urinary system

1. List functions of kidney?

Answer:

• Removal of toxins, metabolic wastes, and excess ions from the blood

• Regulation of:
  

  • Blood volume

  • Chemical composition

  • Blood pH

• Gluconeogenesis during prolonged fasting

• Endocrine functions:
  

  • Renin: regulates blood pressure and kidney function

  • Erythropoietin: stimulates red blood cell production

• Activates Vitamin D (converts calcidiol to calcitriol)

2. Where is kidney present? What can happen to urinary system when peri-renal fat is depleted? What is hilum, renal cortex, renal medulla? What are their roles?

Answer:

• Kidney Location: Retroperitoneal, in the superior lumbar region; the right kidney is lower than the left.

• Perirenal fat depletion: Removes a protective cushion, increasing the risk of kidney injury or displacement.

• Renal Hilum: Medial indentation where ureters, blood vessels, lymphatics, and nerves enter/exit.

• Renal Cortex: Granular superficial region where filtration begins.

• Renal Medulla: Contains renal pyramids separated by columns; involved in concentrating urine .

3. How is renal artery branched ultimately to form afferent arteriole? What is the difference between afferent and efferent arteriole? Why is this important?

Answer:

• Blood flow sequence: Aorta → Renal artery → Segmental artery → Interlobar artery → Arcuate artery → Cortical radiate artery → Afferent arteriole → Glomerulus → Efferent arteriole.

• Afferent vs. Efferent:
  

  • Afferent arteriole brings blood into the glomerulus.

  • Efferent arteriole takes blood away from the glomerulus.

• Importance: Afferent arterioles are wider, maintaining high pressure for filtration; resistance in efferent arterioles reinforces this pressure, aiding in filtration and limiting pressure in peritubular capillaries .

a. How does constriction of afferent arteriole and efferent arteriole regulate filtration and reabsorption?

Answer:

• Afferent constriction: Decreases GFR (glomerular filtration rate), reducing filtration.

• Efferent constriction: Increases glomerular pressure, enhancing filtration but reducing flow into peritubular capillaries, favoring reabsorption .

b. Differentiate between functions of glomerular capillary and peritubular capillary?

Answer:

• Glomerular Capillaries: Specialized for filtration due to high pressure.

• Peritubular Capillaries: Low-pressure vessels ideal for absorption and secretion, surrounding the renal tubule .

c. What is vasa recta?

Answer:

• Long, straight capillaries around juxtamedullary nephrons’ loops of Henle.

• Help form concentrated urine and maintain the osmotic gradient in the medulla .

4. Draw and label parts of nephron. Identify cortex and medulla. Which parts are located where?

Answer:

• Parts of the nephron:
  

  • Renal corpuscle (Glomerulus + Glomerular/Bowman’s capsule)

  • Proximal convoluted tubule (PCT)

  • Loop of Henle (Descending & Ascending limbs)

  • Distal convoluted tubule (DCT)

  • Collecting duct

• Located in the cortex:
  

  • Renal corpuscle (glomerulus + Bowman’s capsule)

  • Proximal convoluted tubule

  • Distal convoluted tubule

• Located in the medulla:
  

  • Loop of Henle (mainly the descending and ascending limbs)

  • Collecting ducts

Note: Cortical nephrons stay mostly in the cortex; juxtamedullary nephrons extend deep into the medulla .

6. Differentiate between cortical and juxtamedullary nephrons. What is vasa recta? What is its role?

• Vasa recta: Long, straight capillaries around the loop of Henle in juxtamedullary nephrons; maintain the medullary osmotic gradient and enable concentrated urine production .

6. What are the 3 major steps in urine formation? Describe what is happening in each.

Answer:

The three major processes in urine formation are:

1. Glomerular Filtration
   

   • A passive process driven by hydrostatic pressure.

   • Water and solutes are filtered from the glomerular capillaries into the glomerular capsule.

   • Large molecules (like proteins) do not pass through.

   • Produces a cell- and protein-free filtrate.

3. Tubular Reabsorption
   

   • A selective transepithelial process that reclaims valuable substances from the filtrate back into the blood.

   • All nutrients (e.g., glucose, amino acids), most water (99%), and ions (Na⁺, Cl⁻) are reabsorbed.

   • Can be active (e.g., Na⁺ transport) or passive (e.g., osmosis for water).

