Urine dilution and concentration

Question 1

Extracellular fluid role

Answer 1

-extracellular fluid is composed of water and solutes and changes in the water component will alter the concentration of solutes in the extracellular fluid.
-Kidney regulates the water component by changing the levels of excretion

Question 2

How do kidneys regulate water excretion?

Answer 2

Kidneys are capable of producing urine with a wide range of osmolarity. So if water is in excess, kidney will excrete urine with low osmolarity (50 mOsm/L) or if water is low then urine excreted with high osmolarity (1400 mOsm/L)
**kidneys can do this by changing the volume of urine without changing the rate of solute excretion

Question 3

Regulation of water excretion

Answer 3

-Antidiuretic hormone
-increase osmolarity=increased ADH (increases permeability of distal and collecting tubules and ducts) =decreased water excretion because more water reabsorbed
-decrease osmolarity=decrease ADH=increase water excretion

Question 4

Diluting urine

Answer 4

-done by reabsorption of more solutes and less water in renal tubules

Question 5

Diluting urine in the proximal tubule

Answer 5

-equal proportion of solutes and water are reabsorbed, so the tubular fluid stays isosmotic

Question 6

Diluting urine in descending limb of loop of henle

Answer 6

As the tubule goes into the medulla, more water is reabsorbed because the medulla is hyperosmotic (more osmolarity than the tubular fluid)
>Since more reabsorption of water, the osmolarity of the tubular fluid increases

Question 7

Diluting urine in the ascending limp of loop of henle

Answer 7

-Na, Cl, K are highly reabsorbed but this section is not permeable to water so the tubular fluid is diluted

Question 8

Diluting urine in the distal and collecting tubules

Answer 8

More solutes are reabsorbed
-under low ADH concentration, these sections stay impermeable to water therefore the tubular fluid is diluted even greater

Question 9

Concentrating urine

Answer 9

We are constantly losing water through respiration, evaporation, perspiration and in urine and feces. But kidneys will reduce water excretion if the intake of water is limited

Question 10

Human body ability to concentrate urine vs. animals in desert

Answer 10

Humans: ~1400 mOsm/L (4-5x osmolarity of plasma)

Desert: 10000 mOsm/L

Question 11

What factors affect urine concentration?

Answer 11

1.ADH- high ADH increases permeability of distal tubule and collecting ducts to water

2. Hyperosmolarity of the renal medullary interstitium (generates osmotic gradient for water reabsorption)

Question 12

Why is the renal medullary interstitium hyperosmotic?

Answer 12

-because of a countercurrent mechanism related to the arrangement of loop of Henle and the vasa recta. It allows solutes to be trapped in the medullary interstitium

Question 13

Normal osmolarity in most tissues and plasma

Answer 13

300 mOsm/L

Question 14

Osmolarity of renal medulla interstitium

Answer 14

-can increase to 1400 mOsm/L

Question 15

How does the renal medulla maintain a high concentration of solutes?

Answer 15

1.Thick segment of loop of Henle generates 200mOsm/L gradient between tubule and medulla by actively pumping Na and the co-transport of other ions (Cl, K) into the interstitial fluid. Impermeable to water here so no reabsorption.

2. Descending limb of loop of Henle is permeeable to water, so as the osmolarity increases in the interstitium, more water is pulled out into the interstitium. So osmolarity in the descending limb increases and this higher osmolarity fluid will then reach thick ascending segment. Pumps in ascending limb continue to pump out and maintain 200 mOsm/L
3. Cycle will continue to increase and repeat until osmolarity of the interstitium increases to 1200-1400 mOsm/L

Question 16

Factors helping to maintain hyperosmotic medullary interstitium

Answer 16

1.Transport of Na, Cl, K from the thick segment of the loop of Henle into medullary interstitium

2. Transport of ions from collecting ducts into the medullary interstitium
3. Diffusion of urea from collecting ducts into the medullary interstitium
4. Reduced transfer of water from collecting ducts into the medullary interstitium

Question 17

Distal tubules role in concentrating urine?

Answer 17

This section is actively reabsorbing NaCl and is impermeable to water so will result in diluting urine

When ADH is present, this section becomes permeable to water. Water will be reabsorbed into the tubular fluids

Question 18

Collecting tubules role in concentrating urine

Answer 18

Some water is reabsorbed from the collecting duct. Increases when more ADH present. The osmolarity of the tubular fluid can become similar to the medullary interstitium (1200 mOsm/L)