Counter Current Mechanism of Kidneys

Question 1

Describe how equilibrium in the thin descending limb of the nephron is created

Answer 1

• passively permeable to water and small solutes like Cl- and urea
• active reabsorption of these solutes increases concentration of solutes within interstitial space
• water and solutes move down their concentration gradients until concentrations within descending tubule and interstitial space have equilibriated

Question 2

Describe the effect the equilibrium of the descending limb and interstitial space has on tubular fluid

Answer 2

• becomes steadily for concentrated (hyperosmotic) as it travels down thin descending limb

Question 3

Describe the events that take place in the thin ascending limb of the nephron in the counter current mechanism

Answer 3

• passively permeable to small solutes
• impermeable to water so water cannot escape
• so solutes move out of tubular fluid and water is retained

Question 4

Describe the effect of the events of the thin ascending limb of the nephron on the tubular fluid

Answer 4

• tubular fluid becomes steadily more dilute (hyposmotic) as it moves up ascending limb of tubule

Question 5

Describe the events that take place at the thick ascending limb of the nephron in counter current mechanism

Answer 5

• active reabsorption of sodium, potassium and chloride
• impermeable to water
• more dilution

Question 6

Describe counter current multiplication and its 2 steps

Answer 6

• the process moves NaCl from tubular fluid into interstitial space deep within kidneys
• the process builds up an osmotic gradient in the interstitial fluid
• step 1, single effect: driven by active transport of NaCl out of tubular fluid in thick ascending limb into interstitial fluid, results in water moving passively down concentration gradient out of tubular fluid in descending limb until equilibrium
• step 2, fluid flow: as urine is continuously being produced, new tubular fluid enters descending limb pushing fluid at higher osmolarity down the tube and gradient is developed
• steps continue over and over until steady state
• the longer the loop, the greater the osmotic gradient

Question 7

Why is urea recycling important to the counter current mechanism?

Answer 7

it contributes to the osmotic gradient generated by the loops of Henle

Question 8

Describe urea recycling

Answer 8

• in inner medullary collecting ducts, ADH increases water and urea permeability
• this allows urea to flow passively down its concentration gradient into the interstitial fluid
• helps further drive water reabsorption