   • Occurs mainly in the PCT, but also in the Loop of Henle, DCT, and collecting duct.

5. Tubular Secretion
   

   • The reverse of reabsorption: substances are moved from the blood into the tubule for elimination.

   • Removes unwanted substances like H⁺, K⁺, NH₄⁺, creatinine, and drug residues.

   • Helps control blood pH and eliminate substances not filtered at the glomerulus.

8. What is juxtaglomerular apparatus (JGA)? What are its components and their functions?

Answer:

• Juxtaglomerular Apparatus (JGA): A structure where the distal portion of the ascending limb of the Loop of Henle contacts the afferent arteriole (and sometimes efferent).

• Function: Regulates filtrate formation and blood pressure.

Components and Functions:

1. Macula densa:
   

   • Tall, packed cells in the ascending limb.

   • Act as chemoreceptors to sense NaCl concentration in filtrate.

3. Granular cells (JG cells):
   

   • Enlarged smooth muscle cells in arteriole walls.

   • Act as mechanoreceptors to detect blood pressure.

   • Release renin when blood pressure is low.

5. Extraglomerular mesangial cells:
   

   • Located between arteriole and tubule.

   • Connected by gap junctions.

   • Possibly relay signals between macula densa and granular cells.

9. What is the filtration membrane? What are the 3 layers and their functions?

Answer:

The filtration membrane is the barrier between blood in glomerulus and filtrate in the capsule.

Three layers:

1. Fenestrated endothelium of glomerular capillaries:
   

   • Allows passage of plasma components but blocks blood cells.

3. Basement membrane:
   

   • Gel-like; negatively charged.

   • Blocks large proteins and repels negatively charged molecules.

5. Foot processes of podocytes (visceral layer):
   

   • Form filtration slits with diaphragms.

   • Prevent macromolecules from entering filtrate.

Function: Allows water and small solutes to pass; prevents plasma proteins and cells from entering urine.

10. How does filtration happen? What is net filtration pressure? How is it calculated?

Answer:

• Filtration happens due to high blood pressure in glomerulus.

• The pressure forces plasma out into Bowman’s capsule.

Net Filtration Pressure (NFP):

• NFP = HPg – (OPg + HPc)

Where:

• HPg: Glomerular hydrostatic pressure (55 mmHg) – pushes filtrate out

• OPg: Blood colloid osmotic pressure (30 mmHg) – pulls water back in

• HPc: Capsular hydrostatic pressure (15 mmHg) – pushes water back

Example:

NFP = 55 – (30 + 15) = 10 mmHg

This 10 mmHg drives filtration.

11. What is GFR? What affects it?

Answer:

• GFR (Glomerular Filtration Rate): Amount of filtrate made per minute by both kidneys.

• Normal: 120–125 mL/min

Factors that affect GFR:

1. Surface area available for filtration.

2. Permeability of the filtration membrane.

3. Net Filtration Pressure (NFP) — controlled by arteriole diameter.

GFR is regulated by:

• Intrinsic controls (autoregulation):
  

  • Myogenic mechanism: afferent arterioles constrict/dilate to keep GFR stable.

  • Tubuloglomerular feedback: macula densa senses NaCl, adjusts arteriole tone.

• Extrinsic controls:
  

  • Sympathetic nervous system: reduces GFR during stress/shock.

  • Renin-Angiotensin-Aldosterone System (RAAS): boosts BP, affects filtration.

12. How does reabsorption occur? What are transcellular and paracellular routes?

Answer:

• Reabsorption: Movement of substances from the filtrate back into the blood (mostly in PCT).

Routes:

1. Transcellular Route:
   

   • Through the tubule cell:
     

     • Across luminal membrane → cytosol → basolateral membrane → interstitial fluid → peritubular capillaries.

3. Paracellular Route:
   

   • Between tubule cells.

   • Limited to water and ions like Ca²⁺, Mg²⁺, K⁺, and Na⁺ in PCT where tight junctions are leaky.

Sodium (Na⁺) plays a key role:

• Na⁺ is actively pumped out of tubule cells (via Na⁺/K⁺ ATPase), allowing passive reabsorption of water and other solutes.

5. Differentiate between parts of nephron and functions of each part. Which parts help in reabsorption and which in secretion?

• Secretion mainly happens in the DCT and collecting duct

• Reabsorption mainly occurs in PCT, Loop of Henle, DCT, and collecting duct

Answer: