The Urinary System

Question 1

Describe the three major functions of the urinary system

Answer 1

• Excretion
  
  • removes organic waste products
• Elimination
  
  • discharge of waste products
• Homeostatic regulation
  
  • blood volume/pressure/pH
  • plasma concentration of ions

Question 2

Describe the organs of the urinary system

Answer 2

• Kidney
  
  • Produces urine
• Ureter
  
  • Transports urine to urinary bladder
• Urinary bladder
  
  • Temporarily stores urine before urination
• Urethra
  
  • Drains urine from bladder
  • Conducts urine to exterior
  • Males: transports semen

Question 3

How is urine eliminated?

Answer 3

• Name: Micturition reflex
• Detrusor muscle: smooth muscle wall of bladder
  
  • Stretch of bladder causes spontaneous action potential and contraction
  • Innervated, controlled by parasympathetic nervous system
• Overactive bladder: drugs targetting muscarinic acetylcholine receptors

Question 4

Name and describe the major functions of each segment of the nephron and collecting system

Answer 4

Nephron

• Proximal convoluted tubule
  
  • Reabsorb water, ions, organic nutrients
  • abundant microvilli
• Distal convoluted tubule
  
  • Secretes ions/toxins
  • Reabsorbs water, sodium/calcium ions
  • fewer microvilli
• Renal corpuscle
  
  • Produces filtrate
• Nephron loop
  
  • Descending: reabsorbs water
  • Ascending: reabsorbs sodium and chloride ions

Collecting System

• Collecting duct
  
  • Reabsorbs water and ions
• Papillary duct
  
  • Delivers urine to minor calyx

Question 5

Describe the functions of glomerular capsule (Bowman’s capsule)

Answer 5

• Surrounds glomerulus
• Forms renal corpuscle with glomerulus
• For glomerular filtration
• Blood filtrate leads into PCT
• Cuboidal cells with many microvilli
  
  • Increases SA for reabsorption

Question 6

Describe the adaptations of proximal convoluted tubule

Answer 6

• Single layer of cuboidal cells with microvilli
• Increases SA for reabsorption

Question 7

Describe the structure of kidney

Answer 7

• Cortex
  
  • outermost layer of kidney
  • contains capillaries
• Medulla
  
  • inner part of kidney
  • consists of renal pyramids separated by renal columns

Question 8

Describe the blood vessels in the renal system

Answer 8

• Afferent arteriole
  
  • entry of blood to nephron for filtration
  • supplies glomeruli
  • pathway: renal corpuscle (glomerulus + Bowman’s capsule) → proximal convoluted tubule → Loop of Henle → distal convoluted tubule → collecting duct
• Efferent arteriole
  
  • exit of blood after filtration

Question 9

Compare between cortical nephron and juxtamedullary nephron

Answer 9

• Cortical nephron
  
  • longer
  • embedded in cortex
  • has a larger number
  • peritubular capillaries surrounds PCT and DCT
  • drains into interlobular veins
• Juxtamedullary nephron
  
  • embedded in medulla
  • vasa recta surrounds loop of Henle

Question 10

Describe glomerular filtration

Answer 10

• Glomerular capillaries and Bowman’s capsule forms a filter for blood
• Fenestrated : large pores between endothelial cells
  
  • Allows plasma molecules to pass
  • Highly permeable
• Pass thru narrow slit diaphragms between pedicles of podocytes
  
  • Lined with negative charges to repel negatively-charged proteins

Question 11

Describe the regulation of glomerular filtration rate (GFR)

Answer 11

• Controlled by extrinsic and intrinsic (autoregulation) factors
• Vasoconstriction: blood flow reduced → GFR reduced
• Vasodilation: blood flow increased → GFR increased

Question 12

Describe sodium ion reabsorption in the PCT

Answer 12

• Reabsorption: the transport of molecules out of tubular filtrate back into blood
  
  • Sodium ions from tubular fluid and glucose pass thru via channel into PCT cell
  • Sodium ions from tubular fluid and H+ ions pass thru counterexchange channel into PCT cell
  • Sodium ions from tubular fluid pass thru via channel into PCT cell
  • Sodium ions pumped out of peritubular fluid via sodium-potassium pump into capillary

Question 13

Describe chloride ion and water reabsorption in the PCT

Answer 13

• Chloride ion follows concentration gradient of sodium ions (passive)
• Osmotic water flow: solute conc. is high in cells → water follows

Question 14

Describe the countercurrent exchange in vasa recta

Answer 14

• Permeable to salt, water and urea
• Recirculates salt
• Reabsorbs water leaving the descending limb
  
  • Due to albumin in blood causing colloid osmotic pressure

Question 15

Describe the countercurrent multiplier system

Answer 15

• Allows descending and ascending limbs of Loop of Henle to build osmolality in medulla
• Thick ascending limb
  
  • Sodium ions and chloride ions pumped out
• Descending limb
  
  • Raised osmolality of filtrate
  • More water diffuses out of filtrate

Question 16

Describe how homeostasis of plasma concentration is maintained by the antidiuretic hormone

Answer 16

• ADH only affects the collecting duct
  1. Responds to dehydration → high plasma osmolality
  2. Detected by osmoreceptors in hypothalamus
  3. ADH secreted by posterior pituitary
  4. Stimulates insertion of aquaporins into plasma membrane of CD
  5. Water drawn out of CD → reabsorbed by vasa recta

Question 17

Describe the role of aldosterone in Na+/K+ balance

Answer 17

• 90% of filtered Na+ and K+ reabsorbed before DCT
• 10% variably reabsorbed in DCT and cortical collecing duct
• Aldosterone controls K+ secretion and Na+ reabsorption
  
  • K+ in urine is secreted, not filtrated