?

Answer 31

These are methods which use ATP directly to move molecules across the membrane. The Na+-K+-ATPase pump uses one ATP molecules to move 3 Na+ outside the cell and 2 K+ into the cell. The second method is endocytosis such as rebasorption of small molecules in the PCT.

Question 32

What is secondary active/coupled transport?

Answer 32

This is where movement of one solute along its electrochemical gradient provides energy for the other solute to move against it. There are 2 methods of movement for this: Symport and Antiport. Symport is where the movement of one molecule down its concentration gradient allows for another to be moved along with it in the same direction. Antiport is where the two molecules are moved in opposite directions.

Question 33

What are transport pathways in renal tubules?

Answer 33

The paracellular pathway allows movement of molecules across the tight junction of cells. The transcellular pathway allows for absorption of molecules along apical membrane of cell and across the cell into the bloodstream through the basolateral membrane. Ions excluding sodium, water and urea are transported paracellularly. Water is also transported transcellularly through aquaporins and sodium is rebasorbed only transcellularly.

Question 34

How does transcellular sodium reabsorption work?

Answer 34

Step 1: Sodium is actively transported out of the epithelial cell from the basolateral membrane through the N+-K+-ATPase transporter, creating a low concentration of sodium within the epithelial cell.
Step 2: Sodium then diffuses from the lumen through the apical membrane into the epithelial cell down its concentration gradient.

Question 35

What is reabsorbed in the early proximal convoluted tubule?

Answer 35

67% Na+, 67% Cl-, 80% HCO3-, 100% Glucose, 67% Water, 100% amino acids, 50% urea is reabsorbed. Drugs, ammonia, bile salts, prostaglandins, vitamins (folate & ascorbate) are secreted.

Question 36

How does sodium and bicarbonate reabsorption work?

Answer 36

Na+-K+-ATPase channel actively transports sodium out creating a downhill concentration gradient. Carbon dioxide freely diffuses in from the tubular fluid into the epithelial cell where it reacts with water to form H+ ions and HCO3- ions. The H+ ions are transported out of the cell through an antiporter in the apical membrane which moves sodium into the cell down its concentration gradient, providing energy for H+ transport into the tubular fluid. A sodium bicarbonate symporter in the basolateral membrane moves both sodium ions and bicarbonate ions into the bloodstream. Angiotensin II regulates Na+ reabsorption by increasing Na+ - H+ antiporters.

Question 37

How is glucose reabsorbed?

Answer 37

The Na+-K+-ATPase pump creates a low conc of sodium in the epithelial cell. A sodium glucose symporter SGLT2 in the apical membrane moves glucose into the cell using energy from sodium’s movement down its concentration gradient. The GLUT2 transporter then allows the glucose through the basolateral membrane into the bloodstream.

Question 38

Describe what is reabsorbed in the different parts of the loop of Henle

Answer 38

In the thin descending limb, water undergoes passive reabsorption and so by the time the tubular fluid reaches the bottom of the loop of Henle it is hyperosmolar as no ionic reabsorption occurs in the thin descending limb. In the thin ascending limb, water is not reabsorbed but passive reabsorption of ions occurs. In the thick ascending limb, active transport of ions occurs and hence at the top of the loop of Henle, the fluid is hypo-osmolar.

Question 39

Describe ionic absorption in the thick ascending limb

Answer 39

Through the paracellular pathway, Na+, Ca2+, K+ and Mg2+ are absorbed. The Na+-K+-ATPase pump found in basolateral membrane and decreases sodium conc inside epithelial cell. Na+K+2Cl- move into cell through symporter in apical membrane down sodium’s concentration gradient. K+ and Cl- transporters in basolateral surface so they diffuse down their concentration gradient.

Question 40

Describe sodium and chloride reabsorption in the early distal convoluted tubule

Answer 40

Water is not reabsorbed here. Na+-K+-ATPase pump creates a conc grad while Na+, Cl- symporter in the apical membrane moves these ions in. Chloride channels in the basolateral membrane allow chloride ions into the bloodstream directly while a potassium, chloride symporter in the basolateral membrane transports the ions into the bloodstream.

Question 41

Describe active calcium reabsorption in the early distal convoluted tubule

Answer 41

The Na+-K+-ATPase pump works. Sodium-calcium antiporters in the basolateral membrane move sodium into the epithelial cell while moving calcium into the bloodstream. Calcium ATPase pumps also move calcium into the bloodstream. Therefore, a low concentration of calcium exists in the cell allowing calcium to passively move into cell from tubular fluid through ion channels in apical membrane.

Question 42

Describe purpose and action of the principal cell

Answer 42

Responsible for sodium reabsorption and potassium excretion. Na+-K+-ATPase pump creates a conc grad for sodium which diffuses into cell from tubular fluid through ion channels in apical membrane. Water is also reabsorbed through aquaporins found in both basolateral and apical membranes. Potassium ion channels exist in both basolateral and apical membrane allowing for concentration to be maintained and hyperkalemia prevented.

Question 43

What hormones act on the principal cell and what is their effect?

Answer 43

Aldosterone regulates Na+ reabsorption by increasing apical Na+ channels & basolateral Na+ - K+ - ATPase pumps.
Anti-Diuretic Hormone regulates water reabsorption by increasing apical aquaporins.

Question 44

Describe purpose and action of the intercalated cell

Answer 44

There are two types of intercalated cells: alpha and beta. Alpha is responsible for HCO3- reabsorption & H+ secretion. Beta is responsible for HCO3- secretion & H+ reabsorption.
[Alpha throws out acid while beta throws out bicarbonate]
Both have a chloride ion channel in the basolateral membrane. However, while an alpha cell has a H+ ATPase pump in the apical membrane, a beta cell has this in the basolateral membrane. Similarly a beta cell has a HCO3-/Cl- antiporter in the apical membrane while an alpha cell has this in the basolateral membrane